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Studying neutron-induced fission at IGISOL-4: From neutron source to yield measurements and model comparisons
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
##### Abstract [en]

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.

##### Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. , p. 109
##### Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1550
##### National Category
Subatomic Physics
##### Identifiers
ISBN: 978-91-513-0052-8 (print)OAI: oai:DiVA.org:uu-328484DiVA, id: diva2:1136346
##### Public defence
2017-10-13, 4001, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
##### Supervisors
Available from: 2017-09-20 Created: 2017-08-28 Last updated: 2017-10-17
##### List of papers
1. A neutron source for IGISOL-JYFLTRAP: Design and characterisation
Open this publication in new window or tab >>A neutron source for IGISOL-JYFLTRAP: Design and characterisation
2017 (English)In: European Physical Journal A, ISSN 1434-6001, E-ISSN 1434-601X, Vol. 53, no 173Article in journal (Refereed) Published
##### Abstract [en]

A white neutron source based on the Be(p,nx) reaction for fission studies at the IGISOLJYFLTRAP facility has been designed and tested. 30 MeV protons impinge on a 5mm thick water-cooled beryllium disc. The source was designed to produce at least 1012 fast neutrons/s on a secondary fission target, in order to reach competitive production rates of fission products far from the valley of stability.

The Monte Carlo codes MCNPX and FLUKA were used in the design phase to simulate the neutron energy spectra. Two experiments to characterise the neutron field were performed: the first was carried out at The Svedberg Laboratory in Uppsala (SE), using an Extended-Range Bonner Sphere Spectrometer and a liquid scintillator which used the time-of-flight (TOF) method to determine the energy of the neutrons; the second employed Thin-Film Breakdown Counters for the measurement of the TOF, and activation foils, at the IGISOL facility in Jyväskylä (FI). Design considerations and the results of the two characterisation measurements are presented, providing benchmarks for the simulations.

##### National Category
Subatomic Physics
##### Identifiers
urn:nbn:se:uu:diva-328569 (URN)10.1140/epja/i2017-12362-x (DOI)000408661200001 ()
##### Funder
Swedish Radiation Safety AuthoritySwedish Nuclear Fuel and Waste Management Company, SKB Available from: 2017-08-26 Created: 2017-08-26 Last updated: 2017-12-01Bibliographically approved
2. A methodology for the intercomparison of nuclear fission codes using TALYS
Open this publication in new window or tab >>A methodology for the intercomparison of nuclear fission codes using TALYS
2017 (English)In: ND 2016: International Conference On Nuclear Data For Science And Technology / [ed] Plompen, A.; Hambsch, FJ.; Schillebeeckx, P.; Mondelaers, W.; Heyse, J.; Kopecky, S.; Siegler, P.; Oberstedt, S., Les Ulis: EDP Sciences, 2017, Vol. 146, article id 04047Conference paper, Published paper (Refereed)
##### Abstract [en]

Codes for the calculation of fission observables are frequently used to describe experimentally observed phenomena as well as provide predictions in cases where measurements are missing. Assumptions in the models, and tuning of parameters within the codes, often result in a good reproduction of experimental data. In this work we propose a methodology, coded in the newly developed program DELFIN (De-Excitation of FIssion fragmeNts), that can be used to compare some of the assumptions of the various models. Our code makes use of the fission fragments information after scission and processes them in an independent and consistent fashion to obtain measurable fission observables (such as ν(A) distributions and Isomeric Fission Yield ratios). All the available information from the models, such as fragments' excitation energies, spin distributions and yields are provided as input to DELFIN that uses the nuclear reaction code TALYS to handle the de-excitation of the fission fragments. In this way we decouple the fragments relaxation from the actual fission models. We report here the first results of a comparison carried out on the GEF, Point-by-Point and FREYA models for thermal fission of 235U and 239Pu and spontaneous fission of 252Cf.

##### Place, publisher, year, edition, pages
Les Ulis: EDP Sciences, 2017
##### Series
EPJ Web of Conferences, ISSN 2100-014X
##### National Category
Subatomic Physics
##### Research subject
Physics with specialization in Applied Nuclear Physics
##### Identifiers
urn:nbn:se:uu:diva-317442 (URN)10.1051/epjconf/201714604047 (DOI)000426429500146 ()978-2-7598-9020-0 (ISBN)
##### Conference
ND 2016: International Conference on Nuclear Data for Science and Technology, September 11-16, 2016, Bruges, Belgium.
##### Funder
Swedish Nuclear Fuel and Waste Management Company, SKBSwedish Radiation Safety Authority Available from: 2017-03-14 Created: 2017-03-14 Last updated: 2018-07-03Bibliographically approved
3. Comparison of Fission Models with the DElFIN code
Open this publication in new window or tab >>Comparison of Fission Models with the DElFIN code
##### Abstract [en]

Nuclear model codes are used to describe aspects of the fission process. The general aim is a better understanding of the states of the fragments right after scission. A successful description of the available experimental data serves as benchmark for the models and determines the reliability of extrapolations to other fissioning systems and energy domains, where no experimental data exist.

The DElFIN code has been developed as a tool to compare and test nuclear fission codes. This can be done using the quantities defined right after scission by the fission models and introducing a transparent and consistent way of handling the fragments' de-excitation. Eliminating any variability in the way the final observables are extracted can help focus on the models' assumptions.

In this work, we present the comparison of the $\bar{\nu}$(A) extracted from DElFIN to using excitation energies from GEF, PbP, FREYA, FIFRELIN and CGMF codes.

##### National Category
Subatomic Physics
##### Identifiers
urn:nbn:se:uu:diva-328570 (URN)
Available from: 2017-08-26 Created: 2017-08-26 Last updated: 2017-08-28
4. Production of Sn and Sb isotopes in high-energy neutron induced fission of natU
Open this publication in new window or tab >>Production of Sn and Sb isotopes in high-energy neutron induced fission of natU
2018 (English)In: European Physical Journal A, ISSN 1434-6001, E-ISSN 1434-601X, Vol. 54, article id 33Article in journal (Refereed) Published
##### Abstract [en]

The first systematic measurement of neutron-induced fission yields has been performed at the upgraded IGISOL-4 facility at the University of Jyvaskyla, Finland. The fission products from high-energy neutron-induced fission of U-nat were stopped in a gas cell filled with helium buffer gas, and were online separated with a dipole magnet. The isobars, with masses in the range A = 128-133, were transported to a tape-implantation station and identified using gamma-spectroscopy. We report here the relative cumulative isotopic yields of tin (Z = 50) and the relative independent isotopic yields of antimony (Z = 51). Isomeric yield ratios were also obtained for five nuclides. The yields of tin show a staggered behaviour around A = 131, not observed in the ENDF/B-VII. 1 evaluation. The yields of antimony also contradict the trend from the evaluation, but are in agreement with a calculation performed using the GEF model that shows the yield increasing with mass in the range A = 128-133.

##### National Category
Subatomic Physics
##### Identifiers
urn:nbn:se:uu:diva-328572 (URN)10.1140/epja/i2018-12462-1 (DOI)000428637900002 ()
##### Funder
EU, FP7, Seventh Framework Programme, 605203Swedish Radiation Safety AuthoritySwedish Nuclear Fuel and Waste Management Company, SKB Available from: 2017-08-26 Created: 2017-08-26 Last updated: 2018-11-06Bibliographically approved

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