934 Dr. J. Chadwick and Mr. E. S. Bieler on the 



§ 7. Variation of the Number o/H particles with the 

 Velocity of the a particle. 



The variation of the number of H particles projected 

 within a given angle with the velocity o£ the incident a-rays 

 can be deduced from the curves of fig. 4 for a range of 

 velocities from 2*02 x 10* to 1-43 x 10 9 cm. per sec. As 

 previously mentioned, these results suggested that the 

 inverse square law w r ould be valid for the collisions of a-rays 

 of still lower velocity. It was found impracticable, how- 

 ever, to determine the whole n, 6 curve for a-rays of very 

 small range ; for, when using the diaphragms A aud B, the 

 '" natural " H particles coming from the source and absorbing 

 screens in the path of the a-rays were more numerous than 

 tho^e produced in the paraffin film. Consequently, the effect 

 of particles of low range was investigated with diaphragm C ; 

 that is, between 21°*4 and 31°'3. 



The results for a-rays of mean range varying from 6'6 to 

 1*0 cm. are collected in the following table. The second 

 column gives the value of (V /V) 2 , where V =l # 92 x 10 9 cm. 

 per sec, the velocity corresponding to a range of 7 cm., and 

 V is the velocity corresponding to the range given in the 

 first column. The third column gives the observed number 

 of H particles per mgm. activity of the source, and the fourth 

 the values of F (31°'3)-F (2'l°'4) after correction for the 

 efficiency of the screen. 



In the case of the counts with a-rays of range 1*6 cm. and 

 l'O cm., a further correction was necessary to take into 

 account the stoppage of « particles by the absorbing screens 

 of gold which were used to cut down the range. This was 

 determined by a second experiment, in which the a particles 

 from a weak source of Ra(B + C) were counted through the 

 same thickness of gold. It was found that the effective 

 number of a, particles in the count at a range of 1*6 cm. was 

 84 per cent, of the number emitted by the source, while in 

 the count at 1 cm. range the effective number was only 

 50 per cent. The numbers observed at these ranges were 

 therefore multiplied by factors of 1*2 and 2*0 respectively. 



For comparison, the values of F(31°-3)-F(21°'4) which 

 would be given by point charges and the inverse square law 

 are shown in the last column. 



It is seen that as the range of the a-rays is diminished, 

 there is a very rapid decrease in the observed number of 

 H particles, until, for an a particle of range about 2 cm., a 



