1.0 Introduction
The term laser is an acronym for light amplification by the stimulated emission of
radiation. The original concept for the laser was developed by Charles Townes in 1955,
who won a Nobel Prize in 1964 for his work on the theory of lasers. The first laser
was built by Theodore Maiman in 1960 using a synthetic ruby crystal as the lasing
medium.
Lasers produce light by a process that involves changes in energy states within the
atoms of certain materials. Atoms that have been promoted to higher energy states
release this energy in the form of light by a process called stimulated emission.
The laser light is amplified by reflecting it back and forth in the lasing medium
with a pair of mirrors. The laser light is then released in a stream or pulse through
a partially transmitting mirror at one end of the cavity.
Light emitted from a laser differs from normal light in several ways; it is extremely
intense, coherent, monochromatic, and highly collimated. Wavelengths are typically
released in the portion of the electromagnetic spectrum that extends through the ultraviolet,
visible, and infrared regions.
Prior to the laser it was not possible to generate light with these characteristics.
The unique properties of lasers have led to a number of applications in industry and
construction where they are used to align, weld, cut, drill, heat, and measure distances.
Lasers are also used in a variety of other fields including, medicine, communications,
energy production, and national defense. Recently lasers are finding increased use
in consumer products such as laser scanners, laser printers, and compact disk and
video players.
2.0 Theory of Operation
Laser light is produced by changes in the energy levels of electrons. Under normal
conditions electrons occupy the lowest energy state in an atom, known as the ground
state. Electrons, however, can move from one energy level to another by the absorption
or emission of energy.
Energy must be pumped into the system to excite the lasing material and promote electrons
to higher energy states. Although electrons can absorb energy from a variety of external
sources, two methods are most commonly used. The first occurs when the energy from
a photon of light is absorbed by an electron. The laser may be optically pumped in
this manner from sources such as flash lamps. The excess energy causes the electrons
to jump to a higher energy level, resulting in a metastable state. Electrons absorb
only those photons that contain the exact amount of energy needed to pump it to another
energy level.
In gas lasers, electrons are pumped to higher energy states by a voltage generator.
In this method, energy is supplied by collisions with electrons that have been accelerated
to a specific energy. In addition to optical and electrical energy, some lasers are
also pumped by chemical and nuclear energy.
Once in a higher energy state the atom can return to the ground state by releasing
excess energy as a photon. The wavelength of light released is approximately equal
to the energy difference between the excited and ground states. This release of a
photon is called spontaneous emission.
Light emitted from phosphorescent materials is an example of spontaneous emission
from a metastable state. These materials are excited to higher energy states by light
from the sun or a lamp. Photons are released when the electrons drop to a lower energy
level. Because the source of energy usually contains many wavelengths, the electrons
can be excited to several energy levels. The released photons will be out of phase
and composed of different wavelengths.
In 1917, Einstein theorized that a photon released from an excited atom could trigger
another excited atom to release an identical photon. These two photons could trigger
other atoms to release photons, resulting in an avalanche of photons. All of these
photons would be of the same frequency, energy, direction, and in phase with the original
triggering photon. This process is termed stimulated emission. The more atoms in an
excited state, the greater the probability of stimulated emission. A population inversion
occurs when the number of atoms in an excited state is greater than in the ground
state.
The key component in making the laser operate properly is the optical cavity. The
purpose of the optical cavity is to provide optimal amplification and stability for
the laser beam. Most of the stimulated photons strike the walls of the optical cavity
and are lost. However, those photons that are released in a direction parallel to
the optical cavity can interact with other atoms causing further stimulated emissions.
By placing mirrors at the end of the optical cavity, photons are reflected back and
forth into the lasing medium; dramatically increasing the number of photons. The mirrors
must be highly reflective and all scattering from other surfaces in the cavity must
be kept low. In a typical laser, one mirror is flat with a reflectance greater than
99.9%. The other mirror is curved with a reflectance of 99% and a transmission of
1%. The beam emerges from the partially transmitting curved mirror. The lasing action
will continue as long as energy is supplied to the lasing medium.
3.0 Lasing Medium
The lasing medium is a substance that can be stimulated to a metastable state by the
addition of energy. The substance must be transparent to the light it produces and
able to exist in a metastable state. The lasing medium may be solid, gaseous, dyes
suspended in a liquid, or atoms or molecules doped into a crystalline structure.
Solid State
This type of laser contains a solid lasing medium embedded with atoms of the lasing
material. Solid state lasers, because of a higher density of lasing atoms, can produce
more power output per unit volume than gas lasers. They are usually pumped optically.
Solid state lasers are usually simple to build and operate.
The first material used to make a laser was a synthetic ruby crystal. This crystal
is made from aluminum oxide and a small amount of chromium oxide. The chromium is
added as an impurity and acts as the active material in the laser. A xenon flashtube
surrounds the crystal. One end of the rod-shaped crystal is silvered and the other
end is partially silvered. A burst of light from the flashtube excites the chromium
atoms and raises them to an excited state. This is defined as a population inversion.
Some of the excited atoms release the excess energy in the form of light photons and
return to the ground state. The photons travel between the reflecting ends of the
rod and trigger other excited atoms through stimulated emission to release photons.
An avalanche of photons results. This stream of photons is discharged through the
partially mirrored end in a pulse of coherent light. The entire sequence takes only
a few milliseconds.
Another example of a solid state laser is the Neodymium:YAG (yttrium aluminum garnet)
laser. The neodymium is added as an impurity to the YAG crystal. The YAG laser is
the most common type of crystal laser in use today. Like the ruby crystal, the YAG
laser is pumped by a flash lamp. It can emit continuous beams or pulses. Continuous
power outputs range from a few milliwatts to greater than 100 watts. Pulsed energies
can result in power levels of 5,000 megawatts per pulse.
Neodymium can also be added to glass to make a neodymium-glass laser. This laser is
similar to the YAG laser but is less expensive and does not conduct heat as well.
Highly energetic pulses of light can be produced from the neodymium-glass laser. Peak
powers of approximately one trillion watts can be generated in a pulse lasting only
a nanosecond.
Gaseous
Many gases will lase and produce highly coherent light when an electric current is
passed through them. The most common type of gaseous laser is the helium-neon. This
laser is commonly used at construction sites, in teaching laboratories, and supermarkets.
The helium-neon laser is constructed of a long tube filled with helium and neon under
low pressure. A few milliamps of direct current applied at several kilovolts is used
to excite the helium atoms. The helium atoms in turn excite the neon atoms to a higher
energy state by means of electron collisions and create a population inversion. Neon
atoms fall to the ground state and release excess energy in the form of a light photon.
Photons travels back and forth between the reflecting end mirrors and stimulate other
neon atoms to release light photons. This results in a continuous stream of laser
light known as a continuous wave. A bright beam of visible red light emerges from
the exit mirror. The Helium-Neon laser can emit light continuously for thousands of
hours. However, they are extremely inefficient, converting only 0.1% of the input
energy to the laser beam.
Other examples of gaseous lasers include argon, krypton, and carbon dioxide lasers.
Argon and krypton both emit a wide range of wavelengths, mostly in the visible region.
The two gases can be mixed to generate most of the visible spectrum. Argon lasers
are commonly used in industry and research and krypton-argon lasers are typically
used in light shows. Helium-neon, argon, and krypton lasers all emit continuous beams.
The most powerful continuous wave laser is a carbon dioxide laser. It can continuously
emit powers from less than a watt to hundreds of kilowatts. Carbon dioxide lasers
can also produce extremely short pulses of even higher powers. The laser is pumped
by an electrical discharge and emits invisible far-infrared light.
Carbon dioxide lasers operate slightly differently from other lasers. Instead of raising
electrons to higher energy levels, the energy used to excite carbon dioxide atoms
causes the atoms to vibrate at a different energy level. The carbon dioxide laser
is widely used in industry to drill, cut, and weld materials because of its high power,
high efficiency (up to 30% compared to 1% for neodymium lasers) and its ease of removing
excess heat.
Excimer
The excimer laser is another example of a gaseous laser. An excimer is a molecule
that can exist only in an electronically excited state. The excimer state does not
exist in nature and exists for only a few nanoseconds in the laboratory. Electrons
deposit energy in the laser gas causing an inert gas (argon, krypton, or xenon) to
react with a halogen (chlorine, bromine, fluorine, or iodine) to form an excimer.
The excimer then breaks up into its constituent atoms and releases the excess energy
as light. These lasers are important because they can produce powerful pulses in the
ultraviolet region. Excimer lasers are used in photochemistry and for marking silicon
wafers used in electronic components.
Chemical Lasers
Gas lasers can also be energized strictly by chemical reactions that produce very
high powered continuous wave or pulsed lasers. All chemical lasers emit infrared radiation.
An example of this type of laser is the hydrogen fluoride laser. An atom of fluorine
and hydrogen combine to produce hydrogen fluoride and energy. The molecules exist
in a vibrationally excited state and the energy can be extracted in the form of a
laser beam. Other examples of chemical lasers include iodine, hydrogen chloride, hydrogen
bromide, deuterium fluoride, and carbon monoxide.
Dye Lasers
Many liquid organic dyes will lase if pumped with ultraviolet light. The dyes are
dissolved in a liquid such as alcohol to form a solution. The energy levels of the
dyes are spaced so close together that they form a continuum. This allows the dye
molecules to release a wide range of wavelengths, mostly in the visible spectrum.
The most important characteristic of a dye laser is its tunability. A single wavelength
in the dye's range can be selected for experimental purposes. Several dyes can be
mixed to create a laser that can be tuned across the entire visible range. Dye lasers
can also produce extremely short pulses on the order of a picosecond. They are usually
very expensive, complex and are used primarily for research purposes.
Semiconductor Lasers
Lasing action can, under certain circumstances, be initiated by passing an electric
current through a semiconductor. Semiconductor lasers, also known as diode lasers,
are similar to light-emitting diodes (LED). These lasers are characterized by small
size, small power, high efficiency, and long life. They are very simple, easy to use,
and inexpensive.
In an LED, light is emitted by spontaneous emission. A diode laser, however operates
predominantly through stimulated emissions. These devices are composed of tiny semiconductor
crystals in which the end facets have been cut to reflect light. The diode laser is
pumped by a high intensity electric current. A very small amount of light of a desired
frequency is produced which stimulates an excited electron to fall to a lower energy
state, giving off laser light.
Unlike other lasers, beam divergence is high, and output power is measured in microwatts.
Semiconductor lasers are made chiefly from gallium-arsenide or gallium aluminum arsenide.
These lasers are used in laser printers, video disks, audio disks, and fiber optics
communication systems.
4.0 Mode of Operation
Lasers can operate in one of the following three modes:
Continuous Wave (CW)
Continuous wave lasers produce a steady stream of photons. Energy is pumped into the
system at a rate that equals the light output. Beam characteristics are easily measured
because the laser has reached a steady state condition. While most CW lasers use a
gas medium, they can be constructed in a wide variety of lasing materials. An extensive
array of wavelengths may be produced. The He-Ne laser was the first CW laser. Power
levels can range from a few milliwatts for He-Ne lasers to several kilowatts for carbon
dioxide lasers.
The output power in a continuous wave laser beam is measured in watts. The power density
of a beam, also called irradiance or flux, is defined as watts per square centimeter.
It is calculated from the output power of the beam and the beam diameter. Power densities
can vary from a few watts to hundreds of watts per square centimeter.
Pulsed
These lasers release their energy in highly concentrated pulses of light. The pulse
can be created by chopping a small portion of a continuous wave beam mechanically
or electrically, or by pumping with short intense flashes of light. The energy is
concentrated in small bursts delivered in 0.1 to 10 milliseconds per pulse. Pulsed
lasers can damage biological tissues by mechanical blast interactions. Even low energies
in the ocular focus region (0.4 to 1.4 um) can produce retinal damage. Pulsed beams
can also be created by chemical means. Terms associated with pulsed beams are choppers,
Q-switched, and mode locked. An example of a laser operating in the pulsed mode is
the ruby laser.
The term used to evaluate a pulsed beam is output energy; measured in joules. The
joule is equal to the power in watts multiplied by the time in seconds. The energy
intensity or radiant exposure within a pulsed beam is expressed in joules per square
centimeter.
Q-Switching
Extremely high power levels can be obtained by using a technique known as "q-switching"
that momentarily stores excess energy. A shutter is placed in the optical path to
prevent laser emission until a very large population inversion has built up. When
the shutter is opened the electrons rapidly fall to the ground state releasing a tremendous
pulse of energy that lasts only a few nanoseconds. Powers in the megawatt range can
be produced by this technique. Q-switching is commonly used with ruby and neodymium
solid lasers.
5.0 Characteristics of Lasers
The light from a laser differs from conventional light in four basic ways: 1) intensity,
2) coherence, 3) collimation, and 4) monochromaticity.
Intensity
Laser light contains a high concentration of energy per unit area of the beam. Lasers
that emit only a few milliwatts of power can produce a highly intense beam 1-2 millimeters
in diameter that will not diverge over a very long distance. Although lasers produce
highly intense light, only a few types of lasers are truly powerful because intensity
is defined as power per unit area. By comparison, an ordinary light bulb is more powerful
than a typical laser, but the light is not collimated and consequently spreads out.
For example, the light irradiance from a 1 mW He-Ne laser can be 6 orders of magnitude
greater than that from a 100 W incandescent lamp at a distance of approximately 20
meters. Some lasers can produce thousands of watts continuously while others can produce
trillions of watts in a nanosecond pulse.
Coherency
Ordinary light is incoherent or out of phase; light waves start at different times
and move in all directions. Laser light, however, is coherent because it is the result
of stimulated emission. All the waves produced by the laser are lined up or in phase
with each other. The crests and troughs of each wave line up exactly and reinforce
each other. The new light wave starts out exactly in phase with the photon that stimulated
it. Holograms are based on this property of laser light.
Collimation
Because laser light is coherent it is highly collimated or directional. Laser beams
are narrow, travel in virtually parallel lines, and will not spread out or diverge
as light from most normal sources. Because of this small divergence the intensity
of laser light, unlike ordinary light, is fairly constant over long distances. This
property of lasers significantly increasing the hazard potential of the beam. The
beam from a typical inexpensive laser will spread only about one meter after traveling
one kilometer. Because laser beams are highly collimated they act as if they come
from a distant point source. The beam can be easily focused to a small point by a
simple lens, dramatically increasing the energy concentration of the beam. The property
of collimation allows laser light to accurately measure distances, aim rifles, and
track flying targets.
