| 1. |
- Gomez L, Ana Maria, 1993-, et al.
(författare)
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Determination of the Plasma Delay Time in PIPS detectors for fission fragments at the LOHENGRIN spectrometer
- 2023
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Ingår i: 15<sup>th</sup> International Conference on Nuclear Data for Science and Technology (ND2022). - : EDP Sciences.
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Konferensbidrag (refereegranskat)abstract
- The VElocity foR Direct particle Identification spectrometer (VERDI) is a 2E-2v fission spectrometer that allows the measurement of the total mass distribution of secondary fission fragments with a resolving power of 1-2 u. It consists of two time-of-flight (ToF) arms, with one Micro Channel Plate (MCP) detector and up to 32 Silicon PIPS (Passive Implanted Planar Silicon) detectors per arm. The MCPs provide the start timing signals and the PIPS detectors provide both the energy and the stopping ToF signals. In real conditions, the PIPS signals are affected by the formation of plasma from the interaction between the heavy ions and the detector material. The plasma contributes to a reduction in signal amplitude, resulting in a Pulse Height Defect (PHD), and introduces a signal delay, known as Plasma Delay Time (PDT). An experiment to characterize the PDT and PHD was performed at the LOHENGRIN recoil separator of the Institut Laue Langevin (ILL). Characteristic fission fragments from the 239Pu(n,f) reaction were separated based on their A/Q and E/Q ratios, allowing the measurement of a wide range of energies from 21 to 110 MeV and masses between 80 and 149 u. Six PIPS detectors were characterized to study their individual responses to the PDT and PHD effects. The signals were recorded in a digital acquisition system to completely exploit the offline analysis capabilities. Achieved combined timing and energy resolutions for fission fragments varied between 72(2) ps and 100(4) ps and 1.4% - 2% (FWHM), respectively. Preliminary PHD and PDT data are presented from the masses A=85, 95, 130 and 143. The PHD trends are strongly correlated with both the ion energy and mass. The PDT, on the other hand, shows a strong variation as a function of the ion kinetic energy but a smaller dependence on the ion mass.
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| 2. |
- Al-Adili, Ali, et al.
(författare)
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Employing TALYS to deduce angular momentum rootmean-square values, J(rms), in fission fragments
- 2020
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Ingår i: ND 2019. - : E D P SCIENCES. - 9782759891061
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Konferensbidrag (refereegranskat)abstract
- Fission fragments exhibit large angular momenta J, which constitutes a challenge for fission models to fully explain. Systematic measurements of isomeric yield ratios (IYR) are needed for basic nuclear reaction physics and nuclear applications, especially as a function of mass number and excitation energy. One goal is to improve the current understanding of the angular momentum generation and sharing in the fission process. To do so, one needs to improve the modeling of nuclear de-excitation. In this work, we have used the TALYS nuclear-reaction code to relax excited fission fragments and to extract root-mean-square (rms) values of initial spin distributions, after comparison with experimentally determined IYRs. The method was assessed by a comparative study on Cf-252(sf) and (235)(nth,f). The results show a consistent performance of TALYS, both in comparison to reported literature values and to other fission codes. A few discrepant Jrms values were also found. The discrepant literature values could need a second consideration as they could possibly be caused by outdated models. Our TALYS method will be refined to better comply with contemporary sophisticated models and to reexamine older deduced values in literature.
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| 3. |
- Al-Adili, Ali, et al.
(författare)
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Isomer yields in nuclear fission
- 2021
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Ingår i: EPJ Web of Conferences. - : EDP Sciences. - 2100-014X. ; 256
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Tidskriftsartikel (refereegranskat)abstract
- The generation of angular momentum in the fission process is still an open question. To shed light on this topic, we started a series of measurements at the IGISOL-JYFLTRAP facility in Finland. Highprecision measurements of isomeric yield ratios (IYR) are performed with a Penning trap, partly with the aim to extract average root-mean-square (rms) quantities of fragment spin distributions. The newly installed Phase-Imaging Ion-Cyclotron Resonance (PI-ICR) technique allows the separation of masses down to tens of keV, which is suffcient to disentangle many isomers. In this paper, we first summarize the previous measurements on the neutron and proton-induced fission of uranium and thorium, e.g. the odd cadmium and indium isotopes (119 ≤ A ≤ 127). The measurements revealed systematic trends as function of mass number, which stimulated further exploration. A recent measurement was performed at IGISIOL and several new IYR data will soon be published, for the first time. Secondly, we employ the TALYS nuclear-reaction code to model one of the newly measured isomer yields. Detailed GEF and TALYS calculations are discussed for the fragment angular momentum distribution in 134I.
