EHS

Radiation Safety-A Workers' Guide

I. Introduction & Purpose:

In the course of performing your duties as a laboratory employee or student at CSUN, you may be asked to work with either radioactive isotopes or radiation-producing machines.

One of the means by which the safe use and handling of radioactive isotopes or radiation-producing machines may be accomplished is for the user to become casually familiar with some of the technical and practical topics associated with the safe use of more common sources of radiation found at CSUN.  State and CSUN requirements will also be presented.

The goal of this handbook is to educate the users so they will be able to identify and deal effectively with the radiation hazards in their immediate work area, thereby providing for their own safety and the safety of those around them.

II. Basics:

The basic building block of all material on Earth is the atom.  The atom is composed of a central nucleus surrounded by an outer cloud of orbital electrons.  The nucleus of the atom is composed of neutrons and protons.  The total number of protons contained within an atomic nucleus is called the atomic number, or Z-number, of that particular type of atom.

Atoms can be either stable or unstable.  Unstable atoms spontaneously decay to a more stable configuration, followed very quickly by the emission of particulate radiation, electromagnetic radiation, or both.  Unstable atoms are termed radioactive.  Depending on the type of decay, an atom may remain an atom of the same element, or it can become an entirely different elemental atom.

Science has defined two categories of radiation.  The ionizing radiation category primarily includes alpha, beta, x-ray, and gamma radiation.  Non-ionizing radiation includes, among others, ultraviolet (UV), infrared (IR), radio frequency (RF), and microwave radiation.  The difference between the two is that ionizing radiation has enough energy to eject orbital electrons from the atoms of the material being irradiated.  This process is called ionization.  Ionization has the potential to lead to biological damage if the material being irradiated is biological tissue.  This manual shall only be concerned with ionizing radiation.

The energy of radiation is measured in electron volts.  The electron volt (eV) is defined as the energy achieved by an electron in traversing an electrical potential difference of 1 volt.  The eV is a very small unit of energy and therefore keV (thousand electron volts) and MeV (million electron volts) are used as units of measurement of the energies associated with radioactive emissions.  The energy of visible light is about 2 to 3 eV.

III. Measuring Radiation-Units:

There are several different units used to describe the amounts, properties and biological effects associated with ionizing radiation.

  1. Activity:  The curie (Ci) and the becquerel (Bq) are used to indicate the activity, or total amount of radioisotope, that is present.
    • The curie is a very large unit of activity and is defined as 3.7 x 1010 disintegrations per second (dps).  Most radioactive sample at CSUN contain amounts of activity which are more appropriately measured in millicuries (10-3 Curies) or microcuries (10-6 Curies).
    • The becquerel is a very small unit of activity and is defined as 1 disintegration per second.  More appropriate units for expressing the activity of a sample in becquerels are megabecquerels (106 becquerels) and gigabecquerels (109 becquerels).
    • The amount of activity in a sample decreases predictably with time.  The amount of time it takes for the activity in a sample to decrease to one-half the initial activity is termed the "half-life" of the isotope.  Radioactive half-lives range from millionths of seconds to billions of years.  The simple mathematical description of the exponential radioactive decay of an isotope is given as follows and can be used to determine the amount of activity remaining in a questionable sample after a particular time, t:

At = A0 x e-(ln 2/T)t

where:

A0        =          initial activity of the isotope
At         =          activity of the isotope at time t
T          =          half-life of the isotope
t           =          elapsed time from the date of the initial activity

When using this formula it is important to express t and T in the same units of time (i.e., days, years, etc.).

Example:   What is the remaining activity 21 days after a sample was known to contain 100 µCi (microcuries) of phosphorus-32?  (The half-life of P-32 is 14.2 days.)

A(21 days) = 100 µCi x e-(ln 2/14.2d) x 21d = 35.9 µCi

  1. Exposure: As gamma or x-ray radiation passes through air, secondary charged particles are liberated from atmospheric molecules by the process of ionization.  The measure of x-ray and gamma radiation based on its ability to cause ionization of molecules is called exposure.  The method used to determine the exposure of gamma or x-ray radiation field is to measure the amount of charge created in a measured volume of air by the ionization process.  The unit is used to indicate the amount of exposure is the roentgen.  The roentgen is only defined for gamma or x-ray radiation in air and is defined as the amount of gamma or x-ray radiation, with photon energy less than 3 MeV, required to produce 1 statcoulomb (SC) of charge in 1 cubic centimeter (cc) of air at standard temperature and pressure (STP).
  1. Absorbed Dose: The primary concern of radiation exposure is the possible biological damage to living organisms.  The amount of biological damage caused by the passage of radiation through a volume of tissue is approximately proportionate to the amount of radiation energy absorbed by the tissue.  For this reason the rad and the gray (Gy) are used to indicate the absorbed dose or amount of radiation energy absorbed in any type of material.  The rad (radiation absorbed dose) is defined as follows:

1 rad = 100 ergs/gram
also,
1 gray (Gy) = 100 rads.

 An exposure of 1.0 roentgen will impart a dose of 0.877 rads to the given volume of air at standard       temperature and pressure (STP).

