W. Younes

Microscopic calculation of 240Pu scission with a finite-range effective force

Hartree-Fock-Bogoliubov calculations of hot fission in

Event-by-event study of prompt neutrons from 239Pu(n,f)

^{240}mathrm{Pu}

Transition from asymmetric to symmetric fission in the 235U(n,f) reaction

have been performed with a newly implemented code that uses the D1S finite-range effective interaction. The hot-scission line is identified in the quadrupole-octupole-moment coordinate space. Fission-fragment shapes are extracted from the calculations. A benchmark calculation for

Cross‐Section Standards for Neutron‐Induced Gamma‐Ray Production in the MeV Energy Range

^{226}mathrm{Th}

Neutron-Induced Partial Gamma-Ray Cross-Section Measurements with GEANIE at LANSCE/WNR

is obtained and compared with results in the literature. In addition, technical aspects of the use of HFB calculations for fission studies are examined in detail. In particular, the identification of scission configurations, the sensitivity of near-scission calculations to the choice of collective coordinates in the HFB iterations, and the formalism for the adjustment of collective-variable constraints are discussed. The power of the constraint-adjustment algorithm is illustrated with calculations near the critical scission configurations with up to seven simultaneous constraints.

Nuclear Science at GEANIE

Employing a recently developed Monte-Carlo model, we study the fission of {sup 240}Pu induced by neutrons with energies from thermal to just below the threshold for second chance fission. Current measurements of the mean number of prompt neutrons emitted in fission, together with less accurate measurements of the neutron energy spectra, place remarkably fine constraints on predictions of microscopic calculations. In particular, the total excitation energy of the nascent fragments must be specified to within 1MeV to avoid disagreement with measurements of the mean neutron multiplicity. The combination of the Monte-Carlo fission model with a statistical likelihood analysis also presents a powerful tool for the evaluation of fission neutron data. Of particular importance is the fission spectrum, which plays a key role in determining reactor criticality. We show that our approach can be used to develop an estimate of the fission spectrum with uncertainties several times smaller than current experimental uncertainties for outgoing neutron energies of less than 2 MeV.

Study of near‐stability nuclei populated as fission fragments in heavy‐ion fusion reactions

Prompt γ rays from the neutron-induced fission of 235U have been studied using the GEANIE spectrometer situated at the LANSCE/WNR “white” neutron facility. Gamma-ray production cross sections for 29 ground-state-band transitions in 18 even-even fission fragments were obtained as a function of incident neutron energy, using the time-of-flight technique. Independent yields were deduced from these cross sections and fitted with standard formulations of the fragment charge and mass distributions to study the transition from asymmetric to symmetric fission. The results are interpreted in the context of the disappearance of shell structure at high excitation energies.

Effect of pre‐equilibrium spin distribution on neutron‐induced reaction cross sections

Gamma‐ray cross‐section standards for neutron‐induced reactions are important in enabling the accurate determination of absolute cross sections from relative measurements of gamma‐ray production. In our work we observed a need for improvement in these standards. In particular there are large discrepancies between evaluations of the natFe(n,n1′γ) cross section for the 847‐keV gamma ray. We have performed (1) absolute cross‐section measurements, (2) measurements relative to the natCr(n,n1′γ) 1434‐keV gamma ray, and (3) comparisons using measured total and elastic scattering cross sections to refine our knowledge of the Fe cross section and the closely linked inelastic channel cross section for Fe. Calculation of integral tests of the cross section libraries may indicate that adjustment of the angular distributions of the neutron elastic and inelastic scattering may be needed.

Gamma‐Ray Cross Section Standards in the MeV Energy Range and 56Fe Inelastic Scattering

GEANIE is the first large-scale Ge detector array used in conjunction with a high-energy neutron spallation source. GEANIE consists of eleven Compton-suppressed planar detectors, nine suppressed and six unsuppressed co-axial detectors. Spallation neutrons are provided by the LANSCE/WNR facility, and reaction neutron energies are determined via time-of-flight. neutron flux is monitored in-beam with a fission chamber. GEANIE at LANSCE/WNR currently emphasizes the measurement of partial gamma-ray cross sections as a function of neutron energy. Absolute cross section measurements require a complete understanding of array performance. Important effects include intrinsic detector efficiency, beam and detector geometry corrections, target attenuation, and deadtime. Measurements and calculations of these effects will be presented for the specific cases of iron and actinide targets. The use of radioactive targets incurs a large deadtime penalty. In order to increase data throughput they are making plans to move to a triggerless data acquisition system. These modifications and other improvements to the electronics for better timing will be discussed.

Cross Sections for γ‐Ray Production in the 191Ir(n,xnγ) Reactions

GEANIE at LANSCE/WNR combines the precision energy resolution of germanium detectors with the advantages of a white source providing neutrons with energies 1 < E{sub n}(MeV) < 250 to address a variety of topics in nuclear physics. The authors present the analysis of two data sets, n+{sup 235}U and n+{sup 92}Mo, acquired at GEANIE during the 1998 beam cycle. These data showcase the breadth of subjects under study at this facility, including the spectroscopy of stable and near-stable nuclei, reaction dynamics, fission studies, and the relative population of isomer and ground states in neutron-induced reactions.