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Indication of anisotropic TKE and mass emission in 234U(n,f)
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
IRMM - JRC - EC.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
IRMM - JRC - EC.
2012 (English)In: Physics Procedia / [ed] Stephan Oberstedt, 2012, 158-164 p.Conference paper, Oral presentation only (Refereed)
Abstract [en]

The neutron-induced fission of 234U has been studied for neutron energies ranging from 200 keV to 5 MeV. Special focus was put around the prominent vibrational resonance in the sub-barrier region around 800 keV incident neutron energy. The aim was to investigate the fission fragment (FF) characteristics and search for fluctuations in energy and mass distributions. The strong angular anisotropy in the case of 234U(n,f) was verified and correlations with changes in energy and mass distributions were found. The TKE around the resonance increases contrary to earlier literature data. Furthermore, the TKE and mass distribution were found to be dependent on emission angle. At the resonance, the TKE was smallest near the 0° emission of the FF. This effect was consistent and coherent with a change in the mass distribution around the resonance. The mass distribution was observed to be less asymmetric near 0° emission. From a fitting analysis based on the Multi-Modal Random Neck-Rupture (MMRNR) model, we found the yield of the standard-1 mode increasing around the resonance. Because the TKE is increasing at larger angles and the mass distribution becomes more symmetric also at larger angles, we conclude that this behavior is due to an increase of the standard-1 mode at these larger angles. Based on the formalism of MMRNR, such difference in angular distribution may be an indication of a different outer barrier height for the standard-1 and standard-2 modes.

Place, publisher, year, edition, pages
2012. 158-164 p.
Series
Physics Procedia, ISSN 1875-3892 ; 31
Keyword [en]
Fission, U-234, Neutron
National Category
Subatomic Physics
Research subject
Physics with specialization in Applied Nuclear Physics
Identifiers
URN: urn:nbn:se:uu:diva-185303DOI: 10.1016/j.phpro.2012.04.021ISI: 000309656300020OAI: oai:DiVA.org:uu-185303DiVA: diva2:571224
Conference
GAMMA-1 Emission of Prompt Gamma-Rays in Fission and Related Topics, nov 22-21, 2011, Navi Sad, Serbia
Available from: 2012-11-21 Created: 2012-11-21 Last updated: 2013-02-11
In thesis
1. Measurements of the 234U(n,f) Reaction with a Frisch-Grid Ionization Chamber up to En=5 MeV
Open this publication in new window or tab >>Measurements of the 234U(n,f) Reaction with a Frisch-Grid Ionization Chamber up to En=5 MeV
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This study on the neutron-induced fission of 234U was carried out at the 7 MV Van de Graaff accelerator of IRMM in Belgium. A Twin Frisch-Grid Ionization Chamber (TFGIC) was used to study 234U(n,f) between En = 0.2 and 5.0 MeV. The reaction is important for fission modelling of the second-chance fission in 235U(n,f). The fission fragment (FF) angular-, energy and mass distributions were determined using the 2E-method highlighting especially the region of the vibrational resonance at En = 0.77 MeV.

The experiment used both conventional analogue and modern digital acquisition systems in parallel. Several advantages were found in the digital case, especially a successful pile-up correction. The shielding limitations of the Frisch-grid, called "grid-inefficiency", result in an angular-dependent energy signal. The correction of this effect has been a long-standing debate and a solution was recently proposed using the Ramo-Shockley theorem. Theoretical predictions from the latter were tested and verified in this work using two different grids. Also the neutron-emission corrections as a function of excitation energy were investigated. Neutron corrections are crucial for the determination of FF masses. Recent theoretical considerations attribute the enhancement of neutron emission to the heavier fragments exclusively, contrary to the average increase assumed earlier. Both methods were compared and the impact of the neutron multiplicities was assessed. The effects found are significant and highlight the importance of further experimental and theoretical investigation.

In this work, the strong angular anisotropy of 234U(n,f ) was confirmed. In addition, and quite surprisingly, the mass distribution was found to be angular-dependent and correlated to the vibrational resonances. The anisotropy found in the mass distribution was consistent with an anisotropy in the total kinetic energy (TKE), also correlated to the resonances. The experimental data were parametrized assuming fission modes based on the Multi-Modal Random Neck-Rupture model. The resonance showed an increased yield from the Standard-1 fission mode and a consistent increased TKE. The discovered correlation between the vibrational resonances and the angular-dependent mass distributions for the asymmetric fission modes may imply different outer fission-barrier heights for the two standard modes.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. 109 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1002
Keyword
Fission, U-234, Neutron, Uranium, Resonance, Ionization Chamber, Frisch-grid
National Category
Subatomic Physics
Research subject
Physics with specialization in Applied Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-185306 (URN)978-91-554-8554-2 (ISBN)
Public defence
2013-01-18, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2012-12-27 Created: 2012-11-21 Last updated: 2013-04-02Bibliographically approved

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Publisher's full texthttp://www.sciencedirect.com/science/article/pii/S1875389212012072#

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Al-Adili, AliPomp, Stephan

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