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Properties of the reaction 238U(n, f) at the vibrational resonances
Örebro University, School of Science and Technology. (Subatomic Physics)
Örebro University, School of Science and Technology. (Subatomic Physics)
2009 (English)In: Nuclear Physics A, ISSN 0375-9474, E-ISSN 1873-1554, Vol. 817, no 1-4, p. 1-34Article in journal (Refereed) Published
Abstract [en]

Recent fission cross-section calculations for the reaction 238U(n, f ), based on an extended statistical model, predict a significant change of fission fragment properties, such as the mean mass by A = 1.5 and a notable increase in total kinetic energy in the region of the vibrational resonance at an incident neutron energy En = 0.9 MeV. This model includes individual fission cross-sections by the asymmetric standard 1 (S1) and standard 2 (S2) as well as the symmetric super-long (SL) mode. In order to verify the model predictions, a dedicated experiment on 238U has been carried out to measure fission-fragment mass yield distributions for incident neutron energies from En = 2.0 MeV down to 0.9 MeV, where the fission characteristics at the vibrational resonance at En = 0.9 MeV were investigated for the first time. The previously reported distinct structure in the angular anisotropy around En = 1.2 and 1.6 MeV wasobserved at En = 0.9 MeV as well. The predicted large changes in fission fragment mass yield and total kinetic energy could not be confirmed. In the resonance the mean total kinetic energy is only about 0.5 MeV higher than at En = 1.8 MeV. At the same time, a slight decrease of the mean heavy fragment mass was observed, probably indicating a slightly increased contribution of the S1 fission mode.

Place, publisher, year, edition, pages
Amsterdam: Elsevier , 2009. Vol. 817, no 1-4, p. 1-34
Keywords [en]
Nuclear reactions 238U(n, f ), E = 0.9–2.0 MeV; measured/analyzed fission fragment mass yields and
National Category
Subatomic Physics Physical Sciences
Research subject
Physics
Identifiers
URN: urn:nbn:se:oru:diva-5485DOI: 10.1016/j.nuclphysa.2008.12.001ISI: 000263449200001Scopus ID: 2-s2.0-58149293406OAI: oai:DiVA.org:oru-5485DiVA, id: diva2:160128
Available from: 2009-02-12 Created: 2009-02-12 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Determination of binary fission-fragment yields in the reaction 251Cf(nth, f) and Verification of nuclear reaction theory predictions of fission-fragment distributions in the reaction 238U(n, f)
Open this publication in new window or tab >>Determination of binary fission-fragment yields in the reaction 251Cf(nth, f) and Verification of nuclear reaction theory predictions of fission-fragment distributions in the reaction 238U(n, f)
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Neutron-induced fission has been studied at different excitation energies of the compound nucleus by measurements on the two fissioning systems, 252Cf* and 239U*.

For the first time, the light fission fragment yields from the reaction 251Cf(nth, f) have been measured with high resolution. This experiment was performed with the recoil mass spectrometer LOHENGRIN at ILL in Grenoble, France. When the results from this work, where the compound nucleus is at thermal excitation, are compared to the spontaneous fission of 252Cf, enhanced emission yields as well as an increased mean kinetic energy is observed around A = 115. This suggests the existence of an additional super-deformed fission mode in 252Cf.

The reaction 238U(n, f) was studied using the 2E-technique with a double Frisch grid ionization chamber. Fission fragment mass, energy and angular distributions were determined for incident neutron energies between 0.9 and 2.0 MeV. The experiments were performed at the Van de Graaff accelerator of IRMM in Geel, Belgium. This is the first measurement of the mass distribution for incident neutron energies around 0.9 MeV. The motivation for studying 238U(n, f) was to verify theoretical predictions of the mass distribution at the vibrational resonance in the fission cross section at 0.9 MeV. However, the predicted changes in fission fragment distributions could not be confirmed.

A precise modelling of the fission process for the minor actinides becomes very important for future generation IV and accelerator driven nuclear reactors. Since fission fragment distributions depend on the excitation of the fissioning system, so does the number of delayed neutrons, which are one of the safety parameters in a reactor.

Place, publisher, year, edition, pages
Örebro: Örebro universitetsbibliotek, 2007. p. 124
Series
Örebro Studies in Physics, ISSN 1652-148X ; 2
Keywords
nuclear reactions, mass energy distributions, fission modes, fission-fragment spectroscopy, 251Cf(nth; f), 238U(n; f)
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:oru:diva-1474 (URN)978-91-7668-548-8 (ISBN)
Public defence
2007-09-24, Hörsal 1, Örebro universitet, Fakultetsgatan 1, Örebro, 10:00 (English)
Opponent
Supervisors
Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2017-10-18Bibliographically approved

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Birgersson, EvertOberstedt, Andreas

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