| 1. |
- Al-Adili, Ali, et al.
(författare)
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Ambiguities in the grid-inefficiency correction for Frisch-Grid Ionization Chambers
- 2012
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Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier. - 0168-9002 .- 1872-9576. ; 673, s. 116-121
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Tidskriftsartikel (refereegranskat)abstract
- 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).
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| 2. |
- Al-Adili, Ali, et al.
(författare)
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Comparison of digital and analogue data acquisition systems for nuclear spectroscopy
- 2010
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Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier BV. - 0168-9002 .- 1872-9576. ; 624:3, s. 684-690
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Al-Adili, Ali, et al.
(författare)
-
First evidence of correlation between vibrational resonances and an anisotropy in the fission mass distribution
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 4. |
- Al-Adili, Ali, et al.
(författare)
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Fragment mass-, kinetic energy- and angular distributions for 234U(n, f) at incident neutron energies from En = 0.2 to 5.0 MeV
- 2016
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Ingår i: Physical review C. - 2469-9985. ; 93:3
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Al-Adili, Ali, et al.
(författare)
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Impact of prompt-neutron corrections on final fission-fragment distributions
- 2012
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Ingår i: Physical Review C. Nuclear Physics. - 0556-2813 .- 1089-490X. ; 86:5, s. 054601-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Al-Adili, Ali, et al.
(författare)
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Indication of anisotropic TKE and mass emission in 234U(n,f)
- 2012
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Ingår i: Physics Procedia. - : Elsevier BV. ; , s. 158-164
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 7. |
- Al-Adili, Ali
(författare)
-
Investigation of 234U(n,f) with a Frisch-grid ionization chamber
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This work treats three topics. The main topic concerns neutron-induced fission of 234U. The main goal is to investigate the fission-fragments properties as a function of the incident neutron energy. The study was carried out using a twin Frisch-grid ionization chamber. The first fluctuations on fragment properties are presented, in terms of strong angular anisotropy oscillation. The second part of the work treats the data-acquisition systems in use, particularly for neutron-induced fission experiments. Modern digital systems are studied and compared with the conventional analogue systems. It was shown that the digital systems are superior in drift stability, pile-up correction and extended the possibilities of offline analysis. The third part of the work concerns the Frisch-grid inefficiency. The Frisch grid was introduced in the chamber to remove the angular dependency from the induced charge. However, the shielding is not perfect and a correction is needed for the small angular dependency. Two contradicting methods have been presented in literature, one adding, and the second subtracting the angular-dependent part from the detected signal. An experiment with Cf(sf) was designed and performed to solve the pending ambiguity. The results support the additive model.
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| 8. |
- Al-Adili, Ali
(författare)
-
Measurements of the 234U(n,f) Reaction with a Frisch-Grid Ionization Chamber up to En=5 MeV
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 9. |
- Al-Adili, Ali, et al.
(författare)
-
Neutron-multiplicity experiments for enhanced fission modelling
- 2017
-
Ingår i: EPJ Web of Conferences. - : EDP Sciences. - 9782759890200
-
Konferensbidrag (refereegranskat)abstract
- 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(A), as a function of the incident neutron energy (En) 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 (Eexc) is manifested through an increase of ν(A) for all fragments or for the heavy ones only. A systematic investigation of ν(A) as a function of En 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 235U(nth,f) at the Van De Graff (VdG) accelerator of the JRC-Geel using a fully digital data acquisition system. Neutrons from 252Cf(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(A) and spectrum in correlation with FF properties are presented.
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| 10. |
- Al-Adili, Ali, et al.
(författare)
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On the Frisch–Grid signal in ionization chambers
- 2012
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Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier. - 0168-9002 .- 1872-9576. ; 671, s. 103-107
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 11. |
- Al-Adili, Ali, et al.
(författare)
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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
- 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.
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| 12. |
- Al-Adili, Ali, et al.
(författare)
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Studying fission neutrons with 2E-2v and 2E
- 2018
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Ingår i: SCIENTIFIC WORKSHOP ON NUCLEAR FISSION DYNAMICS AND THE EMISSION OF PROMPT NEUTRONS AND GAMMA RAYS (THEORY-4). - : EDP Sciences. - 9782759890316
-
Konferensbidrag (refereegranskat)abstract
- 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.
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| 13. |
- Birgersson, Evert, et al.
