Chapter 16 Basic Precautions 16.1 Basic Principles of Radiation Protection The four basic methods used to control radiation exposure are time, distance, shielding, and contamination control. The first three methods apply to all types of radiation sources. The fourth applies only when working with radioactive material sources. Each worker should become familiar with these methods and consciously apply them in a habitual manner. In addition, these methods should be integrated into all written instructions, descriptions, and procedures involving the use of radiation sources. 16.1.1 Time Minimizing the length of time a worker is exposed to a source is frequently the simplest method of limiting exposure. Careful planning of work minimizes the time needed to complete a job, thereby minimizing the exposure. Exposure may be calculated if the exposure rate and time are known: Where: X = Total exposure X_ = Exposure rate t = Time of exposure X = X t 16.1.2 Distance i Exposure rate is inversely proportional to the square of the distance from the source. The relationship can be expressed mathematically and is called the inverse square law, given in Equation 16.2. If the exposure rate from a point source at a given distance is 2 1 X = X 2 1 2 2 x r r known, the exposure rate at other distances can be calculated using the inverse square law. Where: X_ 1 = Exposure rate at the reference point X_ 2 = Exposure rate at the point of interest r 1 = Distance from the reference point to the source r 2 = Distance from point of interest to the source Page 1 of 6
The inverse square law is illustrated in Figure 16.1 which shows that when the distance is doubled, the exposure rate is reduced by (½)² or 1/4. While no bit of matter can occupy a single point in space, a radiation source will behave as if it did under the following conditions: 1. The radiation from the source is emitted isotropically over the full 4π solid angle (equally in all directions); 2. The radioactive atoms are confined to a volume whose dimensions are so small that there is no significant absorption of the emitted radiations; 3. The attenuation of the radiation in the intervening space is negligible; 4. The scatter of the radiation in the intervening space is negligible; and 5. The maximum linear dimensions of the source and the receptor (or object of interest) is small compared with the distance between them. When making radiation measurements, this condition is satisfied when the distance is three times greater than the maximum linear dimension of the source or detector, whichever is larger. Under this condition, the error in the measurement is usually less than 5%. Application of the inverse square law to point sources of alpha and beta particles is complicated by their absorption in air. Application of the inverse square law to indirectly ionizing radiation such as gamma rays and x rays is very useful. While the simplest method of protection may be to limit the time of exposure, it is not necessarily the most effective. Benefits of increasing distance usually outweigh benefits of reducing handling time. For example, suppose forceps are used to increase the distance of the fingertips to a point source from 1 cm to 5 cm but in doing so the handling time is increased by a factor of 2. Distance reduces the exposure rate by a factor of (1/5)² and time increases it by a factor of 2. The exposure received is now (1/25) x (2) =.08 or only 8% of the original exposure. In this case, the increase in distance more than compensates for the increase in handling time. Page 2 of 6
16.1.3 Shielding Alpha Particles - Alpha particles are readily attenuated by most substances. Even the most energetic alpha particles emitted from radionuclides travel only a few inches in air and less than 100 micrometers in water. They generally do not possess sufficient energy to penetrate the dead layer of skin, therefore they present a minimal external radiation hazard. Most alpha emitters also emit electrons, x rays or gamma rays, or have daughter products that emit them. Beta Particles - The ability of a material to absorb beta particle energy is strongly dependent on the number of absorbing electrons in the path of the particle and is only weakly dependent upon the atomic number of the absorber. Ranges of beta particles in matter are frequently reported in the unit of density thickness, a unit directly proportional to the number of electrons per cubic centimeter in the absorber. Density thickness is mathematically expressed as: t d =1000 tl Where: t d = the density thickness of the absorber in mg/cm² ρ = the density of the absorber in g/cm 3 t l = the linear thickness of the absorber in cm Page 3 of 6
