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Fragment-mass distributions in neutron-induced fission of 232Th and 238U at 33, 45, and 60 MeV
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2011 (English)In: Physical Review C. Nuclear Physics, ISSN 0556-2813, E-ISSN 1089-490X, Vol. 83, no 5, 054603- p.Article in journal (Refereed) Published
Abstract [en]

We have measured fission fragment-mass yields for neutron-induced fission of Th-232 and U-238 at energies 32.8, 45.3, and 59.9 MeV. The experiments were done at quasimonoenergetic neutron beams of the Cyclotron Research Center at Louvain-la-Neuve. To detect the fission fragments, a multisection Frisch-gridded ionization chamber was used. The measurement and data analysis techniques are discussed in detail. The obtained mass yields are compared to model calculations with the intermediate-energy nuclear reaction code MCFX. The MCFX code is used to calculate the fraction of fissioning nuclei after cascade, preequilibrium, and statistical reaction stages. The formation of mass distributions is considered as a result of oscillations of the mass-asymmetry degree of freedom in the potential well calculated with the temperature-dependent shell correction method. The experimental results as well as the results of the model calculations demonstrate that the probability of symmetric fission increases with incident neutron energy for both nuclei. The comparison also shows that the symmetric fission is more enhanced for thorium than for uranium with increasing neutron energy. We also compare U-238 results with available experimental data; the Th-232 data were measured for the first time.

Place, publisher, year, edition, pages
2011. Vol. 83, no 5, 054603- p.
National Category
Physical Sciences
URN: urn:nbn:se:uu:diva-134765DOI: 10.1103/PhysRevC.83.054603ISI: 000290309200003OAI: oai:DiVA.org:uu-134765DiVA: diva2:373544
Available from: 2010-11-30 Created: 2010-11-30 Last updated: 2013-02-18
In thesis
1. Fragment Mass Distributions in Neutron-Induced Fission of 232Th and 238U from 10 to 60 MeV
Open this publication in new window or tab >>Fragment Mass Distributions in Neutron-Induced Fission of 232Th and 238U from 10 to 60 MeV
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2010. 58 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 793
ADS, neutron-induced fission, intermediate neutron energies, fragment mass yield
National Category
Subatomic Physics
Research subject
Physics with specialization in Applied Nuclear Physics
urn:nbn:se:uu:diva-134952 (URN)978-91-554-7971-8 (ISBN)
Public defence
2011-01-21, Häggsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 723Available from: 2010-12-21 Created: 2010-12-03 Last updated: 2011-03-21Bibliographically approved

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