Measurement of the Average Energy and Multiplicity of Prompt-Fission-Neutrons from 238U(n,f) and 237 Np(n,f) from 1 to 200 MeV.

Taking advantage of the neutron source of the LANCSE, it has been possible to obtain a measure of the velocity distribution and the number of prompt-neutrons emitted in the neutron-induced fission of {sup 238}U and {sup 237}Np over a broad incident neutron energy range. The mean kinetic energy was extracted and is shown as the function of the incident-neutron energy. We confirm here the observation, for both reactions, of a dip around the second chance fission which is explained by the lower kinetic energy of the pre-fission neutrons. Such a observation is reproduced by Los Alamos model as implemented at Bruyeres le Chatel and by the Maslov model. As far as the neutron multiplicity is concerned, a similar dip is observed. However, such a behavior is not present in data measured by other groups

First Measurements with a Lead Slowing-Down Spectrometer at LANSCE

Abstract. The characteristics of a Lead Slowing-Down Spectrometer (LSDS) installed at the Los Alamos Neutron Science Center (LANSCE) are presented in this paper. This instrument is designed to study neutron-induced fission on ultra small quantities of actinides, on the order of tens of nanograms or less. The measurements of the energy-time relation, energy resolution and neutron flux are compared to simulations performed with MCNPX. Results on neutroninduced fission of 235 U and 239 Pu with tens of micrograms and tens of nanograms, respectively, are presented. Finally, a digital filter designed to improve the detection of fission events at short time after the proton pulses is described

Characteristics of a Lead Slowing-Down Spectrometer Coupled to the LANSCE Accelerator

Abstract A description is given of a lead slowing-down spectrometer (LSDS) installed at the 800-MeV proton accelerator of the Los Alamos Neutron Science Center (LANSCE). The LSDS is designed to study neutron-induced fission on actinides that can only be obtained or used in small quantities. The characteristics of this LSDS (energy-time relation, energy resolution, neutron flux) are presented through simulations with MCNPX and measurements with several different methods. Results on neutron-induced fission of 235 U and 239 Pu with tens of micrograms and tens of nanograms, respectively, are presented. Finally, additional MCNPX calculations have been performed to simulate the measurement of the cross-section for 235m Uðn; fÞ using different target quantities and different initial isomer-to-ground state compositions

First Measurements with a Lead Slowing-Down Spectrometer at LANSCE

Abstract. The characteristics of a Lead Slowing-Down Spectrometer (LSDS) installed at the Los Alamos Neutron Science Center (LANSCE) are presented in this paper. This instrument is designed to study neutron-induced fission on ultra small quantities of actinides, on the order of tens of nanograms or less. The measurements of the energy-time relation, energy resolution and neutron flux are compared to simulations performed with MCNPX. Results on neutroninduced fission of 235 U and 239 Pu with tens of micrograms and tens of nanograms, respectively, are presented. Finally, a digital filter designed to improve the detection of fission events at short time after the proton pulses is described

First Measurements with a Lead Slowing-Down Spectrometer at LANSCE

Abstract. The characteristics of a Lead Slowing-Down Spectrometer (LSDS) installed at the Los Alamos Neutron Science Center (LANSCE) are presented in this paper. This instrument is designed to study neutron-induced fission on ultra small quantities of actinides, on the order of tens of nanograms or less. The measurements of the energy-time relation, energy resolution and neutron flux are compared to simulations performed with MCNPX. Results on neutroninduced fission of 235 U and 239 Pu with tens of micrograms and tens of nanograms, respectively, are presented. Finally, a digital filter designed to improve the detection of fission events at short time after the proton pulses is described

Etude de l'évolution de la réaction 40Ar + nat Ag de 7 à 34 MeV/nucléon à l'aide d'un multidétecteur 4pi de particules chargées

