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- Cannarozzo, Simone
(författare)
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Isomeric yield ratio studies in nuclear reactions and alpha-particle induced fission of Thorium
- 2024
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Despite decades of research since the initial discovery of nuclear fission, numerous unresolved questions still persist. It is known empirically that fission fragments emerge with high angular momentum. The mechanism responsible for the generation of the large angular momenta observed is one of these open questions. Since the characteristics of fission fragments are not directly measurable, experimentally accessible observables are used to derive the angular momenta using nuclear model codes. One of these observables is the yield ratio between fission products produced in different isomeric states, i.e., metastable energy levels of the same nucleus.In this thesis, a study of the level density models implemented in the nuclear model code TALYS is presented. Simulated and experimental isomeric yield ratios of a large number of nuclear reactions is compared. The results show a bias in the models that favours the population of the high-spin states and that this can be produced by the overestimation of the spin width distribution. The reason for this study is to improve the models then used in the angular momentum calculation. Moreover, the isomeric yield ratio measurement of twenty-one FFs is presented. The measurement was performed using the JYFLTRAP system at the University of Jyväskylä. The fission fragments were produced by the 32 MeV alpha-particle induced fission of 232Th. The analysis process, involving different identification and correction methods, and preliminary results are presented.
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| 2. |
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Fission Product Yields Data Current status and perspectives : Summary report of an IAEA Technical meeting
- 2016
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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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| 3. |
- 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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| 4. |
- Gomez L, Ana Maria, 1993-
(författare)
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Simulations And Experiments Of Plasma-Induced Effects In Silicon Detectors
- 2023
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- When an atomic nucleus undergoes fission, two fragments with different mass and kinetic energy are emitted. The highly unstable fission fragments (FFs) evaporate prompt neutrons soon after the nucleus splits. A precise measurement of both, the mass yield distribution of the FFs and the average prompt neutron emission, $\bar{\nu}$, is important not only for current nuclear technologies but also for the development of future technologies such as Generation IV nuclear power plants. Moreover, the experimental determination of the mass yield distributions, both pre- and post-neutron emission, is valuable for testing fission models. Additionally, a precise measurement of the average neutron multiplicity as a function of the FFs mass, , is crucial in the understanding of how the excitation energy is shared between nascent FFs. The VElocity foR DIrect particle identification spectrometer (VERDI) is designed to achieve pre- and post-fission mass distributions with resolutions between 1-2 u. VERDI is a double-energy double-velocity instrument that consists of two arms. On each arm is operated one Microchannel Plate detector (MCP) for the collection of the FFs start time and up to 32 Passive Implanted Planar Silicon (PIPS) detectors for the stop time and energy detection of the FFs. However, challenges in the experimental measurements with VERDI arise due to the high degree of ionization (plasma) in the detector material from the interaction with the FFs. The plasma causes a delay in the charge carriers' migration for the signal start, known as the plasma delay time effect (PDT). Furthermore, the recombination of charge carriers in the plasma causes a shrinking in the signal's height, known as pulse height defect (PHD). This phenomenon leads to inaccuracies in the measurement of FFs mass distributions and increased systematic uncertainties. Previous studies on PDT and PHD have shown varying behaviors across different detector types, which motivated dedicated studies in the type of PIPS detectors used in VERDI. An experimental campaign to characterize the PDT and PHD in PIPS detectors was conducted in the LOHENGRIN recoil separator, which is part of the ILL nuclear facility in Grenoble, France. Measurements of FFs in a range of masses between 80 u and 149 u, with energies between 20 MeV to 110 MeV, were taken to fully characterize six PIPS detectors. The resulting PDT and PHD values were 1 ns to 4 ns and 2 MeV to 10 MeV respectively. The PDT and PHD exhibited consistent energy and mass dependencies across the detectors, which enables the possibility of an event-by-event correction of VERDI data. In this thesis, the basis for discussing the results of the studies of the PDT and PHD effects will be presented.
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