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| 4. |
- Al-Adili, Ali, et al.
(författare)
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Prompt fission neutron yields in thermal fission of U-235 and spontaneous fission of Cf-252
- 2020
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Ingår i: Physical Review C. - : American Physical Society (APS). - 2469-9985 .- 2469-9993. ; 102:6
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Tidskriftsartikel (refereegranskat)abstract
- Background: The sharing of excitation energy between the fission fragments is one of the key issues in studying nuclear fission. One way to address this is by studying prompt-fission neutron multiplicities as a function of other fission observables such as the mass, (nu) over bar (A). These are vital benchmark data for both fission and nuclear deexcitation models, putting constrains on the fragment excitation energy and hence on the competing prompt neutron/gamma-ray emission. Despite numerous detailed studies, recent measurements done at JRC-Geel with the SCINTIA array in the epithermal region show surprisingly strong discrepancies to earlier thermal fission data and the Wahl systematics. Purpose: The purpose was to perform measurements of the prompt-fission neutron multiplicity, as a function of fragment mass and total kinetic energy (TKE), in U-235(n(th), f) and Cf-252(sf), to verify and extend the SCINTIA results. Another goal was to validate the analysis methods, and prepare for planned investigations at excitation energies up to 5.5 MeV. Methods: The experiments were conducted at the former 7 MV Van de Graaff facility in JRC-Geel, using a Twin Frisch-Grid Ionization Chamber and two liquid scintillation detectors. A neutron beam with an average energy of 0.5 MeV was produced via the Li-7(p,n) reaction. The neutrons were thermalized by a 12 cm thick block of paraffin. Digital data acquisition systems were utilized. Comprehensive simulations were performed to verify the methodology and to investigate the role of the mass and energy resolution on measured (nu) over bar (A) and (nu) over bar (TKE) values. The simulation results also revealed that the partial derivative(nu) over bar/partial derivative A and partial derivative(TKE) over bar/partial derivative(nu) over bar are affected by the mass and energy resolution. However, the effect is small for the estimated resolutions of this work. Detailed Fluka simulations were performed to calculate the fraction of thermal neutron-induced fission, which was estimated to be about 98%. Results: The experimental results on (nu) over bar (A) are in good agreement with earlier data for Cf-252(sf). For U-235(n(th), f), the (nu) over bar (A) data is very similar to the data obtained with SCINTIA, and therefore we verify these disclosed discrepancies to earlier thermal data and to the Wahl evaluation. The experimental results on (nu) over bar (TKE) are also in agreement with the data at epithermal energies. For Cf-252(sf) a slope value of partial derivative(TKE) over bar/partial derivative(nu) over bar = (-12.9 f 0.2) MeV/n was obtained. For U-235(n(th), f) the value is (-12.0 +/- 0.1) MeV/n. Finally, the neutron spectrum in the center-of-mass system was derived and plotted as a function of fragment mass. Conclusions: This work clearly proves the lack of accurate correlation between fission fragment and neutron data even in the best-studied reactions. The new results highlight the need of a new evaluation of the prompt-fission multiplicity for U-225(n(th), f).
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| 5. |
- Cannarozzo, Simone, et al.
