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 times of exposure, the irregularities in the curve are not so marked.

Decay of Excited Activity of Radium—short exposure

Fig. 66.

Miss Brooks has recently determined the decay curves of the excited activity of radium for different times of exposure, measured by the α rays. The results are shown in Fig. 67, where the initial ordinates represent the activity communicated to the body from different times of exposure to a constant supply of emanation. It will be observed that in all cases there is a sudden initial drop of activity, which becomes less marked with increasing time of exposure. The activity, several hours after removal, decreases exponentially in all cases, falling to half value in about 28 minutes.

Not only do the curves of variation of the excited activity after removal depend upon the time of exposure to the emanation, but they also depend upon whether the α or β and γ rays are used as a means of measurement. The curves obtained for the γ rays are identical with those from the β rays, showing that these two types