FALSTAFF, an apparatus to study fission fragment properties First arm results

Nuclear fission is a complex process that still need fundamental studies. New measurements, particularly of correlated observables, could allow to develop more sophisticated theoretical models to eventually have truly predictive capabilities for the physics of fission. Moreover, the next generation reactors concepts are mostly foreseen to operate in the fast-neutron energy domain, requiring new high quality nuclear data. In this context, a new experimental setup, called FALSTAFF, dedicated to the study of fission is under development. The FALSTAFF setup aims to investigate the fission of actinides in the fast-neutron energy domain (from a few hundreds of keV to a few MeV). Once completed, this two-arm spectrometer will detect both fragments in coincidence and allow to measure their time of flight (ToF) and kinetic energy. The average neutron multiplicity as a function of the fission fragment mass can then be assessed. The first arm of the FALSTAFF spectrometer was built. It is composed of two main parts: first, two SED-MWPC (Multi-Wire Proportional Counter) detectors are used to measure the time-of-flight as well as the position of the fragments, thus reconstructing their velocity. Second, an axial ionisation chamber gives their kinetic energy and the energy loss profile. This proceeding will describe the FALSTAFF setup as well as the methods that are used to extract the required observables, leading up to the reconstruction of the neutron multiplicity to study the fission process. Then, the recent results obtained with the first arm of FALSTAFF will be presented, exhibiting kinetic energy, velocity and post-evaporation mass distributions. These observables will be displayed for 252 Cf spontaneous fission and some of the improvements recently made will be discussed.