On the Excitation of y Rays by jB Rays. 221 



large extent, especially the softer constituent ; absorbing 

 aluminium-foils were placed between sheet and electroscope 

 close to A, and an absorption curve obtained in the usual 

 way. The electroscope E was of aluminium and contained 

 methyl iodide*, since this gas is known to increase relatively 

 the effects of the soft radiations. If now the metallic sheet 

 is shifted to position B close to the source, a very large 

 increase in ionization is noticed : this must be due to the 

 excitation of secondary rays by the ft particles, which now 

 are allowed to strike the radiator ; a simple geometrical con- 

 sideration shows that it would be impossible to attribute more 

 than a very small part of this secondary effect to primary 

 7-rays striking the radiator in position B under a larger 

 solid angle than in position A. A second absorption curve 

 was obtained with the radiator at B and the absorbing foils 

 in the same position as before, that is at A. By comparing 

 the two curves and subtracting the primary effect from the 

 effect increased by secondary rays, we find the absorption 

 curve for the purely excited radiation. 



The advantages of this method are the following : — 



(a) The emergent secondary radiation is examined, which 

 is known to be much larger than the incident f, especially 

 for thin sheets of light metals. 



(I>) No correction has to be made for scattered radiation, 

 as the scattering of the primary beam must be nearly the 

 same in both cases. 



(r) The effect of the secondary radiation is of the same 

 order as that of the primary rays ; the latter always have to 

 pass through the radiator, whether it is placed at A or B, 

 whereas the secondary rays produced in the surface facing 

 the electroscope and in the nearest layers are not absorbed at 

 all or only to a slight extent by the radiator itself. 



Iron, nickel, copper, and zinc were investigated in this way, 

 the respective sheets having a thickness of 0*043, 0-058, 

 0*022, and 0*025 mm. These thicknesses, out of all avail- 

 able, proved the most suitable in order to obtain a compara- 

 tively large amount of secondary radiation. A large number 

 of measurements had to be made, as the results were not 

 obtained directly but by subtraction of two values, each of 

 which might contain an experimental error of about 1 per 

 cent. The case of copper is represented in fig. 2, where the 

 logarithms of ionization are plotted against the thicknesses of 



* Rutherford and Richardson, Phil. Mag-, vol. xxvi. p. 324 (1913). 

 f J. A. Gray, Proc. Roy. Soc, A. vol. lxxxvi. p. 513 (1912). 



