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  • Al-Adili, Ali, et al. (författare)
  • Ambiguities in the grid-inefficiency correction for Frisch-Grid Ionization Chambers
  • 2012
  • Ingår i: Nuclear Instruments and Methods in Physics Research Section A : Accelerators, Spectrometers, Detectors and Associated Equipment. - Elsevier. - 0168-9002 .- 1872-9576. ; 673, s. 116-121
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>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 <sup>252</sup>Cf 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 <sup>235</sup>U(n<sub>th</sub>, f).</p>
  • Al-Adili, Ali, et al. (författare)
  • Comparison of digital and analogue data acquisition systems for nuclear spectroscopy
  • 2010
  • Ingår i: Nuclear Instruments and Methods in Physics Research Section A : Accelerators, Spectrometers, Detectors and Associated Equipment. - 0168-9002 .- 1872-9576. ; 624:3, s. 684-690
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>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.</p>
  • Al-Adili, Ali, et al. (författare)
  • First evidence of correlation between vibrational resonances and an anisotropy in the fission mass distribution
  • ????
  • Annan publikation (övrigt vetenskapligt)abstract
    • <p>In this work we present evidence of anisotropic emission of fragment masses in <sup>234</sup>U(n,f). The discovered mass anisotropy is correlated with the prominent vibrational resonances at E<sub>n</sub> = 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.</p>
  • Al-Adili, Ali, et al. (författare)
  • Fragment mass-, kinetic energy- and angular distributions for <sup>234</sup>U(n, f) at incident neutron energies from E<sub>n</sub> = 0.2 to 5.0 MeV
  • 2016
  • Ingår i: Physical review C. - 2469-9985. ; 93:3
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>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.</p>
  • Al-Adili, Ali, et al. (författare)
  • Impact of prompt-neutron corrections on final fission-fragment distributions
  • 2012
  • Ingår i: Physical Review C. Nuclear Physics. - 0556-2813 .- 1089-490X. ; 86:5, s. 054601
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>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, ν<sub>tot</sub>, 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, <sup>233,235</sup>U and <sup>239</sup>Pu. 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.</p><p>Purpose: We investigated the consequences of the ν(A,TKE,E<sub>n</sub>) distribution on the fission fragment observables.</p><p>Methods: Experimental data obtained for the <sup>234</sup>U(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.</p><p>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.</p><p>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.</p>
  • Al-Adili, Ali, et al. (författare)
  • Indication of anisotropic TKE and mass emission in <sup>234</sup>U(n,f)
  • 2012
  • Ingår i: Physics Procedia. ; s. 158-164
  • Konferensbidrag (refereegranskat)abstract
    • <p>The neutron-induced fission of <sup>234</sup>U 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 <sup>234</sup>U(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.</p>
  • Al-Adili, Ali, et al. (författare)
  • Neutron-multiplicity experiments for enhanced fission modelling
  • 2017
  • Ingår i: EPJ Web of Conferences. - 978-2-7598-9020-0
  • Konferensbidrag (refereegranskat)abstract
    • <p>The nuclear de-excitation process of fission fragments (FF) provides fundamental information for the understanding of nuclear fission and nuclear structure in neutron-rich isotopes. The variation of the prompt-neutron multiplicity, ν(<em>A</em>), as a function of the incident neutron energy (E<sub><em>n</em></sub>) is one of many open questions. It leads to significantly different treatments in various fission models and implies that experimental data are analyzed based on contradicting assumptions. One critical question is whether the additional excitation energy (E<sub><em>exc</em></sub>) is manifested through an increase of ν(<em>A</em>) for all fragments or for the heavy ones only. A systematic investigation of ν(<em>A</em>) as a function of E<sub><em>n</em></sub> has been initiated. Correlations between prompt-fission neutrons and fission fragments are obtained by using liquid scintillators in conjunction with a Frisch-grid ionization chamber. The proof-of-principle has been achieved on the reaction <sup>235</sup>U(n<sub><em>th</em></sub>,f) at the Van De Graff (VdG) accelerator of the JRC-Geel using a fully digital data acquisition system. Neutrons from <sup>252</sup>Cf(sf) were measured separately to quantify the neutron-scattering component due to surrounding shielding material and to determine the intrinsic detector efficiency. Prelimenary results on ν(<em>A</em>) and spectrum in correlation with FF properties are presented.</p>
  • Al-Adili, Ali, et al. (författare)
  • On the Frisch–Grid signal in ionization chambers
  • 2012
  • Ingår i: Nuclear Instruments and Methods in Physics Research Section A : Accelerators, Spectrometers, Detectors and Associated Equipment. - Elsevier. - 0168-9002 .- 1872-9576. ; 671, s. 103-107
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>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 <em>σ</em>. 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: <em>σ</em>=0.031 and <em>σ</em>=0.083, respectively. The theoretical grid signal was confirmed because the detected grid pulse-height distribution was smaller for the larger <em>σ</em>. By applying the additive GI correction approach, the two grid pulse heights were consistent.</p> <p>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.</p>
  • Al-Adili, Ali, et al. (författare)
  • Possible anisotropy in the emission of fission fragments
  • 2012
  • Ingår i: Conference: 13th international conference on nuclear reaction mechanisms, At Villa Monastero, Varenna, Italy, Volume: pp. 223-225. ; s. 223-225
  • Konferensbidrag (refereegranskat)abstract
    • <p>This study on 234U(n,f) focused on the vibrational resonance at the incident neutron energy En=770 keV. Due to the strong angular anisotropy, Fluctuations of the fission fragment (FF) properties were predicted. The bipolar angular anisotropy was verified in this work and a possible new correlation to anisotropic FF emission has been observed. The mass distribution was found to have the biggest difference in asymmetry, at the vibrational resonance and was less asymmetric in emission along the axis of the beam direction. A corresponding anisotropy in the total kinetic energy was also observed. The observed effect was consistent with the change in the mass distribution. At last, the experimental data were fitted based on the Multi-Modal Random Neck Rupture (MM-RNR) model. The yield of the standard-1 mode was found to increase at the resonance.</p>
  • Al-Adili, Ali, et al. (författare)
  • Studying fission neutrons with 2E-2v and 2E
  • 2018
  • Konferensbidrag (refereegranskat)abstract
    • <p>This work aims at measuring prompt-fission neutrons at different excitation energies of the nucleus. Two independent techniques, the 2E-2v and the 2E techniques, are used to map the characteristics of the mass-dependent prompt fission neutron multiplicity, 7(A), when the excitation energy is increased. The VERDI 2E-2v spectrometer is being developed at JRC-GEEL. The Fission Fragment (FF) energies are measured using two arrays of 16 silicon (Si) detectors each. The FFs velocities are obtained by time-of-flight, measured between micro-channel plates (MCP) and Si detectors. With MCPs placed on both sides of the fission source, VERDI allows for independent timing measurements for both fragments. Cf-252(sf) was measured and the present results revealed particular features of the 2E-2v technique. Dedicated simulations were also performed using the GEF code to study important aspects of the 2E-2v technique. Our simulations show that prompt neutron emission has a non-negligible impact on the deduced fragment data and affects also the shape of 17(A). Geometrical constraints lead to a total-kinetic energy-dependent detection efficiency. The 2E technique utilizes an ionization chamber together with two liquid scintillator detectors. Two measurements have been performed, one of Cf-252(sf) and another one of thermal-neutron induced fission in U-235(n,f). Results from Cf-252(sf) are reported here.</p>
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