a Particles with Liyld Atoms. 551 



We shall now consider the interpretation of the anomalous 

 absorption curve for a range of 7 cm. shown in curve A. 

 The curve is very similar to that to be expected if the 

 hydrogen atoms were thrown forward mainly in the direction 

 of the u particles, and all with the same velocity ; in fact, the 

 absorption curve for a pencil of H atoms is very similar in 

 shape to that for a pencil of homogeneous a rays from 

 radium 0. 



It is well known that the number of a particles counted by 

 the scintillation method in a homogeneous pencil of a rays 

 from radium C remains constant from to 6 cm. of the 

 range, and then rapidly falls to zero in the last centimetre of 

 the range. This end effect is usually ascribed to the scattering 

 of the a particles in their passage through the absorbing- 

 material. Xow if the H particles consist of H atoms carrying- 

 unit positive charge e and projected with a velocity m = 1'6 v, 

 where v is the velocity of the a particle, the average angular 

 scattering per cm. should be proportional to ejmir and should 

 thus be '78 of that suffered by the a. particle for an equal 

 range. Since the H atom has four times the range of the 

 y. particle, the average angular scattering of the H atoms 

 betore absorption should be approximately 2 x -78 = 1*56 that 

 of the a particle. It follows, therefore, that the decrease in 

 the number for a homogeneous beam of H atoms should begin 

 about 6 cm. from the end of the maximum range 28 cm. 



Fig-. 6. 



1 













I 











"^N^ 









\ N D 



\. 





\ c\ 

 b\ x 





















<3 16 19 22 



Range in cms. of air 



25 



28 



This theoretical curve is shown in fio-. 



curve A. Re- 

 membering that the stopping power of the hydrogen column 

 for a rays corresponds to 8 mm. of air, the absorption curve 



