Photoneutron Spectroscopy of Nuclei Near Mass 200

Abstract

<p>A new technique has been developed for the study of photoneutron reactions in the 6 to 11 NeV photon energy range. The technique consists essentially of illuminating a target with a beam of neutron capture gamma rays and observing the neutrons with a high resolution helium-3 detector. Using this method, bismuth, lead, thallium, gold and tantalum targets were studied. Q-values, cross sections and reduced widths were obtained and the systematic variation of the latter two with respect to photon energy, residual excitation energy and neutron energy were investigated. In the case of gold and tantalum, previously undiscovered energy levels in the residual nuclei were determined. Based on these systematics, evidence is presented to suggest that the reactions occur predominantly off resonance and may have substantial direct or semidirect components. Fore-aft neutron emission asymmetry in lead has also been studied and a theoretical model has been derived in order to obtain approximations of the E2/E1 mixing ratios. Direct evidence of parity mixing is presented.</p>