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First evidence of correlation between vibrational resonances and an anisotropy in the fission mass distribution
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Tillämpad kärnfysik.
IRMM - JRC - EC.
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Tillämpad kärnfysik.
IRMM - JRC - EC.
(Engelska)Manuskript (preprint) (Övrigt vetenskapligt)
Abstract [en]

In this work we present evidence of anisotropic emission of fragment masses in 234U(n,f). The discovered mass anisotropy is correlated with the prominent vibrational resonances at En = 0.5 and 0.77 MeV and coincides with a verified strong angular anisotropy. From the outcome of this experimental work one may infer unequal fission barrier heights for different degrees of fission asymmetry.

Nyckelord [en]
U-234, Fission, Neutron, Resonance, Anisotropy
Nationell ämneskategori
Subatomär fysik
Forskningsämne
Fysik med inriktning mot tillämpad kärnfysik
Identifikatorer
URN: urn:nbn:se:uu:diva-185307OAI: oai:DiVA.org:uu-185307DiVA, id: diva2:572815
Tillgänglig från: 2012-11-28 Skapad: 2012-11-21 Senast uppdaterad: 2013-02-11
Ingår i avhandling
1. Measurements of the 234U(n,f) Reaction with a Frisch-Grid Ionization Chamber up to En=5 MeV
Öppna denna publikation i ny flik eller fönster >>Measurements of the 234U(n,f) Reaction with a Frisch-Grid Ionization Chamber up to En=5 MeV
2013 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Uppsala: Acta Universitatis Upsaliensis, 2013. s. 109
Serie
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1002
Nyckelord
Fission, U-234, Neutron, Uranium, Resonance, Ionization Chamber, Frisch-grid
Nationell ämneskategori
Subatomär fysik
Forskningsämne
Fysik med inriktning mot tillämpad kärnfysik
Identifikatorer
urn:nbn:se:uu:diva-185306 (URN)978-91-554-8554-2 (ISBN)
Disputation
2013-01-18, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (Engelska)
Opponent
Handledare
Tillgänglig från: 2012-12-27 Skapad: 2012-11-21 Senast uppdaterad: 2013-04-02Bibliografiskt granskad

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