Introduction to
Radiation Safety
NDT (Nondestructive Testing) Resource Center (USA)
http://www.ndt-ed.org/EducationResources/CommunityCollege/RadiationSafety/introduction/backround.htm
(Accessed 11 March 2011)
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Introduction
Background Information
X-Radiation
Gamma Radiation
Health Concerns
Radiation Theory
Nature of Radiation
Sources of High Energy
Radiation
Radiation for Industrial
Radiography
Radioactive Decay and Half-life
Energy, Activity, Intensity and
Exposure
Interaction of X- and Gamma
Rays with Matter
Ionization
Cell Radiosensitivity
Measures Relative to Biological
Effects
Biological Effects
Biological Effects
Stochastic (Delayed) Effects
-Cancer
-Leukemia
-Genetic Effects
-Cataracts
Nonstochastic (Acute) Effects
Exposure Symptoms
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Safe Use of Radiation
The NRC & Code of Federal
Regulations
Exposure Limits
Controlling Exposure
-Time-Dose Calculation
-Distance-Intensity Calculation
HVL-Shielding
Safety Controls
Responsibilities
Procedures
Survey Techniques
Radiation Safety Equipment
Radiation Detectors
Survey Meters
Pocket Dosimeter
Audible Alarm Rate Meters
Film Badges
Thermoluminescent Dosimeter
Video Clips
References
Quizzes
Introduction to Radiation Safety
Radiography is an important tool in nondestructive evaluation. The method offers a number of
advantages over other NDE methods, but one of its disadvantages is the health risk associated
with the radiation. Health effects can occur due to either long-term low level exposure or shortterm high level exposure.
The primary risk from occupational radiation exposure is an
increased risk of cancer. The amount of risk depends on the
amount of radiation dose received, the time over which the dose is
received, and the body parts exposed. Although scientists assume
low-level radiation exposure increases one's risk of cancer,
medical studies have not demonstrated adverse health effects in
individuals exposed to small chronic radiation doses (i.e., up to
10,000 mrem above background). The increased risk of cancer
from occupational radiation exposure is small when compared to
the normal cancer rate in today's society. The current lifetime risk
of dying from all types of cancer in the United States is
approximately 20 percent (see Figure). If a person received a
radiation dose of 10 rem to the entire body (above background),
his or her risk of dying from cancer would increase by one percent.
Complicating matters further is the fact that Gamma and X-ray
radiation are not detectable by the human body. However, the risks can be minimized when the
radiation is handled and managed properly. The law requires that individuals receive training in
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the safe handling and use of radioactive materials and radiation producing devices. Some of the
topics this training should cover include:
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Health concerns associated with exposure to radioactive materials or radiation.
Precautions or procedures to minimize exposure to radiation.
Purposes and functions of protective devices employed.
The permit conditions and the applicable portions of the Radiation Safety Manual.
Worker’s responsibility to promptly report any condition that may lead to or cause a
violation of the regulations or cause an unnecessary exposure.
Actions to take in the event of an emergency.
Radiation exposure reports that workers have a right to receive.
This material is intended to provide an overview of the fundamental principles and safety
regulations as they apply to radiography. It does not in itself satisfy all of the radiation
safety training required by law.
X-Radiation
X-rays were discovered in 1895 by Wilhelm Conrad
Roentgen (1845-1923) who was a Professor at
Wuerzburg University in Germany. Working with a
cathode-ray tube in his laboratory, Roentgen observed a
fluorescent glow of crystals on a table near his tube. The
tube that Roentgen was working with consisted of a glass
envelope (bulb) with positive and negative electrodes
encapsulated in it. The air in the tube was evacuated, and
when a high voltage was applied, the tube produced a
fluorescent glow. Roentgen shielded the tube with heavy
black paper, and discovered a green colored fluorescent
light generated by a material located a few feet away
from the tube.
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Roentgen's discovery was a scientific bombshell, and was received with extraordinary
interest by both scientists and laymen. Experimenters, physicians, laymen, and
physicists alike set up X-ray generating apparatus to duplicate his experiments. Public
fancy was caught by this invisible ray with the ability to pass through solid matter,
and, in conjunction with a photographic plate, provide a picture of bones and interior
body parts. Scientific fancy was captured by the demonstration of a wavelength
shorter than light. All this work was done with a lack of concern regarding potential
dangers. Such a lack of concern is quite understandable, for there was nothing in
previous experience to suggest that X-rays would in any way be hazardous. Indeed,
the opposite was the case, for who would suspect that a ray similar to light but unseen,
unfelt, or otherwise undetectable by the senses would be damaging to a person? More
likely, or so it seemed to some, X-rays could be beneficial for the body.
Gamma Radiation
Shortly after the discovery of x-rays, another form of
penetrating rays was discovered. In 1896, French
scientist Henri Becquerel discovered natural
radioactivity. Many scientists of the period were
working with cathode rays, and other scientists were
gathering evidence on the theory that the atom could
be subdivided. Some of the new research showed that
certain types of atoms disintegrate by themselves.
Henri Becquerel discovered this phenomenon while
investigating the properties of fluorescent minerals.
One of the minerals Becquerel worked with was a
uranium compound. Uranium ore produces naturally
occurring gamma radiation. Becquerel's discovery
was, unlike that of the x-rays, virtually unnoticed by
laymen and scientists alike. Only a few scientists were
interested in Becquerel's findings. It was not until the
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discovery of radium by the Curies two years later that interest in radioactivity became
widespread.
While working in France at the time of Becquerel's discovery, Polish scientist Marie
Curie became very interested in his work. Marie and her husband, French scientist
Pierre Curie studied radioactive materials, particularly pitchblende, the ore from
which uranium was extracted. They noticed that pitchblende was strangely more
radioactive than the uranium extracted from it. They deduced that the pitchblende
must contain traces of an unknown radioactive substance far more radioactive than
uranium.
Through several years of work, they progressively concentrated the radioactive
substances of several tons of pitchblende ore. Their work resulted in the identification
of two new chemical elements. The first element, they named "polonium," after
Marie's native country, Poland. The other element they named "radium," for its
intense radioactivity. Radium became the initial industrial gamma ray source. The
material allowed radiographs of castings up to 10 to 12 inches thick to be produced.
The couple became well known for their work, but they also became victims of
radiation poisoning. When early scientists were working with naturally occurring
radioactive materials, the effects of radiation on the human body were little
understood, or were ignored in the haste to learn more about this new substance. By
1929, industrial radiation sources were becoming available for radiographing
extremely thick materials. Exposure times were long, and often radiographers were
exposed to excessive doses of radiation.
During World War II and the race to produce a nuclear weapon, much was discovered
about radioactive materials, and manmade isotopes became available. These sources
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were smaller, and considerably stronger than the naturally occurring radioactive
material. Manmade sources were developed to penetrate even thicker materials,
however, they also cause more damage to persons exposed to the radiation. Many
deaths and amputations occurred in this era of early experimentation and use of
isotopes.
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Health Concerns
The science of radiation protection, or "health physics" as it is more properly called, grew out of
the parallel discoveries of x-rays and radioactivity in the closing years of the 19th century. Since
early workers with gamma and x-radiation took few, if any, precautions, serious injuries
inevitably occurred. Radiation burns were recorded within a month of Roentgen’s announcement
of his discovery of x-rays. Becquerel burned himself by carrying a sample of radium in his
pocket. Marie and Pierre Curie received radiation burns on their skin from working with radium,
and it is suspected their lives were cut short because of exposure to large amounts of radiation.
Many developed skin cancer and suffered amputations of fingers and hands as a result of high
exposures of radiation. By the 1922, radiation exposure had caused over 150 deaths.
As early as 1900, five years after Roentgen’s discovery, it was understood that precautions
needed to be taken when working with x- and gamma radiation. The first warning of possible
adverse effects of x-rays came from Thomas Edison, William J. Morton, and Nikola Tesla who
each reported eye irritations from experimentation with x-rays and fluorescent substances.
Studies of the effects of radiation on living tissue were initiated and the development of safe
working practices began. In the 1920’s, the routine use of film badges for personnel monitoring
was introduced and the genetic effects of x- and gamma rays were recognized. Adoption of the
Roentgen as a unit for measuring radiation by the Second International Congress on Radiation
occurred in 1928.
Today, it can be said that radiation ranks among the most thoroughly investigated causes of
disease. Although much still remains to be learned, more is known about the mechanisms of
radiation damage on the molecular, cellular, and organ system than is known for most other
health stressing factors. Moreover, it is the vast accumulation of quantitative dose-response data
that enables health physicists to specify radiation levels so that medical, scientific, and industrial
uses of radiation may continue at levels of risk comparable with the risks associated with any
other technology.
The image on the right shows severe radiation
burns on the back of a man. The man was one of
three woodsmen who found a pair of canisters in
the mountains of the country of Georgia
(formally part of the USSR). The men did not
know the canisters were intensely radioactive
relics that were once used to power remote
generators. Since the canisters gave off heat, the
men carried them back to their campsite to warm
themselves on a cold winter night.
Nature of Radiation
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Radiation is a form of energy. There are two basic
types of radiation. One kind is particulate radiation,
which involves tiny fast-moving particles that have
both energy and mass. Particulate radiation is primarily
produced by disintegration of an unstable atom and
includes Alpha and Beta
particles.
Alpha particles are high energy, large subatomic
structures of protons and neutrons. They can travel only a
short distance and are stopped by a piece of paper or skin.
Beta particles are fast moving electrons. They are a
fraction of the size of alpha particles, but can travel
farther and are more penetrating.
Particulate radiation is of secondary concern to industrial radiographers. Since these
particles have weight and are relatively large, they are easily absorbed by a small
amount of shielding. However, it should be noted that shielding materials, such as
the depleted uranium used in many gamma radiography cameras, will be a
source of Beta particles if the container should ever develop a leak. If a leak were
to occur, the material could be transferred to the hands and other parts of a
radiographer’s body, causing what is known as particulate contamination. This is the
reason periodic “leak” and “wipe tests” are performed on equipment.
The second basic type of radiation is electromagnetic radiation. This kind of radiation
is pure energy with no mass and is like vibrating or pulsating waves of electrical and
magnetic energy. Electromagnetic waves are produced by a vibrating electric charge
and as such, they consist of both an electric and a magnetic component. In addition to
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acting like waves, electromagnetic radiation acts like a stream of small "packets" of
energy called photons. Another way that electromagnetic radiation has been described
is in terms of a stream of photons. The massless photon particles each travel in a
wave-like pattern. Each photon contains a certain amount (or bundle) of energy, and
all electromagnetic radiation consists of these photons. The only difference between
the various types of electromagnetic radiation is the amount of energy found in the
photons. Electromagnetic radiation travels in a straight line at the speed of light (3 x
108 m/s).
Light waves, radio waves, microwaves, X-rays and Gamma rays are some examples
of electromagnetic radiation. These waves differ in their wavelength as shown in the
electromagnetic spectrum image above. Although all portions of the electromagnetic
spectrum are governed by the same laws, their different wavelengths and different
energies allow them to have different effects on matter. Radio waves, for example,
have such a long wavelength and low energy that our eyes cannot detect them and
they pass through our bodies. It takes a special antenna and electronics to capture and
amplify radio waves.
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The wavelength of visible light is on the order of 6,000 angstroms, while the
wavelength of X-rays is in the range of one angstrom and that of Gamma rays is
0.0001 angstrom. This very short wavelength is what gives X-rays and Gamma rays
their power to penetrate materials that light cannot. Unlike light, X- and gamma rays
cannot be seen, felt, or heard. The fact that they cannot be detected with our normal
human senses and can damage our cells is why they must be treated carefully.
Sources of High Energy Radiation
There are many sources of harmful, high energy
radiation. Industrial radiographers are mainly
concerned with exposure from x-ray generators
and radioactive isotopes, but let's start by
considering sources of radiation in general. It is
important to understand that eighty percent of
human exposure comes from natural sources such
as outer space, rocks and soil, radon gas, and the
human body. The remaining twenty percent comes
from man-made radiation sources, such as those
used in medical and dental diagnostic procedures.
One source of natural radiation is cosmic radiation.
The earth and all living things on it are constantly
being bombarded by radiation from space. The sun
and stars emits EM radiation of all wavelengths.
Charged particles from the sun and stars interact
with the earth’s atmosphere and magnetic field to produce a shower of radiation,
typically beta and gamma radiation. The dose from cosmic radiation varies in
different parts of the world due to differences in elevation and the effects of the
earth’s magnetic field.
