D. RADIOACTIVE DECAY

FIGURE 1.3

E. GAMMA AND X-RAYS

F. OTHER MODES OF DECAY

G. BREMSSTRAHLUNG - A TYPE OF X-RAY

To better understand radiation safety procedures, a general understanding of the physical properties of ionizing radiation is useful. For a more complete description of the interaction of ionizing radiation with matter, a radiation safety textbook should be read. Consult the Radiation Safety Officer (RSO), or your Department Safety Advisor (DSA), for additional reading material.

A. STRUCTURE OF THE ATOM

The building blocks of atomic and nuclear structure are the electron, proton, and neutron. In its simplest form the atom has a dense core (nucleus) with electrons traveling in specific orbits or energy levels about the nucleus (See Figure 1.1).

Figure 1.1 Structure of the Atom

NUCLEUS

 

a. Protons - positive charge.

 

b. Neutrons - uncharged.

 

c. The mass of the neutron and proton is as follows (1 proton = 1.00727 amu; 1 neutron = 1.00866 amu). 1 amu = 1/12 of the mass of the carbon nucleus with 6 protons and 6 neutrons.

 

2. ELECTRONS

 

a. Mass equals 0.00055 amu (approximately 1/2000 of the mass of a proton).

 

b. Negatively charged.

 

c. Atom is electrically neutral if total electron charge equals total proton charge.

 

d. Electrons are bound to the positively charged nucleus by electrostatic attraction.

B. ATOMIC NOMENCLATURE

The following terms are commonly used in radiation physics:

X = Chemical symbol
A = Mass number (proton number + neutron number)
Z = Atomic number (proton number)
N = Neutron number (A minus Z)
^AX ⁶⁰Co

With the exception of ²⁰⁹Bi, all nuclei with atomic numbers greater than 82 are unstable. Many nuclei with atomic numbers less than 82 also are unstable. Unstable nuclei undergo transformations which release energy. Each transformation of a parent nucleus is called a disintegration. The disintegration rate is proportional to the number of radioactive atoms and the half-life. The half-life is the time required for half of the radioactive atoms to disintegrate. Each radionuclide is unique in terms of the type and energy of the ionizing radiation it emits, and in the duration of its half-life.

At the University of California, San Francisco (UCSF), many different radionuclides are used. Below, the classes of decay events that we most commonly encounter are identified.

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C. BETA PARTICLES

1. NEGATIVE BETA PARTICLES

 

Commonly used radionuclides at UCSF (³H, ¹⁴C, ³²P, ³⁵S and ⁴⁵Ca) emit beta particles. In beta decay, a neutron is converted to a proton and an electron, and the electron is promptly ejected from the nucleus. An electron emitted from the nucleus of an atom is called a beta particle. Although the correct name for a negatively charged beta-particle is a negatron, the term is so unfamiliar that we will reserve the term "beta-particle" for a negatively charged beta-particle, and the term "positron" to denote a positively charged one.

 

Electrons emitted during beta decay have a continuous energy distribution ranging from zero to a maximum which is characteristic of a particular radioisotope. The maximum energy of a particular beta decay is defined as E_max; the mean energy (E_mean) is approximately E_max/3. The shape of the beta energy spectrum and the values for E_max are characteristic. Modern liquid scintillation counters allow the identification of radionuclides by detecting and measuring the energies of the emitted beta particles.

 

Beta-particles have a finite range in air and other materials (Figure 1.2) linearly related to the E_mean. As a rule of thumb, the range of beta particles in air is about 12 feet per MeV. For example, ³²P has an E_max of 1.7 MeV or a maximum range in air of 12 x 1.7 or approximately 20 feet. The mean range would be approximately 7 feet.

 

2. POSITIVE BETA PARTICLES

 

Some nuclei decay by beta+ (positron) emission. This decay results from a proton converting to a neutron and an electron having a positive charge (positron). The result of this type of decay is the loss of a positive charge in the nucleus of the atom. The most commonly used positron emitters at UCSF are ²²Na, ⁶⁵Zn, ⁶⁸Ga, and ¹¹⁴In.

 

A positron is an example of anti-matter. When matter and anti-matter collide, they annihilate each other, converting their mass directly into electromagnetic energy in the form of x-rays.

The positron's spectral response is similar to that of negatively charged beta-particles. When shielding positron emitters, however, they should be treated as photon-emitters, since their annihilation can result in the generation of 0.51 MeV photons.

Figure 1.2 Penetration Ability of Beta-Particles

D. RADIOACTIVE DECAY

Radioactive decay is the disintegration of the nucleus of an unstable nuclide by spontaneous emission of charged particles and/or photons. The decay rate (i.e. the number of nuclear disintegrations per second) of a radionuclide decreases as an exponential function. The activity of the sample is the curie (Ci). (See Chapter 2.)

The half-life (T_phy) is the time required for a radioactive substance to lose 50% of its activity by decay. Each radionuclide has a unique half-life (a physical property that cannot be modified). Figure 1.3 presents two graphs showing the exponential decay of radioactive gold. The half-lives of some beta-emitters and gamma-emitters are given in Chapter 5.

