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.
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Decay of Excited Activity of Radium.
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.
