| 11. |
- 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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| 12. |
- 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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| 13. |
- Domingo-Pardo, C., et al.
(författare)
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Advances and new ideas for neutron-capture astrophysics experiments at CERN n_TOF
- 2023
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Ingår i: European Physical Journal A. - : Springer. - 1434-6001 .- 1434-601X. ; 59:1
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Tidskriftsartikel (refereegranskat)abstract
- This article presents a few selected developments and future ideas related to the measurement of (n,γ) data of astrophysical interest at CERN n_TOF. The MC-aided analysis methodology for the use of low-efficiency radiation detectors in time-of-flight neutron-capture measurements is discussed, with particular emphasis on the systematic accuracy. Several recent instrumental advances are also presented, such as the development of total-energy detectors with γ-ray imaging capability for background suppression, and the development of an array of small-volume organic scintillators aimed at exploiting the high instantaneous neutron-flux of EAR2. Finally, astrophysics prospects related to the intermediate i neutron-capture process of nucleosynthesis are discussed in the context of the new NEAR activation area.
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| 14. |
- Domingo-Pardo, C., et al.
(författare)
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Compton imaging for enhanced sensitivity (n,gamma) cross section TOF experiments : Status and prospects
- 2023
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Ingår i: 15TH INTERNATIONAL CONFERENCE ON NUCLEAR DATA FOR SCIENCE AND TECHNOLOGY, ND2022. - : EDP Sciences.
-
Konferensbidrag (refereegranskat)abstract
- Radiative neutron-capture cross sections are of pivotal importance in many fields such as nucle-osynthesis studies or innovative reactor technologies. A large number of isotopes have been measured with high accuracy, but there are still a large number of relevant isotopes whose cross sections could not be experimentally determined yet, at least with sufficient accuracy and completeness, owing to limitations in detection techniques, sample production methods or in the facilities themselves. In the context of the HYMNS (High-sensitivitY Measurements of key stellar Nucleo-Synthesis reactions) project over the last six years we have developed a novel detection technique aimed at background suppression in radiative neutron-capture time-of-flight measurements. This new technique utilizes a complex detection set-up based on position-sensitive radiation-detectors deployed in a Compton-camera array configuration. The latter enables to implement gamma-ray imaging techniques, which help to disentangle true capture events arising from the sample under study and contaminant background events from the surroundings. A summary on the main developments is given in this contribution together with an update on recent experiments at CERN n_TOF and an outlook on future steps.
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| 15. |
- Domingo-Pardo, C., et al.
(författare)
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The neutron time-of-flight facility n_TOF at CERN Recent facility upgrades and detector developments
- 2023
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Ingår i: Journal of Physics, Conference Series. - : Institute of Physics (IOP). - 1742-6588 .- 1742-6596. ; 2586
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Tidskriftsartikel (refereegranskat)abstract
- Based on an idea by Carlo Rubbia, the n_TOF facility at CERN has been operating for over 20 years. It is a neutron spallation source, driven by the 20 GeV/c proton beam from the CERN PS accelerator. Neutrons in a very wide energy range (from GeV, down to sub-eV kinetic energy) are generated by a massive Lead spallation target feeding two experimental areas. EAR1, horizonal with respect to the proton beam direction is set at 185 meters from the spallation target. EAR2, on the vertical line from the spallation source, is placed at 20 m. Neutron energies for experiments are selected by the time-of-flight technique (hence the name n_TOF), while the long flight paths ensure a very good energy resolution. Over one hundred experiments have been performed by the n_TOF Collaboration at CERN, with applications ranging from nuclear astrophysics (synthesis of the heavy elements in stars, big bang nucleosynthesis, nuclear cosmo-chronology), to advanced nuclear technologies (nuclear data for applications, nuclear safety), as well as for basic nuclear science (reaction mechanisms, structure and decay of highly excited compound states). During the planned shutdown of the CERN accelerator complex between 2019 and 2021, the facility went through a substantial upgrade with a new target-moderator assembly, refurbishing of the neutron beam lines and experimental areas. An additional measuring and irradiation station (the NEAR Station) has been envisaged and its capabilities for performing material test studies and new physics opportunities are presently explored. An overview of the facility and of the activities performed at CERN is presented in this contribution, with a particular emphasis on the most relevant experiments for nuclear astrophysics.
