Degradation of Gamma-Rat/ Energy. 755 



■probable error, that such extrapolation for large thicknesses 

 of the screen is precarious, and the evidence for any true 

 scattering cannot be considered conclusive. Probably at 

 least 90 per cent, of the secondary radiation at this angle is 

 of the fluorescent type. 



Though we thus find that there is very little of the pene- 

 trating primary gamma radiation present in the secondary 

 rays, it is not impossible that some of the softer components 

 of the unfiltered primary beam may be appreciably scattered, 

 but be so strongly absorbed that they are not detected 

 through the lead screens employed. An upper limit to the 

 amount of such soft scattered radiation that may be present 

 can be assigned in the following manner. If at any specified 

 angle a certain screen suffices to make the ratio I'/ 1 sensibly 

 zero, it is clear that with this screen in position A there is 

 no appreciable scattered radiation entering the ionization 

 chamber, and practically all of the secondary radiation is 

 iluoreseent. The absorption coefficient for the primary rays 

 which excite the fluorescent radiation may then be determined 

 by measuring the decrease in ionization when additional 

 screens are placed at A. Assuming that this absorption 

 coefficient remains constant for all thicknesses of the 

 absorption screen, which experiments on the primary beam 

 show is very nearly the case, the intensity of the fluorescent 

 radiation when no absorption screen is employed is 



v.x 



J = J . e 



where J is the observed intensity for a screen of thickness x, 

 and fju is the linear absorption coefficient. Of the total 

 secondary radiation I observed when no screen is employed, 

 the fraction J /I at least consists of fluorescent radiation. 

 Of the remainder,- (I — J )/I , a part may be fluorescent, 

 •since the true value of jul is presumably greater for small 

 thicknesses of the absorption screen, and the rest will 

 ^represent the truly scattered radiation. Thus the fraction 

 (I — J )/I is an upper limit to the amount of scattered 

 radiation which may be present in the secondary gamma 

 rays when no absorption screens are employed. 



Since theoretical considerations would lead one to expect 

 the scattering to be greater at the smaller angles, an experi- 

 mental determination of the value of this fraction was made 

 for the secondary radiation at 45°. In this experiment 



the window of the ionization chamber consisted oi' 0*15 em. 

 of lead, and the same thickness of lead surrounded the 

 source of gamma rays. This was necessary in order to cut 



