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XCIII. Absorption and Reflexion of the /3-Particles by Matter, 

 By Alois F. Kovarik, Ph.D."" 



RECENTLY considerable discussion has taken place on 

 the law of absorption of the /3-particles. Hahn and 

 Meitnerf have shown that the /3 radiations from radioactive 

 elements are absorbed according to an exponential law, and 

 according to their theory, namely, that radioactive elements 

 emit rays of only one velocity, have concluded that such 

 radiations are homogeneous. W. Wilson J has shown that 

 the /3 rays separated out by a magnetic field, and therefore 

 consisting of practically one velocity, are not absorbed ac- 

 cording to an exponential law. The results of Schmidt §, 

 Crowther ||, and others have confirmed in some respects the 

 experiments of one or the other. The question whether the 

 /3-particles from radioactive elements are homogeneous or 

 complex is still an open one, and obviously quite complicated, 

 involving in its solution several other questions, such as the 

 effect of the reflected radiations and the variation of the 

 ionizing power of the /3-particle with its velocity, and possibly 

 also with the path traversed. 



Since the ft radiations from most of the radioactive materials 

 are absorbed very nearly according to an exponential law, 

 this law becomes very convenient in determining the co- 

 efficients of absorption. The absorption curves are, however, 

 influenced by different experimental arrangements, chiefly 

 because under some conditions the scattered radiation is 

 not properly taken into account in the ordinary methods of 

 measurement. 



The purpose of this investigation was (1) to determine the 

 coefficients of absorption of the /3-particles of different 

 velocities under the most normal conditions; (2) to determine 

 the effect of reflected rays on the coefficients of absorption ; 

 (3) to determine the variation of the amount of reflexion 

 with the velocity as .well as with the reflecting substance. 

 In order to do this, it is necessary to have sources of /3-par- 

 ticles of different velocities, and for this purpose the following 

 were employed : — 



Radium D + E which emits some weak rays probably from 

 RaD but mainly the rays from RaE, whose coefficient of 

 absorption by aluminium is 43*3 (cms.) -1 , corresponding to a 



* Communicated by Prof. E. Rutherford, F.R.S. 



t Hahn and Meitner, Phys. Zeit. ix. x. p. 321 (1908). 



% W. Wilson, Proc. Roy. Soc, A. lxxxii. p. 612 (1909). 



§ H. W. Schmidt, Jahrbuch d. Had. u. Elek. iv. 4 (20), p. 451 (1908). 



|| J. A. Crowther, Proc. Camb. Phil. Soc. vol. xv. p. 442 (1910). 