  1. Quality Factor (QF): All types of radiation do not cause the same degree of biological damage.  The quality factor (QF) is used as an indication of the damage possible by a certain kind of radiation.  The quality factor of beta, gamma and x-ray radiation is 1.  The quality factors of alpha and neutron radiation are approximately 20 and 10 respectively.  This means alpha and neutron radiation are more biologically damaging than beta or gamma radiation for the same absorbed dose.
  1. Dose Equivalent: Because different types of radiation cause varying degrees of biological damage to living tissue, another measurement is needed which reflects this difference.  This measurement is termed the dose equivalent and is used to indicate biological effects to the human body.  The rem and the sievert (Sv) are the units of dose equivalent and are also the units used for radiation protection purposes.  The relationship between the rad and the rem is as follows:

rem = rad x quality factor
also
1 sievert  = 100 rems.

For all practical purpose the amount of rads is equal to the amount of rems for beta, gamma and x-ray radiation.

  1. Deep Dose Equivalent: This is the dose equivalent from external exposure measured at a tissue depth of one (1) cm.
  1. Effective Dose Equivalent: It is, of course, more dangerous for the whole body to receive a particular exposure than if only one organ receives that exposure.  It is possible to translate an organ dose into a whole body dose that would produce the same overall risk; such a translated dose is called the effective dose equivalent.  In other words, the word "effective" implies that any organ dose is normalized to the whole body equivalent.  Note that individual organs and tissues have characteristic weighing factors by which the dose equivalent must be multiplied in order to accurately reflect the health risk.
  1. Committed Effective Dose Equivalent: This dose is important for radionuclides that are taken into the body.  It is the sum, over all organs, of the dose equivalent to those organs from the intake of radioisotopes during fifty (50) years following exposure. 
  1. Total Effective Dose Equivalent: This is the sum of the deep dose equivalent for external exposures and the committed effective dose equivalent for internal exposures.

IV. Radiation Physics:

  1. Modes of Radioactive Decay: There are several modes of decay by which an unstable atom can decay to a more stable configuration.  The most common are:  alpha decay, beta decay, positron emission, gamma and x-ray emission, electron capture, and internal conversion.  The products of these modes of radioactive decay are:  alpha particles, beta-minus particles (ß-) or electrons, beta-plus particles (ß+) or positrons, gamma rays, and x-rays.  Frequently, more than one of these products of decay are emitted during a single decay event.  Let's look at the products of radioactive decay more closely.

  2. Alpha Radiation: Alpha particles are highly charged, heavy, slow-moving particles.  Because of their high electrical charge and slow speed, alpha particles interact very strongly with any material and therefore have a very limited range.  Most alpha particles will not penetrate a sheet of paper, nor will they penetrate our external layer of dead skin.  Because of this fact, alpha radiation is not an external radiation hazard.  However, alpha radiation can be a serious internal radiation hazard, and precautions must be taken to prevent any internal uptake of alpha emitting isotopes, as they tend to accumulate in specific organs in the body and deliver high localized doses.
    • Alpha decay is monoenergetic, meaning that all alpha particles emitted by a particular isotope undergoing a particular nuclear transition have the same energy.  Alpha emission is common only among those elements of high atomic number (83 or higher) and is usually accompanied by one or more other types of radiation.
    • Common alpha emitting isotopes are polonium-210 (Po-210), radium-226 (Ra-226), radon-222 (Rn-222), thorium compounds, and uranium compounds.  It is a good idea to store alpha emitters in such a way that any leakage of the source will be contained.  This is accomplished by storing the alpha emitter in a closed container with a reasonably tight seal and opening it in a fume hood each time it is needed.
  1. Gamma and X-Ray Radiation:  gamma rays are electromagnetic radiation that originates in the nucleus.  X-rays are electromagnetic radiation and are identical to gamma radiation except that they originate in the electron cloud surrounding the nucleus and generally have lower energies than gamma rays.  Gamma and x-ray radiation are emitted in discrete, small packets of energy known as photons.  The properties of gamma ray and x-ray photons are determined by their wavelength and frequency.
    • Photons interact with matter primarily by three processes:  photoelectric effect, Compton scattering and pair production.  A lengthy explanation of these processes is not required, but note that all three eventually produce energetic electrons which ionize or excite the atoms in the absorber.  These interactions permit the detection of gamma rays, or x-rays, and determine the thickness of shielding materials necessary to reduce exposures from gamma or x-ray sources.
    • Attenuation refers to the reduction in the intensity of gamma and x-ray radiation.  Gamma and x-ray photons are very penetrating but they can be effectively attenuated using shielding materials.  The higher the energy of the photon, the more material will be needed to reduce the exposure.  The amount of absorber material needed to attenuate a particular exposure to one-half its original value is called the half-value layer.  The amount of absorber material needed to attenuate a particular exposure to one-tenth its original value is called the tenth-value layer.  Photons with energies less than about 10 keV are attenuated mostly by the skin of the body.  These photons may be an external radiation hazard to the skin.  Higher energy photons penetrate considerable distances into and through the human body.  Photons of this energy are considered an external radiation hazard to the whole body.  The external hazard of gamma and x-ray emitters can be eliminated with lead foil for low energy gamma radiation or lead bricks for high-energy gamma radiation.  Gamma and x-ray emitters are also an internal hazard and precautions must be taken to prevent internal uptakes of either of these.
    • Gamma or x-ray emission seldom occurs alone, being accompanied in almost all cases by another type of radiation.  Common gamma emitters on campus are I-125, I-131, Mn-54, Na-22, Rb-86, Cs-137, and Co-60.  Most sources of x-rays at CSUN are machine-produced.  Also, some isotopes emit x-rays.  Table 1 lists information which can be used to effectively shield some of the more common gamma emitters.