The advantage of using density thickness for the range of beta particles is that the shielding properties of different materials can readily be compared. Figure 16.2 shows the range of beta particles in units of density thickness, and may be used for all materials. The values in Table 16.1 were derived using Figure 16.2 and applying Equation 16.3. Radionuclides such as H-3, C-14, S-35, and Ca-45 that emit only low energy beta particles usually do not require shielding because of their limited range in matter. Higher energy beta emitters such as P-32 and Sr-90/Y-90 should be shielded with low atomic numbered substances such as acrylic or polyethylene to minimize the formation of bremsstrahlung (breaking radiation). Bremsstrahlung is a penetrating type of x ray formed when an electron decelerates in the electromagnetic field of the nucleus of an atom in the shield. Even if low atomic numbered shielding is used, sufficient bremsstrahlung x-rays may be created within the shield to warrant the use of secondary shielding. Proper construction of a composite shield of this type is discussed in the section on shielding of gamma and x rays. TABLE 16.1 RANGE OF BETA PARTICLES IN VARIOUS MATERIALS Radionuclide β-max kev Range mg/cm Air 0.001293 RANGE IN CENTIMETERS FOR GIVEN MATERIAL* Pine 0.45 Water 1.00 Acrylic 1.12 Glass 2.5 Aluminu m 2.7 Cement 2.85 H-3 18.6 0.54 0.42 0.001 0.0005 0.0005 0.- 0002 0.0002 0.0002 C-14 156.5 27 21 0.06 0.027 0.024 0.011 0.010 0.0095 S-35 166.7 31 24 0.07 0.031 0.028 0.012 0.012 0.011 Na-22 545.4 180 139 0.40 0.180 0.161 0.072 0.067 0.063 P-32 1710.3 812 628 1.8 0.812 0.725 0.325 0.301 0.285 * Numbers below materials are densities in units of mg/cm 3 as given by the Radiological Health Handbook. ii Positron Emitters - Positrons are positively charged electrons that are emitted from the nuclei of some atoms when they undergo radioactive decay. A commonly used positron emitter is Na-22. Positrons are categorized as anti-matter and do not exist for extended periods of time. While a positron possesses kinetic energy, it behaves very much like a negatively charged electron or beta particle, causing ionization and excitation of atoms or molecules in the media through which it passes. When a positron comes to rest, it interacts with a local negatively charged electron. In this interaction, both particles are Page 4 of 6
annihilated and their masses are converted into energy as predicted by Albert Einstein's famous equation E = mc². Two 511 kev photons are formed (corresponding to the energy equivalent of the rest masses of the positron and electron) and are emitted 180 from each other. Because positrons have the same properties as beta particles and cause annihilation radiation, positron emitters must be shielded with composite shielding, discussed in the following section on shielding of gamma and x rays. Gamma and X Rays - The effectiveness of gamma-ray and x-ray shielding is strongly dependent upon the atomic number of the absorber and the energy of the photons. Attenuation of a photon source under narrow beam geometry conditions is expressed as follows: I = I o e - t Where: I = Intensity of photons (number of photons per unit area per unit time) passing through a shield that have not interacted with atoms in the shield I 0 = Intensity of photons impinging on the shield μ = Linear absorption coefficient t = Thickness of the shield Narrow beam geometry assumes that all of the photons that interact in a shield are permanently removed from the beam. Broad beam geometry accounts for photons that scatter back into the beam after multiple Compton or elastic scattering, and scattering of photons into the beam that were not part of the original beam. Diagrams of narrow beam and broad beam geometry are shown in Figure 16.3. Since exposure rate is proportional to the number and energies of the photons in the beam, exposure rate may be substituted into Equation 16.4. Under broad beam geometry, an exposure rate buildup factor, B, is introduced to compensate for the extra photons that join the beam. The equation for broad beam geometry is given as follows: X = B X 0 e - t Where: X_ = Exposure rate at the point of interest X_ 0 = Initial exposure rate Page 5 of 6
If equation 16.5 is solved for the thickness of the shield, the following formula is obtained: i. H.J. Moe, Operational Health Physics Training, ANL-88-26, Argonne National Laboratory, 1988, Argonne, IL. ii. Radiological Health Handbook, Public Health Service Publication No. 2016, Revised Edition, January 1970, U.S. Department of Health, Education, and Welfare, Public Health Service, Food and Drug Administration, Bureau of Radiological Health, Rockville Maryland. Page 6 of 6