Determination of Co57(n,xp ) cross sections using the surrogate reaction ratio method

Abstract

The compound nuclei $^{58}\mathrm{Co}^{*}$ and $^{61}\mathrm{Ni}^{*}$ have been populated at overlapping excitation energies by transfer reactions $^{56}\mathrm{Fe}(^{6}\mathrm{Li},\ensuremath{\alpha})^{58}\mathrm{Co}^{*}$ (surrogate of $n+^{57}\mathrm{Co}$) at ${E}_{\mathrm{lab}}$ = 35.9 MeV and $^{59}\mathrm{Co}(^{6}\mathrm{Li},\ensuremath{\alpha})^{61}\mathrm{Ni}^{*}$ (surrogate of $n+^{60}\mathrm{Ni}$) at ${E}_{\mathrm{lab}}$ = 40.5 MeV, respectively. The $^{57}\mathrm{Co}(n,xp$) cross sections in the equivalent neutron energy range of 8.6--18.8 MeV have been determined within the framework of surrogate reaction ratio method using $^{60}\mathrm{Ni}(n,xp$) cross section values from the literature as reference. The proton decay probabilities of the compound systems have been determined by measuring evaporated protons at backward angles in coincidence with projectile-like fragments detected around the grazing angle. The measured $^{57}\mathrm{Co}(n,xp$) cross sections are in good agreement with both the predictions of talys-1.8 statistical model code with default parameters using different microscopic level densities and data evaluation library jeff-3.3 up to equivalent neutron energy $\ensuremath{\approx}12.6$ MeV, while for higher energies the measured $^{57}\mathrm{Co}(n,xp$) cross sections are found to be consistently higher than the predictions. However, the talys-1.8 calculations with modified values of input potential parameters provide a reasonable reproduction of the measured $^{57}\mathrm{Co}(n,xp$) cross sections for the entire neutron energy range. The observed discrepancies at higher energies between the experimental data and the predictions of both the jeff-3.3 library and the talys-1.8 calculations with default parameters indicate the need of new evaluations for this reaction.