TABLE 1 - Common Gamma Emitters

Isotope

Half-Life

Half-Value

Tenth-Value

Exposure Rate

1.

Layer

Layer

@ 1 ft. from .1

mCi

2.

(cm lead)

(cm-lead)

 

(mR/hr

Co-57

270.9 d

0.01

0.05

0.097

Co-60

5.3y

1.5

4.5

1.42

Cr-51

27.7d

0.2

0.7

0.017

Cs-137

30.0y

0.8

2.4

0.36

Fe-55

2.7y

<0.001

<0.002

<0.004

Fe-59

44.5d

1.5

4.5

0.69

l-125

60.2d

0.002

0.006

0.075

l-131

8.0d

0.3

1.1

0.24

Mn-54

312.5d

1.1

3.2

0.51

Na-22

2.6y

0.9

3.6

1.29

 

  1. Beta Radiation:  Beta particles are moderately charged, light, fast moving particles.  Beta emitters radiate beta particles with a wide spectrum of energies.  Unless otherwise stated, the energy of a beta-emitter given in reference literature is the maximum or end-point energy.  The average beta energy is about one-third of the quoted maximum beta energy.
    • Some isotopes are pure beta emitters, which means they emit only beta radiation.  Some examples of pure beta emitters are P-32, H-3, C-14, S-35, and Ca-45.  Table 2 gives values for some of the physical properties of five commonly used pure beta emitters.  Other beta emitters may release both beta radiation and other types of radiation.
    • Beta emitters require special considerations for handling and shielding that are more involved than the straightforward methods used to shield gamma emitters.
    • Beta radiation penetrates matter to varying distances.  The higher the energy of the beta particle, the deeper the penetration into matter.  Depending on the maximum beta energy, the penetration depth may be an external radiation hazard, specifically to the skin and eyes.  The degree of hazard depends on the beta energy of the isotopes and should be evaluated in every case.  Generally, beta emitters whose energies are less than 200 keV, such as tritium (H-3), S-35, and C-14, have limited ranges in tissue and are not considered to be external radiation hazards.  However, in the event of external skin contamination the hazard can be significant as shown on the bottom line of Table 2.  Except for tritium (H-3), the dose rate to the basal cells of the skin is in the range of 1.4 - 9.2 rad/hr, for a skin contamination of 1 µCi/cm2.
    • A beta particle will travel approximately 12 feet/MeV in air.  Therefore, high-energy beta particles, such as P-32 and Sr-90, will travel quite a long distance in air.
    • Beta emitters are also an internal hazard and precautions must be taken to prevent any internal uptake of isotope.
    • Charged particles, including beta particles, lose energy in an absorbing material by excitation, ionization, and radiation.  Radiotive energy losses of charged particles are very important and are termed bremsstrahlung, which in German means "braking radiation".  This process occurs when the charged particle decelerates in an absorber with an attendant creation of x-ray (or bremsstrahlung) radiation.  This radiation is more penetrating than the original beta radiation.  The fraction of beta energy that contributes to the production of bremsstrahlung is directly proportional to both the atomic number of the absorber and the energy of the beta radiation.  To prevent the creation of bremsstrahlung radiation, high-energy beta emitters must be shielded with material having a low atomic number, i.e, Lucite or plastic, about 1 cm thick for P-32.  If bremsstrahlung radiation can be detected through the low atomic number shield, lead should be used to attenuate this radiation.  The lead must be placed on the side of the plastic shield that is away from the source, so as not to create a bremsstrahlung radiation hazard with the lead.  The amount of lead required to attenuate the bremsstrahlung created from P-32 is about 0.3 mm.  Bremsstrahlung produced from P-32 beta radiation is characterized in Table 3.

TABLE 2 - Common Beta Emitters

Properties                   H-3      C-14     S-35     Ca-45   P-32

Half-life                       12.3 y  5715 y 87.4 d  163 d   14.3 d

Maximum                   0.0186 0.156   0.167   0.257   1.71
Beta Energy
(MeV)

Average Beta              0.006   0.049   0.049   0.077   0.695   Energy (MeV)

Range in Air                0.5       23        24        46        610      (Cm)      

Range in Water          0.001   0.029   0.03     0.06     0.8       (Cm)

Fraction                      0.11     0.16     0.37     0.95 transmitted through dead layer of skin

                                                                                                (0.007cm)

Dose Rate                   1.4       1.6       4.0       9.2 to basal cells (rad/hr per µCi/cm2)

Notes: From Healy, 1971.  The dose is from beta particles emitted in all directions equally from contamination on the surface of the skin.  Basal cells are considered to be .007 cm below the surface.

TABLE 3 - Bremsstrahlung from P-32

Absorber

Fraction of Energy

Average

Lucite

0.36%

0.2MeV

Lead

5.0%

0.2MeV

Converted into Bremsstrahlung

Energy of Bremsstrahlung

The products of beta emission can be either the beta-minus (ß-) particle or the positron (ß+) particle.  Associated with positron emission is a special radiation safety problem.  After a positron enters an absorber it begins to slow down and eventually interacts with an orbital electron in the absorber.  When this happens the positron an electron totally annihilate each other and two gamma rays are produced, each with an energy of 0.511 MeV.  This annihilation radiation is more penetrating than the original positron radiation and can be attenuated by using lead shielding.

V. Biological Effects & Risks:

The biological effects and risks associated with exposure to radioactive materials have been studied more thoroughly than any other hazardous agent found in the laboratory environment.

