Neutron Emission from Compound Nuclear Systems of High Angular Momentum

Abstract

Measurements were made of the energy spectra and angular distributions of neutrons emitted with laboratory energies in the range from 1 to 10 Mev in reactions induced by 160-Mev oxygen-16 ions incident on thin targets of aluminum, nickel, copper, and gold. Spectra were obtained (at laboratory angles between 30° and 150°) by analysis of the proton-recoil pulse-height spectra produced by the neutrons in a stilbene scintillation crystal. All spectra and angular distributions at these angles are consistent with the hypothesis of evaporation from compound nuclei. The energy spectra of neutrons emitted from the initial compound nuclei were calculated by use of statistical evaporation theory, both with and without modification to include (in a classical approximation) the effects of conservation of angular momentum. The dominant effect of conservation of angular momentum upon the calculated neutron spectra from the initial compound nuclei is a lowering of the nuclear emission temperature for the cases and parameters chosen. The measured spectra are compared to the spectra calculated both with and without the modification. The angular distributions are symmetric about 90° and peaked forward and backward. The total cross section for neutron production increases with increasing mass of the target atom, and is roughly 70% greater for copper than for nickel. This sudden increase is probably related to the greater neutron excess of copper.