Experimental programme on absolute fission fragment yields with the lohengrin spectrometer: New optical and statistical methodologies

Abstract

Abstract. The study of fission yields has a major impact on the characterization and understanding of the fission process and is mandatory for reactor applications. In the past with the LOHENGRIN spectrometer of the ILL, priority has been given for the studies in the light fission fragment mass range. The LPSC in collaboration with ILL and CEA has developed a measurement program on symmetric and heavy mass fission fragment distributions. The combination of measurements with ionisation chamber and Ge detectors is necessary to describe precisely the heavy fission fragment region in mass and charge. Recently, new measurements of fission yields and kinetic energy distributions are has been made on the 233 U(n th ,f) reaction. The focus of this work has been on the new optical and statistical methodology and the self-normalization of the data to provide new absolute measurements, independently of any libraries, and the associated experimental covariance matrix. Nuclear dataThe accurate knowledge of fission data in the actinide region is important for studies of innovative nuclear reactor concepts. Today, in the framework of nuclear data evaluation, fission models are necessary to increase the consistency and the precision of the libraries. For instance, post neutron fission yields are actually needed in the current and innovative fuel cycles for the calculation of the inventory and the radiotoxicity of the spent fuel, and the estimation of the residual power after shutdown.Moreover, fission yield measurements supply experimental data to put constraints on fission models and improve their predictive power. In this context, since 2007, various experiments from our collaboration have been performed to investigate fission yields at the Lohengrin spectrometer of the ILL with a special focus on the heavy and symmetry mass region, where there are inconsistencies between models (or evaluations) and the few experimental data.Recent measurements from [1-3] of 233 U fission yields in the heavy region with the Lohengrin spectrometer showed an important discrepancy near the symmetry region in comparison with the evaluations. Thus a new methodology has been developed [3,4] to disentangle the dispersion of measurements in the heavy peak mass region in the descent to symmetrical masses. a