BW Whitmore

The energy spectrum of neutrons emitted by a Po---Be source has been determined by the photographic emulsion method. The results agree, on certain assumptions, with the existence of energy levels in the ${\mathrm{C}}^{12}$ nucleus at 2.5, 4.5, and 7.1 Mev. The last two of these are consistently found by other workers, but the existence of the first is sometimes disputed. Possible reasons for the apparent discrepancies in the evidence on this point are discussed.

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.

The energy spectrum of 14-Mev neutrons from the ($T\ensuremath{-}D$) reaction after transmission through 6 cm of lead, or bismuth, has been measured by the photographic emulsion method. Approximate agreement with the Weisskopf theory is found, the nuclear temperatures being 0.8 Mev for lead, 0.9 Mev for bismuth. In lead, there is some indication of a group with a discrete loss of energy of about 5 Mev.

The inelastic scattering of 14-Mev neutrons in carbon and lead has been investigated by the photographic emulsion method. In carbon, evidence of the excitation of the first-excited level is found, as well as excitation corresponding to Weisskopf's evaporation theory. Comparison with previous results for lead gives no evidence for double inelastic scattering in a thick scatterer.

Neutron total cross sections have been measured in the energy range of 20 to 45 MeV for Be, C, Al, Cu, and Zn. The typical accuracy was 1% to 2%. Results are compared with optical model calculations in which the only variable is the surface absorption strength, which is constrained to vary with a linear energy dependence. The calculation is extended to include 13 nuclei with A < 75.