  1. Knowledge Base:  The biological effects of radiation have been studied extensively by many groups worldwide.  Information comes from WWII atomic bomb survivors (the largest group), medical patients, mine workers, accident victims, and animal studies.  These groups yield information primarily about radiogenic cancers caused by high doses of radiation.
  1. Modes of Exposure:  There are two basic modes of radiation exposure.  An acute exposure is generally accepted to be an exposure to a large amount of radiation in a short period of time.  Long term, low level exposure is called chronic exposure, such as that exposure received from background radiation during the course of ones lifetime.  Also, as discussed earlier, exposure can be either external or internal.
  1. Types of Biological Effects:  By studying the knowledge base, it has been determined that exposure to ionizing radiation may cause three types of biological effects.  These are somatic effects, genetic effects, and fetal effects.
    • Somatic effects are experienced directly by the irradiated individual and are either prompt or delayed, depending upon the period of time before the effects are manifest in the exposed individual.  These effects include damage to body tissues and organs which can impair their ability to function normally.  Those symptoms exhibited during the course of the Chernobyl accident were prompt somatic effects.  Delayed somatic effects can be long term, 20-30 years.  The delayed somatic effects of ionizing radiation are an increase in the probability of developing different types of cancers.
    • Genetic effects may be passed on to future generations.  The inherited characteristics in human reproduction are controlled by genes in the reproductive cells.  Radiation can alter genes and thereby produce mutations which may eventually result in anomalies in the offspring.  Several generations may be necessary before such effects become apparent.
    • Fetal effects are those effects which result from the exposure to penetrating radiation of a fetus or embryo inside the abdomen of a pregnant woman.  A number of studies have indicated that the embryo or fetus is more sensitive to radiation than are adults, particularly during the first three months after conception, when a woman may not be aware that she is pregnant.  The main concerns during this period of the pregnancy may be developmental abnormalities during the growth of the fetus.  As the pregnancy progresses, the sensitivity of the fetus to radiation decreases.  The main concerns later on in the course of the pregnancy may be an increase in the risk of leukemia in the first 10 years of the child's life.
  1. Dose Response Curves: Dose response curves are graphical plots of the number of biological effects versus dose.  The three types of biological effects can be studied:  somatic, genetic, and fetal effects.  Dose response curves can then be used to estimate the number of biological effects attributable to a particular radiation dose and thereby estimate the risk associated with a particular dose.
    • Radiation is like most substances that cause cancer in that the effects can be clearly seen at high doses.  Our best estimates of the risks of cancer at low levels of exposure, such as you may be exposed to at CSUN, are derived from data at high dose levels and high dose rates.  Generally, for radiation protection purposes these estimates are made using a linear model.  We have data on health effects at high doses.  Below about 100 rems, studies have not been able to accurately measure the risk, primarily because of the small numbers of exposed people, and because the effect is small compared to differences in the normal incidence of cancer from year to year and place to place.  In order to obtain accurate estimates of the risk for low-level radiation exposure, very large groups of people (many millions) would be needed for a scientific study.
    • Most scientists believe that there is some degree of risk no matter how small the dose.  Some scientists believe that the risk drops off to zero at some low dose, the threshold effect.  A few believe that even very small doses imply a significant risk, while others believe that the same small doses actually decrease the cancer risk: that is, small doses are actually protective against cancer.  The majority of scientists today endorse (for radiation protection purposes) a model wherein risk is approximately proportional to dose even at extremely low doses.  For radiation protection purposes, the USNRC also endorses this model, which shows the number of effects decreasing as the dose decreases.  Estimated risks using this (linear) model are listed in Table 4.

Table 4 - Estimated Risks Associated With Low-Level Radiation Exposure

Biological Effects

Natural Occurrences

Radiation Related

Cancer Cases

2,500 in 10,000

3 in 10,000

Cancer Fatalities

1,640 in 10,000

1 in 10,000

Genetic Effects

1,000 in 10,000

1-10 in 10,000

Fetal Effects

700 in 10,000

1-10 in 10,000

Explanations of the risks are as follows:

  1. Cancer Cases:  Of 10,000 people, 2,500, of them may exhibit some form of cancer during their lifetime.  If 10,000 people are each irradiated with 1 rem of whole body radiation, it is estimated that the radiation may cause 3 additional cases of cancer in the group.
  1. Cancer Fatalities:  Of 10,000 people, 1640 of them may succumb to some form of cancer.  If 10,000 people are each irradiated with 1 rem of whole body radiation, it is estimated that the radiation may cause 1 additional cancer death in the group.
  1. Genetic Effects:  The current incidence of all types of genetic disorders and traits that cause some type of serious handicap at some time during an individual's lifetime is about 1,000 incidents per 10,000 live births.  If each 30-year generation receives 1 rem of whole body radiation, it is estimated that the radiation may cause an additional 1 to 75 genetic disorders in the first generation or 10 to 1,000 disorders at genetic equilibrium about four generations later.
  1. Fetal Effects: The current incidence of all types of fetal effects is about 700 incidents per 10,000 live births.  This includes effects due to measles, alcohol, drugs, etc.  If each child were to receive 1 rem of whole body irradiation before birth, it is estimated that the radiation may cause from 1 to10 additional effects in the group.

