exposure, the variation of activity with time measured by the β and γ rays will thus be represented by the upper curve CC of Fig. 73, where the ordinates represent activity. This curve will be seen to be very similar in shape to the experimental curve for a long exposure which is given in Fig. 68.
Since radium B does not give out rays, the number of α particles expelled from the active deposit per second is proportional to λ1P + λ3R. The activity measured by the α rays, using the electrical method, is thus proportional at any time to λ1P + Kλ3R, where K is a constant which represents the ratio of the number of ions, produced in the testing vessel, by an α particle from C compared with that from an α particle emitted by A.
It will be seen later that, for this particular case, K is nearly unity. Taking K = 1, the activity at any time after removal is proportional to λ1P + λ3R.
Case 1. We shall first consider the activity curve for a short exposure to the radium emanation. The relative values of P, Q, and R at any time corresponding to this case are graphically shown in Fig. 74. The activity measured by the α rays at any time will be the sum of the activities due to A and C separately.
Let curve AA (Fig. 74) represent the activity due to A. This decreases exponentially, falling to half value in 3 minutes. In order to show the small activity due to C clearly in the Figure, the activity due to A is plotted after an interval of 6 minutes, when the activity has been reduced to 25 per cent. of its maximum value. The activity due to C is proportional to λ3R, and in order to represent the activity due to C to the same scale as A, it is necessary to reduce the scale of the ordinates of curve CC in Fig. 72 in the ratio λ3/λ1.