424 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 16 



where g is the geometrical factor taking into account the volume distribution 

 of active material in some appropriate simple shape which approximates the 

 actual organ. The constants / and A are described below. 



Integral gamma-ray dose presents a somewhat more difficult problem 

 for which it is necessary to evaluate a double integral or else establish the 

 isodose curves throughout the organ. In units of gram-roentgens the 

 integral dosage rate and total integral dose delivered to a specified volume of 

 tissue are 



ue~ llR 



dV dV 1 gm r/sec 



I.D. ---- 83 IA P j J ^-dVdV 1 gmr 



or 



I.d. = 83 IAuG gm r/sec 



I.D. = 83 I A UG gm r 



where 83 is a conversion factor (83 ergs absorbed per gram per roentgen) for 

 conversion of energy in ergs to gram roentgens, and G is the integral geometri- 

 cal factor (see Sec. 16.7) and p is the tissue density. If the isodose curves 

 have been determined empirically, the integral dosage rate is 



I.d. = \ S3pdiAVi gm r/sec 



i 



where p = tissue density 



di — dosage rate in r per unit time in volume AVi 

 AVi = volume of tissue between isodose surfaces 



The number A of gamma rays of a particular energy emitted per second 

 per microcurie of radioactive isotope depends upon the complexity of the 

 decay scheme of the radioactive isotope. If there is no branching or inter- 

 nal conversion, each isomeric transition occurs once per disintegration or 

 A — 3.7 X 10 4 gamma rays per second per microcurie of isotope. When 

 decay schemes are complex, branching alone will often make A < 3.7 X 10 4 . 

 Since gamma radiation may be emitted in isomeric transitions in the nucleus, 

 from annihilation of positrons and as x-rays accompanying K capture and 

 internal conversion, the value of A must be determined for each component 

 on the basis of the decay scheme. 



Isomeric transitions (I.T.) are usually the principal source of gamma rays 

 in most isotopes of biological interest. Assuming that branching and 

 internal conversion £I.C.) occur, the number of gamma quanta emitted per 

 second per microcurie is then 



Alt. = 3.7 X 10 4 (1 — })v 7/microcurie/sec 



