Some Theoretical Considerations on the a(y, p)/a(y, n) Ratio in ~He.

Summary. - Recent measurements have confirmed the value 1.7  0.3 for the ratio between 4He(y, p) and (y, n) reaction cross-sections in the excitation energy range 24isospin 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. It has been long recognized (1-3) that the ratio between photoproton and photoneutron reaction cross-sections in self-conjugate nuclei can be used as a tool for the study of isospin mixing and as a test of the charge symmetry of nuclear forces at low energies. 4He is a particularly suitable candidate for this investigation, because of its structural simplicity. The present report concerns a phenomenological model search, which has been undertaken to identify those features which a more complete microscopic treatment must yield to explain the experimental data.