Reflections, however, reduce the collimation of the laser beam and result in beam
divergence. Laser beams are reflected to some extent from all surfaces. If the reflecting
surface is mirror-like, the reflection is termed specular. The reflection is called
diffuse if the reflecting surface is rough. The spreading is greater when the reflection
is from a rough or diffuse spreading surface. Beam intensity can be reduced by a factor
of hundreds to thousands. The roughness of the diffuse reflector is more important
than the color in reducing intensities.
Monochromaticity
Unlike ordinary light, which is composed of all the colors of the spectrum, laser
light is composed of only one color. All the light waves in the beam are composed
of the same wavelength. Each laser produces its own characteristic color of light.
Some lasers are tunable and can be adjusted to produce several different colors, but
they can emit only one color at a time. Because laser light is monochromatic it can
be used to selectively interact with materials, revealing information about the structure
of atoms and molecules. Laser light is approximately 10 million times more monochromatic
than conventional light sources.
6.0 Biological Effects
The eye is extremely vulnerable to injury if exposed to the beams from most types
of lasers. The type of injury depends upon the intensity of light, its wavelength,
and the tissue exposed. Damage results from either temperature or photochemical effects.
Acute exposure may result in corneal or retinal burns. Cataract formation or damage
to the retina may result from chronic exposure to laser light. Retinal damage is of
particular concern from exposure to wavelengths in the visible and near infrared region.
Most sources of incoherent light can be viewed safely because the light reaching the
eye is only a small portion of the total output and the energy is spread over the
entire retina. Laser radiation, however, is composed of coherent light. The beam can
pass through the pupil and focus on a very small spot on the retina, depositing all
its energy on this area. Only visible and near infrared radiation is focused on the
retina. Damage to the retina may result in limited or total blindness if the optic
nerve is injured. Injury may be irreversible and there may be no pain or discomfort
from the exposure.
In addition to heat damage, some very high-powered, short-pulsed lasers such as the
carbon dioxide and the Nd YAG mode-locked laser can cause shock waves that can mechanically
disrupt the retina and cause the eye to hemorrhage.
Damage to the skin is also possible from exposure to laser beams. Acute exposure may
cause injuries ranging from mild reddening to blistering and charring. Skin cancers
may result from chronic exposure to ultraviolet light. The extent and type of damage
depends on the amount of energy deposited and the wavelength of the light. Unlike
injury to the eye, acute damage to the skin is usually repairable.
Ultraviolet radiation (200-400 nanometers)
Exposure to the eye from ultraviolet light in the 200 nm to 315 nm range is absorbed
by the cornea and may cause photokeratitis (corneal inflammation). Unlike the skin,
repeated exposure of the cornea to ultraviolet light does not result in a protective
mechanism. Near ultraviolet light between 315 nm and 400 nm is absorbed largely in
the lens and may cause cataracts. Wavelengths less than 400 nm do not pose a hazard
to the retina.
Exposure to the skin from lasers that emit in the UV region may cause a photochemical
reaction resulting in reddening, aging, and possibly skin cancer. Examples of lasers
operating in the ultraviolet region include the neodymium:YAG-Quadrupled (QSW & CW),
and the Ruby (doubled) laser.
Visible and Near-Infrared Radiation (400 to 1400 nanometers)
Exposure to laser beams in the visible (400 nm - 700 nm) and near-infrared (700 nm
- 1400 nm) regions of the spectrum may damage the retina. Laser beams in this region
are readily transmitted by the eye and focused by the lens to produce an intense concentration
of light energy on the retina. The incident exposure on the cornea can be concentrated
by a factor of approximately 100,000 times at the retina due to this focusing effect.
This energy is converted to heat and may cause a retinal burn resulting in visual
loss or even blindness if the optic nerve is injured. Even low energy laser beams,
if concentrated by a factor of 100,000, can cause damage to the eye. For this reason
wavelengths in the 400 nm to 1400 nm range are termed the ocular hazard region.
Exposure to the skin from laser beams in the visible and infrared regions may cause
photosensitive reactions, skin burns and excessive dry skin. Lasers operating in the
visible region of the spectrum include the ruby, neodymium:YAG (doubled), helium-cadmium,
helium-neon, argon, and krypton. Lasers operating in the near-infrared region include
the neodymium:YAG, gallium arsenide, and helium-neon.
Middle and Far-Infrared Radiation (1,400 -10,000 nanometers)
Laser beams in the middle and far-infrared regions produce injury primarily to the
cornea and to a lesser extent the lens. Damage is usually from heating effects, although
pulsed lasers such as the carbon dioxide laser may cause injury from thermomechanical
effects. Virtually no light reaches the retina beyond 1400 nm. Middle-infrared radiation
between 1,400 nm and 3,000 nm may penetrate deep into the lens causing cataracts.
Far-infrared radiation in the 3000 nm to 10,000 nm range is absorbed by the cornea
and may cause corneal burns and loss of vision.
The major danger to the skin from lasers operating in this region is burn damage.
Lasers operating in this region include: hydrogen fluoride, carbon monoxide, carbon
dioxide, and hydrogen cyanide.
7.0 Laser Safety Standards
Laser safety standards are derived from government mandated regulations and voluntary
standards. Safety rules governing the manufacture of lasers are established by the
Federal Government. Laser products manufactured after August 2, 1976 must conform
to performance standards established by the Food and Drug Administration (FDA), (21
CFR 1040.10). The standard requires that lasers be properly classified and labeled
by the manufacturer. Thus, for most lasers, measurements or calculations to determine
the hazard classification are not necessary. In addition, the standard establishes
certain engineering requirements for each class and requires warning labels that state
maximum output power. Although the performance standard regulates the manufacture
of lasers it does not address the safe use of lasers.
OSHA Regulations (29 CFR 1926.54 and 102) specify generalized rules for the safe use
of lasers in the construction industry. These include user training, posting and labeling
requirements, laser safety goggles, and maximum exposure intensities. Requirements
for the use of laser goggles in general industry are specified in sections 1910.132-133.
The safe use of lasers in general industry (including research laboratories) is covered
under OSHAs General Duty Clause which states that employers must furnish employees
a place of employment that is free from recognized hazards that are likely to cause
death or serious injury.
Extensive recommendations for the safe use of lasers have been developed by the American National Standards Institute (ANSI) and adopted by the American Conference for Governmental Industrial Hygienists (ACGIH). Lasers are classified according to their relative hazards, and appropriate control measures for each class are specified. Compliance with these standards generally eliminates the need for measurement of laser radiation, quantitative analysis of hazard potential, or use of the Threshold Limit Value (TLV). Compliance with ANSI and OSHAs Construction Standard will usually satisfy the requirements of the OSHA General Duty Clause.
8.0 Laser Classification
Since August, 1976 manufacturer's have been required by Federal law to classify lasers.
If the class is not known, it can be determined by measurements and/or calculations.
Lasers are classified according to the ability of the primary or reflected beam to
injure the eye or skin. The appropriate class is determined from the wavelength, power
output, and duration of pulse (if pulsed). Classification is based on the maximum
accessible output power. There are four laser classes, with Class 1 representing the
least hazardous. All lasers, except Class 1, must be labeled with the appropriate
hazard classification.
Class 1
Class 1 laser devices cannot produce damaging radiation levels to the eye even if
viewed accidentally. Prolonged staring at the laser beam however, should be avoided
as a matter of good practice. This class has a power output less than 0.4 uW for CW
lasers operating in the visible range. A completely enclosed laser of a higher classification
is categorized as a Class 1 laser if emissions from the enclosure cannot exceed Class
1 limits. If the enclosure is removed, e.g. during repair, control measures for the
class of laser contained within are required.
Class 2
Class 2 lasers are incapable of causing eye injury within the duration of the blink,
or aversion response (0.25 sec). Although these lasers cannot cause eye injury under
normal circumstances, they can produce injury if viewed directly for extended periods
of time. Class 2 lasers only operate in the visible range (400 nm to 700 nm) and have
power outputs between 0.4 uW and 1 mW for CW lasers. The majority of Class 2 lasers
are helium-neon devices.
Class 3a
Class 3a lasers cannot damage the eye within the duration of the blink or aversion
response. However, injury is possible if the beam is viewed through binoculars or
similar optical devices, or by staring at the direct beam. Power outputs for CW lasers
operating in the visible range are between 1 mW and 5 mW.
Class 3b
Class 3b lasers can produce accidental injuries to the eye from viewing the direct
beam or a specularly reflected beam. Class 3b laser power outputs are between 5 mW
and 500 mW for CW lasers. Except for higher power Class 3b lasers, this class will
not produce a hazardous diffuse reflection unless viewed through an optical instrument.
Class 4
Class 4 lasers are the most hazardous lasers. Exposure to the primary beam, specular
reflections, and diffuse reflections are potentially hazardous to the skin and eyes.
In addition, class 4 lasers can ignite flammable targets, create hazardous airborne
contaminants and usually contain a potentially lethal high voltage supply. The power
output for CW lasers operating in all wavelength ranges is greater than 500 mW. All
pulsed lasers operating in the ocular focus region (400 nm to 1400 nm) should be considered
Class 4.
Unknown Class
Laser classification can be determined by measuring the output irradiance or radiant
exposure using instruments traceable to the National Bureau of Standards. These measurements
should only be performed by qualified personnel. The laser class can also be determined
from calculations. For CW lasers, the wavelength and average power output must be
known. Classification of pulsed lasers requires the following information: wavelength,
total energy per pulse (or peak power), pulse duration, pulse repetition frequency
(PRF), and emergent beam radiant exposure. In addition to the above information laser
source radiance and maximum viewing angle subtended by the laser must be known for
extended-source lasers, such as injection laser diodes. Detailed information on classifying
lasers may be found in the ANSI Guide for the Safe Use of Lasers.
9.0 Control Measures for Laser Radiation
The recommended procedure for using lasers safely is to identify the class involved
and then comply with the appropriate control measures. Control measures are designed
to reduce the possibility of exposure to the eye and skin from hazardous laser radiation.
Preventing ocular exposure from the primary beam and specular reflections is of primary
concern because serious injury to the unprotected eye is possible.
To control potential hazards, priority should be given to the use of engineering controls.
Although commercial laser products manufactured after August 2, 1976 incorporate many
engineering controls, the use of additional controls should be considered to reduce
the risks.The most effective engineering control is to totally enclose the laser and
all beam paths.
The potential for exposure to hazardous laser radiation is probably greatest in the
research laboratory where open and high power laser beams are frequently encountered.
Because researchers need flexibility to change optical arrangements and make adjustments
during experimental procedures, laser beams cannot always be totally enclosed. Generally
in research laboratories where engineering controls cannot be relied upon, administrative
controls, protective eyewear, and properly supervised laser controlled areas must
be rigorously enforced to reduce hazards.
Because OSHA standards are generally non-specific, users are encouraged to adopt the
following requirements and recommendations for the safe use of lasers. These practices
are based on the FDA Performance Standard, OSHA Regulations, and the ANSI Hazard Classification
System. Engineering controls specified by the FDA Performance Standard apply only
to manufacturers of laser equipment but should be incorporated by users when practical.
Protective Housing (All Classes-FDA)- A protective housing shall be provided by the manufacturer to prevent access to
laser radiation which exceeds the intended classification. If the protective housing
is removed, as in research laboratories, control measures shall be based on the maximum
accessible radiation.
Safety Interlock (All Classes-FDA)- Any portion of the protective housing designed to be removed without tools shall
be interlocked to prevent access to radiation in excess of the applicable class.
Optical Viewing Devices and Windows (All Classes)- Viewing optics (including lenses, telescopes, microscopes, and viewing windows)
shall not be used to view the direct beam or specular reflections unless an appropriate
filter is used or adequate laser protective eyewear is worn to reduce laser radiation
to safe levels.
Maximum Exposure (All Classes)- Workers shall not be exposed to light intensities above:
Directing beam at people (All Classes)- The laser beam shall not be directed at people or into potentially occupied areas.
Experiments should be designed so that the laser beam is not at eye level.
Registration (Classes 3a, 3b, 4)- These lasers shall be registered with the Radiation Safety Office.
Reduction of Intensities (Class 2, 3a, 3b, 4)- If full output power is not needed, the laser beam intensity should be reduced to
a less hazardous level by using absorbing filters or beam shutters.
Alignment (Class 2, 3a, 3b, 4)- Alignment of laser optical components (mirrors, lenses, beam deflectors, etc.) shall
be done in a manner that ensures the eyes are not exposed to hazardous levels of radiation.
Firm Laser Mount (Class 2, 3a, 3b, 4)- The laser should be mounted on a firm support to ensure that the beam travels along
its intended path.
Unattended Operation (Class 2, 3a, 3b, 4)- Operating lasers should not be left unattended for appreciable lengths of time.
Lasers should be turned off, or beam shutters or caps should be used when laser transmission
is not required.
Emission Indicator (Class 2, 3a, 3b, 4-FDA)- Laser systems shall incorporate an emission indicator that provides a visible or
audible signal during radiation emission.
Safety Eyewear (Class 2, 3a, 3b, 4)- Laser eyewear designed to protect for the specific wavelengths of the laser shall
be worn when exposure to hazardous levels of laser radiation are possible. Laser safety
eyewear shall be worn in areas where unenclosed Class 3b or 4 lasers are operated.
The optical density of the eyewear shall reduce the laser radiation to the eye to
safe levels. Safety eyewear shall be labeled with the optical density of the lens
and the wavelength that it protects against.
Beam Stop or Attenuator (Class 3a, 3b, 4)- Potentially hazardous beams should be terminated by a permanently attached beam
stop or attenuator. The beam stop should be non-reflecting and fire-retardant.
Training (Class 3a, 3b, 4)- Personnel shall receive training in basic laser safety procedures. Training should
include the potential hazards associated with the use of the laser and control measures
necessary to minimize exposure to laser radiation. Training should be an on-going
program, both for new employees and experienced personnel, and when new equipment
is used. Written standard operating procedures, and a training manual should be presented
to each individual assigned to work with a Class 3 or 4 laser. Documentation of training
should be readily available.
Medical Surveillance (Class 3b, 4)- Personnel operating Class 3b and 4 lasers should receive appropriate eye and skin
examinations prior to laser use. Examinations should be repeated every three years.
Remote Interlock Connector (Class 3b, 4-FDA)- A remote interlock connector shall be provided to allow electrical connections to
an emergency disconnect interlock or a door interlock.
Spectators (Class 3b, 4)- Spectators should not be allowed in a laser controlled area containing an unenclosed
Class 3b or 4 laser unless permission has been obtained from the supervisor, potential
hazards have been explained, and appropriate protective measures taken.
