Nuclear level densities and $\gamma$-ray strength functions in $^{120,124}$Sn isotopes: impact of Porter-Thomas fluctuations

TL;DR

This study measures nuclear level densities and gamma-ray strength functions in $^{120,124}$Sn isotopes using the Oslo method, revealing their consistency with previous data and analyzing Porter-Thomas fluctuations' impact on the results.

Contribution

It provides new measurements of NLDs and GSFs in $^{120,124}$Sn, constrains their functional forms with the Shape method, and investigates Porter-Thomas fluctuations' effects on these nuclear properties.

Findings

NLDs agree with previous measurements in neighboring isotopes.

GSFs are consistent with the generalized Brink-Axel hypothesis.

Large fluctuations in GSFs contribute to uncertainties and deviations at low energies.

Abstract

Nuclear level densities (NLDs) and $\gamma$-ray strength functions (GSFs) of $^{120,124}$Sn have been extracted with the Oslo method from proton-$\gamma$ coincidences in the ($p,p^{\prime}\gamma)$ reaction. The functional forms of the GSFs and NLDs have been further constrained with the Shape method by studying primary $\gamma$-transitions to the ground and first excited states.The NLDs demonstrate good agreement with the NLDs of $^{116,118,122}$Sn isotopes measured previously. Moreover, the extracted partial NLD of 1$^{-}$ levels in $^{124}$Sn is shown to be in fair agreement with those deduced from spectra of relativistic Coulomb excitation in forward-angle inelastic proton scattering. The experimental NLDs have been applied to estimate the magnitude of the Porter-Thomas (PT) fluctuations. Within the PT fluctuations, we conclude that the GSFs for both isotopes can be considered to be independent of initial and final excitation energies, in accordance with the generalized Brink-Axel hypothesis. Particularly large fluctuations observed in the Shape-method GSFs present a considerable contribution to the uncertainty of the method, and may be one of the reasons for deviations from the Oslo-method strength at low $\gamma$-ray energies and low values of the NLD (below $\approx1\cdot10^{3}-2\cdot10^{3}$ MeV$^{-1}$).