Clinicians and researchers must be aware of the biological half-life of a radionuclide in a biological system. The biological half-life (T_bio) is the time required for the body to eliminate one-half of an administered dosage of any substance by the regular processes of elimination. This time is approximately the same for both stable and radioactive isotopes of a particular element.

The effective half-life (T_eff) is the time required for a radioactive element in an animal body to be diminished by 50% as a result of the combined action of radioactive decay and biological elimination. T_eff is computed as follows:

T_effective = T_phy x T_bio / (T_phy + T_bio)

The biological half-life for carbon is about 10-40 days, calcium about 10⁴ days, sulfur about 100-1600 days and for phosphate about 20-1200 days.

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FIGURE 1.3

Graphs showing the exponential decay of a source of 10⁸ atoms of an ¹⁹⁸Au radionuclide with a half-life of 2.70 days. The graph on the left is a linear plot while the one on the right is a semi-logarithmic one.

E. GAMMA AND X-RAYS

Gamma rays and x-rays are types of electromagnetic radiation with about the same wave length. Other types of electromagnetic radiation are radio waves, visible light, and ultraviolet irradiation. Gamma rays and x-rays are very much more energetic (have very much shorter wave lengths) than the other forms mentioned. Sometimes these rays behave like waves and have an energy proportional to their frequency. At other times they are best considered as discrete bundles of energy called photons. Gamma rays and x-rays occupy the same region within the electromagnetic spectrum. They are distinguishable by their origins - gamma-rays result from nuclear transitions (inside the nucleus) and x-rays from the interaction of electrons (outside the nucleus). Gamma rays have discrete wave lengths while x-rays cover a wide band of wave lengths.

Some of the radionuclides used at UCSF emit gamma-rays and/or x-rays. Because of their short wave length, these photons can pass through matter. As they pass through matter, they may be attenuated by their interaction with bound electrons in the matter (photoelectric effect), with "free" electrons in the matter (Compton effect), or with the transfer of their energy to the creation of a positive and negative electron pair (pair production). The importance of each of these effects depends on the atomic number (Z) of the absorbing material and the energy of the photon.

Gamma and x-ray photons with energies between 30 KeV and 30 MeV interact in soft tissue predominantly by Compton scattering. This means a partial energy transfer by the incoming photon through interaction with an orbital electron. The weakened photon continues on until it undergoes another Compton interaction. The Compton electron produces secondary ionizations by ejecting other electrons from their respective orbits. These electrons may have an energy that is higher than chemical bonds and by their interaction may alter chemical structures. The primary cause, approximately 80%, of biological damage is the result of the energetic charged particles produced secondarily by the x-ray or gamma-ray, not the original photon.

The intensity of electromagnetic radiation varies with the distance from the source according to the Inverse Square Law. This means that the intensity is inversely proportional to the square of the distance. I₁X(D₁)² =I₂X(D₂)² where: I= Intensity, D = Distance.

Example: At 1 foot a beam of gamma-rays has 1000 photons crossing a 1 cm square in one second. At 2 feet the beam will consist of 1000/2² = 250 photons/cm²/sec.

The rate at which the intensity (number of photons) decreases also depends on the density of the absorber. Lead, for example, is a more effective absorber of photons than air. If a certain thickness of an absorber reduces the intensity by 50%, twice that thickness reduces it to 25%, three times to 12.5% and so on; then the thickness of that absorber which reduces the intensity by 50% is called the half value layer (HVL). This is an exponential process; that is, as the thickness of the absorber is increased the intensity of the electromagnetic radiation decreases, but statistically never reaches zero. Note the contrast with beta particles. Beta particles can be completely shielded because of their finite path length.

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F. OTHER MODES OF DECAY

There are other modes of radioactive decay besides beta, positron, and gamma decay which include alpha decay, internal conversion, electron capture, and neutron emission. Alpha decay is briefly discussed below. A textbook can be consulted to review the other modes.

An alpha particle is composed of two neutrons and two protons, and is identical to a helium nucleus. Alpha particles are emitted by many heavy radionuclides when they decay. Three familiar alpha-emitting elements are radium, uranium, and plutonium. Alpha energies range from about 4 MeV to 8 MeV. However, the range of alpha particles is very short - a 5 MeV alpha has a range of 0.034 mm in tissue and will not penetrate the skin. Although external exposure is of negligible concern, internal exposure is of very great concert. The concern is due to the very high linear energy transfer of alpha particles. Thus, extreme precautions must be taken to prevent entry inside the body by inhalation, ingestion, or skin puncture.

G. BREMSSTRAHLUNG - A TYPE OF X-RAY

Energetic beta-particles, like those emitted by ³²P, are quickly decelerated when passing through matter. The energy lost to deceleration is emitted in the form of x-rays called "Bremsstrahlung" which translates as "braking radiation". Bremsstrahlung is of concern when shielding beta emitters.

The intensity of bremsstrahlung increases with the increase in energy of the electrons or the mass of the absorbing media. Thus, it is common to use light materials, such as lucite or plastic, to shield beta particles. The shield should consist of a light material (for absorption of the beta radiation) followed by a dense material (such as lead) to absorb the bremsstrahlung.

The bremsstrahlung from a 1 curie source of ³²P solution in a glass container is approximately 10 mrad/hr at 1 foot. Approximately 1 cm of lucite is sufficient to shield ³²P.

