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Accueil du site > Activités scientifiques et techniques > Physique et Radiochimie de l’energie Nucléaire > Physique de l’aval du cycle et de la spallation > Données nucléaires > SOFIA : a next-generation fission experiment

SOFIA : a next-generation fission experiment

A precise description of nuclear fission is of major interest to improve the safety of nuclear reactors and to assess the potentialities of new types of reactors and/or fuels. Fission fragments play a key role in reactors : they are the vector of most of the energy liberated during fission ; they are the source of the delayed neutrons, which the stability of the chain reaction largely depends on ; and they are responsible for the poisoning of the core, a point which may become even more important in the future if high burn-up fuels are used. The role of fission fragments is also key in the case of a reactivity excursion, as their inventory suddenly increases with the core power. Finally, fission fragments play a central role in the residual thermal power of cores, a point acknowledged as the major source of operational risk and tragically emphasized by the Fukushima incident.

Beyond its applicative aspects, fission is a complex process which highlights many nuclear properties. Despite being one of the longest-studied nuclear reactions, no theoretical approach is yet able to describe the whole fission process, i.e. properties of the fissioning system, fission dynamics and fission fragment distributions. Further advances in our understanding of this phenomenon mechanism depend of course on theoretical developments, but also on our ability to access to new observables.

The SOFIA (Studies On FIssion with Aladin) collaboration was started in 2008 in order to realize a new generation of experiment on fission at GSI, taking advantage of the intense 238U beam available at this facility with an energy up to 1 GeV per nucleon. The collaboration brings together CEA/DAM, GSI, IN2P3/IPN, IN2P3/CENBG, USC and Chalmers University. The aim is to realize a complete isotopic identification of both fission fragments, to measure isotopic fission yields and fragments kinematics with a high precision, and to measure the neutrons yields in coincidence with fragments.

A striking feature of SOFIA is the use of the GSI fragment separator, the FRS, as a source of secondary beam. This allows to perform measurement on a very wide range of nuclei, from heavy Neptunium isotopes down to very proton-rich pre-actinides. The other unique feature of SOFIA is the use of an electromagnetic interaction to trigger fission. This results in excitation energy between 10 and 15 MeV, a range reasonably close to the application domain and for which extrapolation to the thermal neutron-induced fission is definitely possible.

A first experiment took place in the summer of 2012 at GSI and was a complete success, leading to the first-ever isotopic yields measurements of several tens of nuclei. Several technical breakthrough have also been achieved, setting new standards for future heavy ions measurements. A second campaign is scheduled for the autumn of 2014, with the study of the fission of 236U as a main objective since it is the analog of the 235U+n system, which is the main nuclear reaction in present nuclear reactors.

IPN has contributed to the SOFIA set-up by providing 3 position detectors (Multi-Wires Proportionnal Counters) and its physicists are actively involved in all stages of the experiment.

Mid-term plans for the SOFIA collaboration are to take advantage of the upcoming FAIR facility, with its higher beam intensity and the improved separation power of the super-FRS. In the long-term, we want to tackle the question of the last physics parameter missing from these studies, the event-by-event measurement of the excitation energy deposited in the fissioning nucleus. for this, we plan to use the future ELISE electron beam interacting with nuclei circulating in the NESR storage ring of the FAIR project.



Institut de Physique Nucléaire Orsay - 15 rue Georges CLEMENCEAU - 91406 ORSAY (FRANCE)
UMR 8608 - CNRS/IN2P3

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