(författare)
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Global comparison between experimentally measured isomeric yield ratios and nuclear model calculations
- 2023
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Ingår i: European Physical Journal A. - : Springer. - 1434-6001 .- 1434-601X. ; 59:12
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Tidskriftsartikel (refereegranskat)abstract
- The level density steers transition probabilities between different states in the decay and de-excitation of excited nuclei. Reliable level density modelling is, therefore, key in describing, e.g., de-excitation of fission fragments, with implications on neutron and gamma-rays multiplicities, and also manifested in the population of isomeric states. We test six currently used level density models and the spin distribution in the level density by comparing calculations with measured isomeric yield ratios. The model calculations are performed with the TALYS code and experimental data for nuclear reactions populating spin isomers are retrieved from the EXFOR database. On average, calculations are in agreement with measured data. However, we find that the population of the high-spin state in an isomeric pair is clearly favoured in all of the six studied level density models. Further studies are then performed on the three used phenomenological level density models, to investigate the significance of their effect. We find that a significant reduction of the spin width distribution improves the agreement between calculated and experimentally observed isomeric yield ratios. This result is independent of the incident particle in the nuclear reaction. The needed reduction of the spin width distribution to comply with empirical data has, e.g., implications for studies in angular momentum generation in fission using isomeric yield rations, calculations of anti-neutrino spectra from nuclear reactors, as well as neutron and gamma-ray multiplicities in nuclear reactor calculations.
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| 6. |
- 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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| 7. |
- Chiodaroli, Marco, et al.
(författare)
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“Every morning I take two steps to my desk…” : students’ perspectives on distance learning during the COVID-19 pandemic
- 2024
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Ingår i: Higher Education. - : Springer Science+Business Media B.V.. - 0018-1560 .- 1573-174X.
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Tidskriftsartikel (refereegranskat)abstract
- During the COVID-19 pandemic, distance learning became the predominant teaching method at most universities, exposing students and teachers alike to novel and unexpected challenges and learning opportunities. Our study is situated in the context of higher physics education at a large Swedish university and adopts a mixed-methods approach to explore how students perceive shifts to distance learning. Quantitative student survey responses comparing distance learning during the pandemic with previous in-person learning are analyzed with k-means cluster analysis and with a random-intercept multilevel linear model. Combined analyses produce a consistent picture of students who report having experienced the greatest challenges. They are on average younger, report being less autonomous in their learning, and find it harder than peers to ask questions to the instructor. They are also less likely to have access to a place where they can study without interruptions. Variation across courses is small with students being largely subjected to the same set of challenges. Qualitative data from semi-structured focus group interviews and open-ended questions supports these findings, provides a deeper understanding of the struggles, and reveals possibilities for future interventions. Students report an overall collapse of structure in their learning that takes place along multiple dimensions. Our findings highlight a fundamental role played by informal peer-to-peer and student-instructor interactions, and by the exchange of what we refer to as “structural information.” We discuss implications for teachers and institutions regarding the possibility of providing support structures, such as study spaces, as well as fostering student autonomy.
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| 8. |
- Gao, Zhihao, et al.
(författare)
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Applying machine learning methods for the analysis of two-dimensional mass spectra
- 2023
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Ingår i: European Physical Journal A. - : Springer Nature. - 1434-6001 .- 1434-601X. ; 59:169
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Tidskriftsartikel (refereegranskat)abstract
- In a measurement of isomeric yield-ratios in fission, the Phase-Imaging Ion-Cyclotron-Resonance technique, which projects the radial motions of ions in the Penning trap (JYFLTRAP) onto a position-sensitive micro-channel plate detector, has been applied. To obtain the yield ratio, that is the relative population of two states of an isomer pair, a novel analysis procedure has been developed to determine the number of detected ions in each state, as well as corrections for the detector efficiency and decay losses. In order to determine the population of the states in cases where their mass difference is too small to reach full separation, a Bayesian Gaussian Mixture model was implemented. The position-dependent efficiency of the micro-channel plate detector was calibrated by mapping it with 133Cs+ ions, and a Gaussian Process was trained with the position data to construct an efficiency function that could be used to correct the recorded distributions. The obtained numbers of counts of excited and ground-state ions were used to derive the isomeric yield ratio, taking into account decay losses as well as feeding from precursors.
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| 9. |
- Gao, Zhihao, et al.
