Fig. 86.
Before considering the explanation of the intermediate portion of the curve further experimental results will be considered.
The curve of decay of the excited activity for a long exposure (24 hours) is shown graphically in [Fig. 86], curve AA. There is at first a rapid decrease for the first 15 minutes to about 50 per cent. of the initial value, then a slower decay, and, after an interval of about 4 hours, a gradual decay nearly to zero, according to an exponential law with the time, falling to half value in 28 minutes.
The curves of variation with time of the excited activity when measured by the β rays are shown graphically in Figs. [87] and [88].
[Fig. 87] is for a short exposure of 1 minute. [Fig. 88] shows the decay for a long exposure of about 24 hours.
Fig. 87.
The curves obtained for the β rays are quite different from those obtained for the α rays. For a short exposure, the activity measured by the β rays is at first small, then passes through a maximum about 36 minutes after removal. There is then a gradual decrease, and after several hours the activity decays according to an exponential law, falling, as in the other cases, to half value in 28 minutes.
The curve shown in [Fig. 88] for the β rays is very similar in shape to the corresponding curve, [Fig. 86], curve AA, for the α rays, with the exception that the rapid initial drop observed for the α-ray curve is quite absent. The later portions of the curve are similar in shape, and, disregarding the first 15 minutes after removal, the activity decays at exactly the same rate in both cases.