The ν\nu-ball γ\gamma-spectrometer

International audienceThe ν -ballspectrometer is an hybrid array combining high purity co-axial germanium detectors from the french-UK loan pool, clover detectors from the GAMMAPOOL, lanthanum bromide (LaBr 3 :Ce) scintillator detectors belonging to the FATIMA collaboration and phoswitches from the PARIS collaboration. The aim was to couple the excellent energy resolution of germanium detectors to the excellent time resolution of the LaBr 3  detectors. We achieved a total photopeak efficiency of 6.7% at 1.3 MeV, and peak-to-total ratio of 50% for the germanium part of the array. Using the digital acquisition system FASTER, we achieved time resolution of about 250 ps for LaBr 3 . This acquisition system made also possible the use of the calorimetry for reaction selection. It makes ν -ball the first fully digital large fast timing spectrometer with time resolution similar to analogue electronics. The construction began in June 2017 and commissioning was performed in early November 2017. From November 2017 to June 2018, more than 3200 h of beam time were provided by the ALTO facility to perform eight experiments during the campaign. Among them, five weeks of beam time were dedicated to γ spectroscopy of fast neutron induced reactions. In this paper all the technical details about the spectrometer are presented. First steps of the data analysis process are also discussed

Prompt fission neutron spectra in the

Prompt fission neutron spectra (PFNS) are crucial to any neutronic simulation of critical nuclear systems. An experimental setup dedicated to the measurements of PFNS of very high accuracy was developed at the Los Alamos Neutron Science Center (LANSCE) some ten years ago. It allows for the measurement of PFNS for neutron induced fission at the Weapon Neutron Research (WNR) neutron source of the LANSCE. A measurement of the PFNS from the 235U(n,f) reaction was realized recently and is currently analyzed. Preliminary results are presented here and are compared to present nuclear data evaluations

Examination of how properties of a fissioning system impact isomeric yield ratios of the fragments

International audienceThe population of isomeric states in the prompt decay of fission fragments—so-called isomeric yield ratios (IYRs)—is known to be sensitive to the angular momentum J that the fragment emerged with, and may therefore contain valuable information on the mechanism behind the fission process. In this work, we investigate how changes in the fissioning system impact the measured IYRs of fission fragments to learn more about what parameters affect angular momentum generation. To enable this, a new technique for measuring IYRs is first demonstrated. It is based on the time of arrival of discrete γ rays, and has the advantage that it enables the study of the IYR as a function of properties of the partner nucleus. This technique is used to extract the IYR of Te134, strongly populated in actinide fission, from the three different fissioning systems: Th232(n,f), U238(n,f), at two different neutron energies, as well as Cf252(sf). The impacts of changing the fissioning system, the compound nuclear excitation energy, the minimum J of the binary partner, and the number of neutrons emitted on the IYR of Te134 are determined. The decay code talys is used in combination with the fission simulation code freya to calculate the primary fragment angular momentum from the IYR. We find that the IYR of Te134 has a slope of 0.004±0.002 with increase in compound nucleus (CN) mass. When investigating the impact on the IYR of increased CN excitation energy, we find no change with an energy increase similar to the difference between thermal and fast fission. By varying the mass of the partner fragment emerging with Te134, it is revealed that the IYR of Te134 is independent of the total amount of prompt neutrons emitted from the fragment pair. This indicates that neutrons carry minimal angular momentum away from the fission fragments. Comparisons with the freya+talys simulations reveal that the average angular momentum in Te134 following U238(n,f) is 6.0ℏ. This is not consistent with the value deduced from recent cgmf calculations. Finally, the IYR sensitivity to the angular momentum of the primary fragment is discussed. These results are not only important to help understanding the underlying mechanism in nuclear fission, but can also be used to constrain and benchmark fission models, and are relevant to the γ-ray heating problem of reactors

Examination of how properties of a fissioning system impact isomeric yield ratios of the fragments

International audienceThe population of isomeric states in the prompt decay of fission fragments—so-called isomeric yield ratios (IYRs)—is known to be sensitive to the angular momentum J that the fragment emerged with, and may therefore contain valuable information on the mechanism behind the fission process. In this work, we investigate how changes in the fissioning system impact the measured IYRs of fission fragments to learn more about what parameters affect angular momentum generation. To enable this, a new technique for measuring IYRs is first demonstrated. It is based on the time of arrival of discrete γ rays, and has the advantage that it enables the study of the IYR as a function of properties of the partner nucleus. This technique is used to extract the IYR of Te134, strongly populated in actinide fission, from the three different fissioning systems: Th232(n,f), U238(n,f), at two different neutron energies, as well as Cf252(sf). The impacts of changing the fissioning system, the compound nuclear excitation energy, the minimum J of the binary partner, and the number of neutrons emitted on the IYR of Te134 are determined. The decay code talys is used in combination with the fission simulation code freya to calculate the primary fragment angular momentum from the IYR. We find that the IYR of Te134 has a slope of 0.004±0.002 with increase in compound nucleus (CN) mass. When investigating the impact on the IYR of increased CN excitation energy, we find no change with an energy increase similar to the difference between thermal and fast fission. By varying the mass of the partner fragment emerging with Te134, it is revealed that the IYR of Te134 is independent of the total amount of prompt neutrons emitted from the fragment pair. This indicates that neutrons carry minimal angular momentum away from the fission fragments. Comparisons with the freya+talys simulations reveal that the average angular momentum in Te134 following U238(n,f) is 6.0ℏ. This is not consistent with the value deduced from recent cgmf calculations. Finally, the IYR sensitivity to the angular momentum of the primary fragment is discussed. These results are not only important to help understanding the underlying mechanism in nuclear fission, but can also be used to constrain and benchmark fission models, and are relevant to the γ-ray heating problem of reactors