Hacxett—The Secondary Radiation excited by y Rays. 207 
Discussion. 
Our ignorance of the exact nature of the y rays is no bar to a theoretical 
discussion of the secondary radiation excited by them. Even the recent addition 
of Bragg’s revolying-couple theory to the earlier ether-pulse theory does not— 
introduce any complication if rightly considered. In Bragg’s theory the y rays 
consist of neutral systems formed by positive and negative particles, revolving 
about each other, and the whole moving in a direction in the plane of their orbit. 
The absorption of the y rays takes place by the collision of the system with an 
atom and the resultant absorption of the positive particle, while the negative 
particle passes on, constituting the secondary radiation on emission from the 
plate. 
But for a discussion of the subject it is not necessary to make use of any 
physical conception of the nature of y rays. A stream of radiation is passing 
through each small element of volume, containing atomic systems of electrons. 
The interaction between the atomic system and the y radiation, whatever may be 
its nature, results in the emission of electrons from the element. Each element 
of volume becomes a source of 6 rays, the intensity of which depends on the 
amount of energy absorbed from the y rays. ‘The energy emitted as 8 rays 
from each element of volume should increase, therefore, as the coefficient of 
absorption of the y rays increases. For the time being, the plate under the 
influence of the rays emits 6 rays as if it were a radio-active substance. A certain 
thickness of a radio-active substance which depends on its coefficient of absorption 
for 6 rays is sufficient to give the maximum effect. The secondary radiation 
here also comes from a certain depth below the surface—certainly not from a 
greater depth than ‘05 cm. of tin, the thickness of forty sheets of tinfoil. ‘The 
intensity of the y rays does not vary more than 1 per cent. in passing through 
such a layer, so that the secondary radiation may be considered as coming from 
a number of sources whose intensity per unit volume is constant. ‘The intensity 
of the radiation emitted depends on two factors—(1) the intensity of the sources 
per unit volume, (2) the thickness of the effective layer. he first factor 
increases with the coefficient of absorption of y rays. The second factor 
diminishes evidently with the coefficient of absorption of 8 rays. Since both 
coefficients are roughly proportional to the density, they are roughly proportional 
to each other, so that the change in the intensity due to the first factor might be 
expected to compensate roughly for the change due to the second factor, which is 
in the opposite direction. An argument much the same as the above has been 
used by Bragg to show that the two factors work against each other, and may be 
expected to balance each other, as they actually do. 
These general considerations explain why pEnsiTy has little effect in 
