P -wave neutron strength-function measurements and the low-energy optical potential

Using the National Bureau of Standards electron linac and underground time-of-flight facility, precise average neutron-transmission measurements have been made in the energy range 1 keV≤E≤600 keV on the elements As, Br, Nb, Rh, Ag, In, Sb, I, La, Ho, Au, and Th. The samples were "thick" in that the s-wave self-protection had to be accounted for at low energies. However, the samples were still sufficiently thin that any errors introduced by neglecting p-wave self-protection were negligible. The average R-matrix theory was employed in the analysis and the $l=0$ scattering length R^' and the p-wave strength function S₁ were extracted from the data. The behavior of S₁ vs mass number A in the region of the $3P$ maximum was found to vary smoothly with no evidence of any splitting of the resonance. Using Moldauer's optical potential, which fits the $l=0$ data well, the behavior of S₁ vs A was calculated. The predicted behavior was found to differ significantly from experiment. In particular, experiment indicates S₁ peaks at a lower mass number and that the maximum is stronger than indicated by the calculations. When the constants of the potential were changed in order to reproduce the observed behavior of S₁, a significant discrepancy with the $l=0$ data resulted. The results presented here imply an orbital angular momentum dependence of the low-energy optical potential.[NUCLEAR REACTIONS As, Br, Nb, Rh, Ag, In, Sb, I, La, Ho, Au, Th; measured average neutron transmission E=1-600 keV; deduced R^', p-wave strength function; optical-model analysis.]