Studies of fission fragment yields via high-resolution gamma-ray spectroscopy

Precise spectroscopic information on the fast neutron induced fission of the U-238(n,f) reaction was recently gained using a new technique which involved coupling of the Miniball high resolution gamma-ray spectrometer and the LICORNE directional neutron source. The experiment allowed measurement of the isotopic fission yields for around 40 even-even nuclei at an incident neutron energy of around 2 MeV where yield data are very sparse. In addition spectroscopic information on very neutron-rich fission products was obtained. Results were compared to models, both the JEFF-3.1.1 data base and the GEE code, and large discrepancies for the S1 fission mode in the Sn/Mo isotope pair were discovered. This suggests that current models are overestimating the role played by spherical shell effects in fast neutron induced fission. In late 2017 and 2018 the nu-ball hybrid spectrometer will be constructed at the IPN Orsay to perform further experimental investigations with directional neutrons coupled to a powerful hybrid Ge/LaBr₃ detector array. This will open up new possibilities for measurements of fission yields for fast neutron-induced fission using the spectroscopic technique and will be complimentary to other methods being developed. (authors)