The photoneutron cross section for ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ has been measured with monoenergetic photons from threshold to 31 MeV. The cross section reaches its maximum value of 1.0 mb from 24.3 to 27.3 MeV and appears to be split into two broad peaks. The integrated cross section is 7.9 MeV mb. The fore-aft asymmetry of the photoneutrons also was measured. This quantity, which is a measure of the interference between the $E1\mathrm{}$ amplitude and the underlying positive-parity contribution, changes from forward to backward peaking at about 27 MeV. The $E2\mathrm{}$-to-$E1\mathrm{}$ amplitude ratio deduced from these data is of the order of 0.04. A comparison of the cross-section results with the considerably larger values for the ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\gamma ,p)$ reaction measured elsewhere shows appreciable ( 15%) isospin mixing throughout the energy region studied, with indications that this mixing might be increasing with energy above 28 MeV.

• Received 13 May 1971

DOI:https://doi.org/10.1103/PhysRevC.4.723