Radioactive material is also found throughout nature. It occurs naturally in soil, water,
plants and animals. The major isotopes of concern for terrestrial radiation are uranium
and the decay products of uranium, such as thorium, radium, and radon. Low levels of
uranium, thorium, and their decay products are found everywhere. Some of these
materials are ingested with food and water, while others, such as radon, are inhaled.
The dose from terrestrial sources varies in different parts of the world. Locations with
higher concentrations of uranium and thorium in their soil have higher dose levels. All
people also have radioactive isotopes, such as potassium-40 and carbon-14, inside
their bodies. The variation in dose from one person to another is not as great as the
variation in dose from cosmic and terrestrial sources.
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There are also a number of manmade radiation sources that present some exposure to
the public. Some of these sources include tobacco, television sets, smoke detectors,
combustible fuels, certain building materials, nuclear fuel for energy production,
nuclear weapons, medical and dental X-rays, nuclear medicine, X-ray security
systems and industrial radiography. By far, the most significant source of man-made
radiation exposure to the average person is from medical procedures, such as
diagnostic X-rays, nuclear medicine, and radiation therapy.
Production of Radiation for Industrial Radiography
Industrial radiography uses two sources of radiation: X-radiation and Gamma
radiation. X-rays and Gamma rays differ only in their source of origin. X-rays are
produced by an X-ray generator, and Gamma radiation is the product of radioactive
atoms. An in depth discussion on radiation production can be found in other areas of
this site, but will be reviewed
briefly in the following
sections.
Production of X-Rays
There are two different atomic
processes that can produce Xray photons. One process
producesBremsstrahlung radiati
on and the other produces K12
shell or characteristic emission. Both processes involve a change in the energy state of
electrons. X-rays are generated when an electron is accelerated and then made to
rapidly decelerate, usually due to interaction with other atomic particles.
In an X-ray system, a large amount of electric current is passed through a tungsten
filament, which heats the filament to several thousand degrees centigrade to create a
source of free electrons. A large electrical potential is established between the
filament (the cathode) and a target (the anode). The cathode and anode are enclosed in
a vacuum tube to prevent the filament from burning up and to prevent arcing between
the cathode and anode. The electrical potential between the cathode and the anode
pulls electrons from the cathode and accelerates them as they are attracted towards the
anode or target, which is usually made of tungsten. X-rays are generated when free
electrons give up some of their energy when they interact with the electrons or
nucleus of an atom. The interaction of the electrons in the target results in the
emission of a continuous Bremsstrahlung spectrum and also characteristic X-rays
from the target material.
Production of Gamma Rays
Gamma radiation is the product of radioactive atoms. Depending upon the ratio of
neutrons to protons within its nucleus, an isotope of a particular element may be stable
or unstable. Over time, the nuclei of unstable isotopes spontaneously disintegrate, or
transform, in a process known as radioactive decay. Various types of radiation may be
emitted from the nucleus and/or its surrounding electrons when an atom experiences
radioactive decay. Nuclides which undergo radioactive decay are called radionuclides.
Any material which contains measurable amounts of one or more radionuclides is a
radioactive material.
There are many naturally occurring radioactive materials, but manmade radioactive
isotopes or radioisotopes are used for industrial radiography. Man-made sources are
produced by introducing an extra neutron to atoms of the source material. For
example, Cobalt-60 is produced by bombarding a sample of Cobalt-59 with an excess
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of neutrons in a nuclear reactor. The Cobalt-59 atoms absorb some of the neutrons and
increase their atomic weight by one to produce the radioisotope Cobalt-60. This
process is known as activation. As a material rids itself of atomic particles to return to
a balance state, energy is released in the form of Gamma rays and sometimes alpha or
beta particles.
Physical size of isotope materials will very slightly between manufacturer, but
generally an isotope is a pellet that measures 1.5 mm x 1.5 mm. Depending on the
activity (curies) desired, a pellet or pellets are loaded into a stainless steel capsule and
sealed. Unlike X-ray tubes, radioactive sources provide a continual source of radiation
that cannot be turned off. Once radioactive decay starts, it continues until all of the
atoms have reached a stable state. The radioisotope can only be shielded to prevent
exposure to the radiation. In industrial radiography, the instruments that are used to
shield the radioisotope so that they can be safely handled and used are commonly
called cameras or exposure devices. Exposure devices will be discussed later in more
detail.
Radioactive Decay and Half-Life
As mentioned previously, radioactive decay is the disintegration of an unstable atom
with an accompanying emission of radiation. As a radioisotope atom decays to a more
stable atom, it emits radiation only once. To change from an unstable atom to a
completely stable atom may require several disintegration steps and radiation will be
given off at each step. However, once the atom reaches a stable configuration, no
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more radiation is given off. For this reason, radioactive sources become weaker with
time. As more and more unstable atoms become stable atoms, less radiation is
produced and eventually the material will become non-radioactive.
The decay of radioactive elements occurs at a fixed rate. The half-life of a
radioisotope is the time required for one half of the amount of unstable material to
degrade into a more stable material. For example, a source will have an intensity of
100% when new. At one half-life, its intensity will be cut to 50% of the original
intensity. At two half-lives, it will have an intensity of 25% of a new source. After ten
half-lives, less than one-thousandth of the original activity will remain. Although the
half-life pattern is the same for every radioisotope, the length of a half-life is different.
For example, Co-60 has a half-life of about 5 years while Ir-192 has a half-life of
about 74 days.
Energy, Activity, Intensity and Exposure
Different radioactive materials and X-ray generators produce radiation at different
energy levels and at different rates. It is important to understand the terms used to
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describe the energy and intensity of the radiation. The four terms used most for this
purpose are: energy, activity, intensity and exposure.
Radiation Energy
As mentioned previously, the energy of the radiation is responsible for its ability to
penetrate matter. Higher energy radiation can penetrate more and higher density
matter than low energy radiation. The energy of ionizing radiation is measured in
electronvolts (eV). One electronvolt is an extremely small amount of energy so it is
common to use kiloelectronvolts (keV) and megaelectronvolt (MeV). An electronvolt
is a measure of energy, which is different from a volt which is a measure of the
electrical potential between two positions. Specifically, an electronvolt is the kinetic
energy gained by an electron passing through a potential difference of one volt. X-ray
generators have a control to adjust the keV or the kV.
The energy of a radioisotope is a characteristic of the atomic structure of the material.
Consider, for example, Iridium-192 and Cobalt-60, which are two of the more
common industrial Gamma ray sources. These isotopes emit radiation in two or three
discreet wavelengths. Cobalt-60 will emit 1.33 and 1.17 MeV Gamma rays, and
Iridium-192 will emit 0.31, 0.47, and 0.60 MeV Gamma rays. It can be seen from
these values that the energy of radiation coming from Co-60 is about twice the energy
of the radiation coming from the Ir-192. From a radiation safety point of view, this
difference in energy is important because the Co-60 has more material penetrating
power and, therefore, is more dangerous and requires more shielding.
Activity
The strength of a radioactive source is called its
activity, which is defined as the rate at which
the isotope decays. Specifically, it is the
number of atoms that decay and emit radiation
in one second. Radioactivity may be thought of
as the volume of radiation produced in a given
amount of time. It is similar to the current
control on a X-ray generator. The International
System (SI) unit for activity is the becquerel
(Bq), which is that quantity of radioactive
material in which one atom transforms per
second. The becquerel is a small unit. In
practical situations, radioactivity is often
quantified in kilobecqerels (kBq) or
megabecquerels (MBq). The curie (Ci) is also commonly used as the unit for activity
of a particular source material. The curie is a quantity of radioactive material in which
3.7 x 1010 atoms disintegrate per second. This is approximately the amount of
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radioactivity emitted by one gram (1 g) of Radium 226. One curie equals
approximately 37,037 MBq. New sources of cobalt will have an activity of 20 to over
100 curies, and new sources of iridium will have an activity of similar amounts.
Once a radioactive nucleus decays, it is no longer possible for it to emit the same
radiation again. Therefore, the activity of radioactive sources decrease with time and
the activity of a given amount of radioactive material does not depend upon the mass
of material present. Additionally, two one-curie sources of Cs-137 might have very
different masses depending upon the relative proportion of non-radioactive atoms
present in each source. The concentration of radioactivity, or the relationship between
the mass of radioactive material and the activity, is called the specific activity.
Specific activity is expressed as the number of curies or becquerels per unit mass or
volume. The higher the specific activity of a material, the smaller the physical size of
the source is likely to be.
Intensity
Radiation intensity is the amount of energy passing through a given area that is
perpendicular to the direction of radiation travel in a given unit of time. The intensity
of an X-ray or gamma-ray source can easily be measured with the right detector.
Since it is difficult to measure the strength of a radioactive source based on
its activity, which is the number of atoms that decay and emit radiation in one second,
the strength of a source is often referred to in terms of its intensity. Measuring the
intensity of a source is sampling the number of photons emitted from the source in
some particular time period, which is directly related to the number of disintegrations
in the same time period (the activity).
Exposure
One way to measure the intensity of x-rays or gamma rays is to measure the amount
of ionization they cause in air. The amount of ionization in air produced by the
radiation is called the exposure. Exposure is expressed in terms of a scientific unit
called a roentgen (R or r). The unit roentgen is equal to the amount of radiation that
produces in one cubic centimeter of dry air at 0°C and standard atmospheric pressure
ionization of either sign equal to one electrostatic unit of charge. Most portable
radiation detection safety devices used by a radiographer measure exposure and
present the reading in terms of roentgens or roentgens/hour, which is known as the
dose rate.
Interaction of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms. But subatomically, matter is
made up of mostly empty space. For example, consider the hydrogen atom with its
one proton, one neutron, and one electron. The diameter of a single proton has been
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measured to be about 10-15 meters. The diameter of a single hydrogen atom has been
determined to be 10-10meters, therefore the ratio of the size of a hydrogen atom to the
size of the proton is 100,000:1. Consider this in terms of something more easily
pictured in your mind. If the nucleus of the atom could be enlarged to the size of a
softball (about 10 cm), its electron would be approximately 10 kilometers away.
Therefore, when electromagnetic waves pass through a material, they are primarily
moving through free space, but may have a chance encounter with the nucleus or an
electron of an atom.
Because the encounters of photons with atom particles are by chance, a given photon
has a finite probability of passing completely through the medium it is traversing. The
probability that a photon will pass completely through a medium depends on
numerous factors including the photon’s energy and the medium’s composition and
thickness. The more densely packed a medium’s atoms, the more likely the photon
will encounter an atomic particle. In other words, the more subatomic particles in a
material (higher Z number), the greater the likelihood that interactions will
occur Similarly, the more material a photon must cross through, the more likely the
chance of an encounter.
When a photon does encounter an atomic particle, it transfers energy to the particle.
The energy may be reemitted back the way it came (reflected), scattered in a different
direction or transmitted forward into the material. Let us first consider the interaction
of visible light. Reflection and transmission of light waves occur because the light
waves transfer energy to the electrons of the material and cause them to vibrate. If the
material is transparent, then the vibrations of the electrons are passed on to
neighboring atoms through the bulk of the material and reemitted on the opposite side
of the object. If the material is opaque, then the vibrations of the electrons are not
passed from atom to atom through the bulk of the material, but rather the electrons
vibrate for short periods of time and then reemit the energy as a reflected light wave.
The light may be reemitted from the surface of the material at a different wavelength,
thus changing its color.
X-Rays and Gamma Rays
X-rays and gamma rays also transfer their energy to matter though chance encounters
with electrons and atomic nuclei. However, X-rays and gamma rays have enough
energy to do more than just make the electrons vibrate. When these high energy rays
encounter an atom, the result is an ejection of energetic electrons from the atom or the
excitation of electrons. The term "excitation" is used to describe an interaction where
electrons acquire energy from a passing charged particle but are not removed
completely from their atom. Excited electrons may subsequently emit energy in the
form of x-rays during the process of returning to a lower energy state.
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Each of the excited or liberated electrons goes on to transfer its energy to matter
through thousands of events involving interactions between charged particles. With
each interaction, the energy may be directed in a different direction. The higher the
energy of a photon, the more likely the energy will continue traveling in the same
direction. As the radiation moves from point to point in matter, it loses its energy
through various interactions with the atoms it encounters. If the radiation has enough
energy, it may eventually make it through the material.
Photon Interaction with Matter is Key
From the previous paragraph, it can be deduced that the energy of X- and Gamma ray
photons is largely responsible for their penetrating power. Einstein linked the energy
of a photon to its frequency and wavelength when he postulated that each photon
carries an energy of the frequency of the wave times Planck’s constant (E = hƒ). The
frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =c/λ
). However, it should be understood that the wavelength or frequency of
electromagnetic radiation does not in itself makes the EM wave more or less
penetrating. The key is its interaction with matter, or more specifically, whether the
photon's energy is right to excite some transition of a charged particle. For instance,
microwaves penetrate glass very easily, but they are strongly absorbed by water.
