£52 Dr. A. F. Kovarik on tlie Absorptio?i and 



The first column gives the thickness of the absorbing 

 layers of aluminium placed directly on top of the thin active 

 layer. The second, third, and fourth columns give the ioni- 

 zation observed in the electroscope. The ionization, when 

 no absorbing matter was on top of the active material, is 

 taken as 100. The remaining columns give similar results 

 when tinfoil was used for which the equivalent absorbing 

 thickness of aluminium is given. This equivalent value of 

 tinfoil in terms of aluminium was found from the latter 

 portions of the absorption curves, where, as will be seen 

 from later work, the absorption curves are similar in form. 

 Several radioactive materials were used in this determination 

 with very similar results. 



It should be stated here that the opening of the electro- 

 scope was covered w T ith a considerable thickness of mica and 

 tinfoil, so that the very easily absorbed radiations are not 

 effective. It will be noticed that the initial portions of the 

 curves rise to a maximum before absorption becomes at all 

 obvious. Furthermore, the percentage increase is greater 

 when tinfoil is used instead of aluminium. For a given 

 source of ft rays, the maximum is reached for equivalent 

 thicknesses of tinfoil and aluminium, but this maximum 

 shifts to the right on using more penetrating /3 rays. The 

 observed effect is therefore a function of both the absorbing 

 material and the velocity of the ft- particles. When the 

 absorbing foils were placed at some distance above the active 

 material, the maximum decreased in magnitude with the 

 increase of the distance until finally the reverse effect ob- 

 served by Crowther showed itself. By placing a perforated 

 diaphragm above the active matter the maximum decreased 

 in magnitude with the decrease of the size of the opening. 



These experiments show that the initial rise to a maximum 

 is undoubtedly due to the scattering of the /3-particles by 

 the thin absorbing foils. In order that scattering may be 

 complete, the /3-particles must pass through a definite thick- 

 ness of the absorbent. In the experiments described the 

 radiation is equal in all directions. Those particles which 

 strike the absorbent normally are scattered less than the 

 oblique ones, provided the absorbing layer has a thickness 

 smaller than that required for complete scattering. Conse- 

 quently, the oblique rays, which did not reach the electro- 

 scope when no absorbing matter was placed over the active 

 material, become scattered when a thin foil is so placed, the 

 degree of scattering depending on the thickness traversed, 

 and therefore on the obliquity of the rays. These scattered 

 /3-particles reach the electroscope in numbers sufficiently 



