| 1. |
- Al-Adili, Ali
(författare)
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Measurements of the 234U(n,f) Reaction with a Frisch-Grid Ionization Chamber up to En=5 MeV
- 2013
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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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| 2. |
- Alhassan, Erwin, 1984-
(författare)
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Nuclear data uncertainty quantification and data assimilation for a lead-cooled fast reactor : Using integral experiments for improved accuracy
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- For the successful deployment of advanced nuclear systems and optimization of current reactor designs, high quality nuclear data are required. Before nuclear data can be used in applications they must first be evaluated, tested and validated against a set of integral experiments, and then converted into formats usable for applications. The evaluation process in the past was usually done by using differential experimental data which was then complemented with nuclear model calculations. This trend is fast changing due to the increase in computational power and tremendous improvements in nuclear reaction models over the last decade. Since these models have uncertain inputs, they are normally calibrated using experimental data. However, these experiments are themselves not exact. Therefore, the calculated quantities of model codes such as cross sections and angular distributions contain uncertainties. Since nuclear data are used in reactor transport codes as input for simulations, the output of transport codes contain uncertainties due to these data as well. Quantifying these uncertainties is important for setting safety margins; for providing confidence in the interpretation of results; and for deciding where additional efforts are needed to reduce these uncertainties. Also, regulatory bodies are now moving away from conservative evaluations to best estimate calculations that are accompanied by uncertainty evaluations.In this work, the Total Monte Carlo (TMC) method was applied to study the impact of nuclear data uncertainties from basic physics to macroscopic reactor parameters for the European Lead Cooled Training Reactor (ELECTRA). As part of the work, nuclear data uncertainties of actinides in the fuel, lead isotopes within the coolant, and some structural materials have been investigated. In the case of the lead coolant it was observed that the uncertainty in the keff and the coolant void worth (except in the case of 204Pb), were large, with the most significant contribution coming from 208Pb. New 208Pb and 206Pb random nuclear data libraries with realistic central values have been produced as part of this work. Also, a correlation based sensitivity method was used in this work, to determine parameter - cross section correlations for different isotopes and energy groups.Furthermore, an accept/reject method and a method of assigning file weights based on the likelihood function are proposed for uncertainty reduction using criticality benchmark experiments within the TMC method. It was observed from the study that a significant reduction in nuclear data uncertainty was obtained for some isotopes for ELECTRA after incorporating integral benchmark information. As a further objective of this thesis, a method for selecting benchmark for code validation for specific reactor applications was developed and applied to the ELECTRA reactor. Finally, a method for combining differential experiments and integral benchmark data for nuclear data adjustments is proposed and applied for the adjustment of neutron induced 208Pb nuclear data in the fast energy region.
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| 3. |
- Andersson, Peter, 1981-
(författare)
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Fast-Neutron Tomography using a Mobile Neutron Generator for Assessment of Steam-Water Distributions in Two-Phase Flows
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis describes the measurement technique of fast-neutron tomography for assessing spatial distributions of steam and water in two-phase flows. This so-called void distribution is of importance both for safe operation and for efficient use of the fuel in light water reactors, which compose the majority of the world’s commercial nuclear reactors. The technique is aimed for usage at thermal-hydraulic test loops, where heated two-phase flows are being investigated under reactor-relevant conditions.By deploying portable neutron generators in transmission tomography, the technique becomes applicable to stationary objects, such as thermal-hydraulic test loops. Fast neutrons have the advantage of high transmission through metallic structures while simultaneously being relatively sensitive to the water/void content. However, there are also challenges, such as the relatively low yield of commercially available fast-neutron generators, the tendency of fast neutrons to scatter in the interactions with materials and the relatively low efficiency encountered in fast-neutron detection.The thesis describes the design of a prototype instrument, FANTOM, which has been assembled and demonstrated. The main design parameters have been optimized to achieve maximal signal count rate in the detector elements, while simultaneously reaching an image unsharpness of ≤0.5 mm. Radiographic projections recorded with the assembled instrument are presented, and the performance parameters of FANTOM are deduced.Furthermore, tomographic reconstruction methods for axially symmetric objects, which is relevant for some test loops, have been developed and demonstrated on measured data from three test objects. The attenuation distribution was reconstructed with a radial resolution of 0.5 mm and an RMS error of 0.02 cm-1, based on data recorded using an effective measurement time of 3.5 hours per object. For a thermal-hydraulic test loop, this can give a useful indication of the flow mode, but further development is desired to improve the precision of the measurements.Instrument upgrades are foreseen by introducing a more powerful neutron generator and by adding detector elements, speeding up the data collection by several orders of magnitude and allowing for higher precision data. The requirements and performance of an instrument for assessment of arbitrary non-symmetric test loops is discussed, based on simulations.
