The effect of isotopic spin impurity on (γ, p) and (γ, n) cross sections

Abstract The presence of small isotopic spin impurity in an excited state of a self-conjugate nucleus can lead to large differences in the neutron and proton emissions from that state. This effect might be expected to be exhibited clearly in reactions involving a T = 1 excited state produced by E1 absorption, e.g. in a giant resonance state of a photonuclear reaction. The photodisintegration data for 12C do in fact suggest that the (γ, p) and (γ, n) cross sections are more dissimilar than can be explained by penetrability considerations. From calculations based on the shell model it is found that, for an excited state appropriate to the giant resonance in 12C, an isotopic spin admixture of order 1% in intensity is required to fit the observed ratio of total cross sections. The angular distributions are sensitive to the details of the excited state wave function; however, if the calculated proton distribution is made to fit the observed distribution in the simplest way, a neutron distribution not very diff...