Move up to slightly higher frequency, and infrared is strongly absorbed by both glass
and water, but both substances transmit visible light. Ultraviolet is stopped by glass,
but not so readily by water.
Ionization and Cell Damage
As previously discussed, photons that interact with atomic particles can transfer their
energy to the material and break chemical bonds in materials. This interaction is
known as ionization and involves the dislodging of one or more electrons from an
atom of a material. This creates electrons, which carry a negative charge, and atoms
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without electrons, which carry a positive charge. Ionization in industrial materials is
usually not a big concern. In most cases, once the radiation ceases the electrons rejoin
the atoms and no damage is done. However, ionization can disturb the atomic
structure of some materials to a degree where the atoms enter into chemical reactions
with each other. This is the reaction that takes place in the silver bromide of
radiographic film to produce a latent image when the film is processed. Ionization
may cause unwanted changes in some materials, such as semiconductors, so that they
are no longer effective for their intended use.
Ionization in Living Tissue (Cell Damage)
In living tissue, similar interactions occur and
ionization can be very detrimental to cells.
Ionization of living tissue causes molecules in the
cells to be broken apart. This interaction can kill
the cell or cause them to reproduce abnormally.
Damage to a cell can come from direct
action orindirect action of the radiation. Cell
damage due to direct action occurs when the
radiation interacts directly with a cell's essential molecules (DNA). The radiation
energy may damage cell components such as the cell walls or the deoxyribonucleic
acid (DNA). DNA is found in every cell and consists of molecules that determine the
function that each cell performs. When radiation interacts with a cell wall or DNA, the
cell either dies or becomes a different kind of cell, possibly even a cancerous one.
Cell damage due to indirect action occurs when radiation interacts with the water
molecules, which are roughly 80% of a cells composition. The energy absorbed by the
water molecule can result in the formation of free radicals. Free radicals are molecules
that are highly reactive due to the presence of unpaired electrons, which result when
water molecules are split. Free radicals may form compounds, such as hydrogen
peroxide, which may initiate harmful chemical reactions within the cells. As a result
of these chemical changes, cells may undergo a variety of structural changes which
lead to altered function or cell death.
Various possibilities exist for the fate of cells damaged by radiation. Damaged cells
can:


completely and perfectly repair themselves with the body's inherent repair
mechanisms.
die during their attempt to reproduce. Thus, tissues and organs in which there is
substantial cell loss may become functionally impaired. There is a "threshold"
dose for each organ and tissue above which functional impairment will
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manifest as a clinically observable adverse outcome. Exceeding the threshold
dose increases the level of harm. Such outcomes are called deterministic effects
and occur at high doses.
repair themselves imperfectly and replicate this imperfect structure. These
cells, with the progression of time, may be transformed by external agents (e.g.,
chemicals, diet, radiation exposure, lifestyle habits, etc.). After a latency period
of years, they may develop into leukemia or a solid tumor (cancer). Such latent
effects are called stochastic (or random).
Exposure of Living Tissue to Non-ionizing Radiation
A quick note of caution about non-ionizing radiation is probably also appropriate
here. Non-ionizing radiation behaves exactly like ionizing radiation, but differs in that
it has a much greater wavelength and, therefore, less energy. Although this nonionizing radiation does not have the energy to create ion pairs, some of these waves
can cause personal injury. Anyone who has received a sunburn knows that ultraviolet
light can damage skin cells. Non-ionizing radiation sources include lasers, highintensity sources of ultraviolet light, microwave transmitters and other devices that
produce high intensity radio-frequency radiation.
Cell Radiosensitivity
Radiosensitivity is the relative susceptibility of cells, tissues, organs, organisms, or
other substances to the injurious action of radiation. In general, it has been found that
cell radiosensitivity is directly proportional to the rate of cell division and inversely
proportional to the degree of cell differentiation. In short, this means that actively
dividing cells or those not fully mature are most at risk from radiation. The most
radio-sensitive cells are those which:




have a high division rate
have a high metabolic rate
are of a non-specialized type
are well nourished
Examples of various tissues and their relative radiosensitivities are listed below.
High Radiosensitivity
Lymphoid organs, bone marrow, blood, testes, ovaries, intestines
Fairly High Radiosensitivity
Skin and other organs with epithelial cell lining (cornea, oral cavity, esophagus,
rectum, bladder, vagina, uterine cervix, ureters)
Moderate Radiosensitivity
21
Optic lens, stomach, growing cartilage, fine vasculature, growing bone
Fairly Low Radiosensitivity
Mature cartilage or bones, salivary glands, respiratory organs, kidneys, liver,
pancreas, thyroid, adrenal and pituitary glands
Low Radiosensitivity
Muscle, brain, spinal cord
Reference: Rubin, P. and Casarett. G. W.: Clinical Radiation Pathology (Philadelphia:
W. B. Saunders. 1968).
Measures Relative to the Biological Effect
of Radiation Exposure
http://www.ndt-ed.org/EducationResources/CommunityCollege/RadiationSafety/quan_units/units.htm
There are five measures of radiation that radiographers will commonly encounter when
addressing the biological effects of working with X-rays or Gamma rays. These measures are:
Exposure, Dose, Dose Equivalent, and Dose Rate A short summary of these measures and their
units will be followed by more in depth information below.



Exposure: Exposure is a measure of the strength of a radiation field at some
point in air. This is the measure made by a survey meter. The most commonly
used unit of exposure is the roentgen (R).
Dose or Absorbed Dose: Absorbed dose is the amount of energy that ionizing
radiation imparts to a given mass of matter. In other words, the dose is the
amount of radiation absorbed by and object. The SI unit for absorbed dose is
the gray (Gy), but the “rad” (Radiation Absorbed Dose) is commonly used. 1
rad is equivalent to 0.01 Gy. Different materials that receive the same exposure
may not absorb the same amount of radiation. In human tissue, one Roentgen
of gamma radiation exposure results in about one rad of absorbed dose.
Dose Equivalent: The dose equivalent relates the absorbed dose to the
biological effect of that dose. The absorbed dose of specific types of radiation
is multiplied by a "quality factor" to arrive at the dose equivalent. The SI unit is
the sievert (SV), but the rem is commonly used. Rem is an acronym for
"roentgen equivalent in man." One rem is equivalent to 0.01 SV. When exposed
to X- or Gamma radiation, the quality factor is 1.
22

Dose Rate: The dose rate is a measure of how fast a radiation dose is being
received. Dose rate is usually presented in terms of R/hour, mR/hour, rem/hour,
mrem/hour, etc.
For the types of radiation used in industrial radiography, one roentgen equals one
rad and since the quality factor for x- and gamma rays is one, radiographers can
consider the Roentgen, rad, and rem to be equal in value.
More Information on Exposure, Dose, Dose Equivalent, and Dose Rate
Exposure
Exposure is a measure of the strength of a
radiation field at some point. It is a measure of
the ionization of the molecules in a mass of air.
It is usually defined as the amount of charge
(i.e. the sum of all ions of the same
sign) produced in a unit mass of air when the
interacting photons are completely absorbed in
that mass. The most commonly used unit of
exposure is the Roentgen (R). Specifically, a
Roentgen is the amount of photon energy
required to produce 1.610 x 1012 ion pairs in
one cubic centimeter of dry air at 0°C. A
radiation field of one Roentgen will deposit
2.58 x 10-4 coulombs of charge in one kilogram
of dry air. The main advantage of this unit is that it is easy to directly measure with a
survey meter. The main limitation is that it is only valid for deposition in air.
Dose or Absorbed Dose
Whereas exposure is defined for air,
theabsorbed dose is the amount of energy that
ionizing radiation imparts to a given mass of
matter. The absorbed dose is used to relate the
amount of ionization that x-rays or gamma rays
cause in air to the level of biological damage
that would be caused in living tissue placed in
the radiation field. The most commonly used
unit for absorbed dose is the “rad” (Radiation
Absorbed Dose). A rad is defined as a dose of
100 ergs of energy per gram of the given
material. The SI unit for absorbed dose is the
gray (Gy), which is defined as a dose of one
23
joule per kilogram. Since one joule equals 107ergs, and since one kilogram equals
1000 grams, 1 Gray equals 100 rads.
The size of the absorbed dose is dependent upon the the intensity (or activity) of the
radiation source, the distance from the source to the irradiated material, and the time
over which the material is irradiated. The activity of the source will determine the
dose rate which can be expressed in rad/hr, mr/hr, mGy/sec, etc.
Dose Equivalent
When considering radiation interacting with living tissue, it is important to also
consider the type of radiation. Although the biological effects of radiation are
dependent upon the absorbed dose, some types of radiation produce greater effects
than others for the same amount of energy imparted. For example, for equal absorbed
doses, alpha particles may be 20 times as damaging as beta particles. In order to
account for these variations when describing human health risks from radiation
exposure, the quantity called “dose equivalent” is used. This is the absorbed dose
multiplied by certain “quality” or “adjustment” factors indicative of the relative
biological-damage potential of the particular
type of radiation.
The quality factor (Q) is a factor used in
radiation protection to weigh the absorbed dose
with regard to its presumed biological
effectiveness. Radiation with higher Q factors
will cause greater damage to tissue. The rem is
a term used to describe a special unit of dose
equivalent. Rem is an abbreviation for roentgen
equivalent in man. The SI unit is the sievert
(SV); one rem is equivalent to 0.01 SV. Doses
of radiation received by workers are recorded in rems, however, sieverts are being
required as the industry transitions to the SI unit system.
The table below presents the Q factors for several types of radiation.
Type of Radiation
X-Ray
Gamma Ray
Beta Particles
Thermal Neutrons
Fast Neutrons
Alpha Particles
Rad
1
1
1
1
1
1
24
Q Factor
1
1
1
5
10
20
Rem
1
1
1
5
10
20
Dose Rate
The dose rate is a measure of how fast a radiation dose is being received. Knowing the
dose rate, allows the dose to be calculated for a period of time. Fore example, if the
dose rate is found to be 0.8rem/hour, then a person working in this field for two hours
would receive a 1.6rem dose.
Biological Effects
The occurrence of particular health effects from exposure to ionizing radiation is a
complicated function of numerous factors including:






Type of radiation involved. All kinds of ionizing radiation can produce health
effects. The main difference in the ability of alpha and beta particles and
Gamma and X-rays to cause health effects is the amount of energy they have.
Their energy determines how far they can penetrate into tissue and how much
energy they are able to transmit directly or indirectly to tissues.
Size of dose received. The higher the dose of radiation received, the higher the
likelihood of health effects.
Rate the dose is received. Tissue can receive larger dosages over a period of
time. If the dosage occurs over a number of days or weeks, the results are often
not as serious if a similar dose was received in a matter of minutes.
Part of the body exposed. Extremities such as the hands or feet are able to
receive a greater amount of radiation with less resulting damage than blood
forming organs housed in the torso. See radiosensitivity page for more
information.
The age of the individual. As a person ages, cell division slows and the body
is less sensitive to the effects of ionizing radiation. Once cell division has
slowed, the effects of radiation are somewhat less damaging than when cells
were rapidly dividing.
Biological differences. Some individuals are more sensitive to the effects of
radiation than others. Studies have not been able to conclusively determine the
differences.
The effects of ionizing radiation upon humans are often broadly classified as being
either stochastic or nonstochastic. These two terms are discussed more in the next few
pages.
Stochastic Effects
Stochastic effects are those that occur by chance and consist primarily of cancer and
genetic effects. Stochastic effects often show up years after exposure. As the dose to
an individual increases, the probability that cancer or a genetic effect will occur also
25
increases. However, at no time, even for high doses, is it certain that cancer or genetic
damage will result. Similarly, for stochastic effects, there is no threshold dose below
which it is relatively certain that an adverse effect cannot occur. In addition, because
stochastic effects can occur in individuals that have not been exposed to radiation
above background levels, it can never be determined for certain that an occurrence of
cancer or genetic damage was due to a specific exposure.
While it cannot be determined conclusively, it often possible to estimate the
probability that radiation exposure will cause a stochastic effect. As mentioned
previously, it is estimated that the probability of having a cancer in the US rises from
20% for non radiation workers to 21% for persons who work regularly with radiation.
The probability for genetic defects is even less likely to increase for workers exposed
to radiation. Studies conducted on Japanese atomic bomb survivors who were exposed
to large doses of radiation found no more genetic defects than what would normally
occur.
