186 Prof. A. Stanley Mackenzie on Secondary Radiation 



\ mm. of lead) cause these transmitted rays in a 9 - 2 mm. 

 plate. Since the reflected rays due to incidence of /3 rays do 

 not increase after £ mm. of lead is used, one would expect 

 the transmitted rays due to the same /3 rays to cease increasing- 

 after about double that amount ; but there seems to be no 

 limit of even that order of magnitude. 



Another important result is shown by the numbers of the 

 second row of the Table, the " transmitted " 7 ray leak. For 

 the smallest thickness used, lead-foil ^ mm. thick, the leak 

 is 2'05 ; as the thickness is increased the leak increases 

 instead of decreasing, as was the case for j3 rays, and as one 

 might at first sight expect. The leak reaches a maximum 

 for a thickness of about f mm. of lead and thereafter 

 decreases steadily, but remains large for a thickness of even 

 15*6 mm. These very penetrating 7 rays are evidently not 

 very largely absorbed by ^ - mm. of lead, and it requires 

 many times that thickness before their maximum effect is 

 brought out. 



The existence of this maximum for the 7 ray leak suggests 

 that there should be a similar one for the /3 ray leak. The 

 values were accordingly plotted, and are found in fig. 2 as 

 curves D and C respectively. The shape of the curve C 

 leads one to expect that there is a maximum for a thickness 

 of about -J- - mm., and I have drawn that part of the curve 

 on that conjecture. It would then be very like the curve 

 whose equation is y=ae~ Klt — he~ x - 1 . 



Referring back to the discussion of the results in Table II., 

 it will be seen that the anomalous behaviour of the air-rays 

 due to 7 rays with increasing thickness of screen S there 

 observed, is precisely the same as that we have just been 

 considering ; in that case also the leak increased with 

 increasing thickness of screen up to 1 mm. and then de- 

 creased, but very slowly, even after a thickness of 11 mm. 

 was used. This gives further evidence that we were there 

 dealing with 7 rays entering the ionization-chamber, and 

 hence that these 7 rays were made by 7 rays striking air. 

 In the case of lead, Table II. shows that /3 rays also can set 

 up these secondary 7 rays. 



It remains to account for the radiation from the back of a 

 thick lead plate the front surface of which is struck by 

 easily absorbable primary /5 rays. It is usually stated that 

 these primary rays are practically all absorbed by 2 mm. of 

 lead, and w T e might have expected some little effect from the 

 back of a 4 mm. plate ; but instead we find a relatively very 

 large effect with 10 and 15 mm. It seems reasonable to 