An experiment using the Amphora 4 pi multidetector of light charged particles has allowed to measure the evolution of some properties of the hot nuclei formed during the 40Ar+Ag reactions at 7, 17, 27 and 34 MeV/nucleon. For the central collisions, the results support the classical image of a hot, heavy (mass 100 to 130 um a) with high spin and emitting a long chain of particles and fragments. The increasing production of out-of-plane fragments suggests a progressive disappearance of the symmetric fission process in favor of a palette of breakup modes of the nucleus composed of several fragments of unequal masses. The study of fragment pair correlations seems to indicate an early emission of lithium nuclei, but on average later than the proton and alpha emission. This implies that the thermal equilibrium of the compound nucleus is so fast that the limits of the instantaneous multifragmentation hypothesis become unclear.Une expérience utilisant le multidétecteur 4 pi de particules légères chargées Amphora a permis de mesurer l'évolution de certaines propriétés des noyaux chauds formés lors des réactions 40Ar+Ag à 7, 17, 27 et 34 MeV/nucléon. Pour les collisions centrales, les résultats supportent l'image classique d'une source chaude, lourde (masse de 100 à 130 uma) à haut spin et émettant une longue chaine de particules et de fragments. La production croissante de fragment hors plan suggère une disparition progressive du processus de fission symétrique au profit d'une palette de modes de cassure du noyau composé en plusieurs fragments de masses inégales. L'étude des corrélations de paires de fragments semble indiquer une émission prompte des noyaux de lithium, mais en moyenne postérieure à l'émission des protons et des alpha. Ceci implique que l'équilibre thermique du noyau composé est si rapide que les limites de l'hypothèse de multifragmentation instantanée deviennent alors peu claires

Etude de l'evolution de la reaction 40^{40}Ar + nat^{nat}Ag de 7 a 34 MeV/nucleon a l'aide d'un multidetecteur 4π\pi de particules chargees

An experiment using the Amphora 4 pi multidetector of light charged particles has allowed to measure the evolution of some properties of the hot nuclei formed during the 40Ar+Ag reactions at 7, 17, 27 and 34 MeV/nucleon. For the central collisions, the results support the classical image of a hot, heavy (mass 100 to 130 um a) with high spin and emitting a long chain of particles and fragments. The increasing production of out-of-plane fragments suggests a progressive disappearance of the symmetric fission process in favor of a palette of breakup modes of the nucleus composed of several fragments of unequal masses. The study of fragment pair correlations seems to indicate an early emission of lithium nuclei, but on average later than the proton and alpha emission. This implies that the thermal equilibrium of the compound nucleus is so fast that the limits of the instantaneous multifragmentation hypothesis become unclear.Une expérience utilisant le multidétecteur 4 pi de particules légères chargées Amphora a permis de mesurer l'évolution de certaines propriétés des noyaux chauds formés lors des réactions 40Ar+Ag à 7, 17, 27 et 34 MeV/nucléon. Pour les collisions centrales, les résultats supportent l'image classique d'une source chaude, lourde (masse de 100 à 130 uma) à haut spin et émettant une longue chaine de particules et de fragments. La production croissante de fragment hors plan suggère une disparition progressive du processus de fission symétrique au profit d'une palette de modes de cassure du noyau composé en plusieurs fragments de masses inégales. L'étude des corrélations de paires de fragments semble indiquer une émission prompte des noyaux de lithium, mais en moyenne postérieure à l'émission des protons et des alpha. Ceci implique que l'équilibre thermique du noyau composé est si rapide que les limites de l'hypothèse de multifragmentation instantanée deviennent alors peu claires

Mo fragment yields in neutron-induced reactions on Uranium 238

International audienceThe GEANIE spectrometer coupled to the pulsed neutron beam of the Los Alamos Neutron Science Center's WNR facility was used to determine gamma-ray cross sections in (n,gamma-f) fast-neutron-induced reactions on Uranium 238

Prompt fission product yields in the 238U(n,f)^{238}\mathrm{U}(n,f) reaction

International audienceBackground: Significant yield discrepancies (500%–600%) were reported recently between experimental results and predictions (from the gef model) and evaluations (from the JEFF-3.1.1 and ENDF/B-VII.1 libraries) for Mo and Sn fission-fragment yields in fast-neutron-induced reactions on U238 using γ-γ-γ coincidence spectroscopy. The model and evaluations also predict Mo and Sn fragments that are on average ∼1 to 2 neutrons richer than the experimental results. Purpose: γ-γ-γ coincidence spectroscopy favors detection of higher-multiplicity γ-ray cascades. An alternative approach is determining the fragment yields using single-γ-ray spectroscopy, as it was attempted here for selected cases where it was feasible. Advantages and drawbacks in both approaches need to be understood and potential systematic errors in the experimental results should be addressed using theoretical models. Methods: Fast neutrons from the LANSCE WNR facility were used to induce fission on U238. The emitted γ rays were measured with the GEANIE spectrometer. Results: The yield of selected even-even fission fragments was determined. The selection was based on the ability to reliably determine excitation functions for the detected γ rays. Conclusions: Our single-γ-ray results provide better agreement between experiment and predictions and evaluations