Radiation-induced hereditary effects have not been observed in human populations,
yet they have been demonstrated in animals. If the germ cells that are present in the
ovaries and testes and are responsible for reproduction were modified by radiation,
hereditary effects could occur in the progeny of the individual. Exposure of the
embryo or fetus to ionizing radiation could increase the risk of leukemia in infants
and, during certain periods in early pregnancy, may lead to mental retardation and
congenital malformations if the amount of radiation is sufficiently high.
More on Specific Stochastic Effects
Cancer
Cancer
Cancer is any malignant growth or tumor caused by abnormal and uncontrolled cell
division. Cancer may spread to other parts of the body through the lymphatic system or the
blood stream. The carcinogenic effects of doses of 100 rads (1 Gy) or more of gamma radiation
delivered at high dose rates are well documented, consistent and definitive.
Although any organ or tissue may develop a tumor after overexposure to
radiation, certain organs and tissues seem to be more sensitive in this respect
than others. Radiation-induced cancer is observed most frequently in the
hemopoietic system, in the thyroid, in the bone, and in the skin. In all these
cases, the tumor induction time in man is relatively long - on the order of 5 to
20 years after exposure.
26
Carcinoma of the skin was the first type of malignancy that was associated with exposure to xrays. Early x-ray workers, including physicists and physicians, had a much higher incidence of
skin cancer than could be expected from random occurrences of this disease. Well over 100 cases
of radiation induced skin cancer are documented in the literature. As early as 1900, a physician
who had been using x-rays in his practice described the irritating effects of x-rays. He recorded
that erythema and itching progressed to hyper-pigmentation, ulceration, neoplasia, and finally
death from metastatic carcinoma. The entire disease process spanned a period of 9 years. Cancer
of the fingers was an occupational disease common among dentists before the carcinogenic
properties of x-rays were well understood. Dentists would hold the dental x-ray film in the
mouths of patients while x-raying their teeth.
Leukemia
Leukemia
Leukemia is a cancer of the early blood-forming cells.
Usually, the leukemia is a cancer of the white blood cells,
but leukemia can involve other blood cell types as well.
Leukemia starts in the bone marrow and then spreads to the
blood. From there it can go to the lymph nodes, spleen, liver,
central nervous system (the brain and spinal cord), testes
(testicles), or other organs. Leukemia is among the most
likely forms of malignancy resulting from overexposure to
total body radiation. Chronic lymphocytic leukemia does not
appear to be related to radiation exposure.
Radiologists and other physicians who used x-rays in their
practice before strict health physics practices were common
showed a significantly higher rate of leukemia than did their
colleagues who did not use radiation. Among American radiologists, the doses associated with
the increased rate of leukemia were on the order of 100 rads (1 Gy) per year. With the increased
practice of health physics, the difference in leukemia rate between radiologists and other
physicians has been continually decreasing.
Among the survivors of the nuclear bombings of Japan, there was a significantly greater
incidence of leukemia among those who had been within 1500 meters of the hypocenter than
among those who had been more than 1500 meters from ground zero at the time of the bombing.
An increase in leukemia among the survivors was first seen about three years after the bombings,
and the leukemia rate continued to increase until it peaked about four years later. Since this time,
the rate has been steadily decreasing.
The questions regarding the leukemogenicity of low radiation doses and of the existence of a
non-zero threshold dose for leukemia induction remain unanswered, and are the subject of
controversy. On the basis of a few limited studies, it was inferred that as little as 1-5 rads (10-50
27
mGy) of x-rays could lead to leukemia. Other studies imply that a threshold dose for radiogenic
leukemia is significantly higher. However, it is reasonable to infer that low level radiation at
doses associated with most diagnostic x-ray procedures, with occupational exposure within the
recommended limits, and with natural radiation is a very weak leukemogen, and that the
attributive risk of leukemia from low level radiation is probably very small.
Genetic Effects
Genetic Effects
Genetic information necessary for the production and functioning of a new organism is contained
in the chromosomes of the germ cells - the sperm and the ovum. The normal human somatic cell
contains 46 of these chromosomes; mature sperm and ovum each carry 23 chromosomes. When
an ovum is fertilized by a sperm, the resulting cell, called a zygote, contains a full complement of
46 chromosomes. During the 9-month gestation period, the fertilized egg, by successive cellular
division and differentiation, develops into a new individual. In the course of the cellular
divisions, the chromosomes are exactly duplicated, so that cells in the body contain the same
genetic information. The units of information in the chromosomes are called genes. Each gene is
an enormously complex macromolecule called deoxyribonucleic acid (DNA), in which the
genetic information is coded according to the sequence of certain molecular and sub-assemblies
called bases. The DNA molecule consists of two long chains in a spiral double helix. The two
long intertwined strands are held together by the bases, which form cross-links between the long
strands in the same manner as the treads in a step-ladder.
The genetic information can be altered by many different chemical and
physical agents called mutagens, which disrupt the sequence of bases in a
DNA molecule. If this information content of a somatic cell is scrambled,
then its descendants may show some sort of an abnormality. If the
information that is jumbled is in a germ cell that subsequently is fertilized,
then the new individual may carry a genetic defect, or a mutation. Such a
mutation is often called a point mutation, since it results from damage to
one point on a gene. Most geneticists believe that the majority of such
mutations in man are undesirable or harmful.
In addition to point mutations, genetic damage can arise through
chromosomal aberrations. Certain chemical and physical agents can cause
chromosomes to break. In most of these breaks, the fragments reunite, and the only result may be
a point mutation at the site of the original break. In a small fraction of breaks, however, the
broken pieces do not reunite. When this happens, one of the broken fragments may be lost when
the cell divides, and the daughter cell does not receive the genetic information contained in the
lost fragment. The other possibility following chromosomal breakage, especially if two or more
chromosomes are broken, is the interchange of the fragments among the broken chromosomes,
and the production of aberrant chromosomes. Cells with such aberrant chromosomes usually
have impaired reproductive capacity as well as other abnormalities.
28
Studies suggest that the existence of a threshold dose for the genetic effects of radiation is
unlikely. However, they also show that the genetic effects of radiation are inversely dependent
on dose rate over the range of 800 mrad/min (8 mGy/min) to 90 rads/min (0.9 Gy/min). The
dose rate dependence clearly implies a repair mechanism that is overwhelmed at the high dose
rate. Geneticists estimate that there are 320 chances per million of a "spontaneous" mutation in a
dominant gene trait of a person. The radiation dose that would eventually lead to a doubling
of the mutation rate is estimated to be in the range of 50-250 rads (0.5-2.5 Gy).
Cataracts
Cataracts
A cataract is a clouding of the normally clear lens of the eye. A much
higher incidence of cataracts was reported among physicists in cyclotron
laboratories whose eyes had been exposed intermittently for long periods
of time to relatively low radiation fields, as well as among atomic bomb
survivors whose eyes had been exposed to a single high radiation dose.
This shows that both chronic and acute overexposure of the eyes can lead
to cataracts. Radiation may injure the cornea, conjunctiva, iris, and the
lens of the eye. In the case of the lens, the principal site of damage is the
proliferating cells of the anterior epithelium. This results in abnormal lens
fibers, which eventually disintegrate to form an opaque area, or cataract,
that prevents light from reaching the retina.
The cataractogenic dose to the lens is on the order of 500 rad of beta or gamma radiation. No
radiogenic cataracts resulting from occupational exposure to x-rays have been reported. From
patients who suffered irradiation of the eye in the course of x-ray therapy and developed
cataracts as a consequence, the cataractogenic threshold is estimated at about 200 rad. In cases
either of occupationally or therapeutically induced radiation cataracts, a long latent period, on the
order of several years, usually elapsed between the exposure and the appearance of the lens
opacity. The cataractogenic dose has been found, in laboratory experiments with animals, to be a
function of age; young animals are more sensitive than old animals.
Nonstochastic (Acute) Effects
Unlike stochastic effects, nonstochastic effects are characterized by a threshold dose
below which they do not occur. In other words, nonstochastic effects have a clear
relationship between the exposure and the effect. In addition, the magnitude of the
effect is directly proportional to the size of the dose. Nonstochastic effects typically
result when very large dosages of radiation are received in a short amount of time.
These effects will often be evident within hours or days. Examples of nonstochastic
29
effects include erythema (skin reddening), skin and tissue burns, cataract formation,
sterility, radiation sickness and death. Each of these effects differs from the others in
that both its threshold dose and the time over which the dose was received cause the
effect (i.e. acute vs. chronic exposure).
There are a number of cases of radiation burns occurring to the hands or fingers.
These cases occurred when a radiographer touched or came in close contact with a
high intensity radiation emitter. Intensity on the surface of an 85 curie Ir-192 source
capsule is approximately 1,768 R/s. Contact with the source for two seconds would
expose the hand of an individual to 3,536 rems, and this does not consider any
additional whole body dosage received when approaching the source.
More on Specific Nonstochastic Effects
Hemopoietic Syndrome
The hemopoietic syndrome encompasses the medical conditions that affect the blood.
Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2
Gy). This disease is characterized by depression or ablation of the bone marrow, and
the physiological consequences of this damage. The onset of the disease is rather
sudden, and is heralded by nausea and vomiting within several hours after the
overexposure occurred. Malaise and fatigue are felt by the victim, but the degree of
malaise does not seem to be correlated with the size of the dose. Loss of hair
(epilation), which is almost always seen, appears between the second and third week
after the exposure. Death may occur within one to two months after exposure. The
chief effects to be noted, of course, are in the bone marrow and in the blood. Marrow
depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete
ablation of the marrow occurs. In this case, however, spontaneous regrowth of the
marrow is possible if the victim survives the physiological effects of the denuding of
the marrow. An exposure of about 700 rads (7 Gy) or greater leads to irreversible
ablation of the bone marrow.
Gastrointestinal Syndrome
The gastrointestinal syndrome encompasses the medical conditions that affect the
stomach and the intestines. This medical condition follows a total body gamma dose
of about 1000 rads (10 Gy) or greater, and is a consequence of the desquamation of
the intestinal epithelium. All the signs and symptoms of hemopoietic syndrome are
seen, with the addition of severe nausea, vomiting, and diarrhea which begin very
soon after exposure. Death within one to two weeks after exposure is the most likely
outcome.
Central Nervous System
A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central
30
nervous system, as well as all the other organ systems in the body. Unconsciousness
follows within minutes after exposure and death can result in a matter of hours to
several days. The rapidity of the onset of unconsciousness is directly related to the
dose received. In one instance in which a 200 msec burst of mixed neutrons and
gamma rays delivered a mean total body dose of about 4400 rads (44 Gy), the victim
was ataxic and disoriented within 30 seconds. In 10 minutes, he was unconscious and
in shock. Vigorous symptomatic treatment kept the patient alive for 34 hours after the
accident.
Other Acute Effects
Several other immediate effects of acute overexposure should be noted. Because of its
physical location, the skin is subject to more radiation exposure, especially in the case
of low energy x-rays and beta rays, than most other tissues. An exposure of about 300
R (77 mC/kg) of low energy (in the diagnostic range) x-rays results in erythema.
Higher doses may cause changes in pigmentation, loss of hair, blistering, cell death,
and ulceration. Radiation dermatitis of the hands and face was a relatively common
occupational disease among radiologists who practiced during the early years of the
twentieth century.
The reproductive organs are particularly radiosensitive. A single dose of only 30 rads
(300 mGy) to the testes results in temporary sterility among men. For women, a 300
rad (3 Gy) dose to the ovaries produces temporary sterility. Higher doses increase the
period of temporary sterility. In women, temporary sterility is evidenced by a
cessation of menstruation for a period of one month or more, depending on the dose.
Irregularities in the menstrual cycle, which suggest functional changes in the
reproductive organs, may result from local irradiation of the ovaries with doses
smaller than that required for temporary sterilization.
The eyes too, are relatively radiosensitive. A local dose of several hundred rads can
result in acute conjunctivitis.
Exposure Symptoms
Listed below are some of the probable prompt and delayed effects of certain doses of
radiation when the doses are received by an individual within a twenty-four hour
period.
Dosages are in Roentgen Equivalent Man (Rem)
• 0-25 No injury evident. First detectable blood change at 5 rem.
• 25-50 Definite blood change at 25 rem. No serious injury.
• 50-100 Some injury possible.
31
• 100-200 Injury and possible disability.
• 200-400 Injury and disability likely, death possible.
• 400-500 Median Lethal Dose (MLD) 50% of exposures are fatal.
• 500-1,000 Up to 100% of exposures are fatal.
• 1,000-over 100% likely fatal.
The delayed effects of radiation may be due either to a single large overexposure or
continuing low-level overexposure.
Example dosages and resulting symptoms when an individual receives an exposure to
the whole body within a twenty-four hour period.
