Experimental programme on absolute fission fragment yields wth the Lohengrin spectrometer at ILL: new optical and statistical methodologies

The study of fission yields has a major impact on the characterization and understanding of the fission process and is mandatory for reactor applications. The mass and isotopic yields of the fission fragments have a direct influence on the amount of neutron poisons that limit the fuel burnup and on the evaluation of the residual power of the reactor after shutdown. The fission yields of the plutonium nuclides are also mandatory for the studies on the fuel multi-recycling. In order to significantly improve the precision of nuclear data, more and more fundamental fission models are used in the evaluation processing. Therefore, tests of fission models become a central issue to achieve a coherent libraries of nuclear data. In this framework, an important investigation in the experimental limits of facilities is required to provide complete sets of data with their coherent variance-covariance matrices. 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, with different fissioning systems, were performed with this facility. The focus has been done on the selfnormalization of the data to provide new absolute measurements, independently of any libraries, and the experimental covariance matrix. To reach precise measurements, a new experimental procedure was developed along with a new analysis method based on metadata. Because of the complex correction scheme from count rates to yields, a classical propagation uncertainty is not possible. The new analysis path gives the mean value of mass yields, its probability density function and the associated experimental variance-covariance matrices. All this information is a first step to bring nuclear data into statistical tests of the underlying hypothesis of fission models.