Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Measurements of the 234U(n,f) Reaction with a Frisch-Grid Ionization Chamber up to En=5 MeV
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
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 [en]
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: urn:nbn:se:uu:diva-185306ISBN: 978-91-554-8554-2 (print)OAI: oai:DiVA.org:uu-185306DiVA: diva2:573126
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
List of papers
1. Comparison of digital and analogue data acquisition systems for nuclear spectroscopy
Open this publication in new window or tab >>Comparison of digital and analogue data acquisition systems for nuclear spectroscopy
Show others...
2010 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 624, no 3, 684-690 p.Article in journal (Refereed) Published
Abstract [en]

In the present investigation the performance of digital data acquisition (DA) and analogue data acquisition (AA) systems are compared in neutron-induced fission experiments. The DA results are practically identical to the AA results in terms of angular-, energy- and mass-resolution, and both compare very well with literature data. However, major advantages were found with the digital techniques. DA allows for a very efficient αparticle pile-up correction. This is important when considering the accurate measurement of fission-fragment characteristics of highly αactive actinide isotopes relevant for the safe operation of Generation IV reactors and the successful reduction of long-lived radioactive nuclear waste. In case of a strong αemitter, when applying the αparticle pile-up correction, the peak-to-valley ratio of the energy distribution was significantly improved. In addition, DA offers a very flexible expanded off-line analysis and reduces the number of electronic modules drastically, leading to an increased stability against electronic drifts when long measurement times are required.

Keyword
Fission, 234-U(n, f), 235-U(n, f), Digital, Analogue, Ionization chambers
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-142438 (URN)10.1016/j.nima.2010.09.126 (DOI)000285370600019 ()
Available from: 2011-01-14 Created: 2011-01-14 Last updated: 2017-12-11Bibliographically approved
2. Ambiguities in the grid-inefficiency correction for Frisch-Grid Ionization Chambers
Open this publication in new window or tab >>Ambiguities in the grid-inefficiency correction for Frisch-Grid Ionization Chambers
Show others...
2012 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, 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
Keyword
Grid Inefficiency, 252Cf(sf), Ionization chambers, Fission
National Category
Physical Sciences
Research subject
Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-172205 (URN)10.1016/j.nima.2011.01.088 (DOI)000301813500016 ()
Available from: 2012-04-02 Created: 2012-04-02 Last updated: 2017-12-07Bibliographically approved
3. On the Frisch–Grid signal in ionization chambers
Open this publication in new window or tab >>On the Frisch–Grid signal in ionization chambers
Show others...
2012 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 671, 103-107 p.Article in journal (Refereed) Published
Abstract [en]

A recent theoretical approach concerning the grid-inefficiency (GI) problem in Twin Frisch–Grid Ionization Chambers was validated experimentally. The experimental verification focused on the induced signal on the anode plate. In this work the investigation was extended by studying the grid signal. The aim was to verify the grid-signal dependency on the grid inefficiency σ. The measurements were made with fission fragments from 252Cf(sf), using two different grids, with 1 and 2 mm wire distances, leading to the GI values: σ=0.031 and σ=0.083, respectively. The theoretical grid signal was confirmed because the detected grid pulse-height distribution was smaller for the larger σ. By applying the additive GI correction approach, the two grid pulse heights were consistent.

In the second part of the work, the corrected grid signal was used to deduce emission angles of the fission fragments. It is inconvenient to treat the grid signal by means of conventional analogue electronics, because of its bipolarity. Therefore, the anode and grid signals were summed to create a unipolar, angle-dependent pulse height. Until now the so-called summing method has been the well-established approach to deduce the angle from the grid signal. However, this operation relies strongly on an accurate and stable calibration between the two summed signals. By application of digital-signal processing, the grid signal's bipolarity is no longer an issue. Hence one can bypass the intermediate summation step of the two different pre-amplifier signals, which leads to higher stability. In this work the grid approach was compared to the summing method in three cases: 252Cf(sf), 235U(n,f) and 234U(n,f). By using the grid directly, the angular resolution was found equally good in the first case but gave 7% and 20% improvements, respectively, in the latter cases.

Place, publisher, year, edition, pages
Elsevier, 2012
Keyword
Grid inefficiency, Ionization chambers, Summing method
National Category
Natural Sciences
Research subject
Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-172203 (URN)10.1016/j.nima.2011.12.047 (DOI)000301474600012 ()
Available from: 2012-04-02 Created: 2012-04-02 Last updated: 2017-12-07Bibliographically approved
4. Impact of prompt-neutron corrections on final fission-fragment distributions
Open this publication in new window or tab >>Impact of prompt-neutron corrections on final fission-fragment distributions
2012 (English)In: Physical Review C. Nuclear Physics, ISSN 0556-2813, E-ISSN 1089-490X, Vol. 86, no 5, 054601- p.Article in journal, Editorial material (Refereed) Published
Abstract [en]

Background: One important quantity in nuclear fission is the average number of prompt neutrons emitted from the fission fragments, the prompt neutron multiplicity, ν . The total number of prompt fission neutrons, νtot, increases with increasing incident neutron energy. The prompt-neutron multiplicity is also a function of the fragment mass and the total kinetic energy of the fragmentation. Those data are only known in sufficient detail for a few thermal-neutron-induced fission reactions on, for example, 233,235U and 239Pu. The enthralling question has always been asked how the additional excitation energy is shared between the fission fragments. The answer to this question is important in the analysis of fission-fragment data taken with the double-energy technique. Although in the traditional approach the excess neutrons are distributed equally across the mass distribution, a few experiments showed that those neutrons are predominantly emitted by the heavy fragments.

