M. Levin — Absorption of the a-Rays from Polonium. 9 



a-particles, like the period of transformation, is a characteristic 

 constant of each product, and the accurate knowledge of these- 

 ranges seems to be of importance at the present state of our 

 knowledge of radio-active transformations. It was also of inter- 

 est to examine the ionization due to the a-rays of polonium at 

 different distances from the source in order to see if it emitted 

 only one type of a-rays. A similar examination of the active 

 deposit of thorium by Halm* had disclosed the unexpected 

 fact that two different a-ray products were present instead of 

 one, as was previously supposed. 



For the experiments a bismuth rod, coated with a thin film 

 of polonium (radio-tellurium), obtained from Sthamer of Ham- 

 burg, was used. 



Measurements by the Scintillation Method. 



First the range in air of the a-particles was measured by the 

 scintillation method in the usual way. The active rod was 

 placed beneath a zinc-sulphide screen, and the distance at 

 which the scintillations just disappeared was measured. Sev- 

 eral layers of aluminium foil, each of about the thickness 

 of *0003 cm , were placed above the rod and the range again 

 measured. The results of one series of experiments are given 

 in Table I. 





Table I. 









Distance between 



Centimeters of air 



Number of 



rod and screen 



corresponding to 



aluminium foils. 



in centimeters. 



one laj 



er of foil. 







3-78 







1 



3-29 





•47 



2 



2-78 





•50 



3 



2-30 





•49 



4 



1-84 





•48 



5 



1-35 





•49 



6 



•82 





•40 



7 



•32 



Mean 



•50 





•49 



Thus the range of the a-particles in air is 3"78 cm . By plot- 

 ting the numbers of layers as abscissae and the ranges as ordi- 

 nates, one obtains approximately a straight line. This line 

 meets the axis of ordinates a little higher than the value 

 obtained by experiment, if no aluminium foil is used, the value 

 found in this way being 3-82.- In another series of experiments, 

 using other samples of aluminium foil, each corresponding in 

 stopping power to about *5o cm of air, the values found for the 

 range in air were 3*76 and 3-79 cms . Adams has given an 

 account of similar measurements with several gases before the 



*L. c. 