OEH&S Radiation Safety Training Manual Chapter 2

CHAPTER 2
UNITS FOR MEASURING IONIZING RADIATION

A. ROENTGEN: THE UNIT OF EXPOSURE

B. RAD: THE UNIT OF ABSORBED DOSE

C. REM: THE DOSE EQUIVALENT UNIT

D. CURIE: THE UNIT OF ACTIVITY

When conducting a survey of the laboratory for radioactive contamination, you note that the instrument reads in mR/hr (milliroentgen/hour). This is a radiation exposure rate measurement. The laboratory has another instrument which reads in counts per minute (cpm). While opening a radioactive vial, you notice that the label describes the contents in microcuries. This is a unit of radioactivity. When reviewing film badge and finger ring records, you note that the results are given in millirems. This is a measure of radiation dose. These units, commonly used at the University of California, San Francisco (UCSF), are discussed below.

A. ROENTGEN: THE UNIT OF EXPOSURE

The roentgen (R) was adopted in l928 as a unit of exposure to medium-energy x-radiation. It is the approximate exposure to one gram of radium located one yard away for one hour (i.e. one gram of radium produces an exposure rate at one yard of approximately one R/hour). Specifically, the roentgen is the quantity of x- or gamma rays that produce 2.58 x 10^{- 4} coulombs/kg of air at standard conditions of temperature and pressure. In measuring the roentgen, a known volume of air is irradiated, and the ions produced (electrical charge) are collected and measured. The choice of air as a standard substance was for convenience. Since air and water have an effective atomic number that is nearly the same as that of tissue, absorption of x-ray energy per gram of soft tissue, water and air is within about 12% of being the same.

However, the roentgen has limitations. By definition it is limited to x- and gamma-rays, and medium of air, and does not include other types of radiation. Further, the definition of the roentgen holds only for lower energy radiations (up to 3 MeV).

B. RAD: THE UNIT OF ABSORBED DOSE

The rad is the unit of absorbed dose and is a measure of the energy deposition per unit mass by all types of ionizing radiation. Chemical and biologic changes in tissue exposed to ionizing radiation depend upon the energy deposited in the tissue rather than the amount of ionization which the radiation produces in air. The rad, an acronym for Radiation Absorbed Dose, is not limited to x- or gamma rays and is not limited to the medium of air.

The rad is specifically defined as the deposition of 100 ergs per gram of absorbing material. As a general rule, the absorbed dose in soft tissue from 1 R of intermediate energy x- or gamma rays is about 1 rad. The rad is being replaced by the Gray (Gy), which is defined as an absorbed energy 100 times greater than a rad (1 Gy = 100 rad = 1 joule/kg). This Manual has retained the older units of rad, rem, curie.

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C. REM: THE DOSE EQUIVALENT UNIT

The rem, an acronym for Roentgen Equivalent Man, was developed in response to evidence that biologic effects per rad of various radiations are often different. The dose equivalent (DE) is defined as the absorbed dose (rads) multiplied by a quality factor (QF), a term that expresses the differences in biologic effectiveness of various types of radiation as compared to x-rays. The QF is a function of the linear energy transfer (LET) of the radiation. The QF for x-rays, gamma-rays, and beta particles with a maximum energy of greater than 30 KeV is 1.0. This category represents a majority of radioactive materials used at UCSF. For information, the QF for neutrons and protons with energies less than 10 MeV is 10 (30 for irradiation of the eyes); for alpha particles from natural radionuclides the QF is 10.

The new unit for dose equivalent is the Sievert (Sv), which is related as 1 Sv = 100 rem.

TO SIMPLIFY THIS MANUAL AND TO MAKE CALCULATIONS EASIER, THE TERMS ROENTGEN, RAD, AND REM ARE CONSIDERED INTERCHANGEABLE.

D. CURIE: THE UNIT OF ACTIVITY

When an excited nucleus emits characteristic neutrons, alpha, beta (positive or negative) particles, and/or gamma rays, the nuclei are said to be radioactive. (Radioactive materials used at UCSF primarily emit beta particles and gamma rays.) Each transformation of a parent nucleus is called a disintegration. Cobalt-60, often used in radiation teletherapy, emits a beta particle followed immediately by two gamma rays. These three radiations are emitted per disintegration.

An important unit in the practical application of radioactivity is the number of disintegrations per unit time (typically seconds or minutes). The quantity of any radionuclide in which the number of disintegrations per second is 3.7 x 10¹⁰ is one curie (Ci).

1 Ci = 3.7 x 10¹⁰ disintegrations per second (dps)

A millicurie is one-thousandth of a curie and microcurie is one-thousandth of a millicurie. The curie is being replaced by the becquerel (Bq) unit defined as: 1 dps. Thus

1 Ci = 3.7 x 10¹⁰ Bq

1 Bq = 2.7 x 10^-11 curies.

Units of conversion are found in the Glossary.

Many UCSF survey meters read in counts per minute (cpm) or counts per second (cps). If the counting efficiency (counts per unit time/disintegrations per unit time) is known for the radioactive material being measured, then the activity of the material can be estimated. The efficiency will vary for each isotope and instrument type.

