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Ambiguities in the grid-inefficiency correction for Frisch-Grid Ionization Chambers
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
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2012 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, Vol. 673, 116-121 p.Article in journal (Refereed) Published
Abstract [en]

Ionization chambers with Frisch grids have been very successfully applied to neutron-induced fission-fragment studies during the past 20 years. They are radiation resistant and can be easily adapted to the experimental conditions. The use of Frisch grids has the advantage to remove the angular dependency from the charge induced on the anode plate. However, due to the Grid Inefficiency (GI) in shielding the charges, the anode signal remains slightly angular dependent. The correction for the GI is, however, essential to determine the correct energy of the ionizing particles. GI corrections can amount to a few percent of the anode signal. Presently, two contradicting correction methods are considered in literature. The first method adding the angular-dependent part of the signal to the signal pulse height; the second method subtracting the former from the latter. Both additive and subtractive approaches were investigated in an experiment where a Twin Frisch-Grid Ionization Chamber (TFGIC) was employed to detect the spontaneous fission fragments (FF) emitted by a 252Cf source. Two parallel-wire grids with different wire spacing (1 and 2 mm, respectively), were used individually, in the same chamber side. All the other experimental conditions were unchanged. The 2 mm grid featured more than double the GI of the 1 mm grid. The induced charge on the anode in both measurements was compared, before and after GI correction. Before GI correction, the 2 mm grid resulted in a lower pulse-height distribution than the 1 mm grid. After applying both GI corrections to both measurements only the additive approach led to consistent grid independent pulse-height distributions. The application of the subtractive correction on the contrary led to inconsistent, grid-dependent results. It is also shown that the impact of either of the correction methods is small on the FF mass distributions of 235U(nth, f).

Place, publisher, year, edition, pages
Elsevier, 2012. Vol. 673, 116-121 p.
Keyword [en]
Grid Inefficiency, 252Cf(sf), Ionization chambers, Fission
National Category
Physical Sciences
Research subject
Nuclear Physics
URN: urn:nbn:se:uu:diva-172205DOI: 10.1016/j.nima.2011.01.088ISI: 000301813500016OAI: oai:DiVA.org:uu-172205DiVA: diva2:513502
Available from: 2012-04-02 Created: 2012-04-02 Last updated: 2013-02-11Bibliographically approved
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.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1002
Fission, U-234, Neutron, Uranium, Resonance, Ionization Chamber, Frisch-grid
National Category
Subatomic Physics
Research subject
Physics with specialization in Applied Nuclear Physics
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)
Available from: 2012-12-27 Created: 2012-11-21 Last updated: 2013-04-02Bibliographically approved

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