912 Mr. F. P. Slater on Excitation of <y Radiation 



four points only over a very wide range of absorption 

 thickness. This soft radiation, which Chadwick (loc. tit.) 

 has already shown to exist when gold is bombarded by 

 a particles, was investigated by another and more suitable 

 method, given in Part II. 



Noiture of Radiation. 



In order to obtain some evidence of the nature of this 

 penetrating radiation, experiments were tried to discover 

 whether it was confined to the same direction as the 

 a particles, or whether, as with the characteristic radiations, 

 a more uniform distribution obtained. Only half of the 

 tube A (fig. 1) was lined with the metal target to be bom- 

 barded, as shown in fig. 1 (a), which is a sectional view of 

 the tube. In the same experiment this tube was rotated on 

 its own axis alternately through 180° and back again to the 

 original position. Thus the metal target was between the 

 active material and the electroscope in one position, and on 

 the side of the active material away from the electroscope 

 in the other position. If the radiation under investigation is 

 emitted mainly in the direction of the incident a particles, 

 greater activity would be induced in the measuring vessel 

 when the target was in the first position than in the second 

 position. Although greater activity was found when the 

 target was in the first position, the increase was so small 

 that much more sensitive detecting apparatus is required to 

 settle the point with certainty. Also, with this arrange- 

 ment the beam of a rays is too indefinite. Not only is the 

 emanation distributed throughout the tube and ^ particles 

 are being fired off in all directions, but some of the gaseous 

 molecules may be between the target and the walls of the 

 containing-tube, so that even in the first position a particles 

 are impinging on the metal at all angles. The test can only 

 be an exceedingly rough one, but since the general direction 

 of the ol particles in the first position is towards the electro- 

 scope, and the reverse holds in the second position, and 

 as the increase is so small, the result seems to favour a 

 more uniform distribution as found in the characteristic 

 radiations. 



The absorption coefficients given above can only be a 

 rough representation of the penetrating power of the radia- 

 tion, owing to the very small intensity obtained. Even 

 with the thinnest absorption plate used the initial activity 

 due to the lead radiation is only 02 div. per minute per 

 milligram of emanation, while for the thicker bases a small 

 error in observation has a very large effect on the result 