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| 4. |
- Bevilacqua, Riccardo, 1974-
(författare)
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Neutron-Induced Light-Ion Production from Iron and Bismuth at 175 MeV
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Radioactive waste management is one of the key issues in sustainability of nuclear energy production. Geological repositories represent today the most appropriate solution to long-term management of high-level radioactive waste. Strategies such as Partitioning and Transmutation of spent nuclear fuel may offer a positive impact on geological repositories, by reducing the mass of transuranium elements to be disposed and the time scale for their radiotoxicity. In this scenario, Accelerator Driven Systems (ADS) may play a relevant role as dedicated burners towards sustainable nuclear energy. The NEXT project at Uppsala University contributes to a European effort to improve nuclear data knowledge for transmutation, providing the first experimental neutron induced data in the 100 to 200 MeV energy region. This thesis presents measurements of double-differential cross sections for production of light-ions in the interaction of 175 MeV quasi-monoenergetic neutrons with Fe and Bi. Results are compared with model calculations obtained with state-of-the-art nuclear reaction codes; TALYS-1.2, a modified version of JQMD, and MCNP6. Special focus in this work is given to pre-equilibrium emission of composite light-ions. A new energy dependence in the mechanisms described by the Kalbach systematics used in TALYS to account for composite particle emission in the pre-equilibrium stage is proposed. Data show also the need to include multiple pre-equilibrium emission of composite particles, a mechanism now included in TALYS only for protons and neutrons. The JQMD code was recently modified to include a surface coalescence model in the quantum molecular dynamics description of the formation of composite particles. Comparisons of the measured data with results from this modified JQMD code confirm the importance of coalescence mechanisms for the description of the emission spectra of composite particles. Finally, the neutron-induced data are compared with MCNP6 calculations, to contribute to the process of validation and verification of the code.
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| 5. |
- Gao, Zhihao
(författare)
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Isomeric yield ratios in nuclear fission
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Isomeric yield ratios (IYR), referring to the relative yield of the high spin states of a nucleus to the total yield of all observed states, is an observable of nuclear fission that has the potential to improve our understanding of the fission dynamics. Apart from that, systematic observations of IYRs can also contribute to other areas, such as the validation of fission models, modelling of the r-process in stellar nucleosynthesis, studies of the antineutrino mixing angle, the safety of present-day nuclear reactors, and the design of advanced reactor systems.With these motivations in mind, an IYR measurement in proton-induced fission was performed at the IGISOL facility in Jyväskylä, Finland. In the measurement, the Penning trap JYFLTRAP was used to separate the excited state from the ground state and to project those onto a position-sensitive MCP detector. The obtained phase images were used to train a Bayesian Gaussian Mixture model to identify the states. After considering corrections for the detector efficiency and radioactive decay, 19 IYRs were obtained.In this thesis, the measurement of IYRs with the IGISOL technique is described, and a systematic study of IYRs in proton-induced fission is presented. Furthermore, the measured ratios are compared with calculations using three different models: the Madland-England (ME) model, the fission model GEF, and the combination of GEF + TALYS. The experimental results show that, in general, the IYR decreases with the spins of measured states. While this, to some degree, confirms the ME model, variations in the IYR between nuclides with the same spin assignments reveal that the model is too simple to predict individual ratios. Furthermore,discrepancies in the IYRs between the measurement and GEF are observed in most cases, indicating a need to optimize the performance of GEF against nuclear data from proton-induced fission. The combination of GEF + TALYS results in an overall under estimation of the observedIYRs, which could be explained by the different assumptions used in GEF and TALYS.To investigate how the angular momenta of the primary fission fragments relate to the IYRs, a surrogate model of GEF has been developed. By reproducing the measured IYR with the calculated ratios from the model, the average angular momentum Jav, is deduced. The Jav for fission products with a mass number greater than 131 show a mass dependency which fits the parameterisation proposed by J. Wilson et al,. For IYRs in the mass region 119 ≤ A < 132, in which no measurements are presented by Wilson, a decrease in the Jav with increasing mass number is observed for the first time.Besides the study of IYRs, a benchmark of a multi-physics simulation model of the ion guide for neutron-induced fission products has been performed using γ-ray spectroscopy data. The results of the benchmark show that the high-energy part of the neutron flux from the simulation with MCNPX is overestimated by about 40 %, while the ion transportation simulated with GEANT4 agrees well with the experimental data. Based on the benchmark, the ion guide can be optimized to achieve high enough intensities of the collected ions to reach reasonable measurement times. In the next step, the addition of electric fields is considered to direct the ions in and to reduce the ion drifting time. However, this task lies outside the scope of this PhD thesis.
