Absorption of Thermal Neutrons in Silver at Low Temperatures

Abstract

WE have measured the absorption curves for neutrons of Fermi's¹ group C using a silver target (2·3 min. activity) as detector, in the centre of a large Dewar vessel containing either liquid hydrogen or water at room temperature. Source, absorber and target were kept at room temperature. The intensity of the C group was determined in the usual way by taking the difference between the whole activity and that obtained by screening the target with 0·3 mm. cadmium. If a Maxwellian distribution for the energies of neutrons of this group be assumed, and if the 1/ absorption law predicted by theory is valid for silver, the ratio of the absorber thickness necessary for equal absorption at different temperatures would be expected to be independent of the absorption itself and to be equal to the square root of the inverse ratio of the temperatures. Experimentally we found that this ratio for 290° K. and 20·4° K. is in fact constant over the whole of the absorption curves; its value, however, was found to be 2·2 ± 0·2, instead of 3·8 to be expected from the T₂/T₁ hypothesis. One of the silver absorbers was measured also at 77° K. in paraffin and the ratio of thickness was also in this case considerably smaller than that given by the theory, the point being nearer to 290° K. than to the 20·4° K. curve. The results of the measurements are given in the table below.