Beam shutters (Class 3b, 4)- Lasers should be equipped with a beam shutter that covers the aperture. The shutter
should completely stop the laser beam. A black, non-reflective material is desirable.
Warning System (Class 3b, 4)- A visible or audible warning device that activates prior to laser emission should
be provided. The delay time should be sufficient to allow personnel to avoid exposure
to the beam.
Key-Switch (Class 3b, 4-FDA)- Class 3b and 4 lasers shall be provided with a keyed switching device. The key shall
be removable and the laser shall be inoperable once the key has been removed.
Enclosed Beam Path (Class 3b, 4)- The entire beam path, including the target area, should be enclosed if possible.
Safety interlocks should be used with the enclosure. If the beam is enclosed the laser
could revert to a less hazardous laser classification.
Laser Controlled Area (Class 3b)- Unenclosed Class 3b laser systems should be operated in a controlled area
10.0 Labels
Warning signs and labels shall be in accordance with the FDA Performance Standard
and ANSI Standards. Signs and labels shall be conspicuously displayed on equipment
and on access doors where applicable. Additional precautionary instructions, such
as eye protection required, should be included on the sign or label.
Class 1: Warning labels and signs are not required for Class 1 lasers. Enclosed Class I
lasers containing more hazardous laser radiation within shall have a warning label
located on the access panel.
Class 2: The label must include the laser hazard symbol and the words "CAUTION- Laser Radiation-
Do Not Stare into Beam", the class, and type of laser. Warning signs on doors are
not required.
Class 3a: Signs and labels shall include the laser hazard symbol and bear the words "CAUTION-
Laser Radiation- Do Not Stare into Beam or View Directly with Optical Instruments
", the class, and type of laser. Entrances to laser areas should be posted with a
warning sign.
Class 3b: Signs and labels shall include the laser hazard symbol and bear the words "DANGER-
Laser Radiation- Avoid Direct Exposure to Beam", the class, and type of laser. Doors
leading to the laser area shall be posted with warning signs.
Class 4: Appropriate warning signs must be posted on equipment and doors leading to the
facility. Signs and labels shall include the laser hazard symbol and bear the words
"Danger- Laser Radiation- Avoid Eye or Skin Exposure to Direct or Scattered Radiation",
the class, and type of laser. Additional precautions or protective actions should
be provided as needed.
11.0 Invisible Radiation
Because infrared and ultraviolet radiation are invisible, special precautions must
be used when working with lasers that emit radiations in these regions. Labels and
warning signs should specify that lasers produce invisible radiation. In addition
to the above requirements, the following recommendations should be used when working
with infrared or ultraviolet lasers:
Infrared: For Class 3 & 4 lasers, the beam path should be terminated with a highly
absorbent, non-reflecting backstop. Caution should be used because metal surfaces
that appear dull can cause specular reflections of infrared radiations. In addition,
for Class 4 lasers, the beam path should be terminated by a fire resistant backstop
such as firebrick. The backstop should be inspected periodically for degradation.
The beam and target area should be shielded with infrared absorbing materials, such
as lucite or plexiglass, to minimize reflections from Class 3b or Class 4 lasers.
Ultraviolet: Exposure to ultraviolet radiation should be minimized by using materials
that attenuate the radiation to safe levels. Special attention should be given to
the possibility of producing undesirable reactions in the presence of ultraviolet
radiation, e.g. ozone.
12.0 Associated Hazards
In addition to the hazards of the laser beam, protection is also necessary from other
hazards associated with the operation of the laser. These hazards include explosions,
electrical shocks, cryogenic liquids, flammable liquids, noise, x-rays, UV radiation,
and airborne contaminants.
Explosion Hazards
Lasers and ancillary equipment may present explosion hazards. High pressure arc lamps
and filament lamps used to excite the lasing medium should be enclosed in housings
that can withstand an explosion if the lamp disintegrates. In addition, the laser
target and elements of the optical train may shatter during laser operation and should
be enclosed in a suitable protective housing. Capacitors may explode if subjected
to voltages higher than their rating and should be adequately contained. High energy
capacitors should be enclosed in one-eighth inch thick steel cabinets.
Electrical Hazards
Lethal electrical hazards may be present, especially around high power laser systems.
Continuous wave lasers use direct current or radiofrequency power supplies and pulsed
lasers employ large capacitor banks for electrical storage. Lasers and associated
electrical equipment should be designed, constructed, installed and maintained in
accordance with the American National Standard Electrical Code. Electrical circuits
greater than 42.5 volts are considered hazardous unless limited to less than 0.5 mA.
To reduce electrical hazards high voltage sources and terminals should be enclosed.
Power should be turned off and all high-voltage points grounded before working on
power supplies. Operators should not stand on metal floors, or in water while working
with live electronic equipment. Accessible, non-current carrying metallic parts of
laser equipment must be grounded. Electrical circuits should be evaluated with respect
to fire hazards.
Cryogenics
Cryogenic liquids (especially liquid nitrogen) may be used to cool the laser crystal
and associated receiving and transmitting equipment. These liquified gases are capable
of producing skin burns and may replace the oxygen in small unventilated rooms. The
storage and handling of cryogenic liquids should be performed in a safe manner. Insulated
handling gloves of quick removal type should be worn. Clothing should have no pockets
or cuffs to catch spilled cryogenics. Suitable eye protection should be worn. If a
spill occurs on the skin, the area should be flooded with large quantities of water.
Adequate ventilation should be present in areas where cryogenic liquids are used.
Flammable Hazards
Flammable solvents, gases, and combustible materials may be ignited by a Class 4 laser
beam. Laser beams should be terminated by a non-combustible material such as a brick.
Combustible solvents or materials should be stored in proper containers, and shielded
from the laser beam or electrical sparks. Lasers and laser facilities should be constructed
and operated to eliminate or reduce any fire hazard. Unnecessary combustible materials
should be removed in order to minimize fire hazards. Laser laboratories should contain
an appropriate fire extinguisher.
Noise
Noise levels in laser laboratories can exceed safe limits because of high voltage
capacitor discharges. Hearing protection may be required.
X-Rays
X-ray production is possible when high voltages exceed 15 kV. Although most laser
systems use voltages less than 8 kV, some research models may operate above 20 kV.
Laser systems capable of producing ionizing radiation must be surveyed by the Safety
Office to ensure that x-ray levels are within legal limits.
Ultraviolet Radiation
Although laser radiation presents the chief hazard, it may not be the only optical
hazard. Laser discharge tubes and pumping tubes may emit hazardous levels of ultraviolet
radiation and should be suitably shielded. Particular care should be used with quartz
tubes. Most lasers now use heat-resistant glass discharge tubes which are opaque in
the UV-B (280 nm-315 nm) and UV-C (100 nm-280 nm) spectrum.
Chemical Hazards
Vaporized target materials, toxic gases, vapors and fumes may be present in a laser
area. Ozone is produced around flash lamps and concentrations of ozone can build up
with high repetition rate lasers. Asbestos fibers may be released from the firebrick
used as backstops for carbon dioxide lasers.
The increasing use of chemical lasers may introduce chemical hazards that are more
dangerous than the laser radiation. For example, fluorides used in fluoride lasers
are highly toxic and demand immediate emergency measures upon contact. He-Cd lasers
may contaminate the laboratory with toxic cadmium vapors if the exhaust gases are
not vented to the outside. The dyes that are the active medium of tunable lasers are
often very toxic and may cause acute or chronic skin problems. Some dyes may be carcinogenic.
Gloves, laboratory coats, and proper eye protection must be worn when handling hazardous
chemicals. An eyewash station and emergency shower shall be available in areas where
there is a possibility that hazardous chemicals may be splashed in the eyes or on
the skin. Food or drink should not be consumed in the laser lab if potential toxic
air contaminants or chemicals are present. Adequate general ventilation or local exhaust
must be maintained in laser installations to ensure that toxic air contaminants are
below acceptable limits.
13.0 Threshold Limit Value
The Threshold Limit Value (TLV) is the amount of laser radiation that nearly all individuals
may be exposed to without experiencing adverse effects. TLVs have been developed for
exposure to the eye and skin. TLVs for the eye are based on possible exposure to the
direct beam or from extended diffuse reflections. No person shall operate a laser
in a manner that exposes the unprotected eye or skin to laser radiation in excess
of the applicable TLV. If engineering controls are not suitable to reduce levels below
the TLV then laser safety eyewear must be worn.
If the laser classification is known it is generally unnecessary to perform calculations
or monitor the laser environment to determine if exposure of the eyes or skin is dangerous.
Appropriate control measures for the classification should be implemented. For example,
if a class 3b or 4 laser is used, the TLV for the eyes may be exceeded from viewing
the direct or specular beam and appropriate eye protection must be worn. Because only
Class 4 lasers can produce hazardous diffuse reflections, calculations for viewing
diffuse reflections from class 1, 2, and 3 systems are not necessary.
The TLV should be determined if the laser has not been classified by the manufacturer
and a quantitative evaluation of the hazard associated with a laser is needed. Monitoring
is often improperly performed and has a high degree of error. Calculations should
be used in most situations to determine if exposures are below the applicable TLV.
The ANSI Guide for the Safe Use of Lasers should be consulted to calculate TLVs and
the latest edition of the ACGIH Threshold Limit Values should be consulted for current
TLVs (available from the Safety Office). Irradiance and radiant exposure are used
to describe exposure limits from direct laser sources. Exposure limits from extended
sources are described in radiance and time-integrated radiance.
14.0 Laser Safety Eyewear
The energy emitted from lasers is highly concentrated and can cause permanent eye
injury. Although engineering controls are preferred to reduce hazards from the laser
beam, it may be necessary to use laser safety eyewear when engineering controls are
inadequate. The eyewear should be matched to the wavelength emitted and provide proper
attenuation for the laser intensity. Laser safety eyewear must be clearly marked with
the optical density of the lens and wavelengths for which protection is provided.
The following factors should be considered when purchasing laser protective eyewear:
Wavelength: The wavelength of the laser output must be known. If the laser emits more than one
wavelength, each wavelength must be considered.
Optical Density: The attenuation of laser light by protective goggles is given by its optical density
(OD). The OD must be sufficient to reduce the laser light to safe levels while transmitting
sufficient ambient light for safe visibility. The OD is measured on a logarithmic
scale, thus a filter that attenuates a beam by a factor of 1,000 has an OD of 3 and
a filter with an OD of 6 attenuates the beam by a factor of 1,000,000. The required
OD is determined from the maximum intensity that an individual could be exposed.
Laser Beam Intensity: The maximum irradiance in watts/square centimeter for CW lasers and the maximum
radiant exposure in joules/square centimeter for pulsed lasers must be known.
Luminous Transmittance: When considering laser safety goggles the visible or luminous transmission must
be considered along with the optical density. Luminous transmission is given in percent
transmission of visible light. Laser goggles with a lower luminous transmittance than
required may result in eye fatigue and accidents. However, proper optical density
should not be sacrificed for increased luminous transmission.
Damage Threshold: The resistance of the lenses to damage from the laser beam must be considered. The
damage can take the form of bubbling, melting, or shattering. The lens must be capable
of absorbing the amount of energy under the most severe operating conditions without
suffering changes in the light transmission characteristics. Laser goggles should
be inspected periodically for pitting, cracking, discoloration and deterioration in
the mountings.
Comfort: Laser safety eyewear should be comfortable and provide a good fit. Spectacle type
frames generally are more comfortable than goggles but do not fit as tight and may
allow unattenuated laser radiation to reach the eyes. However, the increased comfort
of spectacle type frames may increase user acceptance.
Lenses: Lenses in laser safety eyewear can be made of reflective glass or plastic, absorptive glass filters, or absorptive polymeric filters. Reflective lenses can create potentially hazardous specular reflections. These lenses are also heavy and uncomfortable and can scratch easily, allowing potentially hazardous radiation to reach the eye. Absorptive filters are not affected by surface scratches nor do they create potentially hazardous reflections. Polymeric filters, such as polycarbonate, offer several advantages over glass absorptive filters. They are lighter, have better impact resistance, and can withstand higher energy densities. Lenses should be mounted in goggle type frames to ensure maximum protection. No lens material is useful for all wavelengths and for all radiant exposures.
15.0 Medical Surveillance
Personnel who routinely use Class 3b or Class 4 lasers should be enrolled in the Medical
Surveillance Program for examination of the eyes and skin. The examination should
include a medical history, with emphasis on the eyes and skin. Current and past medication
usage should be reviewed, particularly the use of photosensitizing drugs. Visual acuity
should be determined and the structures of the eye that could be injured from the
wavelengths emitted should be examined. For example, workers using laser systems operating
in the wavelength range of 400 to 1400 nm should have retinal examinations. Employees
working with ultraviolet lasers or who have a history of photosensitivity should have
skin examinations. Workers who only occasionally use Class 3b or 4 lasers should have
a visual acuity test.
Medical examinations should be performed prior to employment and following any suspected
laser injury. Eye examinations should be repeated every three years to detect injury
or subtle changes that may have occurred from chronic exposure.
NRC FORM 3
(8-1999)
UNITED STATES NUCLEAR REGULATORY COMMISSION
Washington, DC 20555-0001
NOTICE TO EMPLOYEES
STANDARDS FOR PROTECTION AGAINST RADIATION (PART 20); NOTICES,
INSTRUCTIONS AND REPORTS TO WORKERS; INSPECTIONS (PART 19); EMPLOYEE PROTECTION
WHAT IS THE NUCLEAR REGULATORY COMMISSION?
The Nuclear Regulatory Commission is an independent Federal regulatory agency responsible
for licensing and inspecting nuclear power plants and other commercial uses of radioactive
materials.
WHAT DOES THE NRC DO?
The NRC=s primary responsibility is to ensure that workers and the public are protected
from unnecessary or excessive exposure to radiation and that nuclear facilities, including
power plants, are constructed to high quality standards and operated in a safe manner.
The NRC does this by establishing requirements in Title 10 of the Code of Federal
Regulations (10 CAR) and in licenses issued to nuclear users.
WHAT RESPONSIBILITY DOES MY EMPLOYER HAVE?
Any company that conducts activities by the NRC must comply with the NRC=s requirements.
If a company violates NRC requirements, it can be fined or have it=s license modified,
suspended or revoked.
Your employer must tell you which NRC radiation requirements apply to your work and
must post NRC Notices of Violation involving radiological working conditions.
WHAT IS MY RESPONSIBILITY?
For your own protection and the protection of your co-workers, you should know how
NRC requirements relate to your work and should obey them. If you observe violations
of the requirements or have a safety concern, you should report them.
