MEASUREMENT OF THE ENERGIES OF NUCLEAR STATES EXCITED BY INELASTIC NEUTRON SCATTERING, AND A SEARCH FOR ELECTRIC MONOPOLE TRANSITIONS IN Pb

Abstract

A method of measuring the energies of nuclear states excited by inelastic neutron scattering is described, in which an energy-selective detector is used. The neutron source was Li or H 3 bombarded by protons from a Van de Graaff generator. The ratio of the counting rate produced in this detector by the material under study to that produced by a long counter was plotted against the proton energy. Peaks in this curve occur when the energy of the scattered neutrons is equal to the resonance energy of the selective detector (255 kev). The energies of the excited states responsible for these neutrons were determined with some precision by a step-by-step comparison with those produced by a standard material for which the energies of the excited states are already well known (e.g. Fe 56 ). A weak peak, not corresponding to any known state, was found in Zr. Two states in Fe 56 were used to determine the energies of peaks obtained from Pb. One of these was that due to the well-known state at 2.62 Mev. However, peaks were observed corresponding to both lower and higher energies, none of which agree with those of known excited states. Except for that caused by the 803-kev state of Pb 206 , no other peaks were observed from a target of Pb 206 . Th 232 produced strong peaks corresponding to excited states at 760 and 1080 kev; U 238 produced a peak at 710 kev and another (unresolved from higher energy peaks) at 940 kev. Both peaks near 700 kev from these two nuclei showed unresolved structure.A peak was found from natural Pb corresponding to an excited state at 3.185 Mev. Reasons are given for believing that this state is 0 + . An unsuccessful search was made for annihilation radiation using coincident NaI crystals in a rectilinear arrangement. This apparatus detected readily the pair production from the 0 + state in Zr 90 , but was apparently responsible for a spurious effect in Pb. If pairs are emitted by a 3.185-Mev state in Pb 208 , the cross section for this process for neutrons with energy 0.5 Mev above threshold is probably less than one half of that from Zr 90 for an equal energy excess above threshold.