VI. Alara Program:

The ALARA concept is a vital part of the radiation safety program at CSUN.  ALARA is an acronym meaning "As Low as is Reasonably Achievable".  Simply stated, ALARA is a radiation safety philosophy that seeks to keep doses to radiation workers as low as can be reasonably achieved.  ALARA is achieved through both administrative and practical controls.

  1. Administrative Controls:  Guidance and Regulatory Agencies - Several scientific groups work to provide information and recommendations concerning radiation safety.  These groups are the National Council on Radiation Protection and Measurements (NCRP), the International Commission on Radiation Protection and Measurements (ICRP), the International Atomic Energy Agency (IAEA), and the American National Standards Institute (ANSI).  They only provide recommendations and do not enforce or establish radiation safety policy.  The groups responsible for enforcing radiation policy are the Environmental Protection Agency (USEPA), the Nuclear Regulatory Commission USNRC), the State of California Department of Public Health, and the Department of Transportation (USDOT).
  1. CSUN License - Radioactive materials and radiation producing machines are used in a variety of laboratories at CSUN.  This work is authorized by a broadscope license granted the campus by the State of California, Department of Health Services.  Experienced faculty members on campus are approved for each operation involving radiation sources by the Office of Environmental Health and Safety (EH&S), acting on behalf of the CSUN Radiation Safety Committee (RSC), whose function it is to safeguard the radiological health of the CSUN community.  Each such faculty user is granted an "Ionizing Radiation Use Authorization" (IRUA) which specifics what radioisotopes or machines can be used, where they may be used, how much activity may be used, and what special radiation safety procedures must be followed.
  1. The Radiation Safety Officer (RSO) is responsible for managing the license and the laboratory surveillance program.  The RSO communicates the requirements of the license to the principal investigators and users through the "Radiation Safety Manual," each individual IRUA, lab postings, training sessions, and personal communications.  The "Radiation Safety Manual," besides being an in-depth presentation of the organization and requirements of the radiation safety program at CSUN, also contains very practical information about the use of radioisotopes and radiation producing machines at the campus.  You are strongly encouraged to become familiar with its contents.
  1. Dose Limits for Radiation Workers - Scientists have determined acceptable dose limits for the radiation worker.  The probability of clinically observable harm in an adult working within these limits for an entire lifetime is extremely low, and therefore, deemed acceptable.  Dose limits for radiation workers are listed in Table 5.

    The whole body dose limit is based on a sum of the external dose to the whole body and internal exposures to organs.  This radiation dose is called the Total Effective Dose Equivalent (TEDE).  Limits are also specified for skin, lens of eye, extremities, pregnant females, and members of the public.  Because the risks of undesirable effects may be greater for young people, the dose limits for those under 18 years of age are 10 percent the value listed in Table 5.  Individuals less than 18 years of age are not allowed in radioisotope laboratories at CSUN without prior review and approval of the Radiation Safety Committee.

Table 5 - Dose Limits for Radiation Workers

Portion of Body Affected

Annual Dose Limit

Whole Body

 

Lens of Eye

15 rem

Any Individual Organ or Tissue

Extremities or Skin

50 rem

 

  1. Expected doses at CSUN - The annual whole body exposure you are most likely to receive at CSUN is only a small percentage of the federal dose limit listed in Table 5 (generally less than 50 mrem/yr).  However, even though your expected dose at CSUN will most likely be very small, it is still prudent to do what you can to keep your annual dose ALARA.
  1. Expected Doses from Background Radiation - Each one of us is exposed to radiation every day of our lives.  It comes from natural sources in the food we eat, the building materials in our homes and workplaces, the air we breathe, outer space, and the earth we walk on.  There are also man-made sources of radiation which contribute to our annual dose equivalent.  Some of these sources are medical treatments, radioactive sources used in consumer items, and cigarettes.
  1. Special Concerns in Case of Pregnancy - According to the International Commission on Radiation Protection and Measurements, particular efforts should made to keep the radiation exposure of an embryo or fetus at the very lowest practical level during the entire period of pregnancy.  Also, the National Council on Radiation Protection and Measurements has recommended that the occupational radiation dose of an expectant mother should not exceed 0.5 rem (500 mrem) during the 9-month gestation period.  Therefore, the federal dose limit is a TEDE of 500 mrem.  The Environmental Health and Safety Office is required to take practical steps to ensure that dose rates are kept low in work areas.  However, it is your responsibility to decide whether the exposure you are receiving is sufficiently low to protect your unborn child.  The advice of EH&S should be obtained to determine whether radiation levels in your working area are high enough that an unborn child could receive 0.5 rem (500 mrem) or more before birth.

    If you are pregnant now or are considering becoming pregnant, contact EH&S for more information pertaining to radiation risks.  It is important to do this promptly as the unborn child is most sensitive to radiation during the first three months of pregnancy.

Practical Controls:

  1. Controls of External Radiation Exposure:  External radiation exposure is primarily a problem with neutrons and high-energy beta, gamma and x-ray emitters.  There are several ways to reduce the risk of exposure due to external radiation hazards.  Depending upon your particular situation, some methods may be more appropriate to use than others.
  1. Time - Reduce the amount of time you spend in close proximity to the radioactive source by working quickly and efficiently.  However, do not work so fast as to compromise your results or cause spillage.  Take time to plan your work and possibly perform dry runs.  This will make you more familiar with the required experimental procedures and thereby lower the time required to work in the vicinity of the radioactive source.  Don't loiter in the vicinity of radioactive sources.
  1. Distance - Whenever possible increase the distance between yourself and the radiation source.  Remember, the intensity of a radiation field decreases with the square of the distance from the source, so if you double your distance from a source, the intensity of the radiation field will decrease to one-fourth the initial exposure.
  1. Shielding - Check your work area using a survey meter to see if any shielding is required.  If you are using P-32 or other high energy beta emitters, you might consider using shielding if your survey meter indicates a reading about 10 times background.  As previously discussed, proper shielding of high-energy beta emitters consists of about 1 cm of plexiglass.  Shielding is not required for low energy beta emitters such as S-35 or C-14 as these betas have very limited ranges in air.
    • When using gamma emitters, shielding is required if your meter indicates an exposure reading greater than 2 mR/hr.  Proper shielding for low energy gamma emitters, such as I-125, requires only thin sheets of lead foil.  For medium energy gamma emitters, such as Co-57, about one-fourth inch (6.4 mm) of lead is needed.  High-energy gamma emitters such as Co-60, Na-22, Mn-54, Cr-51, and I-131 require lead bricks to effectively attenuate the gamma radiation.  Use Table I to determine the amount of lead shielding needed for your application.
    • In all shielding applications make sure to shield the source to protect those individuals who may be on the other side of adjoining walls or lab benches.  Notify EH&S if you need assistance in measuring exposure rates, determining shielding requirements or if you need information on where to purchase shielding materials.
  1. Dosimeter Badges - You will most likely be issued a dosimeter if you work with gamma emitters or high-energy beta emitters.  Dosimeters are used to record an estimate of your external radiation exposure.  Most dosimeters can distinguish between penetrating (gamma, x-ray) or non-penetrating radiation (beta).  You may also be issued a finger ring for measuring the radiation exposure to your extremities.  On a routine basis dosimeters and rings are collected and sent out for processing.

You can do several things to help us keep a good record of your dose:

    • Wear your dosimeter any time you work with or near radioactive materials or radiation producing machines.
    • Wear your badge at or near the collar.  If you wear a lead apron, wear the badge on the outside of the apron.  Ring dosimeters should be worn with the sensitive element facing toward the inside of the hand.
    • Do not take your dosimeter home.  Store it in a cool, dry place away from any radioactive sources.
    • Notify EH&S if you accidentally expose your dosimeter to radiation other than on your job, or if you have a medical treatment involving radio pharmaceuticals.
    • When you terminate employment at CSUN, please return your dosimeter(s) to the contact for your group, or to EH&S on the last day of your employment.
  1. Control of Internal Radiation Exposure:  Internal depositions of radioactive compounds may cause high dose rates to internal body organs.  This happens because some isotopes have long residence times in the human body and some isotopes selectively accumulate in particular body organs.
    • In controlling internal radiation exposure it is helpful to know that there are three primary routes of entrance into the human body.  These are inhalation, ingestion and absorption.  All means to prevent the risk of internal radiation exposure seek to prevent the entrance of radioisotopes into the human body via any of these routes.
    • Inhalation is the means whereby volatile forms of radioisotopes are most likely to enter the body.  This is especially true for radioiodine and some forms of tritium (H-3).  This may also be a special concern for fine particulates such as radioactive microspheres.
    • The most effective and straightforward means of eliminating this potential hazard is to perform all work with volatile compounds in a fume hood.  The principal contaminant control factor in the use of fume hoods is the flow of air across the hood face and into the exhaust system.  Studies have shown that clutter i in the hood can greatly reduce effectiveness as can increased face velocities, (the optimal face velocity is about 100-125 linear feet per minute).  Increased effectiveness in eliminating these hazards can be obtained by moving the radioactive source further into the fume hood, checking the fume hood sash level before each use, and verifying that the fume hood velocity has been checked within the last year.  If the fume hood needs to be monitored for compliance, notify EH&S immediately.  If your fume hood malfunctions, immediately cease work and notify CSUN Physical Plant Management (PPM).  Also, do not move your head into the fume hood.
    • Ingestion of isotopes is most likely to occur with a transference of isotope from source to hands to mouth.  Radioisotopes also enter the body by absorption through skin, and through cuts or abrasions.  Absorption is an especially important route of entry into the body for radioactive iodine and some forms of tritium (H-3).  Safe, clean, common sense lab practices can prevent most intakes of radioisotopes by ingestion and absorption.  Some important lab practices are listed below which will lessen the chance of internal exposure from ingestion and absorption:
      • Food and drink should not be consumed in a laboratory, except in EH&S approved "Clean Areas".
      • Face make-up should not be applied while working in laboratory areas.
      • Smoking is not allowed in laboratory areas.
      • Wear proper protective clothing at all times when working with radioisotopes.  This consists of a lab coat or apron, gloves (2 pairs when working with iodine, tritium (H-3), or high concentrations of other isotopes), safety glasses, and shoes.
      • Perform radiation surveys of your work area throughout the workday and promptly decontaminate any "hot spots".
      • Monitor your clothing and body for radioactivity frequently and at the end of each workday.
      • Wash your hands thoroughly with soap and water when finished using radioisotopes, before mealtime and before leaving the laboratory.
      • If you have reason to suspect that you may have had an intake of isotope, notify EH&S immediately.
  1. Bioassay Program - A bioassay is the procedure by which the activity of a particular radioisotope present in the body can be determined.  At CSUN we can most accurately make a determination of intake for radioactive iodine.  Since 30 percent of the activity in an iodine uptake may end up in the thyroid gland, iodine uptake can be monitored directly by placing a scintillation detector probe on the neck directly over the thyroid gland.  The corresponding result indicated on a counter-scaler is directly related to the amount of iodine deposited in the thyroid gland.

    A qualitative assessment of the intake of other isotopes can often be determined by performing a urine test.  A urine sample is submitted to EH&S, mixed with several milliliters of scintillation fluid and then counted in a liquid scintillation counter.
  1. Radiation Detection:  One of the ways to prevent widespread contamination in the laboratory or on your person is to possess and properly use a calibrated radiation survey meter.  There are many types of survey meters.  Each type of survey meter has limitations both in its operation and in the type of radiation that it can most effectively monitor.  You must determine what type of survey meter is needed for you particular situation.  The most common types of meters are discussed below.
  1. Geiger-Mueller Counters - GM counters are the most common type of radiation detector used on campus.  They are easy to use, portable, relatively inexpensive, and excellent for lab surveys.  They are most efficient for detecting high energy beta emitters, such as P-32, but can also be used to measure low energy beta emitters, i.e., C-14 or S-35, if they have a thin end-window.  However, they cannot be used to monitor the presence of tritium (H-3) because the beta particle emitted from tritium (H-3) does not have enough energy to penetrate even thin end-window GM probes.  GM counters can also be used for detecting the presence of gamma and x-ray radiation, from isotopes or man-made sources such as x-ray machines or x-ray diffraction units.  GM counters measure counts per minute (cpm) and must be calibrated every year.
  1. Liquid Scintillation Counters - LSCs are also common on campus.  They are not portable and are primarily used for counting laboratory samples.  They are very sensitive to low energy beta emitters, and are one of the few readily available means to count tritium (H-3) and other very low energy beta emitters.  They can also be used to count many other radionuclides.  LSCs measure counts per minute and typically have very good efficiency, usually very close to the actual activity in the sample.  LSCs can also be used to maintain the periodic wipe tests required by Federal law if your laboratory does not have access to a portable survey meter.
  1. Scintillation Detectors - Scintillation detectors are primarily used to monitor gamma radiation.  They are much more sensitive to gamma and x-ray radiation than GM counters.  They cost a bit more than GM counters, are portable and easy to use.  If the end of the detector crystal is covered with a metal window, the detector may not be very sensitive to low energy gamma and x-ray radiation.  Scintillation detectors measure counts per minute and should be calibrated every year.
  1. Ionization Chambers - Ionization chambers are used primarily to determine the exposure rate from gamma and x-ray emitters and come in very handy when measuring machine produced x-rays.  In general, ion chambers cannot be used to measure the presence of beta radiation.  They cost more than GM counters, are portable, and most models are easy to use.  Ionization chambers measure exposure rate in milliroentgens per hour or roentgens per hour.  They should be calibrated every year.

VII. Special Topics:

  1. Radioactive Spills:  Spills of radioactive materials can happen at any time.  If you have any doubt in your ability or means to effectively clean up a radioactive spill, promptly contact EH&S for assistance at X-2401.  If you have determined that the spill can be managed by individuals in your lab, there are several steps you can take to ensure a timely and thorough clean-up of the contamination:
    • Notify everyone in the area that a spill of radioactive material has occurred.
    • Try to prevent further spread of the spill with paper towels or other absorbent materials, but only if this can be done with minimal risk of spreading the contamination or contaminating yourself.
    • Assemble clean-up materials that include paper towels, plastic bags, gloves, lab coats, radiation survey meter (if needed), and cleaning solution (soapy water works very well most of the time).
    • Determine the extent of the contamination and mark the boundaries with tape, rope, etc.
    • Starting from the least contaminated areas, work inward towards the most contaminated areas of the spill, cleaning all areas as you proceed.  Wipe up the spill in one direction as you clean, folding up the paper towels after each swipe of the contaminated surface.
    • Periodically check the cleaned area with either your survey meter or by taking wipes and counting them on a liquid scintillation counter.  Clean until all removable contamination is cleaned up.  Be aware that widespread amounts of contamination may cause a high background level that can lead to difficulty in localizing areas of contamination.
    • Any non-removable contamination remains greater than twice background, notify EH&S.
    • In the event of personnel contamination, rinse and wash the contaminated area immediately with copious amounts of water using a mild soap.  In some instances several washings may be required.  If the contamination cannot be reduced to background levels, notify EH&S for further assistance.
  1. Using Your Survey Meter:  Survey meters must be used properly if they are to be an effective tool for detecting radioactive contamination.  When monitoring for low energy beta emitters with a GM survey meter, the survey meter probe must be passed slowly and very close to the surface that is being monitored.  Failure to do this could result in contamination being overlooked.  Also, if there is any plastic covering over the probe, the sensitivity of the instrument to low energy beta emitters will be greatly diminished.  Remember, you will not be able to monitor for tritium (H-3) with a GM counter.

