The ranges of   particles in photographic emulsions

Numerous α-particle tracks on Ilford R plates have been measured. From measurements of the tracks of a homogeneous group of α particles of range R , a distribution curve may be drawn, from which a length L , known as the extrapolated length of the tracks, can be determined. The ratio L/R is shown to be constant to within 1 per cent for α particles of various ranges, and hence R can be found for a group of unknown tracks if L is measured. The method is applied to determine the ranges corresponding to the tracks produced by the disintegration of boron and lithium under slow-neutron bombardment. These are found to be 1.14±0.02 cm. in air and 6.64±0.06 cm. in air, respectively. Samarium has also been investigated, and is found to emit α particles of range 1.13±0.02 cm. in air. Other particles from samarium, of longer range, have also been found.