Hackett—The Secondary Radiation excited by y Rays. 208 
with effective screening. ‘To ensure complete absorption of the 6 rays from the 
radium, the opening was covered with a sheet of lead 2 mm. thick. The system 
of blocks (Z) was mounted so that the pencil of y rays travelled upwards at an 
angle of 45° to the vertical, and was incident, therefore, at the same angle on 
the lower surface of a horizontal plate (P), which was supported just over the 
opening. The y rays passing through the plate produce secondary radiation through 
its volume, which emerges at both the lower and upper surfaces of the plate. It 
seemed at first that it was a matter of indifference on which side the radiation 
was observed. With 6 and X rays, it is only possible to measure the radiation 
on the side on which the rays are incident. This method is not applicable here. 
For, though the 8 rays from the radium are entirely stopped by the glass and the 
sheet of lead (JZ) placed over the opening, the pencil of y rays still contains the 
secondary 6 rays which they themselves have produced in passing through the 
lead sheet. These 8 rays produce a tertiary radiation on the lower surface of 
the plate, which vitiates all observations made on this side. In fact, when 
measurements are made on the side of the plate on which the y rays are incident, 
one is really dealing in a large measure with effects produced by 6 rays. 
A method had, therefore, to be devised to enable measurements to be made 
of the radiation emitted at the upper surface of the plate. Its intensity was 
measured by the ionisation it produced in a brass cylinder (A), whose radius was 
5em. and length 20cm. The end nearest the plate (P) was covered with a sheet 
of tinfoil, ‘01 mm. thick; while the other end was stopped by a paraffin cork 
insulating an inner terminal (4), which ran along the axis of the cylinder, and 
was connected to a Dolezalek electrometer. The outside of the cylinder was 
charged to a positive potential of 240 volts, sufficient to give the saturation 
current, and the rate of charge of the inner terminal measured in the usual 
way. 
The cylinder and the emerging pencil of y rays were on the opposite sides 
of the normal to the plate, which made an angle of 45° with the axes of both in a 
vertical plane. This angle was selected merely for the sake of symmetry and 
convenience. 
The secondary radiation from the plate passed through at least 12 cm. of air 
and a sheet of tinfoil before reaching the cylinder, so that, as was mentioned in 
the beginning of the paper, it is only a penetrating radiation which was 
observed. 
THe MetHop oF OBSERVATION. 
The method of observation employed depended on the fact that none of the 
direct pencil of y rays coming through the conical opening passed through 
any part of the cylinder; the latter was brought as near to the plate as was 
2K 2 
