The results are satisfactorily explained if it is supposed:—
(1) That the change B into C (half transformed in 21 minutes) does not give rise to β rays;
(2) That the change C into D (half transformed in 28 minutes) gives rise to β rays.
222. These conclusions are very strongly supported by observations of the decay measured by the β rays for a long exposure. The curve of decay is shown in [Fig. 88] and [Fig. 89], curve I.
Fig. 89.
P. Curie and Danne made the important observation that the curve of decay C, corresponding to that shown in [Fig. 88], for a long exposure, could be accurately expressed by an empirical equation of the form
where λ2 = 5·38 × 10-4 (sec)-1 and λ3 = 4·13 × 10-4 (sec)-1, and α = 4·20 is a numerical constant.
I have found that within the limit of experimental error this equation represents the decay of excited activity of radium for a long exposure, measured by the β rays. The equation expressing the decay of activity, measured by the α rays, differs considerably from this, especially in the early part of the curve. Several hours after removal the activity decays according to an exponential law with the time, decreasing to half value in 28 minutes. This fixes the value of λ3. The constant α and the value of λ2 are deduced from the experimental curve by trial. Now we have already shown ([section 207]) that in the case of the active deposit from thorium, where there are two changes of constants λ2 and λ3, in which only the second change gives rise to a radiation, the intensity of the radiation is given by