(författare)
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Benchmark of a multi-physics Monte Carlo simulation of an ionguide for neutron-induced fission products
- 2022
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Ingår i: European Physical Journal. - : Springer Nature. - 1286-0042 .- 1286-0050. ; 58:2
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Tidskriftsartikel (refereegranskat)abstract
- To enhance the production of medium-heavy,neutron-rich nuclei, and to facilitate measurements of independentyields of neutron-induced fission, a proton-toneutronconverter and a dedicated ion guide for neutroninducedfission have been developed for the IGISOL facilityat the University of Jyväskylä. The ion guide holds thefissionable targets, and the fission products emerging fromthe targets are collected in helium gas and transported to thedownstream experimental stations.Acomputer model, basedon a combination of MCNPX for modeling the neutron production,the fission code GEF, and GEANT4 for the transportof the fission products, was developed. The model willbe used to improve the setup with respect to the productionand collection of fission products. In this paper we benchmarkthe model by comparing simulations to a measurementin which fission products were implanted in foils located atdifferent positions in the ion guide. In addition, the productsfrom neutron activation in the titanium foil and the uraniumtargets are studied. The result suggests that the neutron fluxat the high-energy part of the neutron spectrum is overestimatedby approximately 40%.However, the transportation offission products in the uranium targets agrees with the experimentwithin 10%. Furthermore, the simulated transportationof fission products in the helium gas achieves almost perfectagreement with the measurement. Hence, we conclude thatthe model, after correction for the neutron flux, is well suitedfor optimization studies of future ion guide designs.
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| 10. |
- Gao, Zhihao
(författare)
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Benchmark of simulation of an ion guide for neutron-induced fission products
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Independent yield distributions of high-energy neutron-induced fission are of importance to achieve a good understanding of fission. Even though the mass and charge yield distributions of thermal neutron-induced fission are well known, there are few experimental data for high-energy neutron-induced fission. In addition to basic research on the fission process, independent yield distributions of high-energy neutron-induced fission play a key role in the development of Generation IV fast nuclear reactors.To facilitate measurements of independent fission yields of high-energy neutron-induced fission, a dedicated ion guide and a proton-neutron converter were developed and put to use in experiments at the isotope separator facility IGISOL in Jyväskylä. In parallel, a simulation model of the system was developed in order to optimize the collecting efficiency of fission products in the ion guide. The model uses the Monte Carlo code MCNPX to simulate the neutron production, the fission model code GEF to simulate the fission process, and GEANT4 for ion transportation. In order to benchmark the simulation model, metal foils were inserted in the ion guide with the purpose of collecting fission products. At the same time, nickel, cobalt and indium foils were located between the pn-converter and the ion guide to record the neutron flux from the pn-converter. After the beam was turned off, and after several days of cooling, g-ray spectroscopy measurements of the foils were conducted using a well shielded HPGe detector. Based on the identified g-ray transitions in the spectroscopy data, the productions of corresponding fission products and neutron activation products were calculated, and then used to benchmark the transportation and collection of fission products, as well as neutron production, in the simulations.The conclusion from the benchmark is that the transportation of fission products in the helium gas, as simulated by GEANT4, agrees very well with the measurement, while the transportation of fission products in the uranium targets agrees with the measurement within 10%. The neutron flux at the high-energy part of the neutron spectrum is overestimated by about 40%.Thanks to the benchmark it has been shown that the predictive power of the model is satisfactory and sufficient for the purpose of modeling the ion guide. Furthermore, the parameters involved in the simulations, such as neutron production, distance between the neutron source and the ion guide, volume of the ion guide and so on, play an important role in the optimization of the setup. However, the lower than expected fission rate suggests that the optimization on these parameters may not be enough to achieve a sufficiently high intensity of fission products, especially for nuclei far from the stability line. To achieve a sufficiently high intensity, an electric field guidance, similar to the RF structure of the CARIBU gas catcher presented in G.Savard et al. Nucl. Inst. Meth. B, 376: 246, 2016, to collect fission products is considered.
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