The absolute total cross section for the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$(γ,p${)}^3$H reaction has been measured in the γ-ray energy region between 28.6 and 58.1 MeV with a monochromatic photon beam and a nearly 4π proton detector. The comparison of our results with the most recent data on the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$(γ,n${)}^3$He total cross section provides a mean value of ${\mathit{R}}_{\gamma }$=σ(γ,p)/σ(γ,n) =1.01±0.06 between 28.6 and 42.4 MeV and tends to exclude a strong isospin mixing in the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ due to charge symmetry breaking of the nuclear force.

• Received 17 May 1988

DOI:https://doi.org/10.1103/PhysRevC.38.1990