NEW PHOTOGRAPHIC EMULSIONS SHOWING IMPROVED TRACKS OF IONIZING PARTICLES

Abstract

The photographic emulsion method of registering the tracks of ionizing particles suffers from certain disadvantages that are mainly due to the relatively large size of grains forming the tracks and their large spacing. These disadvantages are mostly removed in new emulsions that have been prepared after numerous tests. The tracks of ionizing particles in these emulsions consist of grains of diameter d ~ 0.2 µ, which often form continuous lines.Grain spacing is no longer the main cause of apparent straggling, except below a range of 3 to 4 mm. of air. The standard deviation in determining energy from a single measurement of range is 3.25 to 3.5% for protons of 0.6 to 2.4 Mev., and 1.3 to 1.9% for α-rays of 4 to 8.5 Mev. The latter deviation is one-half to one-quarter that obtained in commercial emulsions. Ease of finding the tracks, especially of protons, is considerably increased. The tracks of fission fragments have a characteristic appearance and are easily found amidst the faint or invisible tracks of α-rays and protons. Slow electron tracks have occasionally been seen.The action of certain sensitizing and desensitizing chemicals is described and discussed in terms of the theory of the photographic process.The grain spacing is related to the diameter of the grain, the concentration of silver bromide in the emulsion, and the average probability of development of the grains touched by the ionizing particle. A quantitative theory is given for the probability of development in terms of the specific energy loss and a sensitivity parameter of the emulsion. The observed variation of the grain spacing with residual range of protons, α-rays, and fission fragments can be approximately explained by the theory.