SECONDARY RADIATION. 143 



shows the number of nuclei instantly generated behind the lead plates 

 in the two cases. 



Thickness of lead screen ..... o 0.14 0.28 0.56 0.84 1.12 o cm. 



Z> = 6oocm. .VXio" 3 ...... 67 28 28 31 29 31 76 



/;^= 2 oocm. NX io~ 3 ...... 79 44 48 41 44 70 



Again, the X-ray bulb apparently emits this radiation forward as 

 well as rearward, as if the thin auticathode were quite pervious. I 

 found, for instance, for the radiation of the anticathode at 6 meters 

 from the fog chamber 



From the front face (tube directed), ^V)< io~ 3 =42 

 From the rear face (tube reversed), jV/< io~ 3 ==35 



or Si per cent of the former apparently issues from the rear face (fig. 

 80, Chapter III). Even the reversal of the current does not stop the 

 radiation, for about 16 per cent of the normal intensity is still radiated 

 when the concave mirror is made the anode (fig. 80, Chapter III). 



The total efficient radiation may be reduced to a limit by lead screens 

 a few millimeters in thickness, or less ; thereafter it can not be further 

 reduced by lead screens many centimeters in thickness. For instance, 

 when the radiation comes from 600 cm., a single lead plate (thickness 

 0.14 cm.) is more than sufficient to reduce the effective radiation to a 

 minimum, which amounts to (somewhat less than) one-half of the total 

 intensity, at least when estimated in terms, of the number of nuclei 

 produced. (Figs. Si, 83, Chapter III.) If the nucelation comes from 

 200 cm., one plate has the same effect, even though a thickness of 400 

 cm. of air has been removed. The thickness, 0.14 cm., is more than 

 enough to reduce the radiation to the limit in question. This again 

 amounts to a little more than one-half the total intensity. (Fig. 82, 

 Chapter III.) At a distance of 5 cm. no more plates may be needed; 

 but the conditions are now too complicated to be described here, 

 chiefly because persistent nucleil are producible. Moreover, 80 per 

 cent of the total intensity may ultimately escape absorption. Thus 

 the rays from different distances behave alike for the more pervious 

 media and in relation to very dense screens. (Fig. 83, Chapter III.) 



102. Lead-cased fog Chamber. To interpret these surprising results 

 it will be necessary to surround the fog chamber with a casket of lead, 

 having a lid on the side fronting the X-ray bulb ; for even though the 

 lead plates above may efficiently cut off the primary rays, they would 

 leave the secondary radiation free to enter laterally through the broad- 

 sides of the fog chamber. When this was done the results reduced 

 the penetrability of lead to a more reasonable figure, as may be seen 



