Background: The nuclear level density (NLD) and photon strength functions (PSFs) are necessary quantities for calculating the interaction of photons with nuclei, in particular the reaction cross sections. As such, they are important especially in nuclear astrophysics and in the development of advanced nuclear technologies.

Purpose: The presence of a resonancelike structure in the $E1$ PSF at $\gamma$-ray energy of about 5.5 MeV was reported in $\gamma$-soft $A\approx 200$ nuclei from several experimental techniques. However, as data from different experiments are not fully consistent, additional information on PSFs in this region is of great interest. In addition, present PSF models have difficulties to describe resonancelike structures for energies below the neutron separation energy $S_n$. There are also open questions about the energy and parity dependence of the NLD.

Methods: The $\gamma$ rays following the radiative neutron capture on ${}^{195}\mathrm{Pt}s$-wave resonances were measured with the highly segmented $\gamma$-ray calorimeter Detector for Advanced Neutron Capture Experiments at the Los Alamos Neutron Science Center. The $\gamma$-ray energy spectra for different multiplicities were gathered for several resonances of both possible spins.

Results: The $\gamma$-ray energy spectra were analyzed within the statistical model and allowed us to get information about the NLD and PSFs in ${}^{196}\mathrm{Pt}$. Neither the PSFs from any previous experiment nor any available PSFs models are able to describe our spectra. We were able to find PSFs and NLD that reasonably describe experimental spectra and impose various restrictions on these quantities.

Conclusions: The presence of a resonancelike structure in the $E1$ PSF at a $\gamma$-ray energy of about 5.6 MeV is confirmed. The constant temperature energy dependence is favored for a NLD with a significant parity dependence up to an excitation energy of at least 4 MeV. The preferred $M1$ PSF shape is close to a Lorentzian tail of the spin-flip resonance.

• Received 15 November 2019
• Accepted 21 January 2020

DOI:https://doi.org/10.1103/PhysRevC.101.024302