Search for the Fermi-surface anomaly in fast-neutron scattering from yttrium.

Abstract

Neutron differential-elastic-scattering cross sections of yttrium were measured from 4 to 10 MeV, at energy intervals of approximately 500 keV, and at 20 or more angles distributed between 20\ifmmode^\circ\else\textdegree\fi{} and 160\ifmmode^\circ\else\textdegree\fi{}. The results were combined with data previously reported from this laboratory to obtain a detailed data base extending from 1.5 to 10.0 MeV. These data were interpreted using a spherical optical-statistical model, including the surface-peaked real potential predicted by the use of dispersion relations. The volume integral of the real potential, ${J}_{V}$, was found to decrease linearly with energy, and that energy dependence was accounted for using the dispersion relationship linking the real and imaginary potentials. Thus, in the energy range of the present measurements, the neutron interaction with $^{89}\mathrm{Y}$ does not display a Fermi-surface anomaly of the nature reported in the $^{208}\mathrm{Pb}$ region. On the other hand, the energy dependence of ${J}_{V}$ needed to give the correct binding energies for the single-particle and single-hole states did tend toward constant values with decreasing energy, as expected, and perhaps even a decrease is suggested by some of the hole states.