(författare)
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Binary fission-fragment yields from the reaction 251Cf(nth, f)
- 2005
-
Ingår i: Nuclear fission and fission-product spectroscopy. - : American Institute of Physics. - 0735402884 ; , s. 349-352
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Konferensbidrag (refereegranskat)abstract
- The recoil mass spectrometer LOHENGRIN of the Laue-Langevin Institute, Grenoble has been used to measure the light fission-fragment mass yield and kinetic energy distributions from neutron-induced 252Cf*, using 251Cf as target material. ©2005 American Institute of Physics
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| 14. |
- Birgersson, Evert, et al.
(författare)
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Light fission-fragment mass distribution from the reaction 251Cf(nth, f)
- 2007
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Ingår i: Nuclear Physics A. - Amsterdam : Elsevier. - 0375-9474 .- 1873-1554. ; 791:1-2, s. 1-23
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Tidskriftsartikel (refereegranskat)abstract
- For mass numbers A = 80 to 124 the recoil mass spectrometer LOHENGRIN of the Institute Laue-Langevin in Grenoble was used to measure with high resolution the light fission-fragment mass yields and kinetic energy distributions from thermal-neutron induced fission of 252Cf* for the first time, using 251Cf as target material. The obtained mean light fragment mass AL = (107 ± 2) and the corresponding mean kinetic energy Ek,L = (103±2) MeV are within the expected trend. Emission yields around A = 115 are enhanced and the corresponding mean kinetic energy is higher compared to spontaneous fission of 252Cf. This could be explained by the existence of an additional super-deformed fission mode.
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| 15. |
- Birgersson, Evert, et al.
(författare)
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Properties of the reaction 238U(n, f) at the vibrational resonances
- 2009
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Ingår i: Nuclear Physics A. - Amsterdam : Elsevier. - 0375-9474 .- 1873-1554. ; 817:1-4, s. 1-34
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Tidskriftsartikel (refereegranskat)abstract
- Recent fission cross-section calculations for the reaction 238U(n, f ), based on an extended statistical model, predict a significant change of fission fragment properties, such as the mean mass by A = 1.5 and a notable increase in total kinetic energy in the region of the vibrational resonance at an incident neutron energy En = 0.9 MeV. This model includes individual fission cross-sections by the asymmetric standard 1 (S1) and standard 2 (S2) as well as the symmetric super-long (SL) mode. In order to verify the model predictions, a dedicated experiment on 238U has been carried out to measure fission-fragment mass yield distributions for incident neutron energies from En = 2.0 MeV down to 0.9 MeV, where the fission characteristics at the vibrational resonance at En = 0.9 MeV were investigated for the first time. The previously reported distinct structure in the angular anisotropy around En = 1.2 and 1.6 MeV wasobserved at En = 0.9 MeV as well. The predicted large changes in fission fragment mass yield and total kinetic energy could not be confirmed. In the resonance the mean total kinetic energy is only about 0.5 MeV higher than at En = 1.8 MeV. At the same time, a slight decrease of the mean heavy fragment mass was observed, probably indicating a slightly increased contribution of the S1 fission mode.
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| 16. |
- Carlson, Allan D., et al.
(författare)
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Results of a New Evaluation of the Neutron Standards
- 2018
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Ingår i: Reactor Dosimetry. - : ASTM International. - 9780803176614 ; , s. 91-104
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Konferensbidrag (refereegranskat)abstract
- An international effort has produced evaluations of the neutron data standards. Evaluations were obtained for the cross section standards: the H(n,n), 6Li(n,t), 10B(n,067), loB(vx), natc(n,n,) Au(n,y), 235U(n,f), and 238U(n,f) reactions. Also in the evaluation process, the 238U(n,y) and 239Pu(n,f) nonstandard cross sections were evaluated. Many of these are dosimetry cross sections. Evaluations were also obtained for data that are not traditional standards: Maxwellian spectrum averaged cross section for the Au(n,y) cross section at 30 keV, reference cross sections for prompt y-ray production in fast neutron-induced reactions, reference cross sections for very high-energy fission cross sections, the 252Cf spontaneous fission neutron spectrum and the 235U thermal fission neutron spectrum, and the thermal constants. The data and covariances were obtained directly from this evaluation procedure as is required by the dosimetry community.
