The γ -ray strength function in the quasicontinuum has been measured for [231][232][233] 232,233 Pa, and 237-239 U using the Oslo method. All eight nuclei show a pronounced increase in γ strength at ω SR ≈ 2.4 MeV, which is interpreted as the low-energy M1 scissors resonance (SR). The total strength is found to be B SR = 9-11 μ 2 N when integrated over the 1-4 MeV γ -energy region. The SR displays a double-hump structure that is theoretically not understood. Our results are compared with data from (γ , γ ) experiments and theoretical sum-rule estimates for a nuclear rigid-body moment of inertia.

Particle-γ coincidences have been measured to obtain γ -ray spectra as a function of excitation energy for [231][232][233] Th and 237-239 U. The level densities, which were extracted using the Oslo method, show a constant temperature behavior. The isotopes display very similar temperatures in the quasicontinuum, however, the evenodd isotopes reveal a constant entropy increase S compared to their even-even neighbors. The entropy excess depends on available orbitals for the last unpaired valence neutron of the heated nuclear system. Also, experimental microcanonical temperature and heat capacity have been extracted. Several poles in the heat capacity curve support the idea that an almost continuous melting of Cooper pairs is responsible for the constant-temperature behavior.

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It is almost 80 years since Hans Bethe described the level density as a non-interacting gas of protons and neutrons. In all these years, experimental data were interpreted within this picture of a fermionic gas. However, the renewed interest of measuring level density using various techniques calls for a revision of this description. In particular, the wealth of nuclear level densities measured with the Oslo method favors the constant-temperature level density over the Fermi-gas picture. From the basis of experimental data, we demonstrate that nuclei exhibit a constant-temperature level density behavior for all mass regions and at least up to the neutron threshold.PACS. PACS-key 21.10. Ma, 25.20.Lj, 25.40.Hs

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