Page:Radio-activity.djvu/171

 '''93. Charge carried by the [Greek: alpha] rays'''. We have seen that the negative charge carried by the [Greek: beta] particles has been readily measured. Since there is reason to believe (section 228) that four [Greek: alpha] particles are expelled from radium for each [Greek: beta] particle, it is to be expected that the positive charge carried by the [Greek: alpha] particles should be determined still more readily. All the initial experiments, however, made to detect this charge, gave negative results; and, before successful results were obtained, it was found necessary to eliminate some secondary actions, which at first completely masked the effects to be looked for.

In consequence of the importance of this question, a brief account will be given of the methods of measurement adopted and the special experimental difficulties which have arisen.

In the first place, it must be remembered that only a small fraction of the [Greek: alpha] rays, emitted from a layer of powdered radium bromide, escape into the surrounding gas. On account of the ease with which the [Greek: alpha] rays are stopped in their passage through matter, only those escape which are expelled from a superficial layer, and the rest are absorbed by the radium itself. On the other hand, a much larger proportion of the [Greek: beta] rays escape, on account of their greater power of penetration. In the second place, the [Greek: alpha] particle is a far more efficient ionizer of the gas than the [Greek: beta] particle, and, in consequence, if the charge carried by the [Greek: alpha] rays is to be determined by methods similar to those employed for the [Greek: beta] rays (see section 80), the pressure of the gas surrounding the conductor to be charged must be very small in order to eliminate, as far as possible, the loss of charge resulting from the ionization of the residual gas by the [Greek: alpha] rays.

The experimental arrangement used by the writer is shown in Fig. 33.

A thin film of radium was obtained on a plate A by evaporation of a radium solution containing a known weight of radium bromide. Some hours after evaporation, the activity of the radium, measured by the [Greek: alpha] rays, is about 25 per cent. of its maximum value, and the [Greek: beta] rays are almost completely absent. The activity measured by the [Greek: alpha] and [Greek: beta] rays is then slowly regained, and recovers its original value after about a month's interval (see