The polarization of neutrons from the Li 7(p, n)Be 7 reaction (II)

Abstract

The polarization of the Li 7(p, n)Be 7 neutrons has been deduced from measurements of the left-right asymmetry in the scattering of these neutrons from a liquid oxygen analyzer. Angular distributions of the polarization ( P n) were measured at 2.2, 2.6 and 3.0 MeV covering angular ranges (laboratory system) of from 30° to 70°, 30° to 115° and 30° to 70°, respectively. For neutrons emitted at 50°, additional measurements were made at proton energies between 2.1 and 3.1 MeV. At 2.6 MeV P n is close to zero at 105° and 115°, suggesting that at least two states formed by the same initial channel spin and having l 1+l 2 ≧ 2 contribute to the reaction. The polarization at 50° as a function of energy decreases rapidly from about 0.5 at 2.15 MeV to a broad minimum centered at 2.45 MeV where the polarization is about 0.2 and then increases to a constant value near 0.3 at bombarding energies above 2.7 MeV. Polarizations and differential cross sections calculated assuming that the reaction is described by the levels in Be 8 which were postulated by Macklin and Gibbons to account for their measured total cross sections are not consistent with the experimental data. The results of calculations based on several other sets of level parameters are discussed. The unequal values of the reduced widths for emission of protons and neutrons observed for at least one level in Be 8 are explained in terms of the mixing by electromagnetic effects of states in Be 8 with the same spin and parity, but different isobaric spin.