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| 17. |
-
Fission Product Yields Data Current status and perspectives : Summary report of an IAEA Technical meeting
- 2016
-
Proceedings (redaktörskap) (övrigt vetenskapligt/konstnärligt)abstract
- A Technical Meeting on Fission Product Yields Data: current status and perspectives, was held from 23 to 26 May 2016, at the IAEA, Vienna. The purpose of the meeting was to review the current status of Fission Product Yield data, and discuss the progress in measurements, theories, evaluation and covariances. The presentations, technical discussions and recommendations of the meeting are given in detail in this summary report.
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| 18. |
- Göök, Alf, et al.
(författare)
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Neutron Multiplicity Correlations with Fission Fragment Mass and Energy from Pu-239(n,f)
- 2020
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Ingår i: ND 2019. - : EDP Sciences.
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Konferensbidrag (refereegranskat)abstract
- There exists experimental evidence for strong fluctuations of the average neutron multiplicity from resonance to resonance in Pu-239(n,f). These fluctuations have been shown to impact nuclear reactor benchmarks by reducing the criticality. The fluctuating neutron multiplicity can be explained as a consequence of the competition between direct fission and the (n,gamma f) process. However, there is also evidence for fluctuations of the fission fragment mass yields from resonance to resonance. The mass yield fluctuations may also contribute to fluctuations of the neutron multiplicity averaged over all fission fragment masses. In order to model the contribution to the neutron multiplicity fluctuations by the fission fragment mass yield fluctuations new data on the correlations between fission fragment properties and neutron multiplicities are in need. We present experiments carried out to determine prompt neutron multiplicity correlations with fission fragment masses and total kinetic energies in the reaction Pu-239(n,f). The experiment has been performed at the GELINA facility at JRC-Geel. A twin position-sensitive Frisch-grid ionization chamber is used for fission fragment identification via the double kinetic energy technique. An array of scintillation detectors is employed for neutron counting. Correlations between average neutron multiplicities and fission fragment properties have been measured with improved resolution in both mass and TKE, compared to data from the literature.
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| 19. |
- Hambsch, Franz-Josef, et al.
(författare)
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Prompt fission neutron emission from 235U(n,f): thermal and resonance region
- 2015
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Ingår i: Conference: 14th International Conference on Nuclear Reaction Mechanisms - CERN-Proceedings-2015-001, At Villa Monastero, Varenna, Italy. - 9789290834182
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Konferensbidrag (refereegranskat)abstract
- For nuclear modelling and improved evaluation of nuclear data, knowledge of fluctuations of the prompt neutron multiplicity as a function of incident neutron energy is requested for the major actinides 235U and 239Pu. Experimental investigations of the prompt fission neutron emission in resonance-neutron induced fission on 235U are taking place at the GELINA facility of the IRMM. The experiment employs an array of scintillation detectors (SCINTIA) in conjunction with a newly designed 3D position-sensitive twin Frisch-grid ionization chamber. In addition, the mass-dependent prompt neutron multiplicity, (A), has attracted particular attention. Recent, sophisticated nuclear fission models predict that the additional excitation energy, brought into the fission system at higher incident neutron energies, leads to an increased neutron multiplicity only for heavy fragments, as observed in the 237Np(n,f) reaction. A first feasibility study has been performed at the JRC-IRMM VdG accelerator to measure nu(A) for 235U(n,f).
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| 20. |
- Jansson, Kaj, et al.
(författare)
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Measurement of the 6Li(n, α) neutron standard cross-section at the GELINA facility
- 2016
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Ingår i: EPJ Web of Conferences. - : EDP Sciences. - 2100-014X. ; 122
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Tidskriftsartikel (refereegranskat)abstract
- The 6Li(n,α) reaction cross-section is commonly used as a reference cross section. However, it is only considered a neutron standard up to 1MeV. For higher energies, there are discrepancies of several per cents between recent measurements and evaluated data files. In order to extend and establish 6Li(n,α) as a neutron standard above 1MeV these discrepancies must be resolved. Our measurement at the GELINA facility at JRC-IRMM in Geel, Belgium is ongoing. We are using a double twin Frisch-grid setup to detect both α-particles from two 6Li targets and fission products from two 235U reference targets. Our targets have thick backings but are employed in pairs, one forward facing and one backward facing. In this way we still cover, in principle, a solid angle of 4π. We present some preliminary results showing that the existing cross-section data is well reproduced around the resonance at 240 keV. The final data taking will start in the beginning of 2016, when the GELINA facility goes online again after a few months of shut down.
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| 21. |
- Jansson, Kaj, 1987-, et al.
