Selective Absorption of Slow Neutrons

Abstract

IN our letter of April 12¹, we mentioned that the absorption of slow neutrons by iodine was apparently greater when the induced (-ray activity of an iodine detector was used as a measure of the number of neutrons transmitted than when detectors of silver or rhodium were employed. We have now confirmed this selective absorption and have extended the measurements to absorbers of silver and of copper, where analogous though somewhat less marked selectivity is found. The accompanying table shows the activity (expressed as a fraction of that obtained in the absence of the absorber) when hollow cylinders of silver (1.0 gm. cm.²), copper (4.5 gm. cm.²) and iodine (2 gm. cm.²) were placed in turn around various detectors, themselves in the form of hollow cylinders. A layer of cadmium, sufficiently thick to absorb nearly all slow neutrons striking it, was always placed inside each detector during irradiation, so that when the absorber was in place, neutrons reaching the detector must have passed once, and only once, through the absorber. The irradiation was performed within a 4 cm. cavity in a large block of wax, the source being at 9 cm. from the centre of the cavity. It seems clear that different nuclei prefer neutrons of different velocities.