370 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 13 



where p = density of chamber gas 



H m = mass-absorption coefficient for gamma rays in the gas (sum of 

 photoelectric and Compton absorption coefficients) 

 If the gamma rays are monochromatic, with energy E y , the flux of photons 

 crossing the chamber is then/ = F/E y . By taking into account the absorp- 

 tion and the solid angle subtended by the instrument with respect to the 

 source, the emission rate of gamma rays from an external source can be 

 determined. 



b. Solid-wall Ionization Chambers. Gas- wall chambers are not easy to 

 construct and operate, and their usefulness breaks down at energies higher 

 than a few hundred kev. However, ionization chambers with solid walls 

 may be used for x-rays and gamma rays from a few kev to very high energies. 

 Most of the ionization chambers in routine use for radioisotope measurements 

 and dosimetry are of the solid-wall type, a very popular form of which is the 

 so-called "thimble" chamber. The methods for absolute measurement of 

 gamma rays and energy flux with such instruments were elucidated by Gray 

 [21]. To facilitate interpretation, the ionization chambers are constructed 

 in such a way that, for practical purposes, nearly all ionization in the sensitive 

 volume comes from secondaries initiated in the solid electrodes of the instru- 

 ment while the number of secondaries produced in the gas add negligibly 

 small ionization. A simple relationship holds true between the energy E s 

 absorbed per unit mass of the solid medium of the wall and the ionization / 

 produced in a small air-filled cavity of that medium, if the dimensions of the 

 cavity are small compared to the mean distance traveled in the air by the 

 secondary electrons and if the dimensions of the solid are large compared to 

 the distance traveled by the secondary electrons in the solid. This relation- 

 ship, sometimes also called the Bragg-Gray principle, may be written as 



IW E s 



eV P S 



The meanings of W, V, p, and e are the same as above, while S is the relative 

 stopping power per unit mass of the solid medium versus air. With the 

 knowledge of the absorption coefficient p. m of the solid medium of the ioniza- 

 tion chamber the gamma-ray energy flux F at some point near the center of 

 the chamber is then 



£, IWS 



P-m HmVep 



Since gamma rays of a few hundred kev do not lose much energy traversing 

 a few feet of air or through the walls of small ionization chambers, the rate 

 of emission from a point source of monoenergetic gamma rays of energy E y 



