Prompt fissionγ-ray characteristics from neutron-induced fission on ²³⁹Pu and the time-dependence of prompt-γray emission

¹ Ghent University, Department of Physics and Astronomy, Proeftuinstraat 86, 9000 Ghent, Belgium
² European Commission, DG JRC, Directorate G - Nuclear Safety and Security, G.2 SN3S, 2440 Geel, Belgium
³ Extreme Light Infrastructure - Nuclear Physics (ELI-NP) / Horia Hulubei National Institute for Physics and Nuclear Engineering (IFIN-HH), 077125 Bucharest-Magurele, Romania

^* e-mail: stephan.oberstedt@ec.europa.eu

Published online: 6 March 2018

Abstract

Recent years have seen an increased interest in prompt fission γ-ray (PFG) measurements motivated by a high priority request of the OECD/NEA for high precision data, mainly for the nuclear fuel isotopes ²³⁵U and ²³⁹Pu. Our group has conducted a PFG measurement campaign using state-of-the-art lanthanum halide detectors for all the main actinides to a precision better than 3%. The experiments were performed in a coincidence setup between a fission trigger and γ-ray detectors. The time-of-flight technique was used to discriminate photons, traveling at the speed of light, and prompt fission neutrons. For a full rejection of all neutrons below 20 MeV, the PFG time window should not be wider than a few nanoseconds. This window includes most PFG, provided that no isomeric states were populated during the de-excitation process. When isomeric states are populated, PFGs can still be emitted up to 1 yus after the instant of fission or later. To study these γ-rays, the detector response to neutrons had to be determined and a correction had to be applied to the γ-ray spectra.

The latest results for PFG characteristics from the reaction ²³⁹Pu(n_th,f) will be presented, together with an analysis of PFGs emitted up to 200 ns after fission in the spontaneous fission of ²⁵²Cf as well as for thermal-neutron induced fission on ²³⁵U and ²³⁹Pu. The results are compared with calculations in the framework of the Hauser-Feshbach Monte Carlo code CGMF and FIFRELIN.