Assume that the efficiency of a survey meter for measuring ³⁵S is 1%. Assume that 1,000 cpm are measured with the meter. Then, dpm would be computed as 1,000/0.01 or 100,000 dpm. The activity would be computed as 100,000 dpm/60 sec/min = 1,667 Bq or 0.045 microcuries.

OEH&S Radiation Safety Training Manual Chapter 3

CHAPTER 3
MAXIMUM PERMISSIBLE EXPOSURES

CHAPTER 3 Table Of Contents

A. GUIDELINES FOR RADIATION EXPOSURE

B. MAXIMUM PERMISSIBLE DOSE

TABLE 3.1

C. HOW DOES THE MAXIMUM PERMISSIBLE DOSE COMPARE WITH OTHER SOURCES OF RADIATION EXPOSURE?

TABLE 3.2

FIGURE 3.1

D. WHAT IS THE RISK AT THE MAXIMUM PERMISSIBLE DOSE?

TABLE 3.3

E. SPECIAL SAFEGUARDS FOR PREGNANT WOMEN

FIGURE 3.2

TABLE 3.4

A. GUIDELINES FOR RADIATION EXPOSURE

For investigators working with radioactive materials in University of California, San Francisco (UCSF) laboratories, the risk, if any, to low levels of radiation exposure is small. Nevertheless, the risk is real and can only be kept small if the policies and procedures of UCSF, along with the regulations of the State and Federal governments, are carefully followed. UCSF policies and government regulations are based, in part, on three radiation protection principles:

1. Occupational exposure should only take place when the benefit to society warrants the risk. There is little doubt that medically-related research falls into this category.

 

2. Exposure to workers should be As Low As is Reasonably Achievable (ALARA). This has been characterized as the "optimization" of radiation protection by the International Commission on Radiological Protection.

 

3. A "maximum allowable individual dose" must be established to set an upper limit on the risk to individual workers.

UCSF is fully committed to the principle of ALARA. The Radiation Safety Manual spells out this commitment and the ALARA program for the campus. Each authorized user should familiarize themselves with this material. This chapter is devoted to a description of permissible doses.

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B. MAXIMUM PERMISSIBLE DOSE

The maximum permissible doses allowed by state and federal regulations have been set based on current knowledge. Scientific committees composed of the world's leading authorities in radiation science and biology are established to periodically appraise the literature and recommend changes in dose limits, if indicated.

The dose limits consider that damage caused by radiation exposure is dependent upon several factors:

1. The age of the person exposed.

 

2. The absorbed dose.

 

3. The body part exposed.

The occupational radiation dose limits first divide people into two groups: those 18 years and over, and those under 18 years of age. The latter group is limited to the same doses as the general population (i.e. non-radiation workers). Table 3.1 presents a synopsis of the dose limits contained in the Code of Federal Regulations (10CFR20). (The limits are the same throughout the country.)

Review Table 3.1. Note that the hands have a limit 10 times higher than the whole body radiation dose. Radiosensitive tissues, such as the blood forming cells, and the gonads, have the lowest maximum permissible dose.

The regulations limit radiation exposure to members of the public (i.e. those who are not occupational radiation workers) to limits that are one-fiftieth of the occupational values. These lower limits apply to visitors, custodial help, delivery persons, or administrative personnel.

UCSF's commitment to ALARA has resulted in the administrative imposition of even lower limits than those required by regulation. In essence, UCSF is committed to keeping the radiation doses to occupationally exposed workers at levels 25% or more below the State limits. For example, the limit for whole body exposure is 5 rems/year. This is roughly 400 millirems per month. UCSF is committed to keeping whole body exposures below 100 millirems per month. Based on years of monitoring the exposures of UCSF laboratory workers, radiation exposures rarely exceed the detectable limits of the film badge (approximately 10 millirem/month). In fact over 90% of personnel receive less than 100 millirem in one year.

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TABLE 3.1

Summary of Recommendations^a (After Report No. 91, NCRP, 1987a)

A. Occupational exposures (annual)^b

B. Public exposures (annual)

C. HOW DOES THE MAXIMUM PERMISSIBLE DOSE COMPARE WITH OTHER SOURCES OF RADIATION EXPOSURE?

We are continuously irradiated by external ionizing radiation from cosmic and terrestrial sources, and from naturally occurring radioisotopes within our body (i.e. potassium-40 and carbon-14). For example, a person 70 years old will have received, on average, a 9 rem whole body dose from these sources alone. The internal radiation exposure accounts for approximately 20 millirem per year. The cosmic exposure varies by elevation but ranges from about 30 to 120 millirem per year. The terrestrial exposure also varies with mineral deposits and other geological considerations but generally varies form 20 to 120 millirem per year. The external background radiation in San Francisco is approximately 80 millirem/year, or about one-fiftieth of the allowable limit for radiation workers - 80% of the limit for the general public. On the open ocean, the annual dose is approximately 55 mrem/yr and in Denver about 150 mrem/yr (almost twice the level in San Francisco).The average individual in the United States accumulates a dose of 1 rem from natural sources every 12 years. The dose from natural radiation is higher in some states, such as Colorado, Wyoming and South Dakota, primarily because of increased cosmic and terrestrial irradiation. The average individual may receive 1 rem every 8 years or less. However, there are other areas in the world where natural background radiation levels are very much higher. For example, a dose of 1 rem may be received in some areas on the beach at Guarapari, Brazil, in only about 9 days, and some people in Kerala, India get a dose of 1 rem every 5 months.

In addition to natural background radiation, many people receive additional radiation exposure for medical reasons. Medical exposures are intentional and clearly have defined benefits for the individual. For purposes of comparison, the average surface skin dose from one radiographic (P/A view) chest x-ray is 0.027 rem. The estimated average surface skin dose per abdominal x-ray is 0.62 rem. Table 3.2 and Figure 3.1 list annual dose contributions from some of these sources.

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TABLE 3.2

Annual GSD in the U. S. population circa 1980-82

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FIGURE 3.1: Graphic Under Construction

The percentage contribution of various radiation sources to the total average effective dose equivalent in the U. S. population.

Radiation can also be received from natural sources such as rock or brick structures, from consumer products (such as smoke detectors containing radioactive materials), and from air travel. The possible annual dose from working 8 hours a day near a granite wall at the red cap stand in Grand Central Station, New York City, is 0.2 rem, and the average annual dose in the United States from consumer products and air travel is 0.0026 rem.

D. WHAT IS THE RISK AT THE MAXIMUM PERMISSIBLE DOSE?

Death due to radiation exposure requires high exposures. In measuring radiation effect, the concept of the lethal dose 50 (LD₅₀) has been borrowed from pharmacology. The LD₅₀ is defined as the dose of any agent or material that causes a mortality of 50% in the experimental group. The LD₁₀₀ produces a mortality of 100%. For acute whole body human radiation exposure, the LD_50/60 is in the range of 300 to 350 rads. This means 50% mortality within 60 days.

There are variations in the population due to age, sex, degree of health, and sensitivity to radiation exposure. Briefly stated, the young and the old appear to be more radiosensitive than the middle-aged individual. The female appears to have a greater degree of tolerance to radiation than does the male.

The effects from chronic or protracted exposure are less than from acute exposure. Exposure to the sun offers some parallels to radiation exposure. Whole body exposure to the direct sun for several hours can result in a severe sun burn. However, as more of the body is protected (using sun screens, clothing, shade, etc.) the length of exposure can be increased without the effect of sunburn. For example, one can stay out for a few minutes each day (eventually accumulating a total exposure of several hours) and have a very different effect than by receiving an acute dose within several hours. Radiation exposure may also work this way, although experts do not fully agree.

The chief risk to radionuclide users comes from intermittent exposures to very low doses not from an acute exposure to a very high dose. The risks to low doses of radiation are not fully known and so the best principle is to follow is ALARA - the minimum exposure that can be reasonably achieved.

One risk is cancer. The figures for cancer mortality are given in Table 3.3 If the average figure of 300 excess cancers per million people per rad is used, and a scenario of 20 years of exposure at the State limit is assumed, the result would be a total of 6,000 extra cancers per million workers, or a 0.8% increase in extra cases over a thirty year period. If the American Cancer Society's figures that 25% of Americans will contract cancer are used, the maximally exposed worker would increase his/her chances of getting cancer from 25% to 25.8%. Of course, there are only a few thousand UCSF radiation workers, and the average UCSF worker receives an occupational dose of less than 100 mrem/year as opposed to the 5,000 mrem/yr used in this example. The cancer risk from a radiation dose received at this rate may well be zero.

TABLE 3.3

Excess Mortality Estimates - Lifetime Risks per 100,000 Exposed Persons
(extracted from Table 4-2 of BEIR V)

However, these statistical arguments are not very comforting if we, or one of our friends or relatives, develop cancer. The way to avoid even this small risk of a radiation induced-cancer is to stay well below the maximum allowable level by following established policies and procedures.

In 1980, approximately 1.3 million workers were employed in occupations in which they were potentially exposed to radiation. About half of these workers received no measurable occupational dose. In that year, the average worker exposed to a measurable amount of external radiation received an occupational dose equivalent of 0.2 rem to the whole body, based on the readings of individual dosimeters worn on the surface of the body. We estimate (assuming a linear non-threshold model) the increased risk of premature death due to radiation-induced cancer for such a dose is ~2-5 in 100,000 and that the increased risk of serious hereditary effects is about one-third smaller. To put these estimated risks in perspective with other occupational hazards, they are comparable to the observed risk of job-related accidental death in the safest industries, wholesale and retail trades, for which the annual accidental death rate averaged about 5 per 100,000 from 1980 to 1984. The U.S. average for all industries was 11 per 100,000 in 1984 and 1985.

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E. SPECIAL SAFEGUARDS FOR PREGNANT WOMEN

A number of studies have indicated that the embryo/fetus is more sensitive to radiation exposure than the adult, particularly during the first three months after conception. This is also a period when a woman may not be aware she is pregnant. Women who are pregnant or who are considering pregnancy should to be aware of the special needs of their situation. Supervisors and co-workers of fertile women should be aware of the risks to the fetus to avoid creating a situation that might put the embryo/fetus at risk. The National Council on Radiation Protection and Measurements has made two recommendations: a) the maximum dose to the fetus from occupational exposure should not exceed 0.5 rem, and, b) radionuclide workers must know about prenatal exposure risks arising from ionizing radiation. In particular they must know why pregnant women have a lower maximum permissible dose.

