Page:Radio-activity.djvu/181

 Although the scintillations from a particle of pure radium bromide are very numerous, they are not too numerous to be counted. Close to the radium, the luminosity is very bright, but by using a high power microscope the luminosity can still be shown to consist of scintillations. Since the number of scintillations probably bears no close relation to the number of [Greek: alpha] particles emitted, a determination of the number of scintillations would have no special physical significance. The relation between the number of [Greek: alpha] particles and the number of scintillations would probably be variable, depending greatly on the exact chemical composition of the sensitive substance and also upon its crystalline state.

97. Absorption of the [Greek: alpha] rays by matter. The [Greek: alpha] rays from the different radio-active substances can be distinguished from one another by the relative amounts of their absorption by gases or by thin screens of solid substances. When examined under the same conditions, the [Greek: alpha] rays from the active substances can be arranged in a definite order with reference to the amount of absorption in a given thickness of matter.

In order to test the amount of absorption of the [Greek: alpha] rays for different thicknesses of matter, an apparatus similar to that shown in Fig. 17, p. 98, was employed. A thin layer of the active material was spread uniformly over an area of about 30 sq. cms., and the saturation current observed between two plates 3·5 cms. apart. With a thin layer of active material, the ionization between the plates is due almost entirely to the [Greek: alpha] rays. The ionization due to the [Greek: beta] and [Greek: gamma] rays is generally less than 1% of the total.

The following table shows the variation of the saturation current between the plates due to the [Greek: alpha] rays from radium and polonium, with successive layers of aluminium foil interposed, each ·00034 cm. in thickness. In order to get rid of the ionization due to the [Greek: beta] rays from radium, the radium chloride employed was dissolved in water and evaporated. This renders the active compound, for the time, nearly free from [Greek: beta] rays.