Sec. 16.4] INTERNAL DOSIMETRY 419 



The method for calculating beta-particle dose may be illustrated by 

 a simple example. Assume that a colloidal suspension of chromic phosphate 

 containing 2 millicuries of P 32 is administered to a dog and that 75 per cent 

 is rapidly accumulated and fixed in the liver which weighs 300 grams. The 

 only factor affecting the dosage rate is then the reduction in u resulting from 

 radioactive decay. It is assumed that all beta particles are stopped in the 

 liver and that the isotope is uniformly distributed. The initial (/ = 0) 

 dosage rate is 



, (3.7 X 10 7 )(2 X 0.75H6.95 X 10 5 )(60) n . .„ 

 d = (300) (52.5 X 10") = °" 147 rep/min 



After 10 days the dosage rate has diminished to 



d = O.147e-°- 693xl0/14 - 5 = 0.0913 rep/min 



The accumulated dose delivered to the liver during the first ten days is 



D _ (0.147)(14.5)(60 X 24) (J _ ^„ ]X10/1 „ ) _ ^ rep 



Similarly, the total dose (complete decay of the isotope) is 



n (0.147)(14.5)(60 X 24) 



D = — = 4,400 rep 



The integral dosage rate and accumulated integral dose are obtained in 

 units of gram roentgens by multiplying through by the weight of the liver 

 (300 gm). 



16.4. Absorption of Gamma Rays in Tissue. From the point of view of 

 dosimetry the absorption of gamma radiation is concerned primarily with 

 conversion of gamma-ray energy to kinetic energy of secondary electrons. 

 While it is true that the energy given to a single secondary electron is not 

 absorbed in the tissue at the point where the photon interacts with the elec- 

 tron, i.e., the electron causes ionization along a path which at high energies 

 may extend a considerable distance, nevertheless, when radiative equilibrium 

 exists between the primary and secondary radiation, the amount of gamma- 

 ray energy converted to kinetic energy determines directly the density of 

 ionization produced at each point in the tissue. Transfer of energy takes 

 place by three distinct physical processes: the photoelectric effect, Compton 

 scattering, and pair formation. Each of these processes depends in a differ- 

 ent and complicated way on the gamma-ray energy, and the photoelectric 

 effect and pair formation are, in addition, strongly influenced by the atomic 

 composition of the absorber. 



A more detailed description of the interaction of gamma rays with matter 

 is given in Chap. 2, and only those properties important to dosimetry, 