100 - 200 Rem
First Day
First Week
Second Week
Third Week
Fourth Week
No definite symptoms
No definite symptoms
No definite symptoms
Loss of appetite, malaise, sore throat and diarrhea
Recovery is likely in a few months unless complications develop
because of poor health
400 - 500 Rem
First Day
Nausea, vomiting and diarrhea, usually in the first few hours
First Week
Symptoms may continue
Second Week
Epilation, loss off appetite
Hemorrhage, nosebleeds, inflammation of mouth and throat, diarrhea,
Third Week
emaciation
Fourth Week
Rapid emaciation and mortality rate around 50%
The U.S. Nuclear Regulatory Commission
and the Code of Federal Regulations
Since working with ionizing radiation can present significant
safety risks, its use is closely regulated. In the United States, the
Nuclear Regulatory Commission (NRC) is responsible for
protecting workers, the public and the environment from the
effects of radiation. The NRC is an independent agency
established by the Energy Reorganization Act of 1974 to
regulate civilian use of nuclear materials. The NRC is headed by
five commissioners appointed by the President and confirmed by
the Senate for five-year terms.
The Code of Federal Regulations (CFR) is the system used by the US Federal
Government to organize the rules published in the Federal Register by the executive
departments and agencies. The CFR is divided into 50 titles that represent broad areas
32
subject to Federal regulation. Title 10 of the code applies to energy and parts 0
through 50 of Title 10 apply to NRC rules. Part 19, Notices, instructions and reports to
workers: inspection and investigations; Part 20, Standards for protection against
radiation; and Part 34, Licenses for industrial radiography and radiation safety
requirements for industrial radiographic operations, are areas of the Code that are of
primary interest when addressing radiation safety in industrial radiography.
The NRC regulations can be accessed on the Internet
at: www.gpoaccess.gov/cfr/index.html
More than half of the states in the U.S. have entered into "agreement" with the NRC
to assume regulatory control of radioactive material use within their borders. As part
of the agreement process, the states must adopt and enforce regulations comparable to
those found in Title 10 of the Code of Federal Regulations. Some of the requirement
of the Code, such as exposure limits, responsibilities and procedures will be discussed
in the following pages.
Exposure Limits
As discussed in
biological effect
after the discovery
numerous
occupational
by the
Radiological
protection groups.
for radiation
objectives: 1) to
chronic exposure to "acceptable" levels.
the introduction, concern over the
of ionizing radiation began shortly
of X-rays in 1895. Over the years,
recommendations regarding
exposure limits have been developed
International Commission on
Protection (ICRP) and other radiation
In general, the guidelines established
exposure have had two principle
prevent acute exposure; and 2) to limit
Current guidelines are based on the conservative assumption that there is no safe level
of exposure. In other words, even the smallest exposure has some probability of
causing a stochastic effect, such as cancer. This assumption has led to the general
philosophy of not only keeping exposures below recommended levels or regulation
limits but also maintaining all exposure "as low as reasonable achievable"
(ALARA). ALARA is a basic requirement of current radiation safety practices. It
means that every reasonable effort must be made to keep the dose to workers and the
public as far below the required limits as possible.
33
Regulatory Limits for Occupational Exposure
Many of the recommendations from the ICRP
and other groups have been incorporated into
the regulatory requirements of countries
around the world. In the United States, annual
radiation exposure limits are found in Title 10,
part 20 of the Code of Federal Regulations,
and in equivalent state regulations. For
industrial radiographers who generally are not
concerned with an intake of radioactive
material, the Code sets the annual limit of
exposure at the following:
1) the more limiting of:


A total effective dose equivalent of 5
rems (0.05 Sv)
or
The sum of the deep-dose equivalent to
any individual organ or tissue other than
the lens of the eye being equal to 50
rems (0.5 Sv).
2) The annual limits to the lens of the eye, to the skin, and to the extremities, which
are:


A lens dose equivalent of 15 rems (0.15 Sv)
A shallow-dose equivalent of 50 rems (0.50 Sv) to the skin or to any extremity.
The shallow-dose equivalent is the external dose to the skin of the whole-body or
extremities from an external source of ionizing radiation. This value is the dose
equivalent at a tissue depth of 0.007 cm averaged over and area of 10 cm2.
The lens dose equivalent is the dose equivalent to the lens of the eye from an external
source of ionizing radiation. This value is the dose equivalent at a tissue depth of 0.3
cm.
The deep-dose equivalent is the whole-body dose from an external source of ionizing
radiation. This value is the dose equivalent at a tissue depth of 1 cm.
The total effective dose equivalent is the dose equivalent to the whole-body.
Declared Pregnant Workers and Minors
Because of the increased health risks to the rapidly developing embryo and fetus,
pregnant women can receive no more than 0.5 rem during the entire gestation period.
34
This is 10% of the dose limit that normally applies to radiation workers. Persons
under the age of 18 years are also limited to 0.5rem/year.
Non-radiation Workers and the Public
The dose limit to non-radiation workers and members of the public are two percent of
the annual occupational dose limit. Therefore, a non-radiation worker can receive a
whole body dose of no more that 0.1 rem/year from industrial ionizing radiation. This
exposure would be in addition to the 0.3 rem/year from natural background radiation
and the 0.05 rem/year from man-made sources such as medical x-rays.
Controlling Radiation Exposure
When working with radiation, there is a concern for two types of exposure: acute and
chronic. An acute exposure is a single accidental exposure to a high dose of radiation
during a short period of time. An acute exposure has the potential for producing both
nonstochastic and stochastic effects. Chronic exposure, which is also sometimes
called "continuous exposure," is long-term, low level overexposure. Chronic exposure
may result in stochastic health effects and is likely to be the result of improper or
inadequate protective measures.
The three basic ways of controlling exposure to harmful radiation are: 1) limiting the
time spent near a source of radiation, 2) increasing the distance away from the source,
3) and using shielding to stop or reduce the level of radiation.
Time
The radiation dose is directly proportional to the time spent in
the radiation. Therefore, a person should not stay near a source
of radiation any longer than necessary. If a survey meter reads 4
mR/h at a particular location, a total dose of 4mr will be received
if a person remains at that location for one hour. In a two hour
span of time, a dose of 8 mR would be received. The following
35
equation can be used to make a simple calculation to determine the dose that will be
or has been received in a radiation area.
Dose = Dose Rate x Time
(click here for more information on using this formula)
When using a gamma camera, it is important to get the source from the shielded
camera to the collimator as quickly as possible to limit the time of exposure to the
unshielded source. Devices that shield radiation in some directions but allow it pass in
one or more other directions are known as collimators. This is illustrated in the images
at the bottom of this page.
Distance
Increasing distance from the source of radiation
will reduce the amount of radiation received. As
radiation travels from the source, it spreads out
becoming less intense. This is analogous to
standing near a fire. The closer a person stands to
the fire, the more intense the heat feels from the
fire. This phenomenon can be expressed by an
equation known as the inverse square law, which
states that as the radiation travels out from the
source, the dosage decreases inversely with the
square of the distance.
Inverse Square Law: I1/ I2 = D22/ D12
(click here for more information on using this formula)
Shielding
The third way to reduce exposure to radiation is to place something between the
radiographer and the source of radiation. In general, the more dense the material the
more shielding it will provide. The most effective shielding is provided by depleted
uranium metal. It is used primarily in gamma ray cameras like the one shown below.
The circle of dark material in the plastic see-through camera (below right) would
actually be a sphere of depleted uranium in a real gamma ray camera. Depleted
uranium and other heavy metals, like tungsten, are very effective in shielding
radiation because their tightly packed atoms make it hard for radiation to move
through the material without interacting with the atoms. Lead and concrete are the
most commonly used radiation shielding materials primarily because they are easy to
work with and are readily available materials. Concrete is commonly used in the
construction of radiation vaults. Some vaults will also be lined with lead sheeting to
help reduce the radiation to acceptable levels on the outside.
36
37
Half-Value Layer (Shielding)
As was discussed in the radiation theory section, the depth of penetration for a given
photon energy is dependent upon the material density (atomic structure). The more
subatomic particles in a material (higher Z number), the greater the likelihood that
interactions will occur and the radiation will lose its energy. Therefore, the more
dense a material is the smaller the depth of radiation penetration will be. Materials
such as depleted uranium, tungsten and lead have high Z numbers, and are therefore
very effective in shielding radiation. Concrete is not as effective in shielding radiation
but it is a very common building material and so it is commonly used in the
construction of radiation vaults.
Since different materials attenuate radiation to different degrees, a convenient method
of comparing the shielding performance of materials was needed. The half-value
layer (HVL) is commonly used for this purpose and to determine what thickness of a
given material is necessary to reduce the exposure rate from a source to some level. At
some point in the material, there is a level at which the radiation intensity becomes
one half that at the surface of the material. This depth is known as the half-value layer
for that material. Another way of looking at this is that the HVL is the amount of
material necessary to the reduce the exposure rate from a source to one-half its
unshielded value.
Sometimes shielding is specified as some number of HVL. For example, if a Gamma
source is producing 369 R/h at one foot and a four HVL shield is placed around it, the
intensity would be reduced to 23.0 R/h.
Each material has its own specific HVL thickness. Not only is the HVL material
dependent, but it is also radiation energy dependent. This means that for a given
material, if the radiation energy changes, the point at which the intensity decreases to
half its original value will also change. Below are some HVL values for various
materials commonly used in industrial radiography. As can be seen from reviewing
the values, as the energy of the radiation increases the HVL value also increases.
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Half-Value Layer, mm (inch)
Source
Concrete
Steel
Lead
Tungsten
Uranium
Iridium-192
44.5 (1.75)
12.7 (0.5)
4.8 (0.19)
3.3 (0.13)
2.8 (0.11)
Cobalt-60
60.5 (2.38)
21.6 (0.85)
12.5 (0.49)
7.9 (0.31)
6.9 (0.27)
38
Approximate Half-Value Layer for Various Materials when Radiation is from an Xray Source
Half-Value Layer, mm (inch)
Peak Voltage (kVp)
Lead
Concrete
50
0.06 (0.002)
4.32 (0.170)
100
0.27 (0.010)
15.10 (0.595)
150
0.30 (0.012)
22.32 (0.879)
200
0.52 (0.021)
25.0 (0.984)
250
0.88 (0.035)
28.0 (1.102)
300
1.47 (0.055)
31.21 (1.229)
400
2.5 (0.098)
33.0 (1.299)
1000
7.9 (0.311)
44.45 (1.75)
Note: The values presented on this page are intended for educational purposes. Other
sources of information should be consulted when designing shielding for radiation
sources.
Safety Controls
Since X-ray and gamma radiation are not detectable by the human senses and the resulting
damage to the body is not immediately apparent, a variety of safety controls are used to limit
exposure. The two basic types of radiation safety controls used to provide a safe working
environment are engineered and administrative controls. Engineered controls include shielding,
interlocks, alarms, warning signals, and material containment. Administrative controls include
postings, procedures,
dosimetry, and training.
Engineered controls such as
Engineered Controls
shielding and door
interlocks are used to contain the
radiation in a cabinet or a
"radiation vault." Fixed shielding
materials are commonly
high density concrete and/or lead.
Door interlocks are used
to immediately cut the power to
X-ray generating
equipment if a door is
accidentally opened when
X-rays are being produced.
Warning lights are used to
alert workers and the public that
radiation is being
used. Sensors and warning
alarms are often used to
signal that a predetermined
amount of radiation is present. Safety controls should never be tampered with or bypassed.
39
When portable radiography is performed, it is most often not
practical to place alarms or warning lights in the exposure
area. Ropes and signs are used to block the entrance to
radiation areas and to alert the public to the presence of
radiation. Occasionally, radiographers will use battery
operated flashing lights to alert the public to the presence of
radiation. Portable or temporary shielding devices may be
fabricated from materials or equipment located in the area of
the inspection. Sheets of steel, steel beams, or other
equipment may be used for temporary shielding. It is the
responsibility of the radiographer to know and understand the
absorption value of various materials. More information on
absorption values and material properties can be found in the
radiography section of this site.
Administrative Controls
As mentioned above, administrative controls supplement the
engineered controls. These controls include postings, procedures,
dosimetry, and training. It is commonly required that all areas
containing X-ray producing equipment or radioactive materials have
signs posted bearing the radiation symbol and a notice explaining the
dangers of radiation. Normal operating procedures and emergency
procedures must also be prepared and followed. In the US, federal
law requires that any individual who is likely to receive more than
10% of any annual occupational dose limit be monitored for radiation
exposure. This monitoring is accomplished with the use of
dosimeters, which are discussed in the radiation safety equipment
section of this material. Proper training with accompanying
documentation is also a very important administrative control.
