Experimental Evidence for Common Driving Effects in Low-Energy Fission from Sublead to Actinides.

Isotopic distributions of fragments from fission of the neutron-deficient ^{178}Hg nuclide are reported. This experimental observable is obtained for the first time in the region around lead using an innovative approach based on inverse kinematics and the coincidence between the large acceptance magnetic spectrometer VAMOS++ and a new detection arm close to the target. The average fragment N/Z ratio and prompt neutron M_{n} multiplicity are derived and compared with current knowledge from actinide fission. A striking consistency emerges, revealing the unexpected dominant role of the proton subsystem with atomic number between the Z=28 and 50 magic numbers. The origin of nuclear charge polarization in fission and fragment deformation at scission are discussed.

Simultaneous Determination of Neutron-Induced Fission and Radiative Capture Cross Sections from Decay Probabilities Obtained with a Surrogate Reaction.

Reliable neutron-induced-reaction cross sections of unstable nuclei are essential for nuclear astrophysics and applications but their direct measurement is often impossible. The surrogate-reaction method is one of the most promising alternatives to access these cross sections. In this work, we successfully applied the surrogate-reaction method to infer for the first time both the neutron-induced fission and radiative capture cross sections of ^{239}Pu in a consistent manner from a single measurement. This was achieved by combining simultaneously measured fission and γ-emission probabilities for the ^{240}Pu(^{4}He,^{4}He^{'}) surrogate reaction with a calculation of the angular-momentum and parity distributions populated in this reaction. While other experiments measure the probabilities for some selected γ-ray transitions, we measure the γ-emission probability. This enlarges the applicability of the surrogate-reaction method.

First Direct Measurement of Isotopic Fission-Fragment Yields of

A direct and complete measurement of isotopic fission-fragment yields of ^{239}U has been performed for the first time. The ^{239}U fissioning system was produced with an average excitation energy of 8.3 MeV in one-neutron transfer reactions between a ^{238}U beam and a ^{9}Be target at Coulomb barrier energies. The fission fragments were detected and isotopically identified using the VAMOS++ spectrometer at the GANIL facility. The measurement allows us to directly evaluate the fission models at excitation energies of fast neutrons, which are relevant for next-generation nuclear reactors. The present data, in agreement with model calculations, do not support the recently reported anomaly in the fission-fragment yields of ^{239}U, and they confirm the persistence of spherical shell effects in the Sn region at excitation energies exceeding the fission barrier by a few mega-electron volts.

Role of Multichance Fission in the Description of Fission-Fragment Mass Distributions at High Energies.

Fission-fragment mass distributions were measured for ^{237-240}U, ^{239-242}Np, and ^{241-244}Pu populated in the excitation-energy range from 10 to 60 MeV by multinucleon transfer channels in the reaction ^{18}O+^{238}U at the Japan Atomic Energy Agency tandem facility. Among them, the data for ^{240}U and ^{240,241,242}Np were observed for the first time. It was found that the mass distributions for all the studied nuclides maintain a double-humped shape up to the highest measured energy in contrast to expectations of predominantly symmetric fission due to the washing out of nuclear shell effects. From a comparison with the dynamical calculation based on the fluctuation-dissipation model, this behavior of the mass distributions was unambiguously attributed to the effect of multichance fission.

New type of asymmetric fission in proton-rich nuclei.

A very exotic process of ß-delayed fission of 180Tl is studied in detail by using resonant laser ionization with subsequent mass separation at ISOLDE (CERN). In contrast to common expectations, the fission-fragment mass distribution of the post-ß-decay daughter nucleus 180Hg (N/Z=1.25) is asymmetric. This asymmetry is more surprising since a mass-symmetric split of this extremely neutron-deficient nucleus would lead to two 90Zr fragments, with magic N=50 and semimagic Z=40. This is a new type of asymmetric fission, not caused by large shell effects related to fragment magic proton and neutron numbers, as observed in the actinide region. The newly measured branching ratio for ß-delayed fission of 180Tl is 3.6(7) × 10(-3)%, approximately 2 orders of magnitude larger than in an earlier study.