(författare)
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Measurement of the 6Li(n,α)t neutron standard cross-section at the GELINA facility
- 2017
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Ingår i: ND 2016. - Les Ulis : EDP Sciences. - 9782759890200
-
Konferensbidrag (refereegranskat)abstract
- The Li-6(n,alpha)t reaction cross-section is an established standard due to its relatively high crosssection as well as its high Q-value. However, it is only considered a neutron standard up to 1 MeV, because in the neutron energy region 1-3 MeV there exist discrepancies of several per cents between recent measurements [1,2] and evaluated data files [3]. It has been speculated [4] that neglecting of the particle leaking effect might be part of the explanation why there is a disagreement in this region. Based on R-matrix calculations, in the region around 2 MeV, one also expects three excitation levels of Li-7 to significantly influence the cross section [5]. In order to resolve these discrepancies, we perform measurements at the GELINA facility at JRC-Geel with two Frisch-gridded ionisation chambers. The Li-6(n,alpha)t cross section is measured relative to the U-235(n,f) standard. In order to solve previous encountered problems [6], the setup has been modified and moved to a new flight path station. In this proceeding we show that several problems have been eliminated and discuss possible solutions to newly arisen problems, due to the changed experimental conditions. Preliminary results from new data taken during 2016 with the updated setup are presented.
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| 22. |
- Jansson, Kaj, 1987-, et al.
(författare)
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The new double energy-velocity spectrometer VERDI
- 2017
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Ingår i: ND 2016: INTERNATIONAL CONFERENCE ON NUCLEAR DATA FOR SCIENCE AND TECHNOLOGY. - Les Ulis : EDP Sciences. - 9782759890200
-
Konferensbidrag (refereegranskat)abstract
- VERDI (VElocity foR Direct particle Identification) is a fission-fragment spectrometer recently put into operation at JRC-Geel. It allows measuring the kinetic energy and velocity of both fission fragments simultaneously. The velocity provides information about the pre-neutron mass of each fission fragment when isotropic prompt-neutron emission from the fragments is assumed. The kinetic energy, in combination with the velocity, provides the post-neutron mass. From the difference between pre- and post-neutron masses, the number of neutrons emitted by each fragment can be determined. Multiplicity as a function of fragment mass and total kinetic energy is one important ingredient, essential for understanding the sharing of excitation energy between fission fragments at scission, and may be used to benchmark nuclear de-excitation models. The VERDI spectrometer design is a compromise between geometrical efficiency and mass resolution. The spectrometer consists of an electron detector located close to the target and two arrays of silicon detectors, each located 50 cm away from the target. In the present configuration pre-neutron and post-neutron mass distributions are in good agreement with reference data were obtained. Our latest measurements performed with spontaneously fissioning 252Cf is presented along with the developed calibration procedure to obtain pulse height defect and plasma delay time corrections.
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| 23. |
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| 24. |
- Oberstedt, Andreas, et al.
(författare)
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Recent results on the neutron-induced fission cross-section of 231Pa
- 2005
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Ingår i: Proceedings of the 3rd international workshop on nuclear fission and fission-product spectroscopy. - : American Institute of Physics. - 0735402884 ; , s. 27-30
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Konferensbidrag (refereegranskat)abstract
- The cross-section for the neutron-induced fission of 231Pa has recently been measured from the threshold to En = 3.5 MeV. The experimental results are described in terms of extended statistical model calculations.
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| 25. |
- Oberstedt, Stephan, et al.
(författare)
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Light charged particle emission in the reaction 251Cf(nth, f)
- 2005
-
Ingår i: Nuclear Physics A. - : Elsevier BV. - 0375-9474 .- 1873-1554. ; 761:3-4, s. 173-189
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Tidskriftsartikel (refereegranskat)abstract
- High resolution measurements of light charged particles (LCP) emitted in thermal neutron-induced fission of 252Cf ∗ (E=6.2 MeV) have been performed with the recoil mass-separator LOHENGRIN. For this compound nuclear system emission yields of LCPs, their mean kinetic energies and widths have been obtained for 8 isotopes with nuclear charges Z⩾2. For 13 further isotopes the emission yields were estimated on the basis of systematics on their kinetic energy distributions. 34Al and 36Si emission has been observed for the first time in thermal neutron-induced fission.
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| 26. |
- Oberstedt, Stephan, et al.
(författare)
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Neutron-induced fission cross-section of 231Pa
- 2006
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Ingår i: Proceedings of NEMEA-2 Workshop on Neutron Measurements, Evaluations and Applications - 2, October 20-23, 2004, Bucharest, Rumania, EUR Report 22136 EN.
