420 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 16 



essentially the problem of gamma-ray absorption in light elements, are 

 discussed below. 



The photoelectric effect is important only at relatively low energies, 

 < 0.1 mev, and in the energy range of soft x-rays (< 25,000 ev) it is almost 



Ei 



a> .5 

 "5 



o 

 o 



o 

 in 



a 



< 



o 



.05 



.01 



.01 



.05 



.1 .5 1 5 



Gamma Ray Energy in mev 



Fig. 116. Linear-absorption coefficient ix e of water as a function of gamma-ray energy in 

 mev. The absorption coefficient includes the photoelectric effect, scattering absorption, 

 and pair production but does not include the contribution from Compton scattering. 

 [From L. D. Marinelli, E. H. Quimby, and G. J. Hine, Am. J. Roentgenol. Radium Therapy, 

 59, 260 (1948).] 



wholly responsible for absorption of electromagnetic radiation in tissue. 

 It involves the complete absorption of a gamma photon by an atomic electron 

 that recoils from the atom with a kinetic energy E e = E y — I, where / 

 is the ionization potential of the shell, K, L or M, from which the electron is 

 ejected. In light elements composing tissue, / is usually small compared 

 to E y , and for the purposes of dosimetry is neglected. The photoelectric 



