High precision measurement of the half-life of the 391.6 keV metastable level in Pu-239

Materials selection for rare event physics requires high performance detectors and customized analyses. In this context a novel β-γ detection system, composed of a liquid scintillator in coincidence with a high-purity germanium (HPGe) detector, was developed with the main purpose of studying ultratrace contamination of uranium, thorium, and potassium in liquid samples. In the search for U238 contaminations through neutron activation analysis, since the activation product Np239 decays to a relatively long-lived isomeric state of Pu239, it is possible to perform a time selection of these events, obtaining a strong background suppression. Investigating the time distribution of the coincidences between the β-decay of Np239 and the delayed events following the deexcitation of the Pu239 isomeric level at 391.6 keV, a precision measurement of the half-life of this level was conducted. The resulting half-life of the 391.6 keV Pu239 level is 190.2±0.2 ns, thus increasing the precision by about a factor of 20 over previous measurements.