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| 6. |
- Jansson, Kaj, 1987-
(författare)
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Measurements of Neutron-induced Nuclear Reactions for More Precise Standard Cross Sections and Correlated Fission Properties
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- It is difficult to underestimate the importance of neutron cross section standards in the nuclear data field. Accurate and precise standards are prerequisites for measuring neutron cross sections. Two different projects are presented here with the aim of improving on neutron standards.A simulation study was performed for an experiment intended to measure the cross sections of H(n,n), 235U(n,f), and 238U(n,f) relative to each other. It gave the first estimates of the performance of the experimental setup. Its results have aided the development of the experimental setup by setting limits on the target and detector design.A second neutron-standard project resulted in three measurements of 6Li(n,α)t relative to 235U(n,f). Each subsequent measurement improved upon the previous one and changed the experimental setup accordingly. Although, preliminary cross sections were agreeing well with evaluated data files in some energy intervals, the main goal to measure the cross section up to 3 MeV was not reached.Mass yields and energy spectra are important outcomes of many fission experiments, but in low yield regions the uncertainties are still high even for recurrently studied nuclei. In order to understand the fission dynamics, one also needs correlated fission data. One particular important property is the distribution of excitation energy between the two nascent fission fragments. It is closely connected to the prompt emission of neutrons and γ’s and reveals information about how nucleons and energy are transferred within the fissioning nucleus.By measuring both the pre and post neutron-emission fragment masses, the cumbrance of detecting neutrons directly is overcome. This is done using the fission spectrometer VERDI and the 2E-2v method. In this work I describe how both the spectrometer, the analysis method, and the calibration procedures have been further developed. Preliminary experimental data show the great potential of VERDI, but also areas that call for more attention. A previously overlooked consequence of a central assumption was found and a correction method is proposed that can correct previously obtained data as well.The last part of this thesis concerns the efficiencies of the fission product extraction at the IGISOL facility. The methodology of the fission yield measurements at IGISOL are reliant on assumptions that have not been systematically investigated. The presented work is a first step of such an investigation that can also be used as a tool for optimising the setup for measurements of exotic nuclei. A simulation framework connecting three different simulation codes was developed to investigate the produced yield of fission products in a buffer gas. Several different variants of the setup were simulated and the findings were generally accordant with previous estimates. A reasonable agreement between experimental data and the simulation results is demonstrated.
