Evaluation of the Prompt Gamma-ray Spectrum from Spontaneous Fission of ²⁵²Cf

The energy spectra, multiplicities and average energies of the prompt, total and delayed γ-rays accompanying the spontaneous fission of ²⁵²Cf were collected from the literature and dedicated databases. They were carefully analysed for consistency with a view to producing reference data for usage in various applications. This could be accomplished for the prompt fission gamma ray spectrum up to 20 MeV since dozens of measurements exist and reasonably agree. The prompt fission gamma ray spectrum (PFGS) was non-model evaluated by fitting the preselected experimental data with the help of the generalized least-squares (GLS) code GMA. The resulting spectrum could be considered as a reference for the γ-ray energies from 0.1 to 20 MeV with uncertainties varying between ≈ 3 and 25%. This reference gamma spectrum will be a substantial contribution to the precise and complete characterisation of the ²⁵²Cf source since the prompt fission neutron spectrum (PFNS), which has been accepted as a standard for a long time, has comparable uncertainties. The average gamma multiplicity and energy were also surveyed and used to derive the recommended values. The prompt X- and γ-ray energy spectra below ≈ 100 keV and delayed photon spectra in the whole energy range, as well as their multiplicities, are still seldomly and incompletely measured, that excepts an evaluation based on experimental data. The comparison with existing theoretical prompt and delayed ²⁵²Cf(s.f.) γ-spectra or with those presented in the major evaluated cross section libraries explored their incompleteness or deviations from the evaluated PFGS. The existing measurements of the pionic and muonic radioactivity of ²⁵²Cf(s.f.) and ²³⁵U(nth,f) were reviewed and the potential impact of gammas from the π0 decay on the high energy part of the PFGS was investigated. (author)