A Study of the Elastic Scattering of Fast Neutrons by Potassium and lithium

Abstract

The elastic scattering of fast neutrons by 39K and 7Li has been studied at an incident neutron energy of 14.1 MeV, using a neutron spectrometer employing the associated particle time of flight technique. The source neutrons were produced by the 3H(d,n) 4He reaction at an incident deuteron energy of 120 Kev. The time of origin of the source neutron is determined by detecting the associated helium particle, and the neutron energy is determined by measuring electronically its flight time over a fixed flight path. Differential elastic scattering cross section measurements were made on 39K in order to obtain better agreement between optical model predictions and experiment than the one previous measurement by Frasca et al. Improved agreement between theory and experiment was obtained, following the suggestion of Cassola and Koshel that a derivative surface absorption term should be used for potassium. Differential elastic scattering cross section measurements were also made on 7Li in order to investigate the discrepancies in the previous measurements by Armstrong et al, using nuclear emulsions, and Wong et al, using the pulsed beam method, and to investigate the validity of the optical model for light nuclei. A comparison of the angular distributions obtained for both elements was made with the predictions of the nuclear optical model. Finally, in parallel with the work at 14.1 MeV, an associated particle time of flight spectrometer was developed for use with the 2H(d,n) 3He reaction, to enable neutron differential scattering cross section measurements to be made at an incident neutron energy of 2.5 MeV, this being suitable for experiments related to reactor design.