WHAT IF I CAUSE A VIOLATION?
If you engaged in deliberate misconduct that may cause a violation of the NRC requirements,
or would have caused a violation if it had not been detected, or deliberately provided
inaccurate or incomplete information to either the NRC or to your employer, you may
be subject to enforcement action. If you report such a violation, the NRC will consider
the circumstances surrounding your reporting in determining the appropriate enforcement
action, if any.
HOW DO I REPORT VIOLATIONS AND SAFETY CONCERN?
If you believe that violations of NRC rules or the terms of the license have occurred,
or if you have a safety concern, you should report them immediately to your supervisor.
You may report violations or safety concerns directly to the NRC. However, the NRC
encourages you to raise your concerns with the licensee since it is the licensee who
has the primary responsibility for, and is most able to ensure, safe operation of
nuclear facilities. If you choose to report your concern directly to the NRC, you
may report this to an NRC inspector or call or write to the NRC Regional Office serving
your area. If you send your concern in writing, it will assist the NRC in protecting
your identity if you clearly state in the beginning of your letter that you have a
safety concern or that you are submitting an allegation. The NRC=s toll-free SAFETY
HOTLINE for reporting safety concerns is listed below. The addresses for the NRC Regional
Offices and the toll-free telephone numbers are also listed below.
WHAT IF I WORK WITH RADIOACTIVE MATERIAL OR IN THE VICINITY OF A RADIOACTIVE SOURCE?
If you work with radioactive materials or near a radiation source, the amount of radiation
exposure that you are permitted to receive may be limited by NRC regulations. The
limits on your exposure are contained in section 20.1201, 20.1207, and 20.1208 of
Title 10 of the Code of Federal Regulations (10 CAR 20) depending on the part of the
regulations to which your employer is subject. While these are the maximum allowable
limits, your employer should also keep your radiation exposure as far below those
limits as Areasonably achievable.
MAY I GET A RECORD OF MY RADIATION EXPOSURE?
Yes. Your employer is required to advise you of your dose annually if you are exposed
to radiation for which monitoring was required by NRC. In addition, you may request
a written report of your exposure when you leave your job.
HOW ARE VIOLATIONS OF NRC REQUIREMENTS IDENTIFIED?
NRC conducts regular inspections at licensed facilities to assure compliance with
NRC requirements. In addition, your employer and site contractors conduct their own
inspections to assure compliance. All inspectors are protected by Federal law. Interference
with them may result in criminal prosecution for a Federal offense.
MAY I TALK WITH AN NRC INSPECTOR?
Yes. NRC inspectors want to talk to you if you are worried about radiation safety
or have other safety concerns about licensed activities, such as the quality of construction
or operations at your facility. Your employer may not prevent you from talking with
an inspector. The NRC will make all reasonable efforts to protect your identity where
appropriate and possible.
MAY I REQUEST AN INSPECTION?
Yes. If you believe that your employer has not corrected violations involving radiological
working conditions, you may request an inspection. Your request should be addressed
to the nearest NRC Regional Office and must describe the alleged violation in detail.
It must be signed by you or your representative.
HOW DO I CONTACT THE NRC?
Talk to an NRC inspector on-site or call or write to the nearest NRC Regional Office
in your geographical area . If you call the NRC=s toll-free SAFETY HOTLINE during
normal business hours, your call will automatically be directed to the NRC Regional
Office for your geographical area. If you call after normal business hours, your call
will be directed to the NRC=s headquarters Operations Center, which is manned 24 hours
a day.
CAN I BE FIRED FOR RAISING A SAFETY CONCERN?
Federal law prohibits an employer from firing or otherwise discriminating against
you for bringing safety concerns to the attention of your employer or the NRC. You
may not be fired or discriminated against because you:
WHAT FORMS OF DISCRIMINATION ARE PROHIBITED?
It is unlawful for an employer, to fire you or discriminate against you with respect
to pay, benefits, or working conditions because you help the NRC or raise a safety
issue or otherwise engage in protected activities. Violations of Section 211 of the
Energy Reorganization Act (ERA) of 1974 (42 U.S.C 5851) include actions such as harassment,
blacklisting, and intimidation by employers of (i) employees who bring safety concerns
directly to their employer or to the NRC; (ii) employees who have refused to engage
in an unlawful practice, provided that the employee has identified the illegality
to the employer; (iii) employees who have testified or are about to testify before
Congress or in any Federal or State proceeding regarding any provision (or proposed
provision) of the ERA or the Atomic Energy Act (AEA) of 1954; (iv) employees who have
commenced or caused to be commenced a proceeding for the administration or enforcement
of any requirement imposed under the ERA or AEA or who have, or are about to, testify,
assist, or participate in such a proceeding.
HOW DO I FILE A DISCRIMINATION COMPLAINT?
If you believe that you have been discriminated against for bringing violations or
safety concerns to the NRC or your employer, you may file a complaint with the NRC
or the U.S. Department of Labor (DOL). If you desire a personal remedy, you must file
a complaint with the DOL pursuant to Section 211 of the ERA. Your complaint to the
DOL must describe in detail the basis for your belief that the employer discriminated
against you on the basis of your protected activity, and it must be filed in writing
either in person or by mail within 180 days of the discriminatory occurrence. Additional
information is available at the DOL web site at www.osha.gov. Filing an allegation,
complaint, or request for action with the NRC does not extend the requirement to file
a complaint with the DOL. To do so, you may contact the Allegation Coordinator in
the appropriate NRC Region, as listed below, who will provide you with the address
and telephone number of the correct OSHA Regional office to receive your complaint.
You may also check your local telephone directory under the U.S. Government listings
for the address and telephone number of the appropriate OSHA Regional office.
WHAT CAN THE DEPARTMENT OF LABOR DO?
If your complaint involves a violation of Section 211 of the ERA by your employer,
it is the DOL, NOT THE NRC, that provides the process for obtaining a personal remedy.
The DOL will notify your employer that a complaint has been filed and will investigate
your complaint.
If the DOL finds that your employer has unlawfully discriminated against you, it may
order that you be reinstated, receive back pay, or be compensated for any injury suffered
as a result of the discrimination and be paid attorney=s fees and costs.
Relief will not be awarded to employees who engage in deliberate violations of the
Energy Reorganization Act or the Atomic Energy Act.
WHAT WILL THE NRC DO?
The NRC will evaluate each allegation of harassment, intimidation, or discrimination.
Following this evaluation, an investigator from the NRC=s Office of Investigations
may interview you and review available documentation. Based on the evaluation, and,
if applicable, the interview, the NRC will assign a priority and a decision will be
made whether to pursue the matter further through an investigation. The assigned priority
is based on the specifics of the case and it=s significance relative to other ongoing
investigations. The NRC may not pursue an investigation to the point that a conclusion
can be made whether the harassment, intimidation, or discrimination actually occurred.
Even if NRC decides not to pursue an investigation, if you have filed a complaint
with the DOL, the NRC will monitor the results of the DOL investigation.
If the NRC or the DOL finds that unlawful discrimination has occurred, the NRC may
issue a Notice of Violation to your employer, impose a fine, or suspend, modify, or
revoke your employer=s NRC license.
To report safety concerns or violations of NRC requirements by your employer, telephone:
NRC SAFETY HOTLINE
1-800-695-7403
To report incidents involving fraud, waste, or abuse by an NRC employee or NRC contractor,
telephone:
Office of the Inspector General Hotline
1-800-233-3497
REGIONAL OFFICE ADDRESS
U.S. Nuclear Regulatory Commission, Region II
Atlanta Federal Center
61 Forsyth Street, S.W.
Atlanta, GA 30303-3415
Telephone: 800-577-8510
1.0 Program
Introduction
This manual describes the radiation safety program at Radford University for the use
of radioisotopes. Information on the fundamentals of radiation, safety procedures,
methods to reduce exposures, and State, Federal and University regulations concerning
the safe use of radioactive materials are included.
Radioisotope work is regulated by the Nuclear Regulatory Commission and the State
Bureau of Radiological Health. In order to work with radioisotopes, the NRC has issued
a license to the university which contains a number of conditions that must be met.
A copy of the license may be obtained from the Safety Office. This manual is based
upon the Federal and State regulations and contains the specific rules that must be
followed for use of radioactive material at the University. Individuals authorized
to use radioisotopes must comply with all Federal, State, and university regulations
and procedures in order to protect both the user and other personnel from unnecessary
exposure to radiation.
Organization
Radiation Safety Committee
The Radford Radiation Safety Committee has been established by the University with
the authority to regulate the safe use of radioisotopes, x-ray equipment, and non-ionizing
radiation. The Radiation Safety Committee shall perform the following functions:
Members of the committee are appointed by the Chair of the Radiation Safety Committee.
They are selected on the basis of their experience in the safe handling of radioactive
materials, x-ray equipment, or non-ionizing radiation. An individual with administrative
responsibilities shall chair the committee. The Radiation Safety Officer is an ex-officio
member of the committee. At least one user of radioisotopes and one analytical x-ray
user will serve on the committee. Additional members as deemed necessary may be approved
by the Chair. Members will serve for three years and may be re-appointed upon completion
of their terms. A member who misses three consecutive meetings without approval of
the Chair will be considered to have resigned.
A meeting of the committee will be held at least quarterly. Additional meetings will
be convened by the Chair as necessary. Minutes of the meetings shall be recorded and
distributed to the members of the committee and other interested persons. A quorum
shall consist of half the members plus one. The Chair and the Radiation Safety Officer
have the authority to act on behalf of the committee for those occasions that do not
warrant a special meeting of the full committee (e.g., approval of an amendment).
Decisions of the full committee are effective immediately. Decisions made at interim
meetings are tentative, pending approval by the full committee. A member may request
a review by the Radiation Safety Committee if a decision is not unanimous. The decision
will not be implemented pending this review.
Radiation Safety Officer
The Radiation Safety Officer (RSO) will administer the radiation safety program at
the university and advise others in the safe use of ionizing and non-ionizing radiation.
The RSO will ensure that:
Users of Radioactive Materials
Authorized User - An authorized user may independently purchase, possess, and use radioisotopes.
The authorized user is directly responsible for the laboratory and the users on the
authorization. Authorized users must be approved by the Department Head.
Principal User - A principal user works under the indirect supervision of an authorized user and
may supervise other workers and request amendments to the authorization. Principal
users are typically professors appointed by the authorized user to supervise experiments.
General User - A general user works with radioisotopes under the indirect supervision of an authorized
or principal user. General users have no supervisory authority and may not request
amendments. General users may be professors, technicians, or students doing independent
research.
Student User - Student users work with radioisotopes only as part. of a classroom requirement
approved by the Radiation Safety Committee. Student users must be under the direct
supervision of an authorized or principal user.
Responsibilities of Authorized Users
Authorized users are responsible for the safe use of radioisotopes under their control.
Authorized users will ensure that:
Responsibilities of Users
Individuals who use radioisotopes shall:
Obtaining an Authorization
Radiation Safety Training
Users of radioisotopes must receive training and demonstrate competence in the following
radiation safety principles:
Competence will usually be demonstrated by passing a written examination administered
by the RSO. Exceptions to taking the examination may be granted by the Radiation Safety
Committee because of previous training, experience, or education. Individuals wishing
to seek an exemption should submit a request in writing to the RSO or the Chair of
the Radiation Safety Committee.
Individuals who need training in radiation safety principles should call the Radiation
Safety Officer at 831-5860 to obtain a copy of the Radioisotope Safety Manual. A test
will be scheduled after the user has reviewed the training manual. A personnel monitoring
device will be ordered, if necessary, after the individual has passed the test. Radioisotopes
cannot be used until the monitoring device has been issued. An amendment to the authorization
signed by the authorized or principal user must be submitted to the RSO prior to use
of radioactive materials.
Request for Inspection
Any user who believes there has been a violation of the rules presented in this manual
may request an inspection by notifying the Safety Office at Radford University or
the Bureau of Radiological Health. The user's name will be kept anonymous. During
the inspection, safety representatives may confer privately with users. Users may
bring to the attention of safety representatives any past or present condition they
believe may have contributed to or caused a violation. An authorized user shall not
dismiss or discriminate against a user because a complaint was filed. The RSO will
bring any complaints to the attention of the Radiation Safety Committee
Variance
Experiments
Users shall submit protocols to the Radiation Safety Officer before an experiment
can be performed. The documentation for the experiment shall include the following
information and can be initially approved by the RSO, subject to final approval by
the Radiation Safety Committee:
Radiation Protection
Exposure Limits
1. Radiation workers using radioisotopes shall not receive a dose in one calendar
quarter greater than the following limits:
2. Individuals under 18 years of age are not permitted to receive a dose greater than
10% of the above limits.
3. The maximum permissible whole-body dose to a pregnant radiation worker during the
pregnancy shall not exceed 500 millirem.
4. Radiation levels to members of the public shall not exceed 2 mrem/hr or 100 mrem/yr.
ALARA Commitment
Although radiation doses from the use of radioactive materials are very low and current
occupational limits provide a very low risk of injury, Radford University recognizes
that it is prudent to avoid unnecessary exposure. It is therefore the policy of Radford
University to reduce exposures to a level that is as low as reasonably achievable
(ALARA). This will be accomplished through solid radiation protection planning and
practice, as well as a commitment to policies that promote vigilance against unsafe
practices.
Personnel Monitoring
Exposure Records
Exposure records will be maintained by the RSO. The RSO will notify workers at least
annually of their exposure to radiation. The RSO will supply the user with a written
report if a dose in excess of 25% of the occupational limits is received. The RSO
will provide a radiation exposure report to the worker, or an employer, at the request
of the worker.
Pregnant Radiation Workers
Pregnant radiation workers shall wear a whole-body monitoring device at waist level
and be informed of their radiation exposure on a quarterly basis. A pregnant radiation
worker should notify the RSO as soon as her pregnancy is known, limit exposure to
less than 500 mrem, and strive to reduce her exposure to the very lowest practical
level.
Obtaining Radioactive Materials
Record Keeping
1. The following records and documents will be maintained in the Safety Office:
2. Required laboratory survey records will be maintained in individual labs.
3. Authorized users must maintain a running log of each radioactive isotope in his/her
possession, to include: receipt, usage, quantity on hand, and final disposition. All
contamination surveillance records must also be maintained. The proper keeping of
these records is very important to demonstrate appropriate use and disposal of all
isotopes. In order to simplify record keeping, no decay corrections should be performed
on these records.