    It is impossible for your survey meter to detect every disintegration that is emitted from a radioactive source.  Your survey meter indicates counts per minute (cpm) while the source emits disintegrations per minute (dpm).  The response of your meter in cpm is typically only a small fraction of the actual dpm.  The ratio of cpm to dpm is called the efficiency factor of the instrument for that particular isotope.  The GM efficiency factor for low energy beta emitters is quite low, only about 1 to 6 percent.  The GM efficiency factor for high energy beta emitters is high- about 50 percent.  For gamma emitters, a GM counter has an efficiency factor less than 0.1 percent.
  1. Radioactive Waste: The radioactive waste program at CSUN is extensive.  This program needs your cooperation to provide the environmentally safest and most economical means to treat the waste generated by CSUN each year.
    • The program is centered on the operations performed at the waste facility.  These operations include a radioactive waste decay program.  Waste to be decayed is segregated according to isotope and then decayed for 10 half-lives, which is usually long enough to reach background levels.  The waste can then be disposed of as normal waste or retained at the facility for further processing.  These elements of the program, among others, are used to reduce the total amount of activity that has to be shipped to radioactive waste storage locations out of state, thereby saving us many thousands of dollars per year.  You can be a help to the University by trying to minimize the amount of radioactive waste that you generate during the course of your research.  Some of the practical things you can do are:
    • Use smaller animals whenever possible.
    • Use short-lived isotopes whenever possible.
    • Dispose of only those portions of items that are contaminated (cut out spots on bench coat, paper towels, etc.).
    • All radioactive waste must be transferred to EH&S for disposal.  This means that no activity can be put into regular trashcans or poured down the sink.  For waste pick-up, call X-2401.  Someone has to be present in the lab for the pick up.  If you need waste tags, call the above number.
    • All waste for pick up must first be segregated by radioisotope, then further segregated as follows:
      • Dry Solids - All dry solid waste must be placed in plastic bags.  Absolutely no liquids must be contained in dry solid waste.
      • Sharps - All sharps must be placed in a sharps container that must have some means of visual   inspection.  The sharps container must be thick enough to prevent any possibility of puncture.  Sharps include needles, syringes, pipette tips, broken glass, etc.
      • Liquids - Liquid waste includes the primary radioactive liquid and at least the secondary rinse.     Aqueous and organic waste must be segregated.  All liquid waste containers should be stored in             secondary containers large enough to contain the entire volume of liquid contained.
      • Animal Waste - All animal waste has to be double bagged, labeled, and frozen until pick up.
      • Filled Scintillation Vials - Filled vials are only picked up in flats.  Separate glass and plastic vials         into separate flats.
      • Miscellaneous - Contact EH&S for specific instructions for the disposal of gels, high specific activity liquid wastes, or any other special pick-ups.

Before pick affix a "Radioactive Waste" label to each bottle, bag, box or flat of LSC vials.  Copies of the label (Form 107) can be downloaded from the EH&S Website.  Fill out the tag completely, including

VIII. Rules of Thumb:

  1. It requires an alpha particle of at least 7.5 MeV to penetrate the dead layer of skin, 0.07 mm thick.
  1. It requires a beta particle of at least 70 keV to penetrate the dead layer of skin, 0.07 mm thick.
  1. The range of beta particle in air is about 12 feet per MeV: for example, a 3 MeV beta has a range of about 36 feet in air.
  1. The bremsstrahlung x-rays from a 10 mCi P-32 aqueous solution in a glass bottle cause an exposure rate of about 0.1 to 0.2 mR/hr at 1 foot.
  1. For a point source of beta radiation, which can travel at least 1 foot in air, the beta skin dose can be determined as follows.  Multiply the activity, in mCi, by 300.  The result is the beta skin dose at 1 foot from the source in millirad/hr.
  1. For a point source gamma emitter, with an energy between 0.07 and 3 MeV, the exposure rate (mR/hr) at 1 foot is 6 CEN, where C is the activity of the source in millicuries, E is the gamma energy in MeV, and N is the number of gammas per disintegration.
  1. The activity of any radionuclide is reduced to less than 1 percent after 7 half-lives.
  1. For P-32, 8,300 cpm is about equal to 1 mrem/hr beta skin dose when using a Ludlum Model 3 with a pancake probe and which has been calibrated with an electronic pulser.

IX. References:

  • BEIR Committee.  The Effects on Populations of Exposure to Low Levels of Ionizing Radiation, Committee on the Biological Effects of Ionizing Radiation.  National Academy of Sciences, 1980.
  • Brodsky, Allen.  Handbook of Radiation Measurement and Protection, Vol. 1, CRC Press, West Palm Beach, FL, 1978.
  • California State University, Northridge, "Radiation Safety Manual", March 1990.
  • Cember, Herman.  Introduction to Health Physics, Second Edition, Pergamon Press, New York, NY, 1983.
  • Health, Education and Welfare, Department of.  Radiological Health Handbook, Public Health Service, FDA, Bureau of Radiological Health, Revised Edition, Rockville, MD, 1970.
  • Healy.  Surface Contamination:  Decision Levels.  Los Alamos Scientific Laboratory, Los Alamos, New Mexico.  LA-4558-MS, 1971.
  • NCRP.  A Handbook of Radioactivity Measurement Procedures, Number 58, National Council on Radiation Protection and Measurements, Washington, D.C., 1978.
  • Shapiro, Jacob.  Radiation Protection, A Guide for Scientists and Physicians, Third Edition, Harvard University Press, Cambridge, MA, 1990.
  • Stanford University, "Radiation Protection Manual."
  • University of California, Los Angeles, "Technical Information for Radiation Safeguards."
  • University of California, San Diego, "Radiation Safety: A Worker's Guide".  First Edition, August 1989.
  • USNRC, "Instruction Concerning Prenatal Radiation Exposure", USNRC Regulatory Guide 8.13, November 1975.
  • USNRC, "Instruction Concerning Risks From Occupational Radiation Exposure", USNRC Regulatory Guide 8.29, July 1981.