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.

We investigated the 238 U(d,p) reaction as a surrogate for the n + 238 U reaction. For this purpose we measured for the first time the gamma-decay and fission probabilities of 239 U* simultaneously and compared them to the corresponding neutron-induced data. We present the details of the procedure to infer the decay probabilities, as well as a thorough uncertainty analysis, including parameter correlations. Calculations based on the continuum-discretized coupledchannels method and the distorted-wave Born approximation (DWBA) were used to correct our data from detected protons originating from elastic and inelastic deuteron breakup. In the region where fission and gamma emission compete, the corrected fission probability is in agreement with neutron-induced data, whereas the gamma-decay probability is much higher than the neutroninduced data. We have performed calculations of the decay probabilities with the statistical model and of the average angular momentum populated in the 238 U(d,p) reaction with the DWBA to interpret these results.

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.

:We have measured the fission probabilities of 242

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

In the framework of the MICADO (Measurement and Instrumentation for Cleaning And Decommissioning Operations) European Union (EU) project, aimed at the full digitization of low- and intermediate-level radioactive waste management, a set of 32 solid state thermal neutron detectors named SiLiF has been built and characterized. MICADO encompasses a complete active and passive characterization of the radwaste drums with neutrons and gamma rays, followed by a longer-term monitoring phase. The SiLiF detectors are suitable for the monitoring of nuclear materials and can be used around radioactive waste drums possibly containing small quantities of actinides, as well as around spent fuel casks in interim storage or during transportation. Suitable polyethylene moderators can be exploited to better shape the detector response to the expected neutron spectrum, according to Monte Carlo simulations that were performed. These detectors were extensively tested with an AmBe neutron source, and the results show a quite uniform and reproducible behavior.

Since its start in 2001 the n_TOF collaboration developed a measurement program on fission, in view of advanced fuels in new generation reactors. A special effort was made on measurement of cross sections of actinides, exploiting the peculiarity of the n_TOF neutron beam which spans a huge energy domain, from the thermal region up to GeV. Moreover fission fragment angular distributions have also been measured. An overview of the cross section results achieved with different detectors is presented, including a discussion of the 237Np case where discrepancies showed up between different detector systems. The results on the anisotropy of the fission fragments and its implication on the mechanism of neutron absorption, and in applications, are also shown.

No abstract