Page:Radio-activity.djvu/210

 β and γ rays of radium, has recently been made by A. S. Eve. The general experimental method employed is shown in Fig. 45.

The electroscope (Fig. 45) was placed behind a lead screen 4·5 cms. thick, which stopped all the β rays and absorbed the greater proportion of the γ rays from the radium tube placed at R. On bringing near a plate of matter M, the primary rays fell upon it and some of the secondary rays, emitted in all directions, passed into the side of the electroscope, which was covered with aluminium foil of thickness ·05 mm. Before the plate M was placed in position the rate of discharge of the electroscope was due to the natural leak and the γ rays from R, and the secondary radiation from the air. On bringing the radiator M into position, the rate of discharge was much increased, and the difference between the rate of movement of the gold-leaf in the two cases was taken as a measure of the amount of secondary rays from M. The absorption of the secondary rays was tested by placing an aluminium plate ·85 mm. thick before the face of the electroscope.

The secondary rays were found to be fairly homogeneous, for the intensity fell off according to an exponential law with the distance traversed. The value of the absorption constant λ was determined from the usual equation I/I_{0} = e^{-λd}, where d is the thickness of the screen. The table given below shows the results obtained when thick plates of different substances of the same dimensions were placed in a definite position at M. The secondary radiation from fluids was obtained by a slight alteration of the experimental arrangements.

Thirty milligrammes of radium bromide were used, and the results are expressed in terms of the number of scale divisions passed over per second by the gold-leaf.

It will be noticed that the amount of secondary radiation follows in most cases the same order as the densities, and is greatest for mercury. The value of (secondary radiation)/density is not a constant, but varies considerably, being greatest for light substances. The absorption constant of the secondary rays from different radiators is not very different, with the exception