Abstract
In recent years the measurement of prompt fission \(\gamma\)-ray spectra (PFGS) has gained renewed interest, after about forty years since the first comprehensive studies of the reactions 235U(nth, f), 239Pu(nth,f) and 252Cf(sf). The renaissance was initiated by requests for new values especially for \(\gamma\)-ray multiplicity and average total energy release per fission in neutron-induced fission of 235U and 239Pu. Both isotopes are considered the most important ones with respect to the modeling of innovative cores required for the Generation-IV reactors, the majority working with fast neutrons. During the last 5 years we have conducted a systematic study of spectral data for thermal-neutron-induced fission on 235U and 241Pu as well as for the spontaneous fission of 252Cf with unprecedented accuracy. From the new data we conclude that those reactions do not considerably contribute to the observed heat excess and suspect other reactions playing a significant role. Possible contributions may originate from fast-neutron-induced reactions on 238U, which is largely present in the fuel, or from \(\gamma\)-induced fission from neutron capture in the construction material. A first experiment campaign on prompt \(\gamma\)-ray emission from fast-neutron-induced fission on 235,238U was successfully performed in order to test our assumptions. In the following we attempt to summarize, what has been done in the field to date, and to motivate future measurement campaigns exploiting dedicated neutron and photon beams as well as upcoming highly efficient detector assemblies.
Highlights
The energy released in nuclear fission is distributed in kinetic and excitation energy of the two fragments
The excitation energy manifests itself in fragment deformation and intrinsic excitation energy, which subsequently is released by the emission of prompt neutrons and γ-rays and, at a later stage, through β-decay towards isotopes in the valley of stability
An inherent problem of such measurements is the sufficient discrimination of prompt fission neutrons, which may induce the production of γ-rays through inelastic scattering in the detector and the surrounding materials, mixing with the signal from prompt fission γ-rays
Summary
The energy released in nuclear fission is distributed in kinetic and excitation energy of the two fragments. An inherent problem of such measurements is the sufficient discrimination of prompt fission neutrons, which may induce the production of γ-rays through inelastic scattering in the detector and the surrounding materials, mixing with the signal from prompt fission γ-rays. In the end, those data entered in evaluated nuclear data libraries [7,8] at both thermal and 14.6 MeV neutron energies, neglecting a possible dependence on the incoming neutron energy. Up-coming measurement campaigns and their associated instrument developments will be presented
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