2.0 Laboratories
Marking & Labeling
Ventilation
Storage
Work Areas
Fume Hoods
As much work as possible should be done in a radioisotope-rated fume hood. Specifications
for these hoods are as follows:
Equipment & Work Surfaces
Security
Inspections
Surveys
Periodic surveys must be performed to show control of contamination. Additionally,
the affected area must be surveyed any time a contamination incident is suspected.
The recommended surveillance technique is to perform the daily checks with a portable
survey instrument, while the weekly checks would be done with swipe tests analyzed
in scintillation counters.
General
1. Keep a record of survey results. It must include the following information:
Packages
1. The following procedures will be used when opening packages containing licensed
radioactive materials:
2. The RSO or his assistant will perform a wipe test on the exterior of the package
and the source container. The exposure rate at 1 meter and at the package surface
will be measured with a Geiger counter. Sealed sources will be leak tested. Results
of package surveys will be maintained by the RSO.
3. The contamination limit for wipe tests on incoming packages is 220 DPM/100 sq cm.
Exposure rates limits for packages with yellow II or III labels are 200 mr/hr at the
surface or 10 mr/hr at 1 meter from the surface. The dose rate limit for packages
with white I labels is 0.5 mr/hr at the package surface or 0.05 mr/hr at 3 feet.
Unsealed Sources
Contamination from alpha emitters should be non-detectable. Contact the RSO if these limits are exceeded.
Sealed Source Leak Test Procedures
3.0 Handling Radioactive Materials
Efforts should also be made to keep internal radiation exposures ALARA. Radioactive
material can be internally deposited if there is: skin contact, inhalation or ingestion.
The use of good cleanliness practices coupled with adequate contamination surveillance
can avoid skin contamination problems and the associated ingestion or skin absorption
hazard. Airborne radioactivity can pose a significant inhalation problem. Procedures
that generate aerosols or produce volatile or gaseous products should be performed
in closed apparatus. For instance, capped tubes should be vortexed and closed systems
should be used in conjunction with filters or traps when volatile or gaseous products
are expected. If absolute containment is not achievable, the work must be performed
in a fume hood.
Personnel Protective Equipment
Unsealed Sources
Sealed Sources
Phosphorus-32
Airborne Radioactivity
Decontamination
Radioactive contamination must be corrected in a timely fashion. Exceptions to this
are: pipetters, work trays, absorbent paper covered surfaces, or other equipment that
is clearly marked with the caution symbol. Any contamination that is not confined
to protected surfaces must be cleaned immediately.
Equipment
Areas
Radioactive Waste
Major Spills
Initial Response and Notification
1. Major spills are an immediate health hazard, spread over a large area, and contain
removable activity greater than 10,000 disintegrations per minute (dpm).
2. Evacuate all personnel from the area. If possible, quickly surround the spill with
absorbent booms to prevent the spread of contamination. Leave behind clothing and
shoes that may be contaminated. Close doors and windows and lock or otherwise secure
the area. If the spill involves airborne contaminants turn off the ventilation system
in the area.
3. Do not allow unauthorized people into the room. Keep potentially contaminated people
in a protected area. Do not attempt to decontaminate the spill.
4. Report the spill to the Police Department. Provide the following information and
wait in a safe place for emergency personnel to arrive and direct them to the spill:
Name and telephone number of the caller.
Location of the spill.
Name and quantity of materials involved.
Extent of injuries, if any.
Environmental concerns, such as the location of storm drains.
Any unusual features such as foaming, odor, fire, etc.
5. The Police Department will call the Radiation Safety Officer, Vice President for
Business Affairs, Assistant Vice President for Facilities, Assistant Vice President
for Communications, and other appropriate university officials. The Vice President
for Business Affairs will notify the President.
6. A Police Officer will proceed to the site and cautiously evaluate the situation
while waiting for the Radiation Safety Officer. If there is clear danger to the occupants
of the building the officer will evacuate the building.
7. The RSO will determine the severity of the spill and notify the Bureau of Radiological
Health and the Nuclear Regulatory Commission if necessary. Decontamination will be
conducted under the supervision of the senior emergency authority at the scene.
Minor Spills
Initial Response & Notification
Specific emergency procedures
Fires
Loss of Radioactive Material
4.0 Fundamentals of Radioactivity
Atomic Structure
An atom is the smallest division of matter that still displays the chemical properties
of an element. Atoms are composed of a dense positively charged nucleus containing
neutrons and protons. The nucleus is surrounded by a cloud of negatively charged electrons.
In neutral atoms the positive and negative charges are equal. The proton has a mass
of approximately 1 atomic mass units (AMU) and a single positive unit of charge. The
number of protons is known as the atomic number (Z) and defines the particular element.
The neutron has a mass slightly greater than 1 AMU and has no charge. The number of
protons and neutrons is called the mass number, represented by the symbol A. The electrons
circle the nucleus in distinct orbits, called energy shells. These shells are labeled
alphabetically, starting with the letter K, and going outward to the letter Q. An
electrons mass is approximately 1\1840 that of a proton or neutron.
Atoms characterized by a particular atomic number and mass number are called nuclides.
Nuclides with the same number of protons but different numbers of neutrons in the
nucleus are called isotopes of that element. Isotopes of the same element have essentially
the same chemical properties. For example, there are three isotopes of hydrogen. The
simplest nucleus is ordinary hydrogen and consists of a single proton. Adding a neutron
to the nucleus forms an isotope called deuterium. Adding another neutron produce an
isotope called tritium. Tritium has an atomic number (Z) of 1 and a mass number (A)
of 3. Symbolically this is written as H-3. The letter H describes the element as hydrogen
with an atomic number of 1. The 3 stands for the mass number. The number of neutrons
(2) is found by subtracting the atomic number from the mass number.
In addition to varying the number of neutrons and protons the number of electrons
in an atom can change. An atom having the same number of protons and electrons is
electrically neutral. Atoms with an unequal number of protons and electrons are called
ions. A negative ion has more electrons than proton and a positive ion has a deficiency
of electrons.
Radioactive Decay
The stability of an atom depends on the particular combination of neutrons and protons
in the nucleus. An unstable atom has a nuclear energy imbalance due to an improper
combination of neutrons and protons. To achieve stability the nucleus emits this excess
energy in the form of high energy particles or gamma rays. The emission is termed
radiation and the atom is called radioactive. For example, the two lightest isotopes
of hydrogen are stable, while the third is unstable or radioactive. This means that
tritium can spontaneously decay releasing radiation and change into another isotope.
When this happens a negative electron, called a beta particle is emitted and one of
the two neutrons becomes a proton. Thus an unstable isotope has decayed into a stable
one, an isotope of helium. The beta particle is similar to ordinary electrons, except
that it has kinetic energy to ensure conservation of energy.
Stable isotopes with light nuclei tend to have equal numbers of neutrons and protons.
As the number of neutrons and protons increase, however, isotopes begin to have more
neutrons than protons. This is because the protons are confined in a very small space
and strongly repel each other due to their like charges. Since neutrons have no charge,
more of them can be close together. However, nuclear forces prevent too many from
being in a stable nucleus. The largest stable nucleus that has equal numbers of protons
and neutrons is an isotope of calcium with 20 of each.
There can be both stable and unstable isotopes for a given element. Tin has the most
stable isotopes, 10, while there are no completely stable isotopes for elements with
atomic numbers greater than 83. Unstable isotopes decay until the decay product is
stable. This may take more than one step. For example, in a chain decay one unstable
isotope will decay to another unstable one, which will then decay to a stable one.
There are several different ways in which an unstable isotope can decay.
Types of Radioactive Decay
Alpha Particles
Alpha decay occurs when a nucleus emits an energetic helium nucleus consisting of
2 protons and 2 neutrons. The alpha particle has a mass of approximately 4 AMU and
a positive charge of 2 units. Alphas are emitted with discreet energies (monoenergetic),
and typically have energies of 4 to 9 million electron volts (MeV). These particles
are very heavy and have little penetrating power. A single sheet of paper, a few inches
of air, or the top layer of skin will stop an alpha particle. Alphas are not considered
an external hazard, however they are very hazardous if ingested. Alpha particles are
relatively slow and create numerous ionizations along a short path. Alpha decay typically
occurs in heavy nuclides such as uranium, radium, americium, and plutonium.
Beta Particles
Beta decay occurs when a nucleus emits a high speed negatively charged electron. The
beta particle results from the energy released from the transformation of a neutron
into a proton. Their maximum energies range from 0.015 to 3 MeV. Beta particles are
not monoenergetic, but are emitted with an energy that can vary up to a maximum value
for a given isotope. However only a small percentage of beta particles are released
near the maximum energy. The average energy of release is approximately one-third
the maximum energy. The range of beta particles in air is dependent on the maximum
energy and can vary from 6 mm for H-3 to 6 m for P-32. In general the range of a beta
particle in centimeters is approximately equal to the energy in MeV divided by two.
Betas are more penetrating than alphas and can be stopped by several sheets of aluminum
foil, one inch of wood or one-quarter inch of plastic. Beta emitters include H-3,
C-14, P-32, S-35, Cl-36, and Ca-45.
Neutrons
Neutrons are particles with no electrical charge. Neutrons that are loosely bound
in the nucleus can be dislodged by alpha particles. For example in a plutonium-beryllium
(PuBe) source, neutrons are released when alpha particle from plutonium bombard the
beryllium. Neutrons are also released in some heavy elements by nuclear fission processes.
Fission may occur spontaneously in some nuclides, for example Californium-252, but
the major source of neutrons is nuclear-fission reactors.
Gamma Rays
Gamma rays are very short wave electromagnetic radiations emitted by transformations
in the nucleus. They are released as packets of energy called photons, travel at the
speed of light, have no mass or charge, and are very penetrating. The release of gammas
changes the energy state of the nucleus but does not change its structure. Gamma rays
are released at discrete energies (monoenergetic) ranging from a few thousand electron
volts (eV), up to approximately 3 MeV. Gamma rays lose their energy by collisions
with orbital electrons. This results in the ejection of electrons and the creation
of ions. Isotopes that decay by this process include Cr-51 Co-57, Co-60, Cd-109, I-125,
and I-131.
Electron Capture
Electron capture occurs when the nucleus captures an orbital electron. A vacancy is
left in the innermost orbital shell that is filled by an electron falling from a higher
orbit. The excess energy is released as an x-ray photon. I-125 decays in this manner.
Positron Decay
A positron is an electron with a positive charge, the antiparticle to the negative
electron. The positron eventually combines with a negative electron, annihilating
both particles and creating two 0.51 MeV photons. Sodium-22 and fluorine-18 are example
of positron emitters.
Half-Life
The half-life of a radioactive material is the time required for the initial activity
of the substance to decrease by one-half. The half-life for a particular isotope is
a constant and cannot be increased or decreased by any chemical or physical means.
Half-lives range from microseconds to billions of years. Each radioisotope has its
own unique decay rate. For example P-32 has a half-life of 14 days. This means that
one-half will be gone in 14 days and one-half of the remainder will have decayed in
another 14 days. After 7 half-lives less than 1% of the original activity will remain.
A common rule of thumb is that after 10 half-lives have elapsed, all activity is effectively
gone. Since the activity decreases by a factor of 2 as each half-life passes, the
activity after 10 half-lives will diminish by a factor of 1024, or to less than 0.1%.
However, if there was originally a large amount of activity, there may still be considerable
activity remaining even after 10 half-lives. For example, if there were originally
1 Curie of an isotope there would still be approximately 1 mCi remaining after 10
half-lives.
Activity
Radioactive decay occurs when the nucleus releases excess energy in the form of high
speed particles or gamma rays. The number of transformations occurring per unit of
time is called activity. A unit termed the Curie is used to describe the amount of
activity in a radioactive material. The Curie is defined as 3.7 x 10 10 disintegrations
per second (dps) or 2.22 x 10 12 dis/min. It refers to a fairly large amount of activity.
In most cases the amounts of activity used in an experiment would be in the range
of a few microcuries to a few millicuries. A useful quantity that is often used in
laboratories is the amount of activity in a microcurie, 2.22 x 10 6 dis/min.
Activity refers to the number of nuclear disintegrations occurring per unit of time
and is not necessarily equal to the number of particles emitted. The rate of emission
is equal to the activity only when one particle is given off per disintegration. Numerous
particles per disintegration may be given off and careful attention must be paid to
the decay scheme. For example, the decay of 1 microcurie of Bi-212 produces 2.22 x
10 6 disintegrations per minute. This translates to an emission rate per minute of:
(2.22 x 10 6) 0.36 for alpha particles, (2.22 x 10 6) 0.64 for beta particles, and
(2.22 x 10 6) 0.36 for gamma rays. Gamma rays are associated with 72% of the alpha
particles and 16% of the beta particles.
A new unit for activity that is part of the International System (SI) of measurements
is called the Becquerel. Under this system 1 becquerel (Bq) is equal to 1 disintegration
per second. One Curie is equal to 3.7 x 10 10 Bq. Although the Becquerel is widely
used in Europe and the scientific community acceptance in the United States has been
slow.
Specific Activity
Activity does not take into account the volume or mass of the radioactive material
in which the decay process occurs. The term that describes the concentration of activity
is specific activity (SpA). SpA is the number of curies or becquerel per unit mass
or volume and is often expressed in units of Ci/g, mCi/ml, mCi/mm, etc. For example,
if the activity of a radioactive material was 1 mCi and it had a SpA of 10 mCi/ml
then the total volume would be 0.1 ml.
Interactions of Radiation with Matter
The two different kinds of radiation are particulate (alphas, betas and neutrons)
and electromagnetic radiation (gamma rays, x-rays and bremsstrahlung). Each type of
radiation interacts with matter in a unique way.
Charged particles have an electric field, similar to the orbital electrons of an atom.
As a charged particle passes an atom the influence of its electric field can either
remove an electron from the atom or raise an electron to an excited orbital state.
The first process creates an ion pair while the second leaves the atom intact. Both
types require energy which is derived from the kinetic energy of the incident particle.
The kinetic energy of the particle is reduced by the amount of energy transferred
during the interaction. These interactions continue until the particle loses all of
its energy.
Alpha Particles
An alpha particle is a relatively large subatomic particle (4 AMU) that has a charge
of +2. This causes the ionizations per unit length (linear energy transfer) to be
high and the range of the particle to be very short. An alpha loses about 35 electron
volts (eV) for each ion pair it creates in air or soft tissue. A typical alpha creates
more than 100,000 ion pairs before all its energy is lost. Because these particles
are monoenergetic, they have well defined ranges in matter. To illustrate its penetrability,
a 4 MeV alpha has a range of approximately 2.3 cm in air and 0.003 cm in tissue. This
is much less than the thickness of human skin which is approximately 0.1 cm. The greatest
hazard posed by alpha radiation is from ingestion or inhalation, which allows the
radionuclide to be deposited in tissue.
