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

Fast-neutron-induced fission of 238U 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

Determination of the Neutron-Capture Rate of 17C for the R-process Nucleosynthesis

With the R3B-LAND setup at GSI we have measured exclusive relative-energy spectra of the Coulomb dissociation of 18C at a projectile energy around 425 AMeV on a lead target, which are needed to determine the radiative neutron-capture cross sections of 17C into the ground state of 18C. Those data have been used to constrain theoretical calculations for transitions populating excited states in 18C. This allowed to derive the astrophysical cross section n accounting for the thermal population of 17C target states in astrophysical scenarios. The experimentally veri ed capture rate is signi cantly lower than those of previously obtained Hauser-Feshbach estimations at temperatures T9 1 GK. Network simulations with updated neutron-capture rates and hydrodynamics according to the neutrino-driven wind model as well as the neutron-star merger scenario reveal no pronounced in uence of neutron capture of 17C on the production of second- and third-peak elements in contrast to earlier sensitivity studies

Breakup reaction studies of 10Be and 10,11B using a 10Be beam

The structure of 10Be has been investigated by inelastic scattering to states above the breakup threshold using the reaction 12C(10Be,10Be* → 6He+4/He)12C at Ebeam=302 MeV. Excited states in 10Be were observed at 9.6±0.1 and 10.2±0.1 MeV. No evidence was observed for the population of the 4 + member of the ground-state band of 10Be indicating the shell-model-like structure of the ground state. In addition, the decay of 8Be, 10B, and 11B, populated in the two-neutron, proton pickup, breakup and 1p pickup reactions, was reconstructed through the detection of coincident 4He+4He, 4He+6Li, and 4He+7Li particles. Cross sections for the formation of the 8Be, 9Be, 10B, and 11B were also deduced. Contrary to expectations, the two-neutron removal results in the production of 8Be predominantly (80%) in the first excited (2+) state. This suggests that dynamical excitations play an important role in the neutron removal process.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Breakup reaction studies of 10Be and 10,11B using a 10Be beam

The structure of 10Be has been investigated by inelastic scattering to states above the breakup threshold using the reaction 12C(10Be,10Be* → 6He+4/He)12C at Ebeam=302 MeV. Excited states in 10Be were observed at 9.6±0.1 and 10.2±0.1 MeV. No evidence was observed for the population of the 4 + member of the ground-state band of 10Be indicating the shell-model-like structure of the ground state. In addition, the decay of 8Be, 10B, and 11B, populated in the two-neutron, proton pickup, breakup and 1p pickup reactions, was reconstructed through the detection of coincident 4He+4He, 4He+6Li, and 4He+7Li particles. Cross sections for the formation of the 8Be, 9Be, 10B, and 11B were also deduced. Contrary to expectations, the two-neutron removal results in the production of 8Be predominantly (80%) in the first excited (2+) state. This suggests that dynamical excitations play an important role in the neutron removal process.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

High-energy two-neutron removal from Be10

A kinetically complete measurement of the C12(Be10, α+α+n) and (Be10, α+α) reactions has been performed at a beam energy of 30 MeV/nucleon. The charged beam velocity particles were detected in an array of Si-CsI detectors placed at zero degrees, and the neutrons in an 81-element neutron array. The coincident detection of the final-state particles, produced in the breakup of Be10, allowed the reconstruction of the excitation energy in the Be8 and Be9 systems. States in Be8 were identified, in particular the ground and first-excited states; and in Be9, states at 1.68, 2.43, and (2.78, 3.05) MeV were observed. The population of these levels, in particular the 2.43 MeV 5/2- level, suggests that collective excitations play an important role in the neutron removal process. Distorted wave Born approximation and Glauber-type calculations have been used to model the direct neutron removal from the Be10 ground state and the two-step removal via inelastic excitations of the Be10(2+) and Be9(5/2-) excited states. © 2005 The American Physical Society.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Neutron removal and cluster breakup of 14B and 14Be

Measurements of the neutron removal and cluster breakup cross sections for the neutron-rich nuclei 14Be and 14B have been performed at 34.4 and 40.8 MeV/nucleon, respectively. Enhancement of the first chance cluster breakup cross section for 14Be compared to that of 14B provides evidence for a well-developed He cluster structure of the ground state of 14Be. Measurements of both the cross sections and decay-particle velocities suggest that multistep processes play an important role in the excitation and decay of both 14B and 14Be.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Experimental evidence of a ν(1d5/2)2 component to the 12Be ground state

Data have been obtained on exclusive single neutron knockout cross sections from 12Be to study its ground state structure. Preliminary cross sections for the first (0.32 MeV, 1/2-) and second (1.78 MeV, 5/2+, unbound) excited states in 11Be have been obtained, giving evidence of significant admixtures of both ν(1p1/2) 2 and ν(1d5/2)2 configurations in the ground state of 12Be. © Società Italiana di Fisica / Springer-Verlag 2005.SCOPUS: cp.jinfo:eu-repo/semantics/publishe

Evidence of a (1d 5/2) 2 component to the 12Be ground state

Data have been obtained on exclusive single neutron knockout cross sections from 12Be to study its ground state structure. The cross sections for the production of 11Be in its ground state (1/2 +) and first excited state (0.32 MeV, 1/2 -) have previously been measured, indicating a strong (2s 1 2) 2 component to the 12Be ground state. In the present experiment, performed at the GANIL laboratory, cross sections for the first (0.32 MeV, 1/2 -) and second (1.78 MeV, 5/2 +, unbound) excited states in 11Be were measured, which gives information on the admixture of (1p 1 2) 2 and (1d 5 2) 2 components in the ground state of 12Be. A fragmentation beam of 12Be of ∼10000 pps (95% pure) was incident on a carbon target at 41 MeV/u. The beam particles were tracked onto the target, and their energies were measured event-by-event. The beam-like residues were measured in a position sensitive telescope mounted at zero degrees, and neutrons were measured in the DéMoN array. The 1/2 - state of 11Be was identified by measuring coincident 320 keV γ-rays, using four NaI detectors. Full kinematic reconstruction of unbound states in 11Be was performed using coincident neutrons and 10Be ions. Detailed simulations were performed in order to interpret the data, and spectroscopic factors were calculated, using preliminary single particle removal cross sections calculated using a Glauber model. © 2005 American Institute of Physics.SCOPUS: cp.pinfo:eu-repo/semantics/publishe