Measurement of ^136gsI and ^136mI β-decay properties with MTAS

Since most fission products in a nuclear reactor undergo beta decay to reach stability, understanding the beta-feeding patterns and gamma-ray intensities of these beta-decaying fission products is essential for simulating reactor decay heat and ensuring reactor safety. Most of the data available is based on high-resolution gamma spectroscopy, which in general attributes too little decay energy to the gamma rays and too much to beta particles and antineutrinos, affecting the anti-neutrino flux calculations as well. Measurements using a high-efficiency detector such as the Modular Total Absorption Spectrometer (MTAS) can correct this bias in the decay data. We present results from beta-decay measurements using a mixed beam of the ¹³⁶I ground state and 201-keV isomeric state (47 s half life) with MTAS at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory (ORNL). Half-lives, gamma-ray energies, event by event correlations, and collection/counting cycle times were used to help discriminate decays from the ground and isomeric states. A Bayesian analysis of gamma-ray energy spectra was used to extract the beta-feeding intensities for both initial states of ¹³⁶I. Preliminary results will be presented.