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| 16. |
- Dupont, E., et al.
(författare)
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Overview of the dissemination of n_TOF experimental data and resonance parameters
- 2023
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Ingår i: 15TH INTERNATIONAL CONFERENCE ON NUCLEAR DATA FOR SCIENCE AND TECHNOLOGY, ND2022. - : EDP Sciences.
-
Konferensbidrag (refereegranskat)abstract
- The n_TOF neutron time-of-flight facility at CERN is used for nuclear data measurements. The n_TOF Collaboration works closely with the Nuclear Reaction Data Centres (NRDC) network to disseminate the experimental data through the international EXFOR library. In addition, the Collaboration helps integrate the results in the evaluated library projects. The present contribution describes the dissemination status of n_TOF results, their impact on evaluated libraries and ongoing efforts to provide n_TOF resonance parameters in ENDF-6 format for further use by evaluation projects.
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| 17. |
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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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| 18. |
- 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
-
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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| 19. |
- 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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| 20. |
- Gao, Zhihao, et al.
(författare)
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Estimating angular momenta of fission fragments from isomeric yield ratios using TALYS
- 2024
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Ingår i: Physical Review C. - : American Physical Society. - 2469-9985 .- 2469-9993. ; 109:6
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Tidskriftsartikel (refereegranskat)abstract
- Background: Angular momenta of fission fragments considerably higher than that of the fissioning nucleus have been observed in many experiments, raising the question of how these high angular momenta are generated. Wilson et al. have proposed a model for the angular momentum as a function of the mass of fission products based on the assumption that the angular momentum is generated from the collective motion of nucleons in the ruptured neck of the fissioning system. This assumption has caused a lot of debate in the community.Purpose: To estimate the angular momenta of fission fragments based on the observed isomeric yield ratios in 25 -MeV proton -induced fission of 238 U.Method: A surrogate model of the fission code general description of fission observables (GEF) has been developed to generate properties of primary fission fragments. Based on the excitation energy and angular momentum of fission fragments from GEF, an energy -versus -angular -momentum matrix is reconstructed using a set of parameters. With such matrices as input, the reaction code TALYS is used to calculate the deexcitation of the fission fragments, including the population of the isomers, from which the isomeric yield ratios are obtained. By varying one of the parameters, the root -mean -square angular momentum ( J rms ), which determines the angular momentum distribution of the matrix, J rms -dependent isomeric yield ratios are obtained. Considering all primary fission fragments contributing to the isomeric yield ratio for a given fission product, the average angular momentum of those fragments is estimated.Results: Data of 31 isomeric yield ratios in 25 -MeV proton -induced fission of 238 U were analyzed. From the analysis, the average J rms , equivalent to average angular momentum J av , with uncertainties are obtained for 24 fission products, while in seven cases no conclusive result for the angular momentum could be obtained. Furthermore, considering the neutron emissions of the primary fission fragments, the average angular momentum as a function of the average mass number of the primary fission fragment was estimated.Conclusion: A mass dependency of the average angular momentum is observed in the proton -induced fission of 238 U. Moreover, the average angular momenta for mass numbers larger than 131 could be fairly well described by the parametrization proposed by Wilson et al. However, the average angular momenta of 130 Sn and 131 Te cannot be described by Wilson's model, which suggests a different lower limit for the validation of the parametrization in the model. In general, higher average angular momenta for A 132 are observed in the present work compared to those from Wilson et al. This is likely due to the higher excitation energy of the fissioning nuclei in this work. Furthermore, the first systematic observation of the average angular momenta of fission products in the symmetric mass region is presented. In this region, a decreasing trend with mass number is observed, which cannot be explained by the proposal in Wilson's paper. Thus, a different mechanism is needed to explain this observation.
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