Fission fragment deexcitation study by gamma-ray spectrometry with the EXILLexperiment

Nuclear fission is a complex process, still not yet well described by microscopic models, and its main observables are difficult to measure with accuracy. Following the capture of a thermal neutron the compound nucleus fissions in two fragments. Most of the energy of the process is transferred in kinetic energy, whereas the nuclei are let in few tens of MeV excited states. This excitation energy is then released, the majority of it within less than some nanoseconds, by the evaporation of a few neutrons and the emission of -rays in cascade. To overcome the low accuracy of microscopic models in the prediction of fission observables, nuclear technology relies on libraries of evaluated data and semi-empirical models, like the GEF model for fission yield evaluations. Such a strategy requests systematic and accurate experimental data on the few possible observables. Here we will present the advantages of studying yields of fragment correlated pairs and their de-excitations using an array of high-resolution -ray detectors directly placed around an actinide target irradiated by a thermal or cold neutron beam. Such a setup was installed at ILL in 2012 and 2013 (EXILL) and campaigns were performed with 235U and 241Pu targets. This study is largely motivated by the future installation of a permanent -ray detector array at the ILL, which will be coupled to fission fragment detectors in a first construction phase and a fragment separator in a second phase (FIPPS). An important outcome of EXILL data is the observation of the -ray cascade occurring in both fission fragments with an unambiguous determination of the fragments. The cascade is directly linked with the angular momentum of the fragments after scission, which is one of the less precise and less understood properties. With the development of new simulation codes for the neutron evaporation and the -ray cascade like FIFRELIN or KEWPIE, systematic studies and comparisons with the large amount of experimental data resulting from double and triple gamma-ray coincidence analysis become possible. Preliminary results on the gamma–ray cascades measured in the well produced Kr and Ba fragments and comparisons with FIFRELIN simulations will be presented.