The dependance of the photographic action of β-Rays on their velocity

Abstract

The photographic plate has long been used for detecting high speed electrons, but of late several attempts have been made to use it in a more quantitative way for comparing the intensities of two electronic beams. For example, most of the work on β-ray spectra has so far consisted in determining the energies of the various groups of electrons emitted by radioactive bodies by finding the radius of the circle they describe in a uniform magnetic field. This is carried out by placing a photographic plate at some point in their path and noting the point of impact, the only function of the plate being that of a detector. If, however, we wish to compare the number of electrons in two groups of different energies by means of their photographic impression, then we must take into account not only the normal photographic technique of dependance of density on exposure but, in addition, the way the density due to a fixed number of electrons depends on their energy. It has long been an accepted principle that high speed electrons have less photographic action than slow ones, although we have been unable to find any specific experiments demonstrating this fact. Far less were there any measurements of the exact nature of this dependance, and in some experiments recently carried out in this laboratory on the comparison by a photographic method of the intensities of the electronic groups emitted by radium B and C, Ellis and Wooster* were forced to assume the photographic action ran parallel with the total ionisation per centimetre, that is, varied as l/β 2 . This seemed plausible and was used as a temporary means of obtaining approximate values, but, as was pointed out on that occasion, it was a matter that urgently needed investigation.