Fission fragments observables measured at the LOHENGRIN spectrometer

S Julien-Laferrière,J Nicholson,M Rapala,U Köster,O Litaize,L Thombansen,O Serot,O Méplan,M Ramdhane,T Materna,P Mutti,G Kessedjian,D Bernard,A Chebboubi,C Sage

doi:10.1051/epjconf/202023905017

Abstract

Nuclear fission yields are key data for reactor studies, such as spent fuel inventory or decay heat, and for understanding fission process. Despite a significant effort allocated to measure fission yields during the last decades, the recent evaluated libraries still need improvements in particular in the reduction of the uncertainties. Moreover, some discrepancies between these libraries must be explained. Additional measurements provide complementary information and estimations of experimental correlations, and new kinds of measurements enable to test the models used during the nuclear data evaluation process. A common effort by the CEA, the LPSC and the ILL aims at tackling these issues by providing precise measurements of isotopic and isobaric fission yields with the related variance-covariance matrices. Additionally, the experimental program involves a large range of observables requested by the evaluations, such as kinetic energy dependency of isotopic yields and odd-even effect in order to test the sharing of total excitation energy and the spin generation mechanism. Another example is the complete range of isotopic distribution per mass that allows the determination of the charge polarization, which has to be consistent for complementary masses (pre-neutron emission). For instance, this information is the key observable for the evaluation of isotopic yields. Finally, ionic charge distributions are indirect measurements of nanosecond isomeric ratios as a probe of the nuclear de-excitation path in the (E*, J, π) representation. Measurements for thermal neutron induced fission of 241 Pu have been carried out at the ILL in Grenoble, using the LOHENGRIN mass spectrometer. Methods, results and comparison to models calculations will be presented corresponding to a status on fission fragments observables reachable with this facility.

Highlights

An accurate knowledge of fission data in the actinide region is important for studies of innovative nuclear reactor concepts
Despite a real effort on fission yields measurements, current evaluated data still need some improvements on different aspects, such as for instance the uncertainties reduction and the estimation of covariance matrices
We developed different methodologies to obtain absolute isobaric and isotopic yields with the estimation of the covariance matrices associated to the measurements

Summary

Introduction

An accurate knowledge of fission data in the actinide region is important for studies of innovative nuclear reactor concepts. Fission yield measurements supply experimental data to put constraints on fission models and improve their predictive power. In the framework of nuclear data evaluation, these models are necessary to increase the consistency and the precision of the libraries. Despite a real effort on fission yields measurements, current evaluated data still need some improvements on different aspects, such as for instance the uncertainties reduction and the estimation of covariance matrices. A special focus on the heavy and symmetry mass regions is important, since it is where the discrepancies between models (or evaluations) and the few experimental data are mainly observed. We developed different methodologies to obtain absolute isobaric and isotopic yields with the estimation of the covariance matrices associated to the measurements.

The LOHENGRIN spectrometer

Mass yields

Isotopic yields

Indirect data measurements

Nanosecond converted isomeric ratios