Photoneutron Angular Distributions for 4He

Abstract

Differential cross sections for the reaction 4He(γ, n)3He have been measured at laboratory angles of 53°, 68°, 83°, 98°, 128°, and 143° for excitation energies between 22 and 32 MeV by the photoneutron time of flight technique. The photoneutron spectra were used to obtain the angular distribution coefficients in the Legendre series expansion for the differential photoneutron cross sections. Analysis of these coefficients indicates that the photodisintegration process in this nucleus, while dominated by E1 photon absorption followed by channel spin zero (no spin flip) p wave neutron emission, is affected by E1 channel spin one and E2 contributions. In the higher energy region (E > 27 MeV), the amount of E2 absorption observed was ~6% of the total and is dominated by the channel spin one E2 matrix element. The total cross section obtained from the measured differential cross sections yields a value of σ(γ, p)/σ(γ, n) near unity, in agreement with a calculation of this ratio which requires no charge symmetry breaking component of the nuclear force.