Responsibilities
Working safely with radiation is the responsibility
of everyone involved in the use and management of
radiation producing equipment and materials.
Depending on the size of the organization, specific
responsibilities may be assigned to various
individuals and/or committees.
Radiation Safety Officer
All organizations that are licensed to use ionizing
radiation must have a Radiation Safety Officer. The
40
RSO is the individual authorized by the company to serve as point of contact for all
activities conducted under the scope of the authorization. The RSO ensures that
radiation safety activities are being performed in accordance with approved
procedures and regulatory requirements. Some of the common responsible for the
RSO include:
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Ensuring that all individuals using radiation equipment are appropriately
trained and supervised.
Ensuring that all individuals using the equipment have been formally
authorized to use the equipment.
Ensuring that all rules, regulations, and procedures for the safe use of
radioactive sources and X-ray systems are observed.
Ensuring that proper operating, emergency, and ALARA procedures have been
developed and are available to all system users.
Ensuring that accurate records of the use of the sources and equipment are
maintained.
Ensuring that required radiation surveys and leak tests are performed and
documented.
Ensuring that systems and equipment are protected from unauthorized access or
removal.
The minimum qualifications, training, and experience for RSOs for industrial
radiography are as follows: (1) Completion of the training and testing requirements of
Sec. 34.43(a) of Part 10 of the Federal Code of Regulations, (2) 2000 hours of handson experience as a qualified radiographer in industrial radiographic operations, and (3)
Formal training in the establishment and maintenance of a radiation protection
program.
Radiation Safety Committee
Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO.
The RSC often provides oversight of the policies, procedures and responsibilities of
an organizations radiation safety program.
System Users
The individuals authorized to use the X-ray producing system or gamma sources are
responsible for ensuring that:
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All rules, regulations, and procedures for the safe use of the X-ray system are
followed.
An accurate record of the use of the system is maintained.
All safety problems with the system are reported to the RSO and corrected
before further use.
41
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The system is protected from unauthorized access or removal.
Procedures
Written operating procedures must be developed and made available to anyone that
will be working with radiation sources or X-ray producing equipment. These
procedures must be specific to the equipment and its use in a particular application.
Simply making the equipment manufacturers operating instructions available to
workers does not satisfy this requirement. The operating procedure must be followed
at all times unless written permission to deviate is received from the Radiation Safety
Officer.
Standard Operating Procedures
As a minimum, operating procedures must include instructions for the following:
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Appropriate handling and use of licensed sealed sources and radiographic
exposure devices so that no person is likely to be exposed to radiation doses in
excess of the established exposure limits.
Methods and occasions for conducting radiation surveys.
Methods for controlling access to radiographic areas.
Methods and occasions for locking and securing radiographic exposure devices,
transport and storage containers and sealed sources.
Personnel monitoring and the use of personnel monitoring equipment.
Transporting sealed sources to field locations, including packing of
radiographic exposure devices and storage containers in the vehicles,
placarding of vehicles when needed, and control of the sealed sources during
transportation.
The inspection, maintenance, and operability checks of radiographic exposure
devices, survey instruments, transport containers, and storage containers.
The procedure(s) for identifying and reporting defects and noncompliance.
Maintenance of records.
Emergency Procedures
Procedures must also be developed that guide workers in the event of an emergency. A few of
the items that could be covered include:
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Steps that must be taken immediately by radiography personnel in the event a
pocket dosimeter is found to be off-scale or an alarm ratemeter alarms
unexpectedly.
Steps for minimizing exposure of persons in the event of an accident.
The procedure for notifying proper persons in the event of an accident.
Radioactive source recovery procedure if licensee will perform the recovery.
42
Survey Technique
The majority of over exposures in
industrial radiography are the result of
the radiographer not knowing the
location of a gamma emitter and failing
to conduct a proper radiation survey.
Exposure vaults are equipped with
warning lights and safety interlock
switches which provide a margin of
safety for workers. A survey must be
performed occasionally to verify that
vaults are not "leaking" radiation and that
the safety devices are performing properly. However, when conducting
radiography with gamma emitters in the field, the radiographer must rely
heavily on measurements with a survey meter since other safety devices are
uncommon. A series of surveys must be taken and some of the results from
these surveys must be documented when transporting and working with gamma
emitters in the field.
Approaching the Exposure Device
A technician should be thoroughly familiar with the operation of a survey meter
since proper use of the device is essential. Before removing the exposure
device (camera) from storage, the calibration of the survey meter must be
verified and the battery level must be checked. When approaching the exposure
device to remove it from the storage location, the survey meter should be in
hand and operational. The survey meter should be placed next to the exposure
device to verify that the source is contained inside the projector, and to verify
that the survey meter is working properly. Survey meter readings should be
compared to previous readings and recorded.
Transporting the Exposure Device
When transporting the exposure device, it must be stowed securely in the
vehicle. A lockable metal box is often bolted in the rear of the vehicle. A
survey of the over pack, the outside of the vehicle, and the drivers compartment
is then conducted and documented.
Preparing for an Exposure
Once on the job site, the exposure area will be assessed, distance calculations
made for restricted area boundaries, and ropes and signs placed appropriately.
Once this is complete, the radiographer is ready to remove the exposure device
from its storage compartment in the vehicle. The survey meter should be
monitored as the storage compartment is approached and when removing the
exposure device from the compartment. Daily safety checks should then be
made. Once these checks are completed, the radiographer and assistant may
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43
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then move the exposure device to the exposure location. As the cranks and
guide tubes are attached in preparation for the first exposure, the survey meter
should be monitored. Before the source is exposed, the assistant should check
the area for persons who may have crossed into the restricted area, and then
move outside the rope boundary.
Making an Exposure
The radiographer should be at the maximum distance from the exposure device
that the guide tube will allow as he or she quickly cracks the source out of the
exposure device and into place. As the source moves out of the exposure
device, the survey meter will increase to a very high level and then reduce once
the source is inside the collimator. During the exposure, the assistant will
survey the established boundary to determine the levels of radiation present. If
the survey meter indicates levels are higher than calculated, the boundary must
be extended.
Retracting the Source
On retraction of the source, the radiographers will see a rise in readings as the
source moves from the collimator and is retracted into the projector. When the
source is inside the exposure device, the radiographer should approach it while
monitoring the survey meter. If the source is properly retracted, no increase in
the survey meter reading should be seen when approaching the exposure
device. The exposure device should be surveyed on all sides, paying special
attention to the front of the device. The entire length of the guide tube must
then be surveyed.
This process is repeated for each exposure. The survey results must be
documented when the exposure device is returned to the vehicle for
transportation, and when it is placed back into its storage location.
44

Radiation Detectors
Instruments used for radiation measurement
fall into two broad categories:
- rate measuring instruments and
- personal dose measuring instruments.
Rate measuring instruments measure the
rate at which exposure is received (more
commonly called the radiation intensity).
Survey meters, audible alarms and area
monitors fall into this category. These
instruments present a radiation intensity
reading relative to time, such as R/hr or
mR/hr. An analogy can be made between
these instruments and the speedometer of a
car because both are measuring units relative to time.
Dose measuring instruments are those that measure the total amount of
exposure received during a measuring period. The dose measuring instruments,
or dosimeters, that are commonly used in industrial radiography are small
devices which are designed to be worn by an individual to measure the
exposure received by the individual. An analogy can be made between these
instruments and the odometer of a car because both are measuring accumulated
units.
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45
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The radiation measuring instruments commonly used in industrial radiography
are described in more detail in the following pages.
 Survey Meters
The survey meter is the most important resource a
radiographer has to determine the presence and intensity
of radiation. A review of incident and overexposure
reports indicate that a majority of these type of events
occurred when a technician did not have or did not use a
survey meter.
There are many different models of survey meters
available to measure radiation in the field. They all
basically consist of a detector and a readout display.
Analog and digital displays are available. Most of the
survey meters used for industrial radiography use a gas
filled detector.
Gas filled detectors consists of consists of a gas filled
cylinder with two electrodes. Sometimes, the cylinder
itself acts as one electrode, and a needle or thin taut wire along the axis of the
cylinder acts as the other electrode. A voltage is applied to the device so that
the central needle or wire become an anode (+ charge) and the other electrode
or cylinder wall becomes the cathode (- charge). The gas becomes ionized
whenever the counter is brought near radioactive substances. The electric field
created by the potential difference between the anode and cathode causes the electrons of
each ion pair to move to the anode while the positively charged gas atom is drawn to the
cathode. This results in an electrical signal that is amplified, correlated to
exposure and displayed as a value.
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Depending on the voltage applied between the anode and the cathode, the
detector may be considered an ion chamber, a proportional counter, or a
46
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Geiger-Müller (GM) detector. Each of these types of detectors have their
advantages and disadvantages. A brief summary of each of these detectors
follows.
Ion Chamber Counter
Ion chambers have a relatively low voltage between the anode and cathode,
which results in a collection of only the charges produced in the initial
ionization event. This type of detector produces a weak output signal that
corresponds to the number of ionization events. Higher energies and intensities
of radiation will produce more ionization, which will result in a stronger output
voltage.
Collection of only primary ions provides information on true radiation exposure
(energy and intensity). However, the meters require sensitive electronics to
amplify the signal, which makes them fairly expensive and delicate. The
additional expense and required care is justified when it is necessary to make
accurate radiation exposure measurements over a range of radiation energies.
This might be necessary when measuring the Bremsstrahlung radiation
produced by an X-ray generator. An ion chamber survey meter is sometimes
used in the field when performing gamma radiography because it will provide
accurate exposure measurements regardless of the radioactive isotope being
used.
Proportional Counter
Proportional counter detectors use a slightly higher voltage between the anode
and cathode. Due to the strong electrical field, the charges produced in the
initial ionization are accelerated fast enough to ionize other electrons in the gas.
The electrons produced in these secondary ion pairs, along with the primary
electrons, continue to gain energy as they move towards the anode, and as they
do, they produce more and more ionizations. The result is that each electron
from a primary ion pair produces a cascade of ion pairs. This effect is known as
gas multiplication or amplification. In this voltage regime, the number of
particles liberated by secondary interactions is proportional to the number of
ions produced by the passing ionizing particle. Hence, these gas ionization
detectors are called proportional counters.
Like ion chamber detectors, proportional detectors discriminate between types
of radiation. However, they require very stable electronics which are expensive
and fragile. Proportional detectors are usually only used in a laboratory setting.
Geiger-Müller (GM) Counter
Geiger-Müller counters operate under even higher voltages between the anode
and the cathode, usually in the 800 to 1200 volt range. Like the proportional
counter, the high voltage accelerates the charges produced in the initial
ionization to where they have enough energy to ionize other electrons in the
gas. However, this cascading of ion pairs occurs to a much larger degree and
47
continues until the counter is saturated with ions. This all happens in a fraction
of a second and results in an electrical current pulse of constant voltage. The
collection of the large number of secondary ions in the GM region is known as
an avalanche and produces a large voltage pulse. In other words, the size of the
current pulse is independent of the size of the ionization
event that produced it.
The GM counter was

named for Hans Geiger
The electronic circuit of a GM counters counts and
who invented the device
records the number of pulses and the information is
often displayed in counts per minute. If the instrument has in 1908, and Walther
a speaker, the pulses can also produce an audible click. When
the volume of gas in the chamber is completely ionized, ion
collection stops until the electrical pulse discharges. Again, this
only takes a fraction of a second, but this process slightly limits
the rate at which individual events can be detected.

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
Müller who
collaborated with
Geiger in developing it
further in 1928.
Because they can display individual ionizing events, GM counters are generally
more sensitive to low levels of radiation than ion chamber instruments. By
means of calibration, the count rate can be displayed as the exposure rate over a
specified energy range. When used for gamma radiography, GM meters are
typically calibrated for the energy of the gamma radiation being used. Most
often, gamma radiation from Cs-137 at 0.662 MeV provides the calibration.
Only small errors occur when the radiographer uses Ir-192 (average energy
about 0.34 MeV) or Co-60 (average energy about 1.25 MeV).
Since the Geiger-Müller counter produces many more electrons than a ion
chamber counter or a proportional counter, it does not require the same level of
electronic sophistication as other survey meters. This results in a meter that is
relatively low cost and rugged. The disadvantages of GM survey meters are the
lack of ability to account for different amounts of ionization caused by different
energy photons and noncontinuous measurement (need to discharge).