Beta Particles
Beta particles are also charged particles that interact with matter in basically the
same way as alpha particles. Due primarily to the much smaller mass of the beta (1/1840
AMU), there are some differences. For a given energy, their speeds are much greater
and they spend less time in the vicinity of an atom. This results in less interactions
per unit distance. Since they have the same mass as orbital electrons, a large portion
of their energy is given up to a target electron. Betas also lose energy by bremsstrahlung
as their paths are bent by the electric fields of the nucleus and orbital electrons.
The absorption of betas also differs from alpha particles because they are not monoenergetic.
Betas are emitted with energies ranging between 0 and a maximum value. The beta energies
vary because a neutrino is emitted along with the beta and the maximum energy is shared
between them. The interaction between the uncharged neutrino and matter does not transmit
appreciable energy to any material it passes through.
Although a beta will penetrate much more deeply in matter than an alpha, the range
is still not great. For example, the 1.71 MeV beta of P-32 has a range of about 0.8
cm in tissue (1/3 inch). In air the P-32 beta has a much greater range of 610 cm (20
feet). The advantage of using low energy beta emitters can be illustrated by comparing
C-14 and P-32. The C-14 0.156 MeV beta has a range in tissue of 0.04 cm (1/25 inch)
and a range in air of 31 cm (1 foot). Shielding is not necessary for C-14 while considerable
shielding is necessary for P-32.
Bremsstrahlung is another energy loss mechanism for betas in which the beta energy
is converted into x-rays. This occurs when the attractive forces from an atom cause
the beta to rapidly decelerate and change its path. The quantity of bremsstrahlung
increases as the shield density increases. The x-ray energies are determined by the
incident beta energy, but their average energy is 1/3 of the maximum beta energy.
The use of low atomic number shields (e.g. plastic) minimizes the production of bremsstrahlung.
Neutrons
Since neutrons are not charged, they interact differently with matter than charged
particles. They can be scattered, absorbed by the nucleus of the target atoms, or
disrupt chemical bonds. When neutrons are absorbed in a nuclear reaction, prompt gammas
are emitted and charged particles may be emitted. Additionally the element may be
changed when the residual nucleus releases alpha, beta or gamma rays. Since all of
these processes can be highly disruptive to chemical bonds, neutrons can cause severe
radiation damage.
Gammas and X-rays
Gammas and x-rays are electromagnetic radiation. These photons have no electrical
charge and interact with matter differently from particles. Gammas and x-rays are
identical in nature, but are different in origin. Gammas are produced in processes
that involve the nucleus of an atom, while x-rays are produced by interactions that
take place outside of the nucleus. X-rays are emitted with discrete energies or with
a broadspectrum of energies, while gammas are always released with discrete energies.
In passing through matter, energy is transferred from the incident photon to electrons
and nuclei in the target material. An electron can be ejected from the atom with the
subsequent creation of an ion. The amount of energy lost to the electron is dependent
on the energy of the incident photon and the type of material through which it travels.
Gamma and x-ray photons interact with matter in three ways: the photoelectric effect,
Compton scattering, and pair production.
Photoelectric Effect
In the photoelectric effect, an incident photon strikes an orbital electron and is
totally absorbed by the electron. The electron is then ejected from the atom creating
a vacancy in the shell. The ejected electron interacts with other atoms creating ionizations
until it loses its kinetic energy. Electrons in the atom drop from the outer shells
to fill the vacancy in the inner shell forming characteristic x-rays. Photoelectric
absorption occurs predominantly in x-ray photons with energies below 10 keV.
Compton Scattering
Compton scattering occurs when a gamma or x-ray photon scatters from an orbital or
free electron. Unlike the photoelectric process, only part of the photon's energy
is transferred to the electron. The electron is ejected from the atom and the incident
photon is scattered with a reduction in energy. This scattered photon continues to
interact with orbital electrons by additional Compton or photoelectric processes.
The energy of the scattered beta is the difference between the energies of the original
and scattered gammas and x-rays. The probability of Compton scattering increases as
the energy of the photon increase. At approximately 35 keV the probability of interactions
through photoelectric and Compton collisions are about equal.
Pair Production
In pair production the incident photon must possess a minimum energy of 1.02 MeV.
The photon interacts with the electric field around the nucleus and undergoes transformation
into matter, with the creation of an electron and positron (positive electron). The
original gamma disappears with its kinetic energy shared between the electron and
positron. The positron is annihilated in microseconds by interacting with an electron,
creating two 0.511 MeV gamma photons (annihilation radiation). These photons then
interact with matter by Compton and photoelectric collisions.
5.0 Radiation Measurement
Radiation Units
Roentgen (R)
The roentgen is a unit for expressing exposure from x or gamma radiation in terms
of the number of ionizations produced in air. One roentgen of radiation will produce
ionizations equal to one electrostatic unit of charge in one cubic centimeter of dry
air at standard temperature and pressure.
Rad
The roentgen defines a radiation field in air but does not provide a measure of absorbed
dose in ordinary matter or tissue. When radiation passes through an object, part of
the energy will be transferred to the material. This is referred to as the absorbed
dose. In order that absorption properties of the exposed material be taken into account,
a dose unit has been developed called the rad (radiation absorbed dose). In contrast
to the roentgen, the rad is used to express the radiation dose absorbed in any medium
from any type of radiation. One rad is equal to the amount of radiation that results
in the absorption of 100 ergs per gram in any material. It is approximately equal
to the absorbed dose delivered to soft tissue by one roentgen of x or gamma radiation.
Rem
In terms of human exposure, however, another factor must be considered; exposure to
equal doses from different types of radiation do not result in equal damage to biological
tissue. Therefore, in order to account for these varying effects, a unit is used termed
the rem (roentgen equivalent man). The rem estimates the amount of any radiation that
would be necessary to produce the same biological effects in humans as one rad of
x or gamma radiation. The rem is equal to the rad multiplied by a quality factor that
estimates the relative biological effectiveness of different types of radiation. This
biological effectiveness depends upon the number of ionizations created per unit distance
in tissue as the radiation travels through the body. The quality factor for gamma
and beta radiation is one, however, quality factors for other types of radiation can
be as high as twenty (e.g., alpha, particles). Therefore, a dose of 0.05 rads from
alpha particles could do the same biological damage as 1 rad of gamma radiation because
both equal one rem (0.05 rads x 20 quality factor). One advantage of using rem units
is that dosages delivered from different types of radiation become additive.
In summary, the roentgen is a unit of exposure, the rad is a unit of absorbed dose,
and the rem is a unit of biological dose. The rem is the unit that is used to measure
radiation doses to personnel. For practical purposes, however, the roentgen, rad,
and rem are essentially equivalent for gamma and beta rays and can be used interchangeably.
Commonly used subunits are milliroentgen, millirad, and millirem (mR), which are equal
to 1/1000 of these units.
International System (SI)
Recently new radiation protection units consistent with the metric system or International
System of Units (SI system) have been adopted. In order to phase out the use of the
roentgen there is no unit comparable to it in the SI system. The new SI unit for absorbed
dose, which replaces the rad, is the gray (Gy). One gray is equal to 100 rads. The
new unit for the rem is the Sievert (Sv), which is equivalent to 100 rem. Although
SI units are accepted internationally, the roentgen, rad, and rem continue to be widely
used in this country.
Maximum Permissible Dose (MPD)
To protect radiation workers and the public, maximum permissible dose limits have
been established by the National Council on Radiation Protection (NCRP). The MPD is
the dose of radiation, based on current knowledge, that a person can receive without
sustaining appreciable bodily injury. The maximum whole-body dose allowed to radiation
workers is 5 rem per year or 1.25 rem per quarter. The dose limit to the lens of the
eye is 15 rem per year or 3.75 rem per quarter. The dose limit to the skin and extremities
is 50 rem per year or 12.5 rem per quarter, reflecting the decreased sensitivity to
radiation damage of these areas. Radiation workers under the age of 18 are limited
to 10% of these values. The limit for the public has been set at 2 mR/hr or 500 mR/yr.
The dose limit for pregnant radiation workers is limited to 500 mR during the course
of the pregnancy.
Because any amount of radiation is potentially harmful, the NCRP has officially adopted
the ALARA concept. This principal states that all radiation exposures should be kept
"As Low As Reasonably Achievable". Maximum permissible doses have been established
as reasonable risks comparable to other industries. These doses should be thought
of as absolute ceiling levels and not as "safe" limits. Radiation workers should always
strive to reduce doses to ALARA levels by using proper safety procedures and techniques.
Survey Instruments
Radiation survey instruments are used to detect radiation contamination, monitor the
effectiveness of shielding arrangements, and estimate exposure to personnel. There
are two main categories of radiation monitoring devices: gas filled detectors and
scintillation detectors.
Gas detection instruments are based on the principle that ions are produced when radiation
passes through a gas-filled chamber. Electrons liberated in the chamber are attracted
to the center electrode (anode) by a positive voltage potential. Positive ions are
attracted towards the walls (cathode) of the chamber. This produces an electrical
pulse or current which can be detected and recorded by a scaler or ratemeter.
There are three types of gas filled radiation detectors: ionization chambers, proportional
counters, and Geiger-Mueller detectors. The primary difference between these detectors
is the voltage applied to the chamber. The kind of detector used is based on the intensity
and the type of radiation field encountered. Scintillation detectors operate on the
principle that certain materials scintillate or give off light when exposed to radiation.
There are two major types of scintillation detectors, crystal and liquid.
Ionization Chambers
At very low applied voltages, ion pairs created by radiation passing through the chamber
may recombine before they are collected and counted. As the voltage of a gas filled
detector is increased, virtually every ion pair produced by the incident radiation
will be captured. The current flowing through the meter is therefore directly proportional
to the activity of the source. This feature makes ionization detectors very useful
as radiation monitoring devices. Survey instruments operating at this voltage are
called ionization chambers or "cutie pies". Because almost all ion pairs are collected,
this instrument is used when it is necessary to accurately determine exposures. Ionization
chambers, however, are relatively ineffective for measuring rates less than 1 mR/hr,
and are slow to respond to changing fields. For this reason ion chambers are not useful
for detecting contamination and are primarily used to determine exposures in areas
of high radiation intensity.
Proportional Counters
As the voltage of the tube is increased, electrons are accelerated faster and achieve
sufficient energy to create secondary ionizations in the gas. This amplification is
termed an avalanche and dramatically increases the size of the electrical pulse at
the central anode. Gas multiplication can create millions of ion pairs per ionizing
event, in contrast to the ionization chamber which creates one ion pair. Although
an avalanche has occurred, gas amplification is proportional to the energy of the
initiating event in this voltage region. Radiation monitoring devices operating in
this region are called proportional counters. With sufficiently thin windows alpha
particles, which produce a large number of ions in the gas, can be distinguished from
beta particles. In addition, the counter can be used to measure the energies of incoming
gamma rays.
A portable gas flow proportional counter that is often used in laboratories to detect
contamination is called a PAC-4G. Its range is from zero to 500,000 counts per minute(cpm).
The detector is a large flat rectangular area that provides great sensitivity. The
PAC-4G can detect beta energies as low as C-14 but not H-3. The instrument can also
detect contamination from alpha particles.
Geiger-Mueller (GM) Counters
The GM counter is the most widely used area survey instrument for the detection of
low-level radioactive contamination and exposure. It is very sensitive, relatively
inexpensive, and rugged. Radiation passing into a GM tube (typically containing helium,
neon, or argon) creates ions that are accelerated by a high voltage potential of approximately
1200 volts. Secondary ionizations are created from collisions with the accelerated
ions. These ions are also accelerated and achieve sufficient energy to form additional
ions. This process eventually produces an avalanche of billions of ion pairs from
the initial ionization and creates a large electrical pulse at the anode. The magnitude
of the output pulse is independent of the nature of the particle or its energy because
gas amplification has reached its maximum potential.
A major disadvantage of GM counters is their limitation to low radiation fields, typically
below 200 mR/hr. Once ionizations have been initiated in a GM tube it becomes insensitive
for a short time, called the dead time, and will not respond to further ionizing events.
As a result, the number of counts recorded will be less than the true count rate.
This error is relatively small at low radiation intensities, however, in high radiation
fields large errors can be introduced. At very high exposure rates, where events are
interacting with the tube much faster than the dead time, the counter may actually
saturate and read zero.
With sufficiently thin windows, alpha, beta and low energy gamma rays can be detected.
A typical GM counter can be used to count either betas or gammas. The betas enter
the gas through a fragile thin window, typically located at the end of the cylinder.
The window is as thin as 1.5 mg/cm2. The counter can detect betas with energies as
low as 0.030 MeV and even some alphas. If the window is covered by a shield to prevent
charged particles from entering, the response of the counter can be limited to gammas.
This permits characterization of the radiation field. Betas from H-3 cannot be adequately
monitored by any gas filled devices because of their very low energies.
Detector can be purchased in the following configurations: side window, thin end window
or a pancake type window. The side window detector has a relatively thick window which
the radiation must penetrate. Typically, betas of less than 200 KeV are not energetic
enough to be detected, and alphas cannot penetrate the window. This detector would
not be satisfactory for H-3, C-14 or S-35, since their beta energies do not exceed
200 KeV. The 1.71 MeV betas of P-32 could be detected but other probes are normally
used. This type of detector is effective for gammas with energies greater than 50
KeV. Betas and gammas can be differentiated by sliding a built-in metal shield over
the window to completely block out the betas.
The next detector uses a thin end window. Betas with energies as low as 40 KeV can
be detected. This detector can be used to detect the betas from C-14, S-35 and P-32
with efficiencies ranging from 10% (C-14) to 40% (P-32). Betas from H-3 cannot be
detected. Alphas with energies greater than 4 MeV are detectable. Some beta/gamma
discrimination can be achieved by covering the window with a shield which only gammas
can penetrate.
The last type of GM detector uses a large pancake shaped probe. This probe will detect
alphas, betas and gammas similar to the thin end window detector. The pancake probe
has a greater sensitivity than the end window type because the probe's active surface
area is about 3 times larger than the end window.
Scintillation Detectors
Scintillation detectors use a crystal that releases light when exposed to x-rays or
gamma rays. There are two types of scintillation detectors; solid and liquid. Most
solid scintillation crystals are composed of sodium iodide with a small amount of
thallium added as an "activator". The crystal is coupled to a photomultiplier tube
that converts the light flashes to amplified electrical pulses. Amplification factors
of a million or more are achieved. The number of pulses are directly proportional
to the intensity, and the size of the pulse is directly proportional to the energy
of the incident radiation. These pulses are then analyzed by a counter, spectrometer,
oscilloscope, or computer.
