The action of electrons and X rays on photographic emulsions

The measurement of electron densities by means of their photographic effect is discussed, and the results of many past investigations on the photographic action of light photons, x-ray photons and electrons are summarized. Attention is drawn to many points of similarity between high-velocity electrons and x-ray photons, and between low-velocity electrons and light photons. The effect of a gradual increase in the energy of the incident particle on the shape of the blackening curve is also discussed. A theory based on an exponential decrease in the effect of the electrons due to absorption in the emulsion is advanced. The resultant formula is found to depend in a simple fashion on the grain properties of the emulsion, and on the absorption of the beam within it. An extension to variation with grain size is given, and the effect of varying absorption on the shape of the blackening curve is investigated. Previous formulae are compared with the new one, many approximations being found to be similar in nature. Comparison of the formula with experimental results obtained by previous investigators and by the writer allows many of the grain properties of the emulsion to be correctly predicted, as well as the shape of the blackening curve, the range in the emulsion, etc. In particular, the apparent independence of the shape of the characteristic curve for x rays of the wave-length is deduced, and it proves possible to account for observed variations. The quantitative agreement obtained in many cases tends to show the general correctness of the theory. Some possible uses of the theory are appended.