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Konferensbidrag (refereegranskat)
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| 27. |
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| 28. |
- Oberstedt, Stephan, et al.
(författare)
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Shape isomers : a key to fission barriers
- 2008
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Ingår i: Seminar on Fission VI, Corsendonk Priory, Belgium, September 18-21, 2007. - : WORLD SCIENTIFIC. - 9789812791061 ; , s. 233-240
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Konferensbidrag (refereegranskat)abstract
- Quantitative predictions of fission product yields are relevant for the reliable operation of different modern nuclear applications. This concerns the realistic characterizations of the radio-toxicity of the fuel elements after the envisaged extended irradiation, as well as sub-critical assemblies, where the number of delayed neutrons from minor actinides is determined by the characteristic emission yields of the corresponding so-called pre-cursor isotopes. However, to be able to make more reliable quantitative predictions of fission characteristics requires the better understanding of the fission process itself. For this purpose a better knowledge about the distinct structure of the nuclear energy landscape around the fission barrier is indispensable. In particular, the question should be answered, whether the fission barrier is either double- or triple-humped or even multi-humped as been proposed within the multi-modal neck rupture model. Despite quite some effort based on different experimental techniques and theoretical approaches, this question remains still unanswered. There is still no consistent picture of the fission barrier available and hence, different sets of barrier parameters are in use, unable to describe the different observed phenomena in a coherent way. With the systematic investigation of shape isomer population, its decay modes as well as the branching ratio, precise information can be obtained to resolve the puzzling situation. The experimental approach will be discussed and results from first experiments presented.
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| 29. |
- Simutkin, Vasily, 1974-
(författare)
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Fragment Mass Distributions in Neutron-Induced Fission of 232Th and 238U from 10 to 60 MeV
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Since its discovery, the phenomenon of nuclear fission is the object of extensive theoretical and experimental studies. However, we are still far from a complete understanding of the fission process. Nuclear theory can satisfactorily explain the process of neutron-induced fission at thermal neutron energies, but it meets problems at high neutron energies. However, new applications are nowadays developed involving neutron-induced fission in this energy domain. An example of such an application is accelerator-driven systems (ADS) which are dedicated to transmutation of highly radioactive nuclear waste. Conceptual studies of ADS require new nuclear data on neutron-induced reactions within a wide incident energy range. Along with structural, spallation target and other materials, data on neutron-induced fission are especially required for two nuclides, 232Th and 238U. At present, however, there are no published neutron-induced fission yield data for either 232Th or 238U at energies above 20 MeV. In this thesis, I present measurements of fission fragment mass yields at neutron energies from 10 to 60 MeV for 232Th and 238U. The experiment was done at the Louvain-la-Neuve quasi-monoenergetic neutron beam facility. A multi-section Frisch-gridded ionization chamber was used as the fission fragment detector. The fission fragment mass yields were measured at peak neutron energies of 33, 45, and 60 MeV. In addition, data for the neutron-energy intervals 9-11, 16-18, and 24-26 MeV were also extracted from the low-energy tail. The measurement results show that the symmetric fission component increases with incident neutron energy for both uranium and thorium, but it is more enhanced for thorium. The uranium results were compared to the only existing set of experimental data for neutron energies above 20 MeV. Reasonable agreement was found. However, our data show a lower symmetric fission component. For thorium, the present data are the first above 20 MeV. Model calculations with the TALYS code have also been done. This code is based on the multi-modal random neck-rupture model extended for higher excitation energies. We included a phenomenological model into the code and achieved a good description of our experimental results.
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| 30. |
- Tovesson, Fredrik, et al.
(författare)
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Fission fragment properties and the problem of the pulse height defect
- 2002
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Ingår i: Journal of Nuclear Science and Technology. - 0022-3131 .- 1881-1248. ; Supplement 2, s. 673-676
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Tidskriftsartikel (refereegranskat)abstract
- The pulse height defect (PHD) has been investigated for three different counting gases commonly used in ionization chambers. The PHD introduces an underestimation of the kinetic energy of a charged particle detected with an ionization chamber. Thus, in some cases it is of crucial importance to correct for this effect, e.g. when studying fission fragment properties. A now method was used, applying a waveform digitizer, to study the PHD. The fission fragment properties from spontaneous fission of 252Cf where determined using different counting gases and different ways of correcting for the PHD were evaluated.
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