Comparison of Gas Filled Detectors
The graph to the right shows the
relationship of ion collection in a gas
filled detector versus the applied
voltage. In the ion chamber region,
the voltage between the anode and
cathode is relatively low and only
primary ions are collected. In the
proportional region ,the voltage is
higher, and primary ions and a
number of secondary ions
(proportional to the primary ions
originally formed) are collected. In
48
the GM region, a maximum number of secondary ions are collected when the
gas around the anode is completely ionized. Note that discrimination between
kinds of radiation (E1 and E2) is possible in the ion chamber and proportional
regions. Radiation at different energy levels forms different numbers of
primary ions in the detector. However in the GM region, the number of
secondary ions collected per event remains the same no matter what the energy
of the radiation that initiated the event. The GM counter gives up the ability to
accurately measure the exposure due to different energies of radiation in
exchange for a large signal pulse. This large signal pulse simplifies the
electronics that are necessary for instruments such as survey meters.
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Pocket Dosimeter
Pocket dosimeters are used to provide the wearer with an immediate reading of
his or her exposure to x-rays and gamma rays. As the name implies, they are

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commonly worn in the pocket. The two types commonly used in industrial radiography
are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter.

Direct Read Pocket Dosimeter
A direct reading pocket ionization dosimeter is generally of the size and shape
of a fountain pen. The dosimeter contains a small ionization chamber with a
volume of approximately two milliliters. Inside the ionization chamber is a
central wire anode, and attached to this wire anode is a metal coated quartz
fiber. When the anode is charged to a positive potential, the charge is
distributed between the wire anode and quartz fiber. Electrostatic repulsion
deflects the quartz fiber, and the greater the charge, the greater the deflection of
the quartz fiber. Radiation incident on the chamber produces ionization inside
the active volume of the chamber. The electrons produced by ionization are
attracted to, and collected by, the positively charged central anode. This
collection of electrons reduces the net positive charge and allows the quartz
49
fiber to return in the direction of the original position. The amount of
movement is directly proportional to the amount of ionization which occurs.
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By pointing the instrument at a light source, the position of the fiber may be
observed through a system of built-in lenses. The fiber is viewed on a
translucent scale which is graduated in units of exposure. Typical industrial
radiography pocket dosimeters have a full scale reading of 200 milliroentgens
but there are designs that will record higher amounts.During the shift, the
dosimeter reading should be checked frequently. The measured exposure should be
recorded at the end of each shift.

The principal advantage of a pocket dosimeter is its ability to provide the
wearer an immediate reading of his or her radiation exposure. It also has the
advantage of being reusable. The limited range, inability to provide a
50
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permanent record, and the potential for discharging and reading loss due to
dropping or bumping are a few of the main disadvantages of a pocket
dosimeter. The dosimeters must be recharged and recorded at the start of each
working shift. Charge leakage, or drift, can also affect the reading of a
dosimeter. Leakage should be no greater than 2 percent of full scale in a 24
hour period.
Digital Electronic Dosimeter
Another type of pocket dosimeter is the Digital Electronic
Dosimeter. These dosimeters record dose information and
dose rate. These dosimeters most often use Geiger-Müller
counters. The output of the radiation detector is collected
and, when a predetermined exposure has been reached, the
collected charge is discharged to trigger an electronic
counter. The counter then displays the accumulated
exposure and dose rate in digital form.
Some Digital Electronic Dosimeters include an audible
alarm feature which emits an audible signal or chirp with
each recorded increment of exposure. Some models can also
be set to provide a continuous audible signal when a preset
exposure has been reached. This format helps to minimize
the reading errors associated with direct reading pocket ionization chamber
dosimeters and allows the instrument to achieve a higher maximum readout
before resetting is necessary.
 Audible Alarm Rate Meters and Digital Electronic
Dosimeters
Audible alarms are devices that emit a short "beep"
or "chirp" when a predetermined exposure has been
received. It is required that these electronic devices
be worn by an individual working with gamma
emitters. These devices reduce the likelihood of
accidental exposures in industrial radiography by
alerting the radiographer to dosages of radiation
above a preset amount. Typical alarm rate meters
will begin sounding in areas of 450-500 mR/h. It is
important to note that audible alarms are not
intended to be and should not be used as
replacements for survey meters.
Most audible alarms use a Geiger-Müller detector.
The output of the detector is collected, and when a
predetermined exposure has been reached, this
collected charge is discharged through a speaker.
51
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Hence, an audible "chirp" is emitted. Consequently, the frequency or chirp rate
of the alarm is proportional to the radiation intensity. The chirp rate varies
among different alarms from one chirp per milliroentgen to more than 100
chirps per milliroentgen
 Film Badges
Personnel dosimetry film badges are
commonly used to measure and record
radiation exposure due to gamma rays, Xrays and beta particles. The detector is, as the
name implies, a piece of radiation sensitive
film. The film is packaged in a light proof,
vapor proof envelope preventing light,
moisture or chemical vapors from affecting
the film.
A special film is used which is coated with
two different emulsions. One side is coated with a large grain, fast emulsion
that is sensitive to low levels of exposure. The other side of the film is coated
with a fine grain, slow emulsion that is less sensitive to exposure. If the
radiation exposure causes the fast emulsion in the processed film to be
darkened to a degree that it cannot be interpreted, the fast emulsion is removed
and the dose is computed using the slow emulsion.
The film is contained inside a film holder or badge.
The badge incorporates a series of filters to
determine the quality of the radiation. Radiation of
a given energy is attenuated to a different extent by
various types of absorbers. Therefore, the same
quantity of radiation incident on the badge will
produce a different degree of darkening under each
filter. By comparing these results, the energy of the
radiation can be determined and the dose can be
calculated knowing the film response for that
energy. The badge holder also contains an open
window to determine radiation exposure due to
beta particles. Beta particles are effectively
shielded by a thin amount of material.
The major advantages of a film badge as a personnel monitoring device are that
it provides a permanent record, it is able to distinguish between different
energies of photons, and it can measure doses due to different types of
radiation. It is quite accurate for exposures greater than 100 millirem. The
major disadvantages are that it must be developed and read by a processor
(which is time consuming), prolonged heat exposure can affect the film, and
52
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exposures of less than 20 millirem of gamma radiation cannot be accurately
measured.
Film badges need to be worn correctly so that the dose they receive accurately
represents the dose the wearer receives. Whole body badges are worn on the
body between the neck and the waist, often on the belt or a shirt pocket. The
clip-on badge is worn most often when performing X-ray or gamma
radiography. The film badge may also be worn when working around a low
curie source. Ring badges are worn on a finger of the hand most likely to be
exposed to ionizing radiation. A LIXI system with its culminated and
directional beam would be one example where monitoring the hands would be
more important than the whole body.
Thermoluminescent Dosimeter
Thermoluminescent dosimeters (TLD) are often used
instead of the film badge. Like a film badge, it is
worn for a period of time (usually 3 months or less)
and then must be processed to determine the dose
received, if any. Thermoluminescent dosimeters can
measure doses as low as 1 millirem, but under routine
conditions their low-dose capability is approximately
the same as for film badges. TLDs have a precision of
approximately 15% for low doses. This precision improves to approximately
3% for high doses. The advantages of a TLD over other personnel monitors is
its linearity of response to dose, its relative energy independence, and its
sensitivity to low doses. It is also reusable, which is an advantage over film
badges. However, no permanent record or re-readability is provided and an
immediate, on the job readout is not possible.
How it works
A TLD is a phosphor, such as lithium fluoride (LiF) or calcium fluoride (CaF),
in a solid crystal structure. When a TLD is exposed to ionizing radiation at
ambient temperatures, the radiation interacts with the phosphor crystal and
deposits all or part of the incident energy in that material. Some of the atoms in
the material that absorb that energy become ionized, producing free electrons
and areas lacking one or more electrons, called holes. Imperfections in the
crystal lattice structure act as sites where free electrons can become trapped and
locked into place.
Heating the crystal causes the crystal lattice to vibrate, releasing the trapped
electrons in the process. Released electrons return to the original ground state,
releasing the captured energy from ionization as light, hence the name
thermoluminescent. Released light is counted using photomultiplier tubes and
the number of photons counted is proportional to the quantity of radiation
striking the phosphor.
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Instead of reading the optical density (blackness) of a film, as is done with film
badges, the amount of light released versus the heating of the individual pieces
of thermoluminescent material is measured. The "glow curve" produced by this
process is then related to the radiation exposure. The process can be repeated
many times.
Radiation Safety Videos
Working Safely With Gamma Radiation (22MB, mov)
Survey Meter Calibration (8MB, wmv) (16.5MB, wmv)
Dosimeter Calibration (3.2MB, wmv) (6.5MB, wmv)
Alarm Ratemeter Calibration (4MB, wmv) (8MB, wmv)
Radiation Safety References
Andrews, H., Radiation Biophysics, Englewood Cliffs, New Jersey: Prentice
Hall, Inc., 1974
Iddings, F., "Radiation Detection for Radiography", Materials Evaluation,
American Society for Nondestructive Testing, Columbus, OH, August 2001
National Research Council, "Health Exposure to Low Levels of Ionizating
Radiation",BEIR V, Washington D.C., 1990
Shapiro, J., Radiation Protection - A Guide for Scientists and Physicians, 4th
Ed., Cambridge, MA: Harvard University Press, 2002
United Nations Scientific Committee on the Effects of Atomic Radiation
(UNSCEAR),Ionizing Radiation Sources and Biological Effects, New York,
NY, 2000
"Ionizing Radiation Exposure of the Population of the United States", NCRP
Report No. 93, 1987
"Radiation Safety Guide for Users of X-ray Systems", Iowa State University,
Environmental Health and Safety, Ames IA, 2004
"Radiation Safety Study Guide for Users of Analytical X-ray Systems",
Environment, Safety, Health and Assurance, Ames Laboratory, Ames, IA,
April 1996
Code of Federal Regulations, Title 10, Energy, GPO Access web site @
http://www.gpoaccess.gov
 Radiation Safety Quizzes
These quizzes draw from the same database of questions and differ only in the
number of questions presented. Each time a quiz is opened, a new set of
random questions will be produced from the database. The Collaboration for
NDE Education does not record the names of individuals taking a quiz or the
results of a quiz.
 20 Question Radiation Safety Quiz
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35 Question Radiation Safety Quiz
50 Question Radiation Safety Quiz
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Radiation Safety Quiz

~ First name ~
1
~ Last name ~
Which of the following is a rate measuring instrument?
Film badge
Survey meter
TLD
All of the above
2
Radium could be used to radiograph casting as thick as:
2 inches
4 inches
6 inches
12 inches
3
The cathode and anode are parts of:
An iridium camera
A cobalt camera
X-ray system
All of the above
4
Which of the following has the most material penetrating
power?
Iridium 192
55
Cobalt 60
Cobalt 59
Iridium 191
5
Which of the following is used to provide a permanent record of
radiation exposure?
Alarm ratemeter
Survey meter
Dosimeter
Film badge
6
The term used to describe an interaction where electrons acquire
energy from a passing charged particle but are not removed
completely from their atom is called:
Excitation
Radiation
An isotope
Diffraction
7
Which of the following are the two most common industrial
gamma-ray sources?
Ir 192 and Beryllium 54
Radium and Beryllium
Cobalt and Iridium
X-ray and Lead
8
All people have radioactive isotopes inside their bodies.
True
False
The half-life of Iridium 192 is approximately:
56
9
74 days
74 years
5 days
5 years
10
If a technician has been working with an isotope and the
dosimeter reads “off scale”:
All work should be stopped immediately and the film badge
should be turned in
It should be re-zeroed and work should resume
The assistant should take the rest of the radiographs
There is a high chance that the radiographer will get cancer
11
Ionization of living tissue causes:
Cells to multiply faster
The cells to warm up
Molecules in the cells to be broken apart
All molecules to explode
12
How long after the Roentgen’s discovery, were radiation burns
detected?
1 day
1 year
1 month
6 months
13
Which of the following have a positive charge?
Alpha particles
Beta particles
Gamma particles
Only A and B
57
14
Typical industrial radiography pocket dosimeters have a full
scale reading of:
50 milliroentgens
100 milliroentgens
200 milliroentgens
400 milliroentgens
15
A state that has agreed to assume regulatory control of
radioactive material within their borders is called:
An agreement state
An NRC state
A proliferation state
All of the above
16
What does RSO stand for?
Radiation Service Officer
Roentgen Safety Office
Radiation Safety Officer
Radiation Safety Organization
17
The current lifetime risk of dying from all types of cancer in the
United States is approximately:
5%
10%
15%
20%
18
Gamma rays are produced by:
Radioactive atoms
58
Bremstrahlung
K shell disintegration
Television sets
19
Prolonged exposure to heat can effect a:
Alarm ratemeter
Survey meter
Geiger counter
Film badge
20
The most significant source of man-made radiation exposure to
the average person is from:
Gamma rays
Beta rays
Medical procedures
Alpha rays
Copyright � The Collaboration for NDT Education..
