| 1. |
- 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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| 2. |
- Al-Adili, Ali, et al.
(författare)
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Neutron-multiplicity experiments for enhanced fission modelling
- 2017
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Ingår i: EPJ Web of Conferences. - : EDP Sciences. - 9782759890200
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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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| 3. |
- 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
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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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| 4. |
- 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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| 5. |
- Jansson, Kaj, 1987-, et al.
(författare)
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The impact of neutron emission on correlated fission data from the 2E-2v method
- 2018
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Ingår i: European Physical Journal A. - : Springer Science and Business Media LLC. - 1434-6001 .- 1434-601X. ; 54
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Tidskriftsartikel (refereegranskat)abstract
- The double-energy double-velocity (2E-2v) method allows assessing fission-fragment mass yields prior to and after prompt neutron emission with high resolution. It is, therefore, considered as a complementary technique to assess average prompt neutron multiplicity as a function of fragment properties. We have studied the intrinsic features of the 2E-2v method by means of event-wise generated fission-fragment data and found short-comings in the method itself as well as in some common practices of application. We find that the 2E-2v method leads to large deviations in the correlation between the prompt neutron multiplicity and pre-neutron mass, which deforms and exaggerates the so-called “sawtooth” shape of nubar(A). We have identified the treatment of prompt neutron emission from the fragments as the origin of the problem. The intrinsic nature of this deficiency risks to render 2E-2v experiments less interesting. We suggest a method to correct 2E-2v data that can even be applied on existing measurements.
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| 6. |
- 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
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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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| 7. |
- Al-Adili, Ali, et al.
(författare)
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Analysis of prompt fission neutrons in U-235(nth,f) and fission fragment distributions for the thermal neutron induced fission of U-234
- 2016
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Ingår i: CNR*15 - 5th International Workshop On Compound-Nuclear Reactions And Related Topics. - : EDP Sciences.
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Konferensbidrag (refereegranskat)abstract
- This paper presents the ongoing analysis of two fission experiments. Both projects are part of the collaboration between the nuclear reactions group at Uppsala and the JRC-IRMM. The first experiment deals with the prompt fission neutron multiplicity in the thermal neutron induced fission of U-235(n,f). The second, on the fission fragment properties in the thermal fission of U-234(n,f). The prompt fission neutron multiplicity has been measured at the JRC-IRMM using two liquid scintillators in coincidence with an ionization chamber. The first experimental campaign focused on U-235(nth,f) whereas a second experimental campaign is foreseen later for the same reaction at 5.5 MeV. The goal is to investigate how the so-called saw-tooth shape changes as a function of fragment mass and excitation energy. Some harsh experimental conditions were experienced due to the large radiation background. The solution to this will be discussed along with preliminary results. In addition, the analysis of thermal neutron induced fission of U-234(n,f) will be discussed. Currently analysis of data is ongoing, originally taken at the ILL reactor. The experiment is of particular interest since no measurement exist of the mass and energy distributions for this system at thermal energies. One main problem encountered during analysis was the huge background of U-235(nth, f). Despite the negligible isotopic traces in the sample, the cross section difference is enormous. Solution to this parasitic background will be highlighted.
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| 8. |
- Al-Adili, Ali, et al.
(författare)
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Fission Activities of the Nuclear Reactions Group in Uppsala
- 2015
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Ingår i: Scientific Workshop on Nuclear Fission Dynamics and the Emission of Prompt Neutrons and Gamma Rays, THEORY-3. - : Elsevier BV. ; , s. 145-149
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Konferensbidrag (refereegranskat)abstract
- This paper highlights some of the main activities related to fission of the nuclear reactions group at Uppsala University. The group is involved for instance in fission yield experiments at the IGISOL facility, cross-section measurements at the NFS facility, as well as fission dynamics studies at the IRMM JRC-EC. Moreover, work is ongoing on the Total Monte Carlo (TMC) methodology and on including the GEF fission code into the TALYS nuclear reaction code. Selected results from these projects are discussed.
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| 9. |
- Al-Adili, Ali, et al.
(författare)
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Simulations of the fission-product stopping efficiency in IGISOL
- 2015
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Ingår i: European Physical Journal A. - : Springer Science and Business Media LLC. - 1434-6001 .- 1434-601X. ; 51:59, s. 1-7
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Tidskriftsartikel (refereegranskat)abstract
- At the Jyväskylä Ion Guide Isotope Separator On-Line (IGISOL) facility, independent fission yields are measured employing the Penning-trap technique. Fission products are produced, e.g. by impinging protons on a uranium target, and are stopped in a gas-filled chamber. The products are collected by a flow of He gas and guided through a mass separator to a Penning trap, where their masses are identified. This work investigates how fission-product properties, such as mass and energy, affect the ion stopping efficiency in the gas cell. The study was performed using the Geant4 toolkit and the SRIM code. The main results show a nearly mass-independent ion stopping with regard to the wide spread of ion masses and energies, with a proper choice of uranium target thickness. Although small variations were observed, in the order of 5%, the results are within the systematic uncertainties of the simulations. To optimize the stopping efficiency while reducing the systematic errors, different experimental parameters were varied; for instance material thicknesses and He gas pressure. Different parameters influence the mass dependence and could alter the mass dependencies in the ion stopping efficiency.
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| 10. |
- Alhassan, Erwin, et al.
(författare)
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Benchmark selection methodology for reactor calculations and nuclear data uncertainty reduction
- 2015
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Ingår i: Annals of Nuclear Energy. - 0306-4549 .- 1873-2100.
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Tidskriftsartikel (refereegranskat)abstract
- Criticality, reactor physics and shielding benchmarks are expected to play important roles in GEN-IV design, safety analysis and in the validation of analytical tools used to design these reactors. For existing reactor technology, benchmarks are used for validating computer codes and for testing nuclear data libraries. Given the large number of benchmarks available, selecting these benchmarks for specic applications can be rather tedious and difficult. Until recently, the selection process has been based usually on expert judgement which is dependent on the expertise and the experience of the user and there by introducing a user bias into the process. This approach is also not suitable for the Total Monte Carlo methodology which lays strong emphasis on automation, reproducibility and quality assurance. In this paper a method for selecting these benchmarks for reactor calculation and for nuclear data uncertainty reduction based on the Total Monte Carlo (TMC) method is presented. For reactor code validation purposes, similarities between a real reactor application and one or several benchmarks are quantied using a similarity index while the Pearson correlation coecient is used to select benchmarks for nuclear data uncertainty reduction. Also, a correlation based sensitivity method is used to identify the sensitivity of benchmarks to particular nuclear reactions. Based on the benchmark selection methodology, two approaches are presented for reducing nuclear data uncertainty using integral benchmark experiments as an additional constraint in the TMC method: a binary accept/reject and a method of assigning file weights using the likelihood function. Finally, the methods are applied to a full lead-cooled fast reactor core and a set of criticality benchmarks. Signicant reductions in Pu-239 and Pb-208 nuclear data uncertainties were obtained after implementing the two methods with some benchmarks.
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