Thermal neutron capture gamma rays from sulfur isotopes: Experiment and theory

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Abstract

We have carried out a systematic investigation of $\ensuremath{\gamma}$ rays after thermal neutron capture by all stable sulfur isotopes ($^{32}\mathrm{S}$, $^{33}\mathrm{S}$, $^{34}\mathrm{S}$, and $^{36}\mathrm{S}$). The measurements were made at the internal target facility at the Los Alamos Omega West Reactor. We detected a larger number of $\ensuremath{\gamma}$ rays: \ensuremath{\sim}100 in $^{33}\mathrm{S}$, \ensuremath{\sim}270 in $^{34}\mathrm{S}$, \ensuremath{\sim}60 in $^{35}\mathrm{S}$, and \ensuremath{\sim}15 in $^{37}\mathrm{S}$. Before developing detailed level schemes, we culled and then consolidated the existing information on energies and ${J}^{\ensuremath{\pi}}$ values for levels of these nuclides. Based on the current data, we have constructed detailed decay schemes, which imply that there are significant populations of 26 excited states in $^{33}\mathrm{S}$, 70 states in $^{34}\mathrm{S}$, 20 states in $^{35}\mathrm{S}$, and 7 states in $^{37}\mathrm{S}$. By checking the intensity balance for these levels and by comparing the total intensity of primary transitions with the total intensity of secondary $\ensuremath{\gamma}$ rays feeding the ground state, we have demonstrated the relative completeness of these decay schemes. For strongly populated levels, the branching ratios based on the current measurements are generally better than those available from previous measurements. In all four cases, a few primary electric dipole ($E1$) transitions account for a large fraction of the capture cross section for that particular nuclide. To understand and explain these transitions, we have recapitulated and further developed the theory of potential capture. Toward this end, we reviewed the theory relating off-resonance neutron capture to the optical-model capture. We studied a range of model-dependent effects (nature and magnitude of imaginary potential, surface diffuseness, etc.) on the potential capture cross section, and we have shown how experimental data may be analyzed using the expression for channel capture suitably modified by a factor that takes into account the model-dependent effects. The calculations of cross sections for most of the primary transitions in the sulfur isotopes are in good agreement with the data. Some discrepancies for weaker transitions can be explained well by an interfering compound-nucleus contribution to capture. This contribution is of the magnitude expected from statistical surveys of resonance capture data. Estimates of the cross section due to the valencecapture mechanism in $s$-wave resonances show that this cross section should dominate the more complicated compound-nucleus contributions.

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