The photoproton and photoneutron cross sections for ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ are evaluated in the light of several recent measurements, and are found to be substantially different in the energy region below 30 MeV. Because these cross sections are dominated by $\Delta S=0\mathrm{}$, $\Delta T=1\mathrm{}$ $E1\mathrm{}$ transitions, this result implies strong isospin mixing in the four-nucleon system. Furthermore, since available continuum shell-model calculations including Coulomb effects predict only small differences in the photoproton and photoneutron cross sections except near threshold, this result suggests that a nonzero charge asymmetry is present in the nuclear force.

NUCLEAR REACTIONS ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\gamma , p){}_{}{}^3{}_{}{}^{}\mathrm{H}$, ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\gamma , n){}_{}{}^3{}_{}{}^{}\mathrm{He}$, evaluated cross sections, cross-section ratio; isospin mixing, charge asymmetry.

• Received 1 June 1982

DOI:https://doi.org/10.1103/PhysRevC.27.1866