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

Measurement of prompt X-rays in

Prompt K X-ray emission yields in the fission induced by neutrons on 238U have been measured for the first time for incident energies ranging from below 1MeV up to 400MeV. Results are used to investigate the evolution with incident neutron energy of the fragment elemental distribution and the X-ray emission probability per element. The progressive increase of the symmetric fission probability with neutron energy is observed in qualitative agreement with Wahl systematics for the primary fission fragment charge yields

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

International audienceBackground: Significant yield discrepancies (500%–600%) were reported recently between experimentalresults 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 $^{238}$U using $\gamma$ -$\gamma$ -$\gamma$ coincidence spectroscopy. The model and evaluations also predict Mo and Sn fragments that are on average $\sim$1 to 2 neutrons richer than the experimental results.Purpose: $\gamma$ -$\gamma$ -$\gamma$ coincidence spectroscopy favors detection of higher-multiplicity γ -ray cascades. An alternative approach is determining the fragment yields using single-$\gamma$-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 $^{238}$U. The emitted γrays were measured with the GEANIE spectrometer.Results: The yield of selected even-even fission fragments was determined. The selection was based on theability to reliably determine excitation functions for the detected γ rays.Conclusions: Our single-γ -ray results provide better agreement between experiment and predictions andevaluations