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| 7. |
- Mattera, Andrea, 1985-
(författare)
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Studying neutron-induced fission at IGISOL-4 : From neutron source to yield measurements and model comparisons
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fission yields represent the probability of producing a certain nuclide in a fission event, and are important observables for fission research. For applications, accurate knowledge of the yields is fundamental at all stages of the fuel cycle of nuclear reactors, e.g., for reactivity calculations, or to estimate (spent) fuel inventory. Fission yields also help in the basic understanding of the fission process, for nucleosynthesis models, and for radioactive ion beam production.This thesis was developed in the framework of the AlFONS project, the objective of which was to measure neutron-induced fission yields of relevance for partitioning and transmutation of spent fuel. The work is performed at the IGISOL-4 facility in JYFL (University of Jyväskylä).The first part of this thesis work is dedicated to the development and characterisation of a suitable 9Be(p(30MeV),nx) neutron source for IGISOL-4. The neutron energy spectrum and the neutron yield from a 5mm thick converter were studied with Monte Carlo simulations. Two characterisation campaigns that validated the MCNPX code were also performed. At the maximum current available from the cyclotron at JYFL, a total neutron yield between 2 and 5×1012 neutrons/(sr s) can be obtained. This satisfies the design goal for studies of fission yields.The neutron source was used in the measurement of fission yields from high-energy neutron-induced fission of natU at IGISOL-4, discussed in the second part of this thesis. The fission products were online-separated with a dipole magnet. The isobars, with masses in the range A = 128-133, were identified using γ-spectroscopy. Data for the relative yields of tin and antimony, as well as isomeric yield ratios for five nuclides will be reported. The yields show trends not observed in the ENDF/B-VII.1 evaluation, and only in part confirmed by the GEF model.The final part of this thesis concerns a study of the performance of different nuclear model codes, that aim at describing the states of the fission fragments right after scission. Reproduction of experimental data serves to benchmark the models and it indicates, to some extent, how reliably the results can be extrapolated to regions where no data exist.A methodology to compare and test these models has been developed, which was implemented in the DEℓFIN code. DEℓFIN takes the excited fission fragments, defined by the model under test, and de-excites them in a standardised way using the nuclear model code TALYS. Eliminating any variability in the way the final observables are extracted helps focusing on each model's assumptions. DEℓFIN was tested on five models, and interesting features in the prompt neutron multiplicity were found for some of them. This study will promote a better understanding of the ideas used in the development of fission models.
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| 8. |
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| 9. |
- Rakopoulos, Vasileios
(författare)
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Isomeric yield ratio measurements with JYFLTRAP : In quest of the angular momentum of the primary fragments
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, isomeric yield ratios of twenty nuclides produced in the fission of natU and 232Th by protons at 25-MeV and natU by high-energy neutrons were studied. The experiments were performed at the IGISOL-JYFLTRAP facility at the University of Jyväskylä. It is the first time that direct ion counting is used for the determination of the intensities of the states of interest, thus avoiding dependency on knowledge of nuclear decay schemes and properties. This was possible due to the superior resolution of a Penning trap which was utilized for this work. Two different techniques were employed, namely the sideband cooling technique and the phase-imaging ion-cyclotron-resonance technique. With the former, a mass resolving power of m/δm = 105 can be routinely achieved, while the latter, which was recently implemented at JYFLTRAP, offers an increase in the mass resolving power by a factor of ten. In addition, isomeric yield ratios were also determined by means of γ-ray spectroscopy.From a comparison of the same isomeric pair from two different reactions, a dependency on the fissioning system can be observed. This indicates an effect of the fission mode to the yield ratio. Moreover, the evolution of the odd-A isotopes of Cd and In in the mass range A = 119 - 127 exhibit two distinguishably different trends. The ratios for the isotopes of In decrease with increasing mass, while the ratios for the isotopes of Cd are almost constant until mass number A = 125, where an increase can be noticed.The origins of the angular momentum in the fission fragments is one of the long-standing questions regarding the fission process. Surprisingly, fission fragments have been observed to carry a considerable amount of angular momentum, even from fissioning systems with very low (or even zero) angular momentum. So far, the angular momentum can only be inferred from other fission observables, such as the isomeric yield ratios. In this work, a methodology was developed in order to deduce the root-mean-square angular momentum (Jrms) of the primary fragments by employing the nuclear reaction code TALYS.Lower values of Jrms for the more spherical nuclei, near the closed-shell neutron configuration at N = 82, and higher ones for fragments with odd proton number have been deduced, in agreement with other studies. Moreover, a correlation between the angular momentum of the primary fragments with the electric quadrupole moments of the products was observed for the isotopes of In. The data can be used to gain insight into scission configuration and as guide for models that propose mechanisms for the generation of the angular momentum. Furthermore, the observed correlation is an indication of the role that the repulsive Coulomb force, together with the shape of the nascent fragment, play in the generation of the fragments’ angular momentum.
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| 10. |
- 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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