Origin of the high-energy structures in the prompt-fission γ-ray spectrum of 252Cf

Background: Enhanced production of prompt gamma rays peaked at 4 and 6 MeV in the mass region of fission fragments around 132Sn is a known phenomenon for 30 years. Various contradictory interpretations of their origin can be found in the literature: either discrete or statistical gamma rays originating from the light or heavy fragment. Purpose: The present work aims at lifting this uncertainty by looking at the prompt gamma spectrum in coincidence with isomeric decays and its evolution with the excitation energy of the isomeric nucleus. Method: The experimental setup made of a nearly 4 pi NaI array coupled to a twin Frisch-gridded ionization chamber allowed the detection of both fragments and the measurement of their kinetic energy with a resolution of 570 keV, leading to a mass resolution of 0.68 u. in the case of neutronless fission events, and about 4 u. for standard fission events. Fifteen isomers were measured near the double shell closure Z = 50, N = 82. Results: The study of the prompt gamma spectrum in coincidence with these isomers revealed that the aforementioned structures are incompatible with a discrete nature. The comparison of the prompt gamma spectra in coincidence with two isomers of 134Te differing by 4.1 MeV in excitation energy unambiguously attributes these structures to the decay of the heavy fragments. Conclusions: From the study of the photon emission probability we interpret these structures as arising from the competition between the "strength-driven" decay of the heavy fragments and the "density-driven" decay of the light fragments.