S. J. Rose

Constant-temperature level densities in the quasicontinuum of Th and U isotopes

Particle-gamma coincidences have been measured to obtain gamma-ray spectra as a function of excitation energy for 231-233Th and 237-239U. The level densities, which were extracted using the Oslo method, show a constant temperature behavior. The isotopes display very similar temperatures in the quasi-continuum, however, the even-odd isotopes reveal a constant entropy increase Delta 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.

Scissors resonance in the quasicontinuum of Th, Pa, and U isotopes

The gamma-ray strength function in the quasi-continuum has been measured for 231-233Th, 232,233Pa and 237-239U using the Oslo method. All eight nuclei show a pronounced increase in gamma strength at omega_SR approx 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 mu_N^2 when integrated over the 1 - 4 MeV gamma-energy region. The SR displays a double-hump structure that is theoretically not understood. Our results are compared with data from (gamma, gamma) experiments and theoretical sum-rule estimates for a nuclear rigid-body moment of inertia.

Investigation of the U238(d,p) surrogate reaction via the simultaneous measurement of γ-decay and fission probabilities

We investigated the U238(d,p) reaction as a surrogate for the n+U238 reaction. For this purpose we measured for the first time the γ-decay and fission probabilities of U∗239 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 coupled-channels 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 γ emission compete, the corrected fission probability is in agreement with neutron-induced data, whereas the γ-decay probability is much higher than the neutron-induced data. We have performed calculations of the decay probabilities with the statistical model and of the average angular momentum populated in the U238(d,p) reaction with the DWBA to interpret these results.

Evidence for the dipole nature of the low-energy γ enhancement in 56Fe

The γ-ray strength function of 56Fe has been measured from proton-γ coincidences for excitation energies up to ≈11u2009u2009MeV. The low-energy enhancement in the γ-ray strength function, which was first discovered in the (3He,αγ)56Fe reaction, is confirmed with the (p,pγ)56Fe experiment reported here. Angular distributions of the γ rays give for the first time evidence that the enhancement is dominated by dipole transitions.

Anomalies in the Charge Yields of Fission Fragments from the U(n,f)238 Reaction

Fast-neutron-induced fission of ^{238}U at an energy just above the fission threshold is studied with a novel technique which involves the coupling of a high-efficiency γ-ray spectrometer (MINIBALL) to an inverse-kinematics neutron source (LICORNE) to extract charge yields of fission fragments via γ-γ coincidence spectroscopy. Experimental data and fission models are compared and found to be in reasonable agreement for many nuclei; however, significant discrepancies of up to 600% are observed, particularly for isotopes of Sn and Mo. This indicates that these models significantly overestimate the standard 1 fission mode and suggests that spherical shell effects in the nascent fission fragments are less important for low-energy fast-neutron-induced fission than for thermal neutron-induced fission. This has consequences for understanding and modeling the fission process, for experimental nuclear structure studies of the most neutron-rich nuclei, for future energy applications (e.g., Generation IV reactors which use fast-neutron spectra), and for the reactor antineutrino anomaly.

Shell-gap-reduced level densities in Y 89 , 90

Particle-

La137,138,139(n,γ) cross sections constrained with statistical decay properties of La138,139,140 nuclei

ensuremath{gamma}

Statistical properties of Pu243 , and Pu242(n,γ) cross section calculation

coincidences from the

Experimental level densities of atomic nuclei

^{89}mathrm{Y}(p,{p}^{ensuremath{}}ensuremath{gamma})^{89}mathrm{Y}

Low-energy enhancement and fluctuations of γ-ray strength functions in 56,57Fe: test of the Brink–Axel hypothesis

and