The tracks of <i>α</i> -particles and protons in photographic emulsions

Abstract

1—It has been known for some twenty-five years that when an α -particle passes through a photographic emulsion, the silver bromide grains which it encounters are rendered developable. Kinoshita examined the question in some detail in 1910, and came to the conclusion that, for the emulsions tested, every grain struck by an α -particle can be developed. During the following years Reinganum, Michl, Mayer and others worked on the subject, and showed that after development the track of each separate α -particle is visible, under a high magnification, as a row of developed grains in a straight line. If a spot of radioactive matter, such as radium C, be placed on the surface of a plate, α -particles are emitted in all directions, and produce a halo consisting of a number of tracks radiating from a centre. Photographs of these haloes, and of the individual tracks, were published inter alia by Kinoshita and Ikeuti, and Walmsley and Makower. Some of the large-angle deflections reported by the latter are probably tracks of two separate α -particles arising from the same spot of contamination in the emulsion. In 1925 it was observed by Blau that tracks could also be produced by natural protons ejected from a layer of paraffin by α -particles. It was found, however, that not every grain encountered necessarily becomes developable; owing, doubtless, to the feebler ionization along the path of a proton. This is true at least for the less sensitive emulsions. The protons which result from the disintegration of aluminium when bombarded by α -particles have also been recorded in this way by Blau, and Blau and Wambacher. Furthermore Blau and Wambacher have published results obtained with protons ejected by the passage of neutrons through paraffin, and through the gelatine of the emulsion.