produced by ft and 7 Rays at High Pressures. 181 



The theory for a large shallow ionization-chamber has been 

 worked out by Bragg (Phil. Mag. Sept. 1930). The lower 

 plate was supposed to be composed of some dense material, 

 and the upper plate of some material that would reflect very 

 few ft rays. Hence, using Bragg's notation, we have : — 



I. The wall effect, i. e., the ionization produced in the 

 chamber by the emergent ft rays, 



= M(l-e-A' D )/\; 

 where 



D = depth of chamber x density, 



X = weight absorption coefficient of ft rays in the plate, 

 \'= „ „ . „ „ in the gas, 



k' = „ „ „ 7 rays in the gas, 



k = „ ,, „ „ in the plate. 



II. The gas effect 



= k'I)I-k'I(l-e-*»)l\'. 

 The total ionization in the chamber 



^WMX'--)}- 



In the present experiments it was found impossible to 

 reproduce such simple conditions. A complete expression 

 representing the action of the y rays is very complex, and 

 the value of the constants too uncertain to make such an 

 expression of any value in the present case. It is necessary 

 to take into consideration the effect of the ft rays set up in 

 the sides of the vessel due to the scattered 7 rays and 

 the effect of multiple reflexion on the resultant ionization. 

 Again, the indirect ionization produced by the 7 rays in the 

 gas will depend, to a certain extent, on the lower and upper 

 plates. A lead plate will always return more ft rays into the 

 ionization-chamber than will a plate of aluminium. There 

 is in fig. 7 a slight difference in the final slope of the curves, 

 but the pressure has not been raised sufficiently high to 

 determine how far this is dependent on the plates. 



From figs. 3 and 8 it is seen that the ft ray curve due to 

 Ur X is similar to that obtained by plotting the differences 

 in the ionization currents when the bottom plate is changed 



