Degradation of Gamma- Ray Energy. 753 



front of the ionization chamber. Supplementary tests 

 showed that when the primary rays entered directly into 

 the ionization chamber, the absorption was nearly the same 

 whether the lead screen was at A or at B *. Let us suppose 

 that the primary beam consists of any number of components 

 of different wave-lengths \ ]? X 2 , . .., that I 1? I 2 , ... are the 

 intensities of these components in the primary beam, c ]9 c 2 , ... 

 the fractions of each component scattered into the ionization 

 chamber, k u k 2 . . . . the fractions of the respective energies 

 transmitted through the absorption screen when placed at A, 

 and ki,k 2 'i ... the corresponding fractions when placed at 13. 

 Then it is clear that, when the absorption screen is placed 

 at A, ihe intensity of the beam scattered into the ionization 

 chamber is 



I = c x hj.i + cjc 2 l 2 + . . . = %c s k s I s , 



and, when placed at B, the intensity is 



I' = cA'Ii + c 2 k 2 % + . . . = Xc s k s 'J s , 



But for all wave-lengths, k s is very nearly equal to kj . 

 Hence I is nearly equal to J' : that is, for truly scattered 

 radiation, the observed intensity of the secondary radiation 

 should be approximately the same whether the absorbing- 

 plate is in the position of A or B. 



If, on the other hand, the primary radiation excites in the 

 radiator hi a fluorescent radiation which is more readily 

 absorbed than the primary rays, the observed intensity of 

 the secondary radiation will be less when the absorption 

 screen is in the position B ; for if k v is the fraction of the 

 primary radiation transmitted through the absorption screen, 

 while k s is the corresponding transmission factor for the 

 fluorescent radiation, the ratio I'/I of the intensity of the 

 fluorescent radiation when the screen is at B to that when 

 the screen is at A is obviously k s jk p . Thus the effect of any 

 fluorescent radiation will be to make the fraction I'/I iess 

 than unity. 



If all the secondary radiation is of the fluorescent type, 

 the ratio k s /k p , and hence also of I'/I, should become in- 

 definitely small as the thickness of the absorption screen is 



* The supplementary experiment referred to showed that for the 

 gamma rays from radium (J 'filtered through 2 mm. of lead, and using- an 

 absorption screen of 1 cm. of lead, the value of k was - o7 and of h' was 

 0-52. The difference is doubtless due to the difference in the amount of 

 secondary radiation reaching the ionization chamber in the two cases. 

 This difference will be relatively less important for softer radiation, but 

 will be relatively somewhat more prominent for greater thicknesses of 

 the screen. 



