New DDEP recommendations for the radioactive decay scheme of $^{137}$Cs

International audienceCaesium-137 is a non-naturally occurring fission product, in particular one of the many radionuclides produced in uranium-based nuclear reactors. It decays through β- transitions, populating both the ground state and the first two excited states of $^{137}$Ba. With a half-life of approximately 30 years, and a well-known and intense 662 keV gamma-ray emission, this radionuclide is relatively easy to detect in the environment as an unambiguous trace of human activity. In addition, due to its simple decay scheme and limited number of emitted radiations, the $^{137}$Cs is widely used in laboratories as a gamma calibration standard. The decay of the $^{137}$Cs has been intensively investigated since the early days of Nuclear Physics, with more than one hundred studies reported over the years. However, despite such abundant literature, some ambiguities remain on the decay parameters for this important radionuclide. In particular, the previous DDEP evaluation has pointed out inconsistencies between the numerous half-life measurements, leading to an evaluated value (T$_{1/2}$ = 30.05 ± 0.08 a) having a much larger uncertainty than the most precise measurement (T$_{1/2}$ = 30.174 ± 0.011 a). A comparable evaluated value (T$_{1/2}$ = 30.08 ± 0.09 a) is also recommended by in the latest ENSDF evaluation confirming the complexity of the half-life evaluation of $^{137}$Cs. In the scope of the DDEP project, a complete re-evaluation of the $^{137}$Cs decay scheme has been performed, taking advantage of the measurements published after the above-mentioned evaluations. A new evaluated half-life, T$_{1/2}$ = 30.018 ± 0.022 a, is recommended and a revision of beta decay branching ratios is proposed, leading to a reevaluation of the 662 keV gamma transition probability. The consistency of the deduced decay scheme will be discussed as well as the needs for new measurements