216 Hacxert— The Secondary Radiation excited by y Rays. 
The values given under [)g’, \g”, Xp” | have been measured very carefully in 
this way. The results of these observations for lead, copper, and aluminium, 
which were taken as typical substances, are exhibited in the following table :— 
Taste LV. 
Substance. | p' | Ag’ | ry’ | p" Ap” | Ay” ja” Ap" ay” 
Uranium oxide, "44 313 | | 
Lead, . ; “54 234 1:090 6 On eho. —- “69 141 *660 
Copper, . 6 “59 200 0:°765 “68 | 176 _ 64 168 °370 
Aluminium, . *62 183 0°184 "62 183 141 62 183 °122 
| 
Intensity of 100 83 50 
y rays, 
Thickness of — 2 mm. lead 10 mm. lead 
lead sereen, 
For the 8 rays from uranium oxide, p=°‘77; eg = 100. 
The observations fall into three groups depending on the type of y rays used 
to excite the secondary rays. In all the experiments the radium was contained 
in a sealed glass tube, which absorbed nearly all the 8 rays, and of course some of 
the very easily absorbed y rays. In the first series the y rays were incident 
directly on the under-surface of the plate, thick enough to absorb all the B rays 
(2mm. for lead, 6mm. for aluminium). The secondary radiation from the upper 
surface is therefore excited by an easily absorbed type of y rays. In the other 
two series the y rays traversed the lead screens before falling on the plate. 
Before discussing these results, it should be pointed out that the method of 
measurement is only approximate. It would be an accurate method if the lead 
frustrum 5 cm. thick absorbed all the y rays; but, as was remarked earlier, it 
only absorbs 95 per cent. The error involved is small. Let 
S = intensity of secondary rays excited by y rays; 
F = fraction transmitted through four additional sheets of tinfoil ; 
s&f= similar magnitudes for the secondary rays excited by the rays after 
passing through the lead frustrum. 
These observations, therefore, only give / when there is no variation in the 
penetrating power of the secondary rays. Where such a variation exists, the 
