Sec. 16.4] INTERNAL DOSIMETRY 421 



cross section per electron and per atom and the linear- and mass-absorption 

 coefficients are designated by r e , r , ri, and r m , respectively. 



Compton scattering is usually the most important absorption process for 

 gamma- ray energies 0.1 < E y < 3 mev associated with radioactive isotopes. 

 Each photon that collides with an electron undergoes a deflection from its 

 initial direction and a partial loss of energy, the amount of which is con- 

 tributed to kinetic energy of the recoil electron. The absorption coeffi- 

 cient a e , <r a , <n, or <T m is the sum of a scattering and a scattering-absorption 

 coefficient a = s o- + a a. In nearly all internal-dose problems the coefficient 

 a a alone should be used since only this term represents the transfer of energy 

 to recoil electrons. 



Pair production cannot occur for gamma-ray energies less than 1.02 mev 

 (2m c 2 ) and does not contribute appreciably to gamma-ray absorption in 

 tissue -except for energies greater than several mev. When pair production 

 occurs, the amount of energy transformed to kinetic energy of recoil electrons 

 is E y — 2m c 2 since the amount of energy 2m c- is required to create the 

 positron and negatron. This amount of energy is subsequently regained 

 in the form of two gamma rays, each of 0.511 mev, on annihilation of the 

 positron. The contribution of annihilation radiation to the dose is greatest 

 for primary gamma-ray energies of 5 to 10 mev, but Mayneord [15] has 

 shown that the effect produced by the annihilation radiation is at most only 

 a few per cent of the dose delivered by the primary radiation and can, there- 

 fore, usually be neglected. The absorption coefficients for pair formation 

 are designated by k with appropriate subscripts e, a, I, and m as for r and a. 



Except in limited energy ranges the absorption of gamma rays must be 

 represented by all three interaction processes. The total absorption coeffi- 

 cient is then ji = r + a o- + k. The units of fi, r, a, and k are largely a matter 

 of convenience, but those commonly used are the cross section per atom 

 fx„, the linear-absorption coefficient m in centimeters, and the mass coeffi- 

 cient Hm in square centimeters per gram. These are simply related by 



fJ-l = Pfim = pNna 



where p = density of absorber, gm per cc 

 N — number of atoms per gm 

 Although the absorption coefficient \l is a complicated function of energy, 

 in tissuelike substances it changes very slowly for energies greater than 

 about 0.1 mev. Nevertheless, the variation in p. with energy is sufficiently 

 great so that if the radiation consists of several monoenergetic components 

 with very different energies each component should be considered separately 

 in computing its contribution to tissue dose. The actual dose is then 

 obtained as the sum of the separate contributions. If, however, the energies 

 of the components are greater than 0.1 and are not very different, the com- 



