122677490
On current ambiguity in the interpretation of fission at intermediate excitation energy
Abstract Various approaches are currently used to interpret experimental data on fission. We critically examine a wide set of observables measured for fission of Po 206 , 210 nuclei at medium excitation energy, and illustrate the ambiguity in current analysis. Dynamical calculations based on the four-dimensional Langevin equation using a macroscopic potential energy landscape are performed, and found to consistently describe available measurements. This observation calls into question the robustness of recent analysis based on statistical-model calculations and concluding, on the contrary, to substantial shell effects at the fission saddle point in Po 206 , 210 . The inconsistency in interpretation reached by the two approaches shows that, depending on the system, the conclusion can be strongly model-dependent. Although this may not be surprising, it emphasizes the today still limited reliability of firmly extracting fundamental nuclear properties from customary analysis.
55265914
Transfer reactions in inverse kinematics: An experimental approach for fission investigations
Inelastic and multinucleon transfer reactions between a U-238 beam, accelerated at 6.14 MeV/u, and a C-12 target were used for the production of neutron-rich, fissioning systems from U to Cm. A Si telescope, devoted to the detection of the targetlike nuclei, provided a characterization of the fissioning systems in atomic and mass numbers, as well as in excitation energy. Cross sections and angular and excitation-energy distributions were measured for the inelastic and transfer channels. Possible excitations of the targetlike nuclei were experimentally investigated for the first time, by means of gamma-ray measurements. The decays from the first excited states of 12C, B-11, and Be-10 were observed with probabilities of 0.12-0.14, while no evidence for the population of higher-lying states was found. Moreover, the fission probabilities of U-238, Np-239 and Pu-240,Pu-241,Pu-242 and Cm-244 were determined as a function of the excitation energy.
119392837
Fission-Fragment Mass Distribution and Particle Evaporation at low Energies
Fusion-fission dynamics is investigated with a special emphasis on fusion reactions at low energy for which shell effects and pairing correlations can play a crucial role leading in particular to multi-modal fission. To follow the dynamical evolution of an excited and rotating nucleus we solve a 2-dimensional Langevin equation taking explicitly light-particle evaporation into account. The confrontation theory-experiment is demonstrated to give interesting information on the model presented, its qualities as well as its shortcomings.
116938056
Observation of the lowest energy gamma-ray in any superdeformed nucleus : 196Bi
New results on the superdeformed $^{196}$Bi nucleus a re reported. We have observed with the EUROBALL IV $\gamma$-ray spectrometer array a superdeformed trans ition of 124 keV which is the lowest observed energy $\gamma$-ray in any superdeformed nucleus. We have de velopped microscopic cranked Hartree-Fock-Bogoliubov calculations using the SLy4 effective force and a realistic surface p airing which strongly support the $K^\pi=2^-$($\pi$[651]1/2$\otimes \nu$[752]5/2) assignment of this su perdeformed band.
5034444
Study of the multi-nucleon transfer reactions of 136Xe + 198Pt for producing exotic heavy nuclei
The feasibility of the multi-nucleon transfer (MNT) reaction in the 136 Xe + 198 Pt system was investigated for synthesizing neutron-rich heavy nuclei around mass number A 200 of astrophysical interest. Identification of the projectile-like fragments (PLF's) was successfully performed by using VAMOS++ spectrometer. The isotopic distributions of the PLF's produced by two-proton stripping and pickup reactions were compared with each other, demonstrating the properties of the MNT reactions observed in the present system. The two-proton pickup channels strongly populated than the two- proton stripping channels were observed. The isotopic distributions for di erent total kinetic energy loss (TKEL) ranges are also discussed. The overall enhancement of the strength of the two-proton pickup compared with the two-proton stripping appears to be related to the MNT reactions with relatively moderate energy dissipation.
15709360
New signatures on dissipation from fission induced by relativistic heavy-ion collisions
Fissile nuclei with small shape distortion relative to the ground-state deformation and with low angular momentum were produced in peripheral heavy-ion collisions. Under the conditions of small shape distortions and low angular momentum, the theoretical description of the fission process can be considerably simplified, and the relevant information on dissipation can be better extracted than in conventional experiments based on fusion-fission reactions. In addition, this experimental approach induces very high excitation energies, a condition necessary to observe transient effects. The experimental data were taken at GSI using a set-up especially conceived for fission studies in inverse kinematics. This set-up allowed determining three observables whose sensitivity to dissipation was investigated for the first time: the total fission cross sections of 238U at 1 A GeV as a function of the target mass, and, for the reaction of 238U at 1 A GeV on a (CH2)n target, the partial fission cross sections and the partial charge distributions of the fission fragments. The comparison of the new experimental data with a reaction code adapted to the conditions of the reactions investigated leads to clear conclusions on the strength of dissipation at small deformation where the existing results are rather contradictory.