Because scintillation crystals are solid, rather than gaseous, their higher density
and atomic number makes them very efficient and sensitive instruments for the measurement
of x-rays and gamma rays. Portable scintillation detectors are even more sensitive
than GM counters because of their increased efficiency. The crystal in a solid scintillation
detector can be thin or thick. The thin crystal has an energy range of approximately
10-60 KeV, while the thick crystal has a range from about 50 KeV to 1 MeV. Scintillation
detectors, however, are not as rugged as Geiger counters because the crystal is hygroscopic
and can absorb water from the atmosphere.
Liquid scintillation detectors use organic compounds that give off light when radioactive
materials are added to a liquid scintillation cocktail (LSC). Material from a swipe
is dissolved or suspended in the solution, and almost all of the emitted radiation
passes through some portion of the scintillator. The light is detected by photomultiplier
tubes and analyzed and counted in a manner similar to solid scintillation detectors.
The liquid scintillation counter is primarily used for detection of beta contamination.
Detection efficiencies range from approximately 40% (H-3) to almost 100% (P-32). The
instrument can also detect alphas (up to 100% efficiency) or gammas (up to 20% efficiencies).
Radiation Monitoring Techniques
Two types of instruments are commonly used to monitor contamination of personnel,
equipment, and areas. Portable survey instruments provide direct measurement capabilities.
Fixed instruments such as liquid scintillation counters provide an indirect means
to determine contamination by analyzing paper wipes of test areas. While portable
instruments are faster, fixed instruments offer greater sensitivity.
Before a portable survey instrument is used, several quality checks should be made.
The calibration sticker should be checked to ensure that the instrument is not due
for recalibration. The batteries should be checked to ensure the instrument will be
powered properly. Finally, the instrument response should be tested with a check source.
Most instruments have a response time selector. This will vary the response from slow (10-15 seconds to reach 70% of true readings) to fast (1-3 seconds). The fast response times will greatly reduce the survey time. After the proper response time is selected, turn on the instrument to its most sensitive scale (e.g. X1 or X0.1) and determine the background readings for that scale. Once the background is determined, the monitoring must be performed slowly at a rate of approximately 1 inch per second and very close to the surface without touching. If the probe has a window, this must be directed at the surface being monitored. However, small or pointed objects can puncture the thin windows if care is not used. If a reading above background is indicated, the probe movement should be stopped to determine the extent over background. The response time should be changed to slow to get a more accurate reading. Since the clean limit is 220 DPM, the actual value can be calculated as follows:
The other method of monitoring uses paper wipes that are analyzed in a fixed instrument
such as a liquid scintillation counter (LSC). A piece of dry filter paper is rubbed
on the area to be tested, using moderate pressure. An area of 100 square centimeters
(16 square inches) should be tested. The filter paper is placed in a LS vial, fluid
added, and counted. The results are calculated in the same manner as the portable
instrument except counting efficiencies are usually much higher.
Personnel Monitoring
Personnel monitoring is used to detect and measure radiation exposure to workers.
The purpose of personnel monitoring is to document the exposure an individual receives
to determine if radiation limits have been exceeded, and to aid in keeping doses as
low or reasonably achievable. Personnel monitors are relatively inexpensive, reasonably
reliable, and portable. They are usually worn on the belt, shirt pocket, collar, or
finger. Personnel monitoring devices designed to measure low energy radiation should
not be worn in a shirt pocket.
Personnel monitoring is required if there is a possibility that a radiation worker
will receive greater than 10% of the maximum permissible dose (MPD).
Film Badges
The film badge consists of one or more photographic emulsions contained in light tight
envelopes inside a plastic holder. These emulsions have varying degrees of sensitivities
to x-rays, gamma rays, beta particles, and neutrons. Windows and filters are built
into the badge to distinguish between different types of radiation. An estimation
of radiation energies can also be made. The film is developed and the density of the
exposed film is proportional to the exposure received by the film badge. The degree
of darkening is compared with film exposed to known quantities of radiations. Film
can also discriminate between primary beam exposure and scattered radiation. Scatter
radiation produces a fuzzy image while a distinct image is produced from primary beam
exposure.
Film badges have several advantages that have made them the most popular form of personnel
monitoring devices. They are inexpensive, reasonably accurate and sensitive, provide
a permanent record of exposure, and supply information on the type and approximate
energy of the radiation exposure.
Film badge monitors also possess several disadvantages that have led to a decline
in their popularity. Film badges are not as accurate or reliable as other personnel
monitoring devices, nor do they detect exposures from low energy photons very well.
In addition, film badges are relatively insensitive, the lower limit of detection
is typically between 20 to 30 mR. Artificially high readings can result from false
darkening as a result of improper handling, heat, humidity and age. In order to limit
false darkening, film badges should not be worn for periods in excess of a month.
Pocket Dosimeters
Pocket dosimeters are small, pencil shaped ionization chambers that directly measure
ionizations due to radiation. The chamber is charged before use and a scale is adjusted
to read zero at this voltage potential. As radiation passes through the chamber, ions
are created. These ions are collected at the electrodes of the chamber and discharge
the dosimeter. This discharge is directly proportional to the quantity of radiation
entering the chamber and read on a voltage meter that is calibrated in radiation units.
Some pocket dosimeters can be read directly from an internal scale while others must
be inserted into a dosimeter reader.
The major advantage of a pocket dosimeter is its ability to supply an immediate readout
of radiation exposure. Because of this, pocket dosimeters are typically used only
in high radiation area. It is important to ensure that the dosimeters can detect the
energy of the radiation field being emitted. Pocket dosimeters are also reasonably
accurate and sensitive, however, they are expensive, easily damaged, and give false
readings due to charge leakage.
Thermoluminescent Dosimeters
The newest type of personnel monitoring device, which is rapidly becoming the most
popular, is the thermoluminescent dosimeter (TLD). This device looks similar to a
film badge but uses a calcium fluoride or lithium fluoride crystal to measure radiation
exposure. When radiation strikes the crystal, electrons absorb the energy and are
promoted to higher energy states in the crystalline lattice. Upon heating, these excited
electrons fall back into their original energy states releasing the stored energy
as ultra-violet light. The amount of this light is directly proportional to the radiation
dose received by the crystal. The light is then quantified by a photomultiplier tube.
TLD monitors have several advantages and few disadvantages. They are more accurate,
reliable, and sensitive than film badges and pocket dosimeters, and their response
to low as well as high energy photons is more uniform. TLD monitors are also relatively
sensitive; radiation doses as low as 5 mR can be detected. TLD monitors are not influenced
by normal heat and moisture, which allows the monitors to be worn for as long as three
months without loss of information. TLD monitors are also reusable, having lost the
"memory" of the previous radiation exposure when heated. They can also be read quickly
so that radiation doses can be obtained within a few minutes.
The primary disadvantage of TLD monitors is cost. The monitoring program can be twice
as expensive as film badge monitoring. However, this cost can be reduced because TLD
monitors can be read every three months instead of monthly. Costs are expected to
be lowered in the future and TLD monitors will probably replace film badges as the
method of choice in personnel dosimetry programs.
6.0 Reducing Radiation Exposure
Because any amount of radiation is potentially harmful every effort should be made
to reduce doses to a level that is as low as reasonably achievable. Time, distance,
shielding, and substitution represent four practical methods laboratory personnel
can use to minimize external radiation exposure.
Time
The dose of radiation received is directly proportional to the amount of time spent
in a radiation field. Thus, reducing the time by one-half will reduce the radiation
dose by one-half. Users should always work quickly, efficiently and spend as little
time as possible around radioisotopes. Personnel can reduce time by ensuring that
all tools and materials are in place before the radioactive material is brought out
of storage. The use of practice runs without radioactive materials is also recommended
to increase familiarization with the technique and increase operator speed.
Distance
Radiation exposure decreases rapidly as the distance between the worker and the radiation
source increases. Maximizing distance represents one the simplest and most effective
methods for reducing radiation exposure to workers. For example, distance can be maximized
by using long handled tools to keep radioactive materials well away from the body
and storing radioactive materials as far from workers as possible.
The decrease in exposure from a point source of x or gamma radiation can be calculated
by using the inverse square law. This law states that the amount of radiation at a
given distance from a point source varies inversely with the square of the distance.
For example, doubling the distance from a radiation source will reduce the dose to
one-fourth of its original value, and increasing the distance by a factor of three
will reduce the dose to one-ninth of its original value. For example, if the dose
rate at one foot from a source is 20 mR/hr, then the dose rate at two feet (twice
the distance) will be 5 mR/hr.
In contrast to x or gamma radiation, beta particles have a finite range in air. Low
energy beta emitters such as H-3, C-14, S-35 do not pose an external radiation exposure
problem when the material is handled in containers. Higher energy beta emitters such
as P-32 do pose an external hazard. Since the energy distribution of betas is from
zero to some maximum (dependent upon the isotope), the average energy is approximately
one-third of the maximum. Once the distance from a beta source exceeds 4 inches, dose
rate reduction follows the inverse square law as the separation distance increases.
Shielding
Radiation exposure can also be decreased by placing a shielding material between a
worker and the source of radiation. Materials with high densities and high atomic
numbers are the most effective shielding choice for protection from x and gamma rays.
The energy of the photons is reduced by Compton and photoelectric interactions in
the shielding material. Thus, substances such as lead, concrete, and steel are very
practical shielding materials because of the abundance of atoms and electrons that
can interact with the photon. As the energy of the gammas increase, the thickness
of shielding must increase to provide comparable stopping power. The amount of shielding
necessary to reduce the radiation intensity to a desired level can be calculated from
the half-value layer of the material. The half-value layer is the thickness of the
material necessary to reduce the radiation intensity to one-half of its original value.
Seven half-value layers will reduce the radiation level approximately 100 times, and
ten half-value layers will reduce the radiation level by a factor of 1000.
An example of a shielding device for gammas is a benchtop shield. The upright portion
shields the whole body while an angled top piece of leaded glass shields the face.
This angled top feature allows for optimum viewing while keeping exposures low. This
type of shield should be used when stock solutions of gamma emitters such as Zn-65
or Fe-59 are manipulated. When low energy gamma emitters such as I-125 are used, lead
foil can effectively reduce the emissions.
The shielding principles applied to beta particles are different from those applied
to x and gamma rays. Since beta particles have a finite range, shields are designed
to totally stop all betas. While gamma shields rely on high density, beta shielding
materials must be made from low density materials. The incidence of "bremsstrahlung"
increases to unacceptable levels if beta shields are composed of materials with an
atomic number higher than aluminum (13).
Bremsstrahlung results in the conversion of beta energy into x-rays and these secondary
x-rays can pose a greater hazard than the original betas. For this reason beta shields
are commonly composed of plexiglass, glass, aluminum, or water. Plexiglass bench top
shields provide protection for the body and eyes and plexiglass or aluminum blocks
are used as tube holders to protect the hands. Several thicknesses of tygon tubing
provide excellent hand shielding when a tube must be held. Plexiglass cylinders or
PVC pipe can shield liquid waste or other containers of radioactive solutions.
Substitution
The isotope selection process is another effective method to reduce potential radiation
exposures. The areas to be considered are: the radioactive half-life, the energy and
type of emissions, the quantity of isotope, and the chemical form of the isotope.
The half-life of the isotope selected can affect waste management. Generally, shorter
lived isotopes are preferred over longer lived. Since the University stores waste
with half-lives less than 90 days until decayed to background, this category of waste
causes minimal monetary impact because it is not shipped. This waste also poses minimal
environmental impact because it is not sent to a radioactive waste burial facility.
The energy and type of emissions from the perspective isotopes must be considered.
Selection of low energy beta or gamma emitters is preferred because radiation hazards
are proportionally related to the energy. Beta emitters are preferred over gamma emitters
because betas require less shielding. The radiation hazard is also proportionally
related to the quantity (radioactivity) of the isotope to be used. The use of small
activities is preferred. The chemical form selected for the experiment can also affect
the radiation hazards associated with the work. It is preferred to avoid the use of
compounds that are or produce volatile or gaseous compounds.
Several examples can be used to illustrate the selection process. When considering
the use of phosphorus, two isotopes are feasible. P-32 has a short 14 day half-life
but emits high energy betas (1.710 MeV). P-33 has a longer half-life (25 days) but
emits low energy betas (0.248 MeV). P-33 would be the most desirable isotope to use,
however, availability is limited. Recently, a substitute for P-32 has been found for
use in a number of molecular biology procedures. The substitute is S-35, with its
87 day half-life and low 0.168 MeV beta. The low energy improves resolution of autoradiographs
and requires no shielding or remote handling.
A common use of iodine involves studies with iodinated hormones. Two isotopes of iodine,
I-125 and I-131, are feasible. The low x and gamma radiation of I-125 makes it more
acceptable than I-131 (high beta and gamma emitter).
Exposure of the human body to ionizing radiation can result in harmful biological effects. The nature and severity of the effects depends primarily on the dose of radiation absorbed and the rate at which it is received. Exposure to radiation can result in radiation burns and sickness, cancer, genetic defects, and abnormalities in unborn children. Very large doses of radiation to the whole body can result in death. These effects have been observed in people exposed to radiation in a variety of situations including therapeutic x-rays, radiation accidents, and the Japanese A-bomb survivors.
Radiation Burns
Radiation burns were first noted within a month of Roentgen's discovery of x-rays.
Within a year, it was widely known that radiation workers had to take precautions
to avoid injury. Today, great efforts are made to protect workers from accidental
exposure but radiation induced injuries still occur. Severe local injury may result
when a worker is exposed to a high dose of radiation for a short period of time. Symptoms
may range from reddening of the skin, swelling and blistering, to tissue death and
amputation of the affected area.
For example accidental exposure to the primary beam from analytical x-ray equipment
may result in high radiation doses to localized areas of the body. The smallest dose
to the skin that will result in visible damage is approximately 300 rem. Reddening
of the skin, called erythema, may occur 2-3 weeks after the exposure in highly susceptible
individuals. Usually the dose must exceed 600 rem before radiation burns become apparent.
These burns are equivalent to first-degree thermal burns similar to a mild sunburn.
There are no initial symptoms from the over-exposure and the worker may be unaware
of an injury. These types of injury, however, are typically not seen in radioisotope
laboratories.
