Delayed neutron measurements for²³²Th neutron-induced fission

Abstract

Abstract. Delayed neutrons (DN) play an important role in nuclear reactor physics. Innovative critical reactor studies bring to light the need of new DN yields data. For the Th fuel cycle, according to the OECD recommendation, DN of the 232 Th is needed with an accuracy of 5%. In the literature, significant discrepancies were observed for energies below 4 MeV and data are dispersed around 14 MeV. Therefore, a programme has been undertaken by CEA in order to measure DN yields from 232 Th with incident neutron energies from 2 to 16 MeV. In this paper, the experimental setup will be described and preliminary results obtained at the PTB Ion Accelerator Facility of Braunschweig for incident neutron beam energies of 2, 3, 4, 6, 7, 10 and 16 MeV will be presented. MotivationsDelayed neutrons (DN) emitted after fission are of prime interest for several topics. Among others, they allow the control of a nuclear reactor. With the development of reactors of new generation and/or new fuels, additional requirements appear in determining DN yields or to measure more accurately existing data. For the innovative thorium cycle, data on DN yields of 232 Th are needed. Unfortunately, available data in the literature are scarce and important discrepancies are observed as can be seen in Figure 1. Moreover, inside the recent ISTC project, results obtained between 3 and 5 MeV seem to indicate a quasi constant yield in this energy region which deviates from the energy dependence predicted by Yoshida et al. [1]. These observations have motivated physicists of CEA (Commissariatà l'énergie atomique et auxénergies alternatives) to undertake an experimental program for measurements of DN yields of 232 Th for the incident neutron energy range from 2 MeV up to 16 MeV.The experimental procedure, the neutron production facility and the detection setup will be described in the section 2. In section 3, analysis and simulations will be presented. Preliminary results will be shown in section 4. Experiment MethodologyEmission of delayed neutrons follows the beta decay of some fissions fragments called precursors. There are more than 200 different fragment isotopes involved which are usually lumped in 6 groups This is an Open Access article distributed under the terms of the Creative Commons Attribution-Noncommercial License 3.0, which permits unrestricted use, distribution, and reproduction in any noncommercial medium, provided the original work is properly cited.