The study of fission fragment de-excitation is important for both nuclear applications and fundamental nuclear physics. For modelling innovative nuclear reactor (GEN-IV) cores, a better understanding of the released heat during fission of the major isotopes (235U,239Pu) is crucial. Present knowledge on the released heat states that around 10% of all energy released in fission comes from gamma-rays, and about 40% of these are prompt, i.e. within a few ns after fission. Since currently evaluated data from the 1970s show a deviation from benchmark calculations of up to 28%, an urgent request for new prompt fission gamma -ray spectra (PFGS) measurements was put high on the OECD Nuclear Energy Agency (OECD/NEA) Nuclear Data High Priority Request List (HPRL) targeting at an uncertainty of 7.5%.
In response to the HPRL, our group executed a dedicated measurement program on prompt fission gamma-rays employing state-of-the-art lanthanide halide detectors (cerium-doped LaBr3 and LaCl3 as well as CeBr3) with superior timing and good energy resolution. These detectors allow clean discrimination between prompt gamma-rays and other gamma-rays (both isomeric gamma-rays and from inelastically scattered neutrons). Our results from 252Cf(SF) , 235U(nth,f), 241Pu(nth,f) ,240,242Pu(SF), and, most recently, 239Pu(nth,f) provide PFGS characteristics (average number of photons per fission, average total energy per fission and mean photon energy) with uncertainties as low as 2%.
Prompt fission gamma-ray spectral data provide also important information about the nuclear de-excitation. The present challenge in nuclear modelling is the description of the competition of prompt neutron and gamma emission during the de-excitation process. Since fission is a unique tool for producing very neutron-rich isotopes, far from the valley of stability, we measured fragment-mass resolved prompt fission gamma-ray spectra and also investigated the time dependence of prompt gamma-ray emission to allow benchmarking nuclear theory.