Some theoretical considerations on the σ(γ, p)/σ(γ, n) ratio in⁴He

Summary

Recent measurements have confirmed the value 1.7 ± 0.3 for the ratio between⁴He(γ, p) and (γ, n) reaction cross-sections in the excitation energy range 24 ≤E _x ≤ 30 MeV. This large deviation from unity implies isospin symmetry breaking an order of magnitude larger than that predicted from direct Coulomb effects using conventional shell model estimates. We present here a phenomenological model search using continuum calculations, undertaken to identify those features of the problem that more complete future treatments must yield to explain the experimental data.