Resonance-Averaged (n,γ) Spectra and their Applications to Nuclear Structure

Abstract

Because they populate final states without regard to the details of the nucleonic configurations, neutron-capture γ rays are ideally suited for locating essentially all the low-lying nuclear states whose spins lie within a few units of that of the capturing state. With the reduction of the Porter-Thomas fluctuations through the use of neutrons having an energy distribution broad enough to average over a finite (and large) number of capture resonances, the primary capture γ -ray intensities can provide rather definite spin-parity information for those states which they are observed to populate. The power of these studies is greatly enhanced through the analysis of such resonance-averaged spectra at two distinct neutron energies, ~ 2 and ~ 24 keV, obtained from Sc and Fe neutron “filters”, respectively. In this paper, the use of resonance-averaged primary γ -ray spectra as a tool for low-energy nuclear spectroscopy is discussed and its strengths and limitations pointed out. Examples are drawn from studies of 2- and 24-keV neutron capture in samples of ¹⁵⁴, ¹⁵⁶, ¹⁵⁷, ¹⁵⁸Gd and ²³⁸U.