Radiation Safety Quiz
~ First name ~
1
~ Last name ~
Which of the following have no charge?
Alpha particles
Beta particles
59
Gamma particles
Only A and B
2
Radium could be used to radiograph casting as thick as:
2 inches
4 inches
6 inches
12 inches
3
The half-life of Iridium 192 is approximately:
74 days
74 years
5 days
5 years
4
Which of the following is packaged in a light proof, vapor proof
envelope?
Alarm ratemeter
Survey meter
Geiger counter
Film badge
5
Some individuals are more sensitive to the effects of radiation
than others. This is called:
Somatic effects
Genetic effects
Biological effects
All of the above
A symptom from an overexposure to radiation which occurs
years later is called:
60
6
Somatic effects
Biological effects
Latent effects
Genealogy effects
7
Which of the following has the shortest wavelength?
Radio waves
Light waves
Gamma rays
Alpha rays
8
Which of the following have a positive charge?
Alpha particles
Beta particles
Gamma particles
Only A and B
9
Which of the following is a rate measuring instrument?
Survey meter
Audible alarm
Area monitor
All of the above
10
The Curies discovered a radioactive element which they called:
Uranium
Cobalt
Pitchblende
Radium
When radiation interacts with a cell wall or DNA:
61
11
The cell becomes energized
The radiation shifts to a different waveform
The cell becomes radioactive
The cell either dies or becomes a different kind of cell such as a
cancer cell
12
Cobalt 60 is produced by:
Exposing Cobalt 59 to high energy x-rays
Changing the number of protons in the nucleus
Converting an element to a compound
Adding an extra neutron
13
In general, the denser the material:
The more radiation is reflected
The better shielding it will provide
The lower the atomic number
The faster the exposure time
14
In the 1920's, what was used to personnel monitor individuals in
order to determine their radiation exposure?
Film badge
Geiger counter
Alarm ratemeter
Ionization chambers
15
Typical industrial radiography pocket dosimeters have a full
scale reading of:
50 milliroentgens
100 milliroentgens
200 milliroentgens
62
400 milliroentgens
16
Which of the following have a negative charge?
Alpha particles
Beta particles
Gamma particles
Only A and B
17
All people have radioactive isotopes inside their bodies.
True
False
18
Generally an isotope pellet is approximately:
1 inch in diameter
1.5 mm in diameter
1.5 cm in diameter
1.5 m in diameter
19
The term used to describe an interaction where electrons acquire
energy from a passing charged particle but are not removed
completely from their atom is called:
Excitation
Radiation
An isotope
Diffraction
20
The half-life of Cobalt 60 is approximately:
74 days
74 years
5 days
63
5 years
21
Gamma rays are produced by:
Radioactive atoms
Bremstrahlung
K shell disintegration
Television sets
22
The target of an x-ray tube is often made from:
Balsa wood
Cobalt
Tungsten
Aluminum
23
The instrument used to measure external radiation exposure for
the day from gamma and x-rays worn by the radiographer is
called a:
Alarm ratemeter
Pocket dosimeter
Geiger counter
Survey meter
24
Prolonged exposure to heat can effect a:
Alarm ratemeter
Survey meter
Geiger counter
Film badge
25
Which of the following is an example of electromagnetic
radiation?
64
Radio waves
Light waves
Gamma rays
All of the above
26
The most significant source of man-made radiation exposure to
the average person is from:
Gamma rays
Beta rays
Medical procedures
Alpha rays
27
X-rays are produced:
From a decaying isotope
By a nuclear generator
By high speed protons
By an X-ray generator
28
If a technician has been working with an isotope and the
dosimeter reads “off scale”:
All work should be stopped immediately and the film badge
should be turned in
It should be re-zeroed and work should resume
The assistant should take the rest of the radiographs
There is a high chance that the radiographer will get cancer
29
Radiation is a form of:
Atomic waveform
Energy
Decayed plutonium
Active isotopes
65
30
TLD stands for:
Thermoluminescent Dosimeter
Thermal Latent Distance
Time Life Dosimeter
Translucent Latent Dosimeter
31
A dosimeter which is off scale would be considered:
Fully charged
Fully discharged
Broken
Transparent
32
Which of the following has the longest wavelength?
Radio waves
Proton rays
Gamma rays
Photon rays
33
The strength of a source is called:
Sequence radiation
Refraction
Activity
Curies
34
Which of the following is used to provide a permanent record of
radiation exposure?
Alarm ratemeter
Survey meter
Dosimeter
66
Film badge
35
Which of the following has the most material penetrating
power?
Iridium 192
Cobalt 60
Cobalt 59
Iridium 191
Copyright � The Collaboration for NDT Education..
Radiation Safety Quiz
~ First name ~
1
~ Last name ~
Which of the following have a negative charge?
Alpha particles
Beta particles
Gamma particles
Only A and B
2
How long after the Roentgen’s discovery, were radiation burns
detected?
1 day
1 year
1 month
67
6 months
3
Prolonged exposure to heat can effect a:
Alarm ratemeter
Survey meter
Geiger counter
Film badge
4
Some individuals are more sensitive to the effects of radiation
than others. This is called:
Somatic effects
Genetic effects
Biological effects
All of the above
5
Depending upon the ratio of neutrons to protons within its
nucleus:
An isotope is always unstable
An isotope is always stable
An isotope can be stable or unstable
The x-ray source can emit a wide variety of energy levels
6
ALARA stands for:
As Low As Responsibly Acceptable
Alarm Loss Activated Radiation Activated
As Low As Reasonable Achievable
As Low As Reasonably Attenuated
7
Typical industrial radiography pocket dosimeters have a full
scale reading of:
68
50 milliroentgens
100 milliroentgens
200 milliroentgens
400 milliroentgens
8
Which of the following have no charge?
Alpha particles
Beta particles
Gamma particles
Only A and B
9
How much radiation is a radiographer allowed to receive in a
calendar year?
15 rem
10 rem
50 rem
5 rem
10
The device that measures the total amount of exposure received
during a measuring period is called a:
Survey meter
Geiger counter
Dose measuring instrument
All of the above
11
The instrument used to measure external radiation exposure for
the day from gamma and x-rays worn by the radiographer is
called a:
Alarm ratemeter
Pocket dosimeter
Geiger counter
69
Survey meter
12
The dislodging of one or more electrons from an atom is called:
Ionization
Refractology
Half-life
Isotope
13
Which of the following is a personal monitoring device?
Alarm ratemeter
Survey meter
Geiger counter
Film badge
14
What does a collimator do?
It reduces the exposure time by ionizing the radiation before it
hits the film
It holds the film in place during an exposure
It provides shielding from radiation
There is no such thing as a collimator
15
In the 1920's, what was used to personnel monitor individuals in
order to determine their radiation exposure?
Film badge
Geiger counter
Alarm ratemeter
Ionization chambers
16
Most radiation overexposure accidents occur when:
The survey meter is not used correctly
70
The film badge is not used correctly
The alarm ratemeter is not used correctly
All of the above
17
Generally an isotope pellet is approximately:
1 inch in diameter
1.5 mm in diameter
1.5 cm in diameter
1.5 m in diameter
18
Which of the following has the longest wavelength?
Radio waves
Proton rays
Gamma rays
Photon rays
19
The half-life of Cobalt 60 is approximately:
74 days
74 years
5 days
5 years
20
Which of the following would be the most effective shielding
material?
Wood
Paper
Concrete
Tungsten
One source of natural radiation is:
71
21
Iridium 192
Cobalt 60
Cosmic radiation
Nuclear power plants
22
The most significant source of man-made radiation exposure to
the average person is from:
Gamma rays
Beta rays
Medical procedures
Alpha rays
23
The strength of a source is called:
Sequence radiation
Refraction
Activity
Curies
24
Gamma rays are produced by:
Radioactive atoms
Bremstrahlung
K shell disintegration
Television sets
25
Which of the following is an example of electromagnetic
radiation?
Radio waves
Light waves
Gamma rays
All of the above
72
26
What are the two types of radiation?
Particulate and electromagnetic
Alpha and Beta
Alpha and Gamma
Positive and negative
27
If a technician has been working with an isotope and the
dosimeter reads “off scale”:
All work should be stopped immediately and the film badge
should be turned in
It should be re-zeroed and work should resume
The assistant should take the rest of the radiographs
There is a high chance that the radiographer will get cancer
28
The primary risk from occupational radiation exposure is the
increased risk of:
Cancer
Blindness
Abrasions
None of the above
29
Cobalt 60 is produced by:
Exposing Cobalt 59 to high energy x-rays
Changing the number of protons in the nucleus
Converting an element to a compound
Adding an extra neutron
30
X-rays were discovered in 1895 by:
Robert Conrad
Marie Curie
73
Pierre Curie
Wilhelm Roentgen
31
Radioisotopes such as Iridium 192 and Cobalt 60 are used in an
industrial setting and are carried in a container called a:
Camera
Vault
Safe
Overpack
32
The term used to describe an interaction where electrons acquire
energy from a passing charged particle but are not removed
completely from their atom is called:
Excitation
Radiation
An isotope
Diffraction
33
At what radiation level will an alarm ratemeter begin to sound?
200 mr/hr
300-400 mr/hr
450-500 mr/hr
700-1000 mr/hr
34
Which of the following are the two most common industrial
gamma-ray sources?
Ir 192 and Beryllium 54
Radium and Beryllium
Cobalt and Iridium
X-ray and Lead
74
35
Which of the following has the most material penetrating
power?
Iridium 192
Cobalt 60
Cobalt 59
Iridium 191
36
The time required for one half of the amount of unstable
material to degrade into a more stable material is called:
Half-time
Half-life
Half value layer
Half-sign
37
Radiation is a form of:
Atomic waveform
Energy
Decayed plutonium
Active isotopes
38
The current lifetime risk of dying from all types of cancer in the
United States is approximately:
5%
10%
15%
20%
39
Who discovered natural radioactivity?
Wilhelm Roentgen
Marie Curie
75
Henri Becquerel
None of the above
40
Ionization of living tissue causes:
Cells to multiply faster
The cells to warm up
Molecules in the cells to be broken apart
All molecules to explode
41
When radiation interacts with a cell wall or DNA:
The cell becomes energized
The radiation shifts to a different waveform
The cell becomes radioactive
The cell either dies or becomes a different kind of cell such as a
cancer cell
42
Which of the following have both energy and mass?
Alpha particles
Beta particles
Gamma particles
Only A and B
43
A symptom from an overexposure to radiation which occurs
years later is called:
Somatic effects
Biological effects
Latent effects
Genealogy effects
A dosimeter which is off scale would be considered:
76
44
Fully charged
Fully discharged
Broken
Transparent
45
The advantages of a TLD over other personnel monitors is its:
Resistance to heat
Sensitivity to alpha particles
Linearity of response to dose
Ability to activate an audible alarm
46
Which of the following is used to provide a permanent record of
radiation exposure?
Alarm ratemeter
Survey meter
Dosimeter
Film badge
47
Which of the following produces a permanent record of
radiation exposure?
Alarm ratemeter
Survey meter
Geiger counter
Film badge
48
A state that has agreed to assume regulatory control of
radioactive material within their borders is called:
An agreement state
An NRC state
A proliferation state
All of the above
77
49
If a person received a radiation dose of 10 rem to the entire body
(above background), his or her risk of dying from cancer would
increase by one percent to a high of:
5%
10%
15%
21%
50
Which of the following is a rate measuring instrument?
Survey meter
Audible alarm
Area monitor
All of the above
Copyright � The Collaboration for NDT Education..
78
What is NDT?
The field of Nondestructive Testing (NDT) is a very broad, interdisciplinary field that plays a critical role in assuring that structural components and systems perform their
function in a reliable and cost effective fashion. NDT technicians and engineers define and implement tests that locate and characterize material conditions and flaws that
might otherwise cause planes to crash, reactors to fail, trains to derail, pipelines to burst, and a variety of less visible, but equally troubling events. These tests are performed
in a manner that does not affect the future usefulness of the object or material. In other words, NDT allows parts and material to be inspected and measured without
damaging them. Because it allows inspection without interfering with a product's final use, NDT provides an excellent balance between quality control and costeffectiveness. Generally speaking, NDT applies to industrial inspections. The technologies that are used in NDT are similar to those used in the medical industry, but
nonliving objects are the subjects of the inspections.
http://www.ndt-ed.org/EducationResources/CommunityCollege/RadiationSafety/introduction/backround.htm
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