Radiation Sickness
Radiation burns occur when a large dose of radiation is received by a small part of
the body. Severe damage and tissue death may occur but the exposed person usually
survives. If a large dose of radiation is delivered to the whole body of an individual
in a short period of time, severe illness or death may occur. The sequence of events
that follows exposure to high levels of radiation to the whole body is termed radiation
sickness or the "acute radiation syndrome".
Radiation doses to the whole body greater than 100 rem delivered within a few hours,
are usually necessary to produce noticeable symptoms. Changes in the blood, however,
can be observed from exposures as low as 25 rem. Symptoms usually become apparent
within a few hours or days depending on the dose received. The first stage of radiation
sickness is often characterized by nausea, vomiting, and diarrhea. Following this
initial period of sickness, symptoms may subside and the individual may feel well.
This stage can last from hours to weeks, and while no symptoms are present, changes
are occurring in the internal organs.
Following this asymptomatic period, other symptoms may appear. Loss of hair and appetite,
fatigue, fever, severe diarrhea, vomiting, internal bleeding, and death may occur,
depending on the dose received. If a whole-body dose of 400-500 rem is received, approximately
50% of those exposed will die within 30 days if untreated. Recovery is likely with
medical care although the exposed individual will suffer several months of illness.
If the radiation dose is spread over several weeks, a person may survive a whole-body
dose or large as 1000 to 2000 rem. Exposure to a dose in excess of 700 rem to the
entire body in a short period of time will likely result in death within a few weeks.
This dose of radiation kills cells in the bone marrow and the body can no longer produce
enough red blood cells to survive. Doses of the magnitude necessary to cause radiation
sickness are not typically seen in radioisotope laboratories.
Cancer
Exposure to chronic small doses of radiation over long periods of time can result
in delayed effects that may become apparent years after the initial exposure. These
effects may also occur after acute exposure to high doses and include carcinogenesis,
life span shortening, and cataract formation.
The principle delayed effect from chronic exposure to radiation is an increased incidence
of cancer. Ionizing radiation is a well known carcinogenic agent in animals and humans
and has been implicated as capable of inducing all types of human cancers. Those types
of cancer with the strongest association with radiation exposure include leukemia,
cancer of the lung, bone, female breast, liver, skin, and thyroid gland.
By 1905 it was widely known that exposure to radiation could cause cancer. Many of
the early researchers who were exposed to large repeated doses of radiation died from
fatal skin cancer and leukemia. Marie and Pierre Curie, for example, both developed
leukemia, probably from their experiments with radium.
Further evidences that ionizing radiation can induce cancer in humans has been demonstrated among radiation workers, children exposed in-utero to diagnostic x-rays, patients receiving therapeutic x-rays and internal radiation exposure, individuals exposed to fallout, and the Japanese A-bomb survivors. Some of these evidences are summarized below:
It is not known how radiation induces cancer. Several theories have been proposed
to explain the carcinogenic properties of radiation. Cancer is characterized by the
uncontrolled growth of cells. According to one theory, radiation damages the chromosomes
in the nucleus of a cell resulting in the abnormal replication of that cell. Another
theory postulates that radiation decreases the overall resistance of the body and
allows existing viruses to multiply and damage cells. A third theory suggests that
as a result of irradiation of water molecules in the cells, highly reactive and damaging
agents called "free radicals" are produced which may play a part in cancer formation.
Approximately 25% of all adults between the ages of 20 and 65 will develop cancer
during their lifetime. It is not known what an individual's chances are of developing
cancer from exposure to ionizing radiation. However, risk estimates can be made based
on statistical increases in the incidence of cancer among populations exposed to large
doses of radiation.
The Nuclear Regulatory Commission (NRC) has adopted a linear, non-threshold model
for calculating the cancer risks associated with low level radiation exposure. According
to the NRC, this model neither seriously underestimates nor overestimates the risks
involved from radiation exposure. Using this model, the risks decrease proportionally
to the does of radiation. Thus, a worker who receives 5,000 mR/yr is assumed to incur
ten times the risk as a worker who receives 500 mR/yr. Because no threshold is assumed,
theoretically all radiation exposures have the potential to cause cancer. Based on
this model, the best risk estimates available today are that an additional 3 cancers
would occur in a group of 10,000 radiation workers exposed to 1,000 mR each. This
should be compared to the 2,500 cancer cases that would be expected to occur from
other causes. It is important to realize that these risk estimates are extrapolated
from high doses and may not apply to low doses. Increases in cancer have not been
clearly demonstrated at levels below the occupational limit of 5,000 mR/yr.
Recent controversial studies have suggested that linear extrapolation from high doses
may significantly underestimate the actual risks involved from chronic low doses of
radiation. Other studies have indicated that extrapolation may overestimate these
risks. Both sets of data, however, lack sufficient validity to be used with confidence
for the estimation of cancer risks at this time.
Genetic Effects
Radiation exposure to the reproductive cells can alter the genetic code, resulting
in damaged or defective genes that can be passed on to future generations. It has
been known since 1927 that radiation can cause genetic defects in the descendants
of insects. Experiments with other animals have shown similar results. These studies
demonstrated that radiation does not increase the types of mutations seen in nature,
only the frequency.
Genetic mutations, however, have not been demonstrated in human populations exposed
to radiation. For example, studies of the children of the A-bomb survivors in Japan
have not detected any more genetic defects than expected. It is very difficult to
determine if a person has a particular genetic defect. Usually there are no easily
detectable signs and several generations and large populations may be necessary before
the mutation becomes visible. Most effects will probably be seen in subsequent generations
as minor impairments that lead to higher spontaneous abortions, shorter life spans,
increases in diseases, and ill health. Serious genetic defects usually do not manifest
themselves because the person does not survive to reproduce.
Based on the irradiation of animals, the following inferences can be made regarding genetic effects in humans:
Because genetic mutations are usually undesirable, the level of genetic defects in
the population should be kept as low as possible. This can be accomplished by avoiding
any unnecessary radiation exposure.
The risk of a genetic defect in a child of a person exposed to one rem of radiation
is approximately one-third that of developing cancer. Thus, there would be about one
chance in 10,000 that the child would have a genetic defect. Because genetic defects
are less likely than cancer, and not as serious, the risk of developing cancer from
radiation exposure is more significant.
Teratogenic Effects
Radiation exposure to a pregnant woman may be harmful to the unborn child. Malformations
induced in the embryonic or fetal stages of development are termed teratogenic effects.
Embryological and fetal tissue are composed of rapidly dividing unspecialized cells
that are highly sensitive to damage from ionizing radiation. Radiation exposure in-utero
can result in spontaneous abortions, congenital abnormalities, impairment of growth
and mental functions, and increased incidences of leukemia.
The effects of radiation exposure during pregnancy are dependent on the stage of pregnancy
and the dose of radiation received. The most critical stage of pregnancy is the first
trimester. This includes the time a woman may not even be aware she is pregnant.
During this period, rapid cell division is occurring and the major body organs are
forming. Exposure during the first trimester may result in embryonic death or congenital
malformations. During the later stages of fetal growth, functional changes such as
learning disorders or leukemia are possible. As the fetus ages, however, it becomes
more resistant to radiation.
Evidences of embryological and fetal effects in humans have been demonstrated in the
Japanese A-bomb survivors and children exposed to diagnostic x-rays while in-utero.
An increase in mental retardation and small head circumference was observed in the
children of the A-bomb survivors. Irradiation of the fetus from diagnostic x-rays
has been associated with an increased incidence of leukemia in children. The highest
risk occurred to those x-rayed during the first trimester. Because of the sensitivity
of the developing fetus to radiation, a pregnant radiation worker should limit her
whole-body exposure to 500 mR during the course of the pregnancy.
Conclusion
The health risks associated with exposure to ionizing radiation are smaller that the
risks involved in many of our daily activities. These risks are also comparable to
those encountered in other professions. This small but real increase in health risks
calls for a weighing of the benefits versus the risks associated with the use of radiation.
Radiation workers are benefited because their livelihood is derived from the use of
radiation and students and researchers receive the benefit of a valuable educational
and research tool. However, if the same information can be obtained by using methods
that reduce the exposure and risks then they should be employed. Because the biological
effects of exposure to low level ionizing radiation are not fully understood, it is
prudent to maintain radiation doses at a level that is as low as reasonably achievable
(ALARA concept).
Glossary
Activity: the number of atoms decaying per unit of time.
Airborne radiation area: any room, enclosure, or operating area where airborne radioactive materials exist
in concentrations above the maximum permissible concentration (MPC) specified in 10
CFR 20; or any room enclosure or operating area where airborne radioactive material
exists in concentrations that, averaged over the number of hours in any week when
individuals are in the area, exceed 25% of the MPC's specified in 10 CFR 20.
Alpha particle: a helium nucleus consisting of two neutrons and two protons, with a mass of 4 AMU
and a charge of +2.
As low as reasonably achievable (ALARA): basic radiation protection concept to reduce doses to the lowest possible levels
through the proper use of time, distance and shielding.
Atom: the smallest division of matter that still displays the chemical properties of an
element.
Atomic mass unit (AMU): one twelfth of the arbitrary mass assigned to carbon 12. It is equal 1.6604 x 10
24 gm.
Becquerel: a unit of activity equal to one disintegration per second.
Beta particle: a charged particle emitted from the nucleus of an atom, with a mass and charge equal
to that of the electron.
Bremsstrahlung: German for braking radiation. It is incidental photon radiation caused by the deceleration
of charged particles passing through matter.
Chain decay: a process by which an unstable atom decays to another unstable atom, repeating the
process until the atom becomes stable.
Code of Federal Regulations (CFR): title 10 contains the regulations established by the NRC. Part. 19 deals with the
rights of employees to be informed of any radiation hazards associated with their
working conditions, and the rights of the worker to complain about any working conditions
that may be unsafe. Part. 20 is the basic regulatory guide which establishes the standards
for protection against ionizing radiation.
Compton scattering: interaction process for x or gamma radiation where an incident photon interacts
with an orbital electron of an atom to produce a recoil electron and a scattered photon
with energy less than the incident photon.
Curie: a unit of activity equal to 3.7 E10 disintegrations per second.
Decay, radioactive: the disintegration of the nucleus of an unstable atom caused by the spontaneous
emission of charged particles and/or photons.
Electron: elementary particle with a unit negative charge and a mass of 1/1840 AMU.
Energy shells: labels given to the different orbits of the negatively charged electrons circling
the nucleus of an atom.
Gamma: electromagnetic radiation with a very short wave length and no mass or charge.
Half-life: the time required for the initial activity to decrease by half.
High radiation area: any area accessible to personnel where there exists radiation at such levels that
a major portion of the body could receive a dose over 100 mR in any one hour.
Isotopes: atoms with the same number of protons, but different numbers of neutrons.
Monoenergetic: where all the particles or photons of a given type of radiation (alpha, beta, neutron,
gamma, etc.) originate with and have the same energy.
Neutrino: a particle with no mass or charge, but has energy associated with it.
Neutron: an atomic particle with a mass of 1 AMU and no charge.
Nuclear Regulatory Commission (NRC): Federal agency charged with the responsibility of regulating the use of radioactive
material.
Nucleus: the central part of an atom that has a positive charge, and is composed of protons
and neutrons.
Pair production: an absorption process for x and gamma radiation where the incident photon is annihilated
in the vicinity of the nucleus of the absorbing atom, producing an ion pair (beta
and positron). This process only occurs for incident photon energies exceeding 1.02
MeV.
Photoelectric effect: process by which a photon ejects an electron from an atom. All the energy of the
photon is absorbed in ejecting the electron and in imparting energy to it.
Photon: energy emitted in the form of electromagnetic radiation, such as x-rays and gamma
rays.
Positron: particle equal in mass to the electron and having an equal but positive charge.
Proton: a particle with a positive charge and a mass of 1 AMU.
Quality factor: a term to express the varying effects of different types of radiation when assessing
doses to tissue.
Rad: an amount of absorbed radiation dose of 100 ergs per gram of matter.
Radiation area: any area accessible to personnel where there exists radiation at such levels that
a major portion of the body could receive a dose of over 5 mR in any one hour or a
dose over 100mR in any 5 consecutive days.
Rem: stands for radiation equivalent man, the dose in rems is equal to the dose in rads
multiplied by the quality factor.
Restricted area: any area where access is controlled by the University to protect individuals from
exposure to radiation and radioactive materials.
Roentgen: the amount of x or gamma radiation which will cause ionization of one electrostatic
unit of charge in 1 cubic centimeter of dry air at standard temperature and pressure.
Specific activity: total activity of a given nuclide per gram of a compound, element, or radioactive
nuclide.
Tenth value: the thickness of a given material that will decrease the amount of radiation to
one-tenth of the original value.
X-ray: penetrating electromagnetic radiation similar to visible light, but having extremely
short wave lengths.
NOTICE TO EMPLOYEES
Standards for protection against radiation notices, instructions and reports to workers,
inspections.
Your employer's responsibility
Your employer is required to:
Your responsibility as a worker
Reports on your Radiation exposure history
The Department of Health regulations require that your employer give you a written
report if you receive an exposure in excess of any applicable limit as set forth in
the regulations or in the license. The basic limits for exposure to employees are
set forth in Section D.101, D.103 and D.104 of the regulations. These sections specify
limits on exposure to radiation and exposure to concentrations of radioactive material
in air and water.
If you work where personnel monitoring is required pursuant to Section D.202:
Inspections
All licensed or registered activities are subject to inspection by representatives
of the Department of Health. In addition, any worker or representative of workers
who believes that there is a violation of the Rules and Regulations for Ionizing Radiation
or the terms of the employer's license or registration with regard to radiological
working conditions in which the worker is engaged may request an inspection by sending
a notice of the alleged violation to the Bureau of Radiological Health. The request
must set forth the specific grounds for the notice, and must be signed by the worker
or the representative of the workers. If requested, anonymity will be maintained.
During inspections agency inspectors may confer privately with the workers and any
worker may bring to the attention of the inspector any past or present condition which
he believes contributed to or caused any violation as described above. No licensee
or registrant shall discharge or in any manner discriminate against any worker because
such worker has filed any complaint or instituted or caused to instituted any proceeding
under these regulations or has testified or is about to testify in any such proceeding
or because of the exercise by such worker on behalf of himself or others of any option
afforded by this part.
Inquiries
Inquiries dealing with matters outlined above can be sent to the Bureau of Radiological
Health, 910 James Madison Building, 109 Governor Street, Richmond, Virginia 23219,
Telephone 800-786-5932 or the Radiation Safety Office, Radford University, Radford,
Virginia 24142, Telephone 540-831-5860.