The Appendix of the UCSF Radiation Safety Manual contains a reprint of the U.S. Nuclear Regulatory Commission (NRC), Regulatory Guide 8.13, Instruction Concerning Prenatal Radiation Exposure. In addition, this section contains the UCSF pregnant personnel policy. Each authorized user should read and become familiar with this material.

The prediction that an unborn child would be more sensitive to radiation than an adult is supported by observations for relatively large doses. The National Academy of Sciences noted that doses of 25-50 rems to a pregnant human may cause growth disturbances in offspring. Such doses substantially exceed, of course, the maximum permissible occupational exposure limits.

Concern about prenatal exposure (i.e., exposure of a child while in its mother's uterus) at the permissible occupational levels is primarily based on the possibility that cancer (especially leukemia) may develop during the first 10 years of the child's life. According to a report by the National Academy of Sciences, the incidence of leukemia among children from birth to 10 years of age in the United States could rise from 3.7 to 5.6 cases per 10,000 children exposed to 1 rem in utero, an increase of 50%.

FIGURE 3.2: Graphic Under Construction

The Academy also estimated that an equal number of other types of cancers could result from this level of radiation. Although other scientific studies have shown a much smaller effect from radiation, women employees should be aware of any possible risk so that they can take steps they think appropriate to protect their offspring. Efforts should be made to keep the radiation exposure of an embryo/fetus the lowest practicable level during the entire period of pregnancy.

The employer should take practicable steps to minimize the radiation exposure of a potential mother. The advice of the Radiation Safety Office can be obtained to determine if radiation levels in working areas are high enough that a baby could receive 0.5 rem or more before birth.

The following facts should be noted in making a decision about continuing to work with ionizing radiation:

1. If you are planning on becoming pregnant or think you may be pregnant, discuss the matter with your supervisor or Principal Investigator so that appropriate appraisal of the potential radiation exposure may be made.

 

2. In most cases of occupational exposure, the actual dose received by the unborn baby is less than the dose received by the mother because some of the dose is absorbed by the mother's body.

 

3. At the present occupational exposure limit, the actual risk to the unborn baby is quite small, even though experts disagree about the exact level of risk.

 

4. There is no need to be concerned about a loss of your ability to bear children. The radiation dose required to produce such effects is many times larger than the State dose limits for adults.

 

5. Even if you work in an area where you receive only 0.5 rem per three-month period, in nine months you could receive 1.5 rem and the unborn baby could receive more than 0.5 rem, the full-term limit suggested by the NCRP. Therefore, if you decide to restrict your unborn baby's exposure as recommended by the NCRP, be aware that the 0.5 rem limit to the unborn baby applies to the full nine-month pregnancy.

To put the risk due to radiation in perspective, a table of the effects of various risk factors on the outcome of pregnancy is included (Table 3.4).

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TABLE 3.4

Effect and Frequency of Certain Maternal Factors on Pregnancy Outcome

OEH&S Radiation Safety Training Manual Chapter 4

CHAPTER 4
BIOLOGICAL EFFECTS OF RADIATION

CHAPTER 4 Table Of Contents

A. SOMATIC AND GENETIC EFFECTS

B. INCREASE IN CANCER INCIDENCE

C. GENETIC DAMAGE

D. EXPOSURE OF UNBORN CHILDREN

The fact that ionizing radiation produces biological damage has been known for many years. The first case of human injury was reported in the literature just a few months following Roentgen's original paper in 1895 announcing the discovery of x-rays. The first case of radiation induced cancer was reported seven years later. Early human evidence of the harmful effects of ionizing radiation, as a result of high exposures, became available in the 1920s and 30s through the experience of radiologists, miners exposed to airborne activity, and workers in the radium industry. However, the long term biological significance of smaller, repeated doses of radiation was not widely appreciated until later. Most of our knowledge of these effects has accumulated since World War II.

A. SOMATIC AND GENETIC EFFECTS

Biological effects can be conveniently subdivided into two groups:

1. Genetic effects which occur in the reproductive cells and may be inherited.

2. Somatic effects which arise from damage to all cells in the body and are observable in the individual affected.

Genetic effects are essentially long term in nature since they are manifested in offspring. In discussing somatic effects, it is convenient to further subdivide them into early (or acute) effects and late (or chronic) effects. The terms acute and chronic are also used to describe the period during which the radiation exposure is carried out. An acute exposure takes place within seconds, minutes or hours and the early (or acute) effects may be seen within minutes, hours or up to a few weeks later. A chronic exposure may extend over weeks, months or years, it may not be continuous and the late (chronic) effects may be produced during or after the irradiation.

Somatic effects may also be categorized as non-stochastic or stochastic. In some irradiations, the biological response increases in severity as the dose increases. Skin, for example, may only show a slight reddening (erythema) at low doses, but will exhibit severe gross tissue damage at high doses. Such a response is termed non-stochastic and usually exhibits a threshold dose below which the response is not observed. Other irradiations produce a response such as leukemia where the severity is independent of dose, the disease is either contracted or it is not. The probability of inducing the response does depend upon the dose. Such a response is termed stochastic.

