QRPA prediction of the nuclear level densities and de-excitation photon strength functions

Radiative capture cross section strongly depends on the electromagnetic de-excitation probability, hence on the so-called photon strength function (PSF) and nuclear level density (NLD). Microscopic models for both PSF and NLD have been developed for the past decades but remain affected by fundamental assumptions, like the independent particle approximation for NLD and the assumption that the de-excitation PSF is equal to the photoabsorption PSF. The present study goes beyond the existing models by providing a common framework based on QRPA estimates of nuclear excitations that are used to derive NLD by coupling QRPA bosons as well as the de-excitation PSF by calculating all possible transition probabilities between QRPA states. The present framework is found to reproduce experimental data related to both NLD and PSF accurately, provided QRPA excitation energies are globally reduced by an energy-dependent shift. More specifically, the resulting NLDs, also extended to odd- A and odd-odd nuclei, are shown to be in good agreement with experimental s-wave resonance spacings and the photoabsorption PSF with photodata. Interestingly, the estimated E1 de-excitation PSF is found to be close to the photoabsorption PSF for excitation energies below typically 10–12 MeV, validating the Brink hypothesis in this energy region, but to significantly differ above, especially for spherical nuclei. In contrast, the M1 de-excitation PSF deviates strongly from the photoabsorption one both at low photon energies where an enhancement, the so-called “upbend”, is clearly predicted and at higher photon energies where it lies far below the spin-flip component. For spherical nuclei, the de-excitation PSF increases the radiative neutron capture cross section appreciably, while a modest impact is obtained for deformed nuclei.