Fission Fragment Properties in the _1hn2_1hn3_1hn8U (n,f) Reaction at Incident Neutron Energies from 1 MeV to 500 MeV PDF Download
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Languages : en
Pages : 8
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Languages : en
Pages : 8
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Author: A. B. Smith
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Category : Fission products
Languages : en
Pages : 44
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The distribution of the kinetic energy of fragments emitted as a result of the neutron-induced fission of Th-232 has been determined. Incident neutron energies of 1475 plus or minus 35 kev and 1600 plus or minus 35 kev were used. The distributions determined at the two incident neutron energies are identical. The measured total average fragment kinetic energy was l55 plus or minus 4.5 Mev. The most probable fragment mass ratio is 1.47 plus or minus 0.05, and the average kinetic energies of the light and heavy fragments are 95 plus or minus 2 and 60 plus or minus 3 Mev, respectively. The experimental results were related to the known systematics of neutron-induced and spontaneous fission. The effect of collective nuclear rotations at the saddle point is discussed, with particular emphasis on fission from specific rotational bands.
Author: Esther Leal Cidoncha
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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.
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Languages : en
Pages : 38
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In this study, Fission Product Yields (FPY) have historically been one of the most observable features of the fission process. They are known to have strong variations that are dependent on the fissioning species, the excitation energy, and the angular momentum of the compound system. However, consistent and systematic studies of the variation of these FPY with energy have proved challenging. This is caused primarily by the nature of the experiments that have traditionally relied on radiochemical procedures to isolate specific fission products. Although radiochemical procedures exist that can isolate all products, each element presents specific challenges and introduces varying degrees of systematic errors that can make inter-comparison of FPY uncertain. Although of high importance in fields such as nuclear forensics and Stockpile Stewardship, accurate information about the energy dependence of neutron induced FPY are sparse, due primarily to the lack of suitable monoenergetic neutron sources. There is a clear need for improved data, and to address this issue, a collaboration was formed between Los Alamos National Laboratory (LANL), Lawrence Livermore National Laboratory (LLNL) and the Triangle Universities Nuclear Laboratory (TUNL) to measure the energy dependence of FPY for 235U, 238U and 239Pu. The measurements have been performed at TUNL, using a 10 MV Tandem Van de Graaff accelerator to produce monoenergetic neutrons at energies between 0.6 MeV to 14.8 MeV through a variety of reactions. The measurements have utilized a dual-fission chamber, with thin (10-100 [mu]g/cm2) reference foils of similar material to a thick (100-400 mg) activation target held in the center between the chambers. This method allows for the accurate determination of the number of fissions that occurred in the thick target without requiring knowledge of the fission cross section or neutron fluence on target. Following activation, the thick target was removed from the dual-fission chamber and gamma-ray counted using shielded HPGe detectors for a period of 1-2 months to determine the yield of various fission products. To the extent possible all irradiation and counting procedures were kept the same to minimize sources of systematic errors. FPY have been determined at incident neutron energies of 0.6, 1.4, 2.4, 3.5, 4.6, 5.5, 8.9 and 14.8 MeV.
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.
Author:
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Category :
Languages : en
Pages : 28
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This report is a description of fission and its application.
Author: H. Märten
Publisher: Nova Publishers
ISBN: 9781560720232
Category : Science
Languages : en
Pages : 420
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Contents: Fission Fragment Distributions: Experiment and Theory -- Fission Barriers, Fission Channels, Fission Valleys; Fragment Charge Distributions in Low Energy Fission; Double-Energy, Double-Velocity Measurement of Fission Fragments from Thermal Neutron Induced Fission; Odd-Even Neutron and Proton Effects in Low Energy Nuclear Fission; Energy Balance in MeV Neutron Induced Fission; Formation of the Fragment Mass and Energy Distributions in Fission of Nuclei Lighter than Radium; A New Approach to Determine Elemental Yield, Charge Polarisation and Odd-even Effects in Fission; Fundamental Fission Problems -- Dissipation and Friction in Nuclear Fission; Influence of Diabaticity on Fission Fragment Mass Asymmetry; Space Parity Violation in Nuclear Fission.
Author:
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ISBN:
Category : Nuclear fission
Languages : en
Pages : 496
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Author: Atomic Energy of Canada Limited
Publisher:
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Category : Nuclear energy
Languages : en
Pages : 428
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Author: Oleksiy Kosinov
Publisher:
ISBN:
Category :
Languages : en
Pages : 155
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