Studies in both early and late effects of ionizing radiation are of great importance in the establishment of guidelines for minimizing the risk inherent in the use of ionizing radiation. The first radiation protection standards were devised to protect workers from acute radiation effects. The present standards recommended by the International Commission on Radiological Protection (ICRP) are largely based on the incidence of late stochastic effects, such as cancer, for radiation workers and on genetic effects for the general public.

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B. INCREASE IN CANCER INCIDENCE

While the relationship between acute effects and radiation levels is well known, the situation for late effects, both somatic and genetic, is more obscure. The difficulty arises in part because the effects are so small. Since so many of the population (16-25%) die of cancer, small effects due to low levels of chronic radiation exposure are impossible to measure. As a consequence, data must be extrapolated from cancer incidence rates in individuals who received extremely high exposures, such as the victims of nuclear weapons, accidents, or experimental medical procedures. An additional problem in making an accurate assessment is the factor of age at the time of exposure. The time of onset can be delayed for 30 years or more after the exposure (latent period). To estimate the possible risks to us as users of radiation, information is needed about the properties of radionuclides, the measurement of radiation exposure, and the other topics presented in this Manual.

C. GENETIC DAMAGE

Genetic effects occur when there is radiation damage to the germ cells carried by the parents, due to radiation exposure of either parent. These effects may show up as birth or other defects in the children of the exposed parents or in succeeding generations. From animal studies it is estimated that the risk of producing serious genetic effects is about one-third the risk of producing cancer. However, it is difficult to apply animal data to humans. Damage to germ cells should not be confused with damage to the cells of an embryo/fetus from in utero irradiation.

D. EXPOSURE OF UNBORN CHILDREN

While the risks of cancer or genetic damage are barely significant for a prudent worker, the unborn child is at a higher risk. The more rapidly dividing cells of the embryo/fetus are more sensitive to the effects of radiation than slowly dividing cells such as brain or bone cells. Cells in the unborn child are dividing very rapidly. Furthermore, the child has its whole life ahead during which delayed effects might occur.

Women who work with radioactivity and are considering pregnancy should carefully read the material presented in Chapter 3, Section E of this manual. Supervisors and co-workers of fertile women should also be familiar with this material to be sure that situations that might put the embryo/fetus at risk are avoided.

OEH&S Radiation Safety Training Manual

CHAPTER 5
SAFETY HAZARDS ASSOCIATED WITH COMMONLY USED RADIONUCLIDES

CHAPTER 5 Table Of Contents

A. INTERNAL RADIONUCLIDE HAZARDS

1. INHALATION
2. INGESTION
3. ABSORPTION
4. PUNCTURE

B. EXTERNAL EXPOSURE TO RADIONUCLIDES

1. RADIONUCLIDES ON THE SKIN

TABLE 5.1
2. EXTERNAL SOURCE OF BETA-EMITTERS
3. EXTERNAL SOURCE OF GAMMA-EMITTERS
TABLE 5.2
TABLE 5.3
4. RADIATION EXPOSURE FROM STORED RADIONUCLIDES
TABLE 5.4

Working with radioactive materials involves some potential risks. Therefore, precautionary measures must be taken to ensure the safety of personnel working with such materials as well as the public. The following summarizes the procedures and methods used in protection against internal or external radiation hazards.

The hazard from radionuclides can be divided into internal and external exposures. The severity of the hazard depends on a number of factors including the energy of the radionuclide, the type of radiation emission (e.g. beta or gamma radiation), the activity, and the chemical form.

A. INTERNAL RADIONUCLIDE HAZARDS

The possibility of a radioisotope inadvertently entering the body exists. Once this occurs, the protection techniques are somewhat limited; so emphasis has to be placed on preventing radionuclides from entering the body. The possible pathways into the body are inhalation, ingestion, absorption, and puncture.

1. INHALATION

Airborne radioactive materials can enter the body through inhalation. In biomedical applications, this is not a major problem since most isotopes are used as bound chemicals. However, the use of HTO (tritiated water), ³⁵S in some labeling reactions, and Na¹²⁵I can create potential problems. Adequate protection can be obtained by performing operations involving potential airborne radioactivity in approved fume hoods. These hoods are designed to maintain a negative air flow and have a face velocity of at least 100 linear feet per minute. The air from these hoods is vented to the outside. The evaluation by the Radiation Safety Office of the procedure ensures that the air flow is sufficient to keep environmental concentrations well within acceptable limits. Some sterile hoods are not suitable for use with volatile radioisotopes because they operate under a positive pressure.

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2. INGESTION

Ingestion is possible when unsealed sources of radioactive materials are used. Ingestion can arise from direct consumption of a radionuclide (!), by placing contaminated fingers in or close to the mouth, or from the consumption of contaminated food. Food can be contaminated by coming in contact with the radionuclides or with other contaminated items such as plates, utensils, or even hands. This potential can be eliminated by following the guidelines listed below.

a. Do not store food or beverages where radioactive materials or contaminated items are stored or used.

b. Do not eat, drink, smoke, or apply cosmetics in areas where radioactive materials are used.

c. Wear disposable gloves when handling radioisotopes or contaminated articles.

d. Label all containers used for radioisotopes or contaminated items.

e. Segregate and clearly label radioactive or non-radioactive waste.

