Angular Distribution of Fragments in the Fission of Uranium 238 by Neutrons of Various Energies PDF Download
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Author: A. A. Varfolomeev
Publisher:
ISBN:
Category :
Languages : en
Pages : 4
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Author: A. A. Varfolomeev
Publisher:
ISBN:
Category :
Languages : en
Pages : 4
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Author: Richard W. Lamphere
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Category : Nuclear fission
Languages : en
Pages : 182
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Author: Joseph L. Fowler
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Category : Nuclear energy
Languages : en
Pages : 12
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Author: Esther Leal Cidoncha
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Category :
Languages : en
Pages : 0
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Neutron-induced fission cross sections of 238U and 235U are used as standards in the fast neutron region up to 200 MeV. A high accuracy of the standards is relevant to experimentally determine other neutron reaction cross sections. Therefore, the detection effciency should be corrected by using the angular distribution of the fission fragments (FFAD), which are barely known above 20 MeV. In addition, the angular distribution of the fragments produced in the fission of highly excited and deformed nuclei is an important observable to investigate the nuclear fission process. In order to measure the FFAD of neutron-induced reactions, a fission detection setup based on parallel-plate avalanche counters (PPACs) has been developed and successfully used at the CERN-n_TOF facility. In this work, we present the preliminary results on the analysis of new 235U(n,f) and 238U(n,f) data in the extended energy range up to 200 MeV compared to the existing experimental data.
Author: Keith Alan Griffioen
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Category : Cross sections (Nuclear physics)
Languages : en
Pages : 332
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Author: Kenneth Gen Wah Inn
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Category : Uranium
Languages : en
Pages : 68
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Author: John S. Wahl
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Category : Plutonium
Languages : en
Pages : 180
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Author:
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ISBN:
Category :
Languages : en
Pages : 8
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Prompt gamma-ray and x-ray spectroscopy techniques are being employed to study fission induced on 238U targets by neutrons of energy from 1 MeV to 150 MeV. Data are acquired using the GEANIE high resolution gamma-ray spectrometer at the LANSCE/WNR unmoderated spallation neutron source. Information on the pre-decay yields (excitation functions and isotopic distributions) were extracted from gamma and gamma-gamma coincidence data for a wide range of nucleides. Charge yields were extracted by measuring prompt x-rays from a thin, fission-sensitive target using photovoltaic cells specially designed. The experimental results are presented and compared to model calculations.
Author: O. V. Lozhkin
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Category :
Languages : en
Pages : 4
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Author: Lou Sai Leong
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Category :
Languages : en
Pages : 0
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The present knowledge of angular distributions of neutron-induced fission is limited to a maximal energy of 15 MeV, with large discrepancies around 14 MeV. Only 238U and 232Th have been investigated up to 100 MeV in a single experiment. The n_TOF Collaboration performed the fission cross section measurement of several actinides (232Th, 235U, 238U, 234U, 237Np) at the n_TOF facility using an experimental set-up made of Parallel Plate Avalanche Counters (PPAC), extending the energy domain of the incident neutron above hundreds of MeV. The method based on the detection of the 2 fragments in coincidence allowed to clearly disentangle the fission reactions among other types of reactions occurring in the spallation domain. I will show the methods we used to reconstruct the full angular resolution by the tracking of fission fragments. Below 10 MeV our results are consistent with existing data. For example in the case of 232Th, below 10 MeV the results show clearly the variation occurring at the first (1 MeV) and second (7 MeV) chance fission, corresponding to transition states of given J and K (total spin and its projection on the fission axis), and a much more accurate energy dependence at the 3rd chance threshold (14 MeV) has been obtained. In the spallation domain, above 30 MeV we confirm the high anisotropy revealed in 232Th by the single existing data set. I'll discuss the implications of this finding, related to the low anisotropy exhibited in proton-induced fission. I also explore the critical experiments which is valuable checks of nuclear data. The 237Np neutron-induced fission cross section has recently been measured in a large energy range (from eV to GeV) at the n TOF facility at CERN. When compared to previous measurements, the n TOF fission cross section appears to be higher by 5-7 % beyond the fission threshold. To check the relevance of n TOF data, we simulate a criticality experiment performed at Los Alamos with a 6 kg sphere of 237Np. This sphere was surrounded by enriched uranium 235U so as to approach criticality with fast neutrons. The simulation predicts a multiplication factor keff in better agreement with the experiment (the deviation of 750 pcm is reduced to 250 pcm) when we replace the ENDF/B- VII.0 evaluation of the 237Np fission cross section by the n TOF data. We also explore the hypothesis of deficiencies of the inelastic cross section in 235U which has been invoked by some authors to explain the deviation of 750 pcm. The large distortion that should be applied to the inelastic cross sections in order to reconcile the critical experiment with its simulation is incompatible with existing measurements. Also we show that the nubar of 237Np can hardly be incriminated because of the high accuracy of the existing data. Fission rate ratios or averaged fission cross sections measured in several fast neutron fields seem to give contradictory results on the validation of the 237Np cross section but at least one of the benchmark experiments, where the active deposits have been well calibrated for the number of atoms, favors the n TOF data set. These outcomes support the hypothesis of a higher fission cross section of 237Np.
