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

Materials selection for rare event physics requires high performance detectors and customized analyses. In this context a novel beta-gamma 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 U-238 contaminations through neutron activation analysis, since the activation product Np-239 decays to a relatively long-lived isomeric state of Pu-239, 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 beta(-) decay of Np-239 and the delayed events following the deexcitation of the Pu-239 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 Pu-239 level is 190.2 +/- 0.2 ns, thus increasing the precision by about a factor of 20 over previous measurements.