Abstract
Background: The photon strength functions (PSFs) and nuclear level density (NLD) are key ingredients for calculation of the photon interaction with nuclei, in particular the reaction cross sections. These cross sections are important especially in nuclear astrophysics and in the development of advanced nuclear technologies. Purpose: The role of the scissors mode in the M1 PSF of (well-deformed) actinides was investigated by several experimental techniques. The analyses of different experiments result in significant differences, especially on the strength of the mode. The shape of the low-energy tail of the giant electric dipole resonance is uncertain as well. In particular, some works proposed a presence of the E1 pygmy resonance just above 7 MeV. Because of these inconsistencies additional information on PSFs in this region is of great interest. Methods: The γ-ray spectra from neutron-capture reactions on the U234, U236, and U238 nuclei have been measured with the total absorption calorimeter of the n_TOF facility at CERN. The background-corrected sum-energy and multi-step-cascade spectra were extracted for several isolated s-wave resonances up to about 140 eV. Results: The experimental spectra were compared to statistical model predictions coming from a large selection of models of photon strength functions and nuclear level density. No combination of PSF and NLD models from literature is able to globally describe our spectra. After extensive search we were able to find model combinations with modified generalized Lorentzian (MGLO) E1 PSF, which match the experimental spectra as well as the total radiative widths. Conclusions: The constant temperature energy dependence is favored for a NLD. The tail of giant electric dipole resonance is well described by the MGLO model of the E1 PSF with no hint of pygmy resonance. The M1 PSF must contain a very strong, relatively wide, and likely double-resonance scissors mode. The mode is responsible for about a half of the total radiative width of neutron resonances and significantly affects the radiative cross section.3 MoreReceived 24 August 2021Accepted 31 January 2022DOI:https://doi.org/10.1103/PhysRevC.105.024618Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic transitionsModels & methods for nuclear reactionsNeutron physicsNuclear reactionsRadiative captureResonance reactionsPropertiesA ≥ 220TechniquesNuclear data analysis & compilationNuclear Physics
Highlights
Nuclear level densities (NLDs) and photon strength functions (PSFs), called γ -ray or radiation strength functions, represent average properties of the nucleus in the regime of excitation where individual levels and transition probabilities by γ decay are not readily accessible by experimental or theoretical means
Neutron capture experiments on 234U, 236U, and 238U were performed with the total absorption calorimeter (TAC) [27,32] consisting of 40 BaF2 crystals, lined with carbon shells doped with 10B, in a 4π configuration surrounding the capture sample placed in the neutron beam
According to Ref. [58], the experimental γ have similar values for all three isotopes. It shows that the CT NLD model with fixed parameters in conjunction with fixed PSFs yields very similar values of γ for a broad range of initial excitation energies, which is a consequence of the NLD functional form. This indicates that the experimental similarity of γ for all three nuclei can be reached for the same PSFs only if the NLD is almost identical in all three nuclei
Summary
The photon strength functions (PSFs) and nuclear level density (NLD) are key ingredients for calculation of the photon interaction with nuclei, in particular the reaction cross sections. These cross sections are important especially in nuclear astrophysics and in the development of advanced nuclear technologies. Results: The experimental spectra were compared to statistical model predictions coming from a large selection of models of photon strength functions and nuclear level density. After extensive search we were able to find model combinations with modified generalized Lorentzian (MGLO) E 1 PSF, which match the experimental spectra as well as the total radiative widths. The mode is responsible for about a half of the total radiative width of neutron resonances and significantly affects the radiative cross section
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
