Scattering of D—D Neutrons

Abstract

EXPERIMENTS hitherto performed on the scattering cross-sections of various substances for D—D neutrons have led to fundamentally different results. Most observers have found that the cross-section σ increases regularly with the atomic number, and some have deduced from their results the variation with Z of ξ, the ‘sticking factor’. Kikuchi and Aoki and collaborators¹, however, have observed a periodic variation of σ with Z. The detectors used in all these experiments have been either sensitive to fast neutrons of wide range of energy, or sensitive to slow neutrons. Since the 170 min. period of phosphorus is excited only by neutrons of energy greater than about 2 Mev., it is an ideal detector for D—D neutrons, especially as our experiments have shown that there is no observable elastic scattering of these neutrons by elements of widely differing atomic number such as sulphur, iron and lead. The disadvantage of the use of phosphorus, however, lies in its relatively low intensity of excitation, so that the detector had to be placed near (9 cm.) the target. In consequence the geometry was not ideal, and only relative cross-sections could be obtained. In spite of this, however, the mean free path in paraffin wax was the same to within experimental error as that observed by Booth and Hurst², among others, using better geometry and an indium foil as a slow neutron detector.