Title: Investigation of Mass-TKE Distributions of Fission Fragments from the 235U(n,f)-Reaction in Resonances
Citation: Proceedings of the 13th International Seminar on Interaction of Neutrons with Nuclei (ISINN-13) - Neutron Spectroscopy, Nuclear Structure, Related Topics p. 351-359
Publisher: Joint Institute for Nuclear Research, 2006
Publication Year: 2006
JRC Publication N°: JRC34313
URI: http://publications.jrc.ec.europa.eu/repository/handle/JRC34313
Type: Contributions to Conferences
Abstract: The aim of the present work was to obtain experimental data on the incident neutron energy dependence of the relative weights of the two main asymmetric fission modes, which make up the mass distribution of the 235U(n,f)-reaction. This dependence of the fission modes as a function of incident neutron energy in 235U(n,f) was discovered in Ref. [1]. But so far it is the only measurement where fission mode fluctuations in resonance neutron induced fission were detected. The statistical significance of the data of Ref. [1] below 8 eV was not high enough to draw conclusions on the physical nature of the fluctuations. Based on the progress in fission cross-section and the multi-modal model calculations reported in Ref.[2] it was found reasonable to analyse the data of the measurement reported in Ref. [3]. Measurements of fission fragment (FF) mass and total kinetic energy (TKE) distributions of 235U(n,f) were carried out at the Dubna IBR-30 time of flight spectrometer in the resonance neutron energy range from thermal up to 35 eV. In the energy range from thermal to 8.77 eV individual resonances could be resolved, but for the higher incident neutron energies the resolution degraded due to the rather large (4 s) width of the neutron pulse. The data analysis was carried out in two steps. At first the experimental difficulties were investigated i.e. the very close proximity to the source, the alignment of the ionisation chamber was checked and necessary improvements were made. At the final step FF mass yield and TKE distributions were evaluated for various incident neutron energy intervals and the difference of these values obtained for thermal and resonance neutron energies was constructed. In contrast to Ref. [1] we realised true relative measurements (with respect to thermal energy). The data analysis confirmed the main characteristics of the asymmetric fission modes obtained in Ref. [1] from thermal neutron induced fission of 235U as well as the scale of the fluctuation of the fission mode weights in different neutron resonances (~15% for the standard I and ~4% for the standard II mode). Some differences between the results of the present work to the results obtained in Ref. [1] could be explained by the worse neutron energy resolution of the present investigation as well as due to the data analysis, where new calibration results obtained after the publication of Ref. [1] were used. References 1. F.-J. Hambsch, H.-H. Knitter, C. Budtz-Jorgensen, and J.P. Theobald, Nucl. Phys. A491, 56 (1989). 2. F. -J. Hambsch, Proc. 12th Int. Seminar on Interaction of Neutrons with Nuclei (ISINN-12), Dubna, Russia, May 26-29, 2004. 3. Sh. S. Zeinalov, M. Florek, W.I. Furman, V.A. Kriatchkov and Yu. S. Zamyatnin, Proc. 7th Int. Seminar on Interaction of Neutrons with Nuclei (ISINN-7), Dubna, Russia, May 25-28, 1999.
JRC Institute:Institute for Reference Materials and Measurements

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