Purpose: We investigated the consequences of the ν(A,TKE,En) distribution on the fission fragment observables.

Methods: Experimental data obtained for the 234U(n, f) reaction with a Twin Frisch Grid Ionization Chamber, were analyzed assuming two different methods for the neutron evaporation correction. The effect of the two different methods on the resulting fragment mass and energy distributions is studied.

Results: We found that the preneutron mass distributions obtained via the double-energy technique become slightly more symmetric, and that the impact is larger for postneutron fission-fragment distributions. In the most severe cases, a relative yield change up to 20–30% was observed.

Conclusions: We conclude that the choice of the prompt-neutron correction method has strong implications on the understanding and modeling of the fission process and encourages new experiments to measure fission fragments in coincidence with prompt fission neutrons. Even more, the correct determination of postneutron fragment yields has an impact on the reliable assessment of the nuclear waste inventory, as well as on the correct prediction of delayed neutron precursor yields.

Keyword
Fission, Neutron
National Category
Subatomic Physics
Research subject
Physics with specialization in Applied Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-185076 (URN)10.1103/PhysRevC.86.054601 (DOI)000310685400003 ()
Available from: 2012-11-21 Created: 2012-11-19 Last updated: 2017-12-07Bibliographically approved
5. Indication of anisotropic TKE and mass emission in 234U(n,f)
Open this publication in new window or tab >>Indication of anisotropic TKE and mass emission in 234U(n,f)
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.

Series
Physics Procedia, ISSN 1875-3892 ; 31
Keyword
Fission, U-234, Neutron
National Category
Subatomic Physics
Research subject
Physics with specialization in Applied Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-185303 (URN)10.1016/j.phpro.2012.04.021 (DOI)000309656300020 ()
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
6. First evidence of correlation between vibrational resonances and an anisotropy in the fission mass distribution
Open this publication in new window or tab >>First evidence of correlation between vibrational resonances and an anisotropy in the fission mass distribution
(English)Manuscript (preprint) (Other academic)
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.

Keyword
U-234, Fission, Neutron, Resonance, Anisotropy
National Category
Subatomic Physics
Research subject
Physics with specialization in Applied Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-185307 (URN)
Available from: 2012-11-28 Created: 2012-11-21 Last updated: 2013-02-11
7. Fragment mass-, kinetic energy- and angular distributions for 234U(n, f) at incident neutron energies from En = 0.2 to 5.0 MeV
Open this publication in new window or tab >>Fragment mass-, kinetic energy- and angular distributions for 234U(n, f) at incident neutron energies from En = 0.2 to 5.0 MeV
Show others...
2016 (English)In: Physical review C, ISSN 2469-9985, Vol. 93, no 3, 034603Article in journal (Refereed) Published
Abstract [en]

This work investigates the neutron-induced fission of U-234 and the fission-fragment properties for neutron energies between E-n = 0.2 and 5.0 MeV with a special highlight on the prominent vibrational resonance at E-n = 0.77 MeV. Angular, energy, and mass distributions were determined based on the double-energy technique by means of a twin Frisch-grid ionization chamber. The experimental data are parametrized in terms of fission modes based on the multimodal random neck-rupture model. The main results are a verified strong angular anisotropy and fluctuations in the energy release as a function of incident-neutron energy.

Keyword
234U, Neutron, Fission, Resonance, Frisch-Grid
National Category
Subatomic Physics
Research subject
Physics with specialization in Applied Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-185332 (URN)10.1103/PhysRevC.93.034603 (DOI)000371409000006 ()
Available from: 2012-11-29 Created: 2012-11-22 Last updated: 2016-04-13Bibliographically approved

Open Access in DiVA

fulltext(7091 kB)614 downloads
File information
File name FULLTEXT01.pdfFile size 7091 kBChecksum SHA-512
a0c54c83ebaa5dbbee7308d45fb46d48cfb13bc3aaa1be6753d15b0c02ad4520b24c4a62cf9cfaf53a875b850e7a4bb1df7539d5b0596fc58f5f5f7857c3caae
Type fulltextMimetype application/pdf
Buy this publication >>

Authority records BETA

Al-Adili, Ali

Search in DiVA

By author/editor
Al-Adili, Ali
By organisation
Applied Nuclear Physics
Subatomic Physics

Search outside of DiVA

GoogleGoogle Scholar
Total: 614 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

isbn
urn-nbn

Altmetric score

isbn
urn-nbn
Total: 1876 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf