Photographic Plates for Use in Nuclear Physics

Abstract

The registration of charged particles by the photographic emulsion is considered from the standpoint of their space rate of energy loss on passage through matter. On the basis of the highest-energy particles that can be recorded as recognizable tracks, an energy-loss value of 0.013 Mev per centimeter of air path is taken as an approximate threshold sensitivity value of the best present-day nuclear-particle emulsions. This energy-loss value corresponds to alpha-particles of energy g400 Mev, deuterons of 100 Mev, protons of 50 Mev, and electrons of 20 kev. Some evidence is cited to show that these limiting energy values are in accord with experience.Range-energy curves for alpha-particles, protons, deuterons, and mesons in high silver halide concentration emulsions are given, based on constant stopping-power values of the emulsion. A relative stopping-power value of 1800 is used for alpha-particles, and a value of 2000 is used for protons, deuterons, and mesons.Data on the composition, physical characteristics, and sensitivity, and also recommended uses of some of the commercially available nuclear-particle emulsions are presented.A theoretical calculation of the stopping power of the photographic emulsion relative to that of air is given. Curves of stopping power versus energy for alpha-particles and protons in a high silver halide content emulsion are calculated and compared with published experimental values.Grain density and its variation in the recorded tracks of nuclear particles in the photographic emulsion are discussed. It is pointed out that the grain density for low-energy particles approaches a maximum value determined by the grain population of the unexposed emulsion. A qualitative discussion is given to show why grain spacing varies with particles of differing ionizing power.Finally, the mechanism of latent-image formation is considered from the standpoint of ion-pair production in the grain. Using the energy-loss value of 0.013 Mev per centimeter as a threshold limit and a value of 7.6 ev for the energy required to produce an ion pair in silver bromide, it is shown that about 150 ion pairs must be produced in a single grain to form the latent image.