116 Prof. E. Kutherford on Uranium Radiation and 

 Thickness of Aluminium foil *0005 cm. 



Number of Layers 



Leak per min. in. 



Eatio. 



of Aluminium foil. 



scale-divisions. 







182 





1 



77 



•42 



2 



33 



•43 



3 



146 



•44 



4 



9-4 



•65 



12 



7 





The aluminium foil in this case was about '0005 cm. thick, 

 so that after the passage of the radiation through '002 cm. of 

 aluminium the intensity of the radiation is reduced to about 

 2V of its value. The addition of a thickness of *001 cm. of 

 aluminium has only a small effect in cutting down the rate 

 of leak. The intensity is, however, again reduced to about 

 half of its value after passing through an additional thickness 

 of '05 cm., which corresponds to 100 sheets of aluminium 

 foil. 



These experiments show that the uranium radiation is 

 complex, and that there are present at least two distinct types 

 of radiation — one that is very readily absorbed, which will be 

 termed for convenience the a radiation, and the other of a 

 more penetrative character, which will be termed the ft 

 radiation. 



The character of the j3 radiation seems to be independent 

 of the nature of the filter through which it has passed. It 

 was found that radiation of the same intensity and of the same 

 penetrative power was obtained by cutting off the a radiation 

 by thin sheets of aluminium, tinfoil, or paper. The /S radiation 

 passes through all the substances tried with far greater facility 

 than the a radiation. For example, a plate of thin cover- 

 glass placed over the uranium reduced the rate of leak to ^ 

 of its value; the j3 radiation, however, passed through it 

 with hardly any loss of intensity. 



Some experiments with different thicknesses of aluminium 

 seem to show, as far as the results go, that the /3 radiation is 

 of an approximately homogeneous character. The following 

 table gives some of the results obtained for the ft radiation 

 from uranium oxide : — 



