Collisions of a. Particles with Hydrogen Nuclei. - 927 



away will be ,= — 9 . n If the thickness of the paraffin 



J 2irr 2 sm 0. r 



sheet AA' is such that it is equivalent in hydrogen content 

 to t cm. of hydrogen gas, then the number of H particles 

 projected from the elementary ring at P to unit area at S, 

 placed at right angles to RS, is 



Q/2 . sin 0/2 . d(0/2) . cos 0/2.t. sec 0/2 . F ^ . 

 ' 2irr 2 sin 



= 8^.*-sec0/2.F(0).rf(0/2). 



For the whole angular ring of angular limits 0\\2, 2 /2 the 

 number of H particles falling on unit area of the zinc- 

 sulphide screen is 



i ! -9-j.*. sec 0/2. F(0) .d(0j2). 



] e ± /2 



)irr 



For our purpose, it is sufficient to take mean values of r' 

 and t .sec 0/2. Calling these r 2 and t, the above number is 



! -i.[F(0 2 ) -J? (<?,)]• 



lQjrr 2 



If the number of H particles observed on the screen be 

 referred to a source of Ra(B-f-C), of 1 nig, 7-ray activity, 

 Q is then the number of a particles emitted per second by 

 1 mg. Ra. 



If the paraffin wax is in the form of a circular sheet, i. e. 

 01/2 = 0, the number of H particles observed on the ZnS 

 screen is directly proportional to F(# 2 ). The simplest 

 method of experiment is therefore to use circular sheets of 

 wax of different angular limits 2 j2, 2 /2, etc. The obser- 

 vations give immediately points on the curve n=F(0) 

 corresponding to the various angles 2 , 2 , etc. This 

 method, however, has the disadvantage that TI particles 

 projected at very different angles, and therefore with very 

 different velocities, fall upon the screen during the same 

 observation, and it is doubtful if in such a mixture of 

 bright and weak scintillations all the weak scintillations 

 would be counted. It was considered advisable, therefore, 

 to use circular sheets for the smaller angles only ; for angles 

 greater than 20°, annular rings of wax of suitable angular 

 limits were used. The use of these has the further advantage 

 that the " natural" H particles emitted by the source of 

 a-rays can be prevented from reaching the ZnS screen. 