118470196
Production of medium-mass neutron-rich nuclei in reactions induced by Xe 136 projectiles at 1 A GeV on a beryllium target
Production cross sections of medium-mass neutron-rich nuclei obtained in the fragmentation of {sup 136}Xe projectiles at 1 A GeV have been measured with the FRagment Separator (FRS) at GSI. The measured cross sections are compared to {sup 238}U fission yields and model calculations to determine the optimum reaction mechanism to extend the limits of the chart of the nuclides around the r-process waiting point at N=82.
118399911
High Rate Proton Irradiation of 15mm Muon Drifttubes
Future LHC luminosity upgrades will significantly increase the amount of background hits from photons, neutrons and protons in the detectors of the ATLAS muon spectrometer. At the proposed LHC peak luminosity of 5*10^34 1/cm^2s, background hit rates of more than 10 kHz/cm^2 are expected in the innermost forward region, leading to a loss of performance of the current tracking chambers. Based on the ATLAS Monitored Drift Tube chambers, a new high rate capable drift tube detecor using tubes with a reduced diameter of 15mm was developed. To test the response to highly ionizing particles, a prototype chamber of 46 15mm drift tubes was irradiated with a 20 MeV proton beam at the tandem accelerator at the Maier-Leibnitz Laboratory, Munich. Three tubes in a planar layer were irradiated while all other tubes were used for reconstruction of cosmic muon tracks through irradiated and non-irradiated parts of the chamber. To determine the rate capability of the 15mm drift-tubes we investigated the effect of the proton hit rate on pulse height, efficiency and spatial resolution of the cosmic muon signals.
206538240
A Shorter 146Sm Half-Life Measured and Implications for 146Sm-142Nd Chronology in the Solar System
A New Lease on Half-Life Radiometric dating relies on measuring the abundance of long-lived radionuclides relative to the abundance of their radiogenic decay products—a process determined by the original radionuclide's half-life. For primordial radionuclides that decay slowly, such as 146Sm decaying to 142Nd, this method provides the timing of some of the earliest processes in solar system history. Using accelerator mass spectrometry, Kinoshita et al. (p. 1614) provide a revised estimate for the 146Sm half-life of ∼68.7 million years, which is 30% shorter than the previously accepted value. This shorter half-life suggests that reductions need to be made in the estimated ages for differentiation of Earth's mantle and the solidification of the Moon's magma ocean and for other more recent processes. Mantle differentiation of Earth, the Moon, and Mars occurred earlier and over a shorter time scale than previously estimated. The extinct p-process nuclide 146Sm serves as an astrophysical and geochemical chronometer through measurements of isotopic anomalies of its α-decay daughter 142Nd. Based on analyses of 146Sm/147Sm α-activity and atom ratios, we determined the half-life of 146Sm to be 68 ± 7 (1σ) million years, which is shorter than the currently used value of 103 ± 5 million years. This half-life value implies a higher initial 146Sm abundance in the early solar system, (146Sm/144Sm)0 = 0.0094 ± 0.0005 (2σ), than previously estimated. Terrestrial, lunar, and martian planetary silicate mantle differentiation events dated with 146Sm-142Nd converge to a shorter time span and in general to earlier times, due to the combined effect of the new 146Sm half-life and (146Sm/144Sm)0 values.
119287201
High-precision measurement of total fission cross sections in spallation reactions of 208 Pb and 238 U
Total cross sections for proton- and deuteron-induced-fission of 208Pb and 238U have been determined in the energy range between 500 MeV and 1 GeV. The experiment has been performed in inverse kinematics at GSI Darmstadt, facilitating the counting of the projectiles and the identification of the reaction products. High precision between 5 and 7 percent has been achieved by individually counting the beam particles and by registering both fission fragments in coincidence with high efficiency and full Z resolution. Fission was clearly distinguished from other reaction channels. The results were found to deviate by up to 30 percent from Prokofiev's systematics on total fission cross sections. There is good agreement with an elaborate experiment performed in direct kinematics.
118688563
Isotopic fission fragment distributions as a deep probe to fusion-fission dynamics
During the fission process, the atomic nucleus deforms and elongates up to the two fragments inception and their final separation at the scission deformation. The evolution of the nucleus energy with deformation defines a potential energy landscape in the multidimensional deformation space. It is determined by the macroscopic properties of the nucleus, and is also strongly influenced by the single-particle structure of the nucleus, which modifies the macroscopic energy minima. The fission fragment distribution is a direct consequence of the deformation path the nucleus has encountered, and therefore is the most genuine experimental observation of the potential energy landscape of the deforming nucleus. Very asymmetric fusion-fission reactions at energy close to the Coulomb barrier, produce well-defined conditions of the compound nucleus formation, where processes such as quasi-fission, pre-equilibrium emission and incomplete fusion are negligible. In the same time, the excitation energy is sufficient to reduce significantly structural effects, and mostly the macroscopic part of the potential is responsible for the formation of the fission fragments. We use inverse kinematics combined with a spectrometer to select and identify the fission fragments produced in 238U+12C at a bombarding energy close to and well-above the Coulomb barrier. For the first time, the isotopic yields are measured over the complete atomic-number distribution, between Z=30 and Z=63. In the experimental set-up, it is also possible to identify transfer-induced reactions, which lead to low-energy fission