The curves obtained by means of the γ rays are identical with those obtained for the β rays. This shows that the β and γ rays always occur together and in the same proportion.
For increase of the time of exposure from 1 minute to 24 hours the curves obtained are intermediate in shape between the two representative limiting curves, Figs. [87] and [88]. Some of these curves have already been shown in [Fig. 68].
Fig. 88.
220. Explanation of the curves. It has been pointed out that the rapid initial drop for curves A and B, [Fig. 86], is due to a change giving rise to α rays, in which half of the matter is transformed in about 3 minutes. The absence of the drop in the corresponding curves, when measured by the β rays, shows that the first 3-minute change does not give rise to β rays; for if it gave rise to β rays, the activity should fall off at the same rate as the corresponding α-ray curve.
It has been shown that the activity several hours after removal decays in all cases according to an exponential law with the time, falling to half value in about 28 minutes. This is the case whether for a short or long exposure, or whether the activity is measured by the α, β, or γ rays. This indicates that the final 28-minute change gives rise to all three types of rays.
It will be shown that these results can be completely explained on the supposition that three successive changes occur in the deposited matter of the following character[[315]]:—
(1) A change of the matter A initially deposited in which half is transformed in about 3 minutes. This gives rise only to α rays.
(2) A second “rayless” change in which half the matter B is transformed in 21 minutes.
(3) A third change in which half the matter C is transformed in 28 minutes. This gives rise to α, β, and γ rays.