Table 5.1 gives the Annual Limit of Intake (ALI) of radionuclides that an individual may ingest without exceeding a body dose of 5000 mrem/yr, a bone surface dose of 50 rem/yr, and a thyroid dose of 15 rem/yr. As a simple rule of thumb; the more energetic the particle, the less that can be ingested. Finally, iodide uptake is clearly the major hazard which is why bioassays are required of users of ¹³¹I and ¹²⁵I.

3. ABSORPTION

Disposable gloves should be worn because some radionuclides can be absorbed through the skin.

4. PUNCTURE

Radionuclides can also enter the body via punctures in the skin so it is important that sharp objects which could be contaminated with radionuclides be handled with utmost care. Any wounds received while working with radioisotopes should be checked for possible contamination immediately.

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B. EXTERNAL EXPOSURE TO RADIONUCLIDES

1. RADIONUCLIDES ON THE SKIN

A matter of interest to users of radioactive materials is having contamination on the skin. Approximately 50% of the ionizing radiations will be captured by the body. Low energy beta-emitters, such as ³H, have such a short path length that they do not penetrate dead skin. Thus, unless the skin is cut or abraded there is no direct risk from skin irradiation by ³H. Slightly higher energy emitters such as ¹⁴C, ³⁵S, and ⁴⁵Ca pose a slight risk since only 10-40% can cross the dead layer of skin.

In Table 5.2 the properties of the common beta-emitters are listed. The estimated dose in mrad/hr is calculated for a situation in which one uCi of radionuclide is deposited on one square cm of skin. Note that these calculations only give the radiation dose to which the basal cells are subjected. High energy emitters can damage internal organs. How long would it take to reach the yearly limit if the radionuclide remained on the skin?

The yearly limits allowed by the State are:

	Rems/year
Whole body	5
Hands, forearms, feet, ankles	50
Lens of eye	15

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TABLE 5.1

Annual Limits of Intakes (ALI) 10CFR20 App B

Radionuclide	Form	Target Organ	ALI uCi Ingestion
3H	Water	Total Body	80,000
	5-3H-CdR	Hematopoietic,Stem Cell Nuclei & Spermatogonia
	All other DNA	Hematopoietic, Stem Cell
	& RNA & RNA	Nuclei & Spermatogonia
	Precursors
14C	Soluble	Total Body	2000
	Inorganic
	DNA Precursors
	RNA Precursors
22Na	Soluble	Total Body	400
32P	Soluble incl	Total Body	600
	DNA Precursors
35S	Soluble	Total Body	10,000
36Cl	Soluble	Total Body	2000
45Ca	Soluble	Bone surfaces	30,000
51Cr	Soluble	Total Body	40,000
86Rb	Soluble	Total Body	500
99Tc	Soluble	Total Body	4000
111In	Soluble	Total Body	4000
125I	Soluble	Thyroid	40

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If the radionuclide were on the extremities, the limit could be reached in about 24 hours; if on skin of the remainder of the body, 6 hours. If the contamination is detected, as it should be, the radionuclide should be removed as soon as possible. The danger comes from inadvertent contamination. High energy radionuclides should be readily detected by lab monitors. Wearing disposable gloves coupled with careful surveillance procedures after experiments will avoid skin contamination.

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2. EXTERNAL SOURCE OF BETA-EMITTERS

The risk to external exposure from low energy beta-emitters is small when they are handled away from the body. Beta- particles have a finite range in air. Thus, when at least two feet separates the user from ¹⁴C, ³⁵S, and ⁴⁵Ca, there is no exposure. If these radionuclides are contained in a vial, very little radiation will escape through the walls.

Higher energy beta-particles such as ³²P are a different case. They have a range of 20 feet in air. The dose rate from a 1 mCi source of ³²P at 1 cm is 200 rad/hr. As illustrated in Table 5.2, ³²P will penetrate about 1 cm through biological tissue. The major risk from external exposure is to the eye, a radiosensitive organ for which the maximum permissible dosage is 15 rem/yr. If the 1 mCi source were held 1 cm from the eye, the maximum dose would be reached in 4.5 min. Use of a lucite shield (1 cm thick) will practically eliminate the exposure hazard.

3. EXTERNAL SOURCE OF GAMMA-EMITTERS

The exposure rate for gamma emitters can be calculated for each radionuclide using the Specific Gamma Ray Constant which has units of R/hr per mCi at 1 cm. Data for three of the most commonly used gamma-emitters are given in Table 5.3, and for a much wider range of emitters in Table 5.4.

Consider two examples:

a. ¹²⁵I has a Gamma Constant of 0.7 (Table 5.3) which means a 1 mCi source would produce 0.7 R/hr at 1 cm, a 2 mCi source, 1.4 R/hr at 1 cm, and a 1 mCi source would produce 0.007 R/hr at 10 cm.

 

b. A 1 mCi ⁶⁰Co source will produce an exposure of 13.2 R/hr at 1 cm and 0.132 R/hr at 10 cm, in the absence of shielding.

The best safety rule to apply when using gamma-emitters is to maximize distance, minimize the time of exposure, and use shielding, as possible. Fortunately, the hands are the body part that most often come near to an unprotected gamma-emitter source, and the hands are relatively radiation-resistant.

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TABLE 5.2

Properties of Some Commonly Used Beta-Emitters

3H

14C