S- and p-wave neutron spectroscopy : Part VIII. Subshell effect on nuclear level spacing near A = 50

Abstract

The neutron total cross sections of separated isotopes of several even A targets near A = 50 have been measured with high resolution using the Li 7( p, n)Be 7 reaction as the neutron source for a 20° collimator. The measurements on Ca 40, Cr 52, Fe 54, and Fe 56 are characterized by a number of narrow l > 0 resonances superimposed on much wider but easily distinguishable s-wave resonances. The Ti 46, Ti 48, and Ti 50 nuclei exhibit a large enough proportion of these l > 0 resonances to complicate identification of s-wave resonances. Since the usefulness of area analysis is limited by strong resonance-resonance and resonance-potential interference, a multilevel formula obtained from the R-matrix formalism has been used to fit the wide s-wave resonances. After correcting the resulting 1 2 + level spacings to zero angular momentum and for variations in excitation energy, two other important influences on level spacing were observed; the neutron excess effect observed earlier by Newson and the subshell effects predicted by Rosenzweig. The former phenomenon has been verified beyond reasonable doubt and the latter which results from the influence of ground-state subshell occupation on the level spacing at higher excitation energy is consistent with Rosenzweig's predictions. The moment of inertia of some of these nuclei when excited to the neutron separation energy have been determined. They apparently fluctuate widely around the rigid body value but on the average are slightly higher. We have also found the distributions of widths and spacings to be in reasonable good agreement with the respective Porter-Thomas and Wigner predictions.