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

Abstract

Materials selection for rare event physics requires high performance detectors and customized analyses. In this context a novel ${\hat{I}}^2$-${\hat{I}}^3$ detection system, comprised of a liquid scintillator in coincidence with a HPGe, was developed with the main purpose of studying ultra trace contamination of uranium, thorium and potassium in liquid samples. In the search for ${}^{238}\text{U}$ contaminations through neutron activation analysis, since the activation product ${}^{239}\text{Np}$ decays to a relatively long-lived isomeric state of ${}^{239}\text{Pu}$, 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 ${}^{239}\text{Np}$ and the delayed events following the de-excitation of the ${}^{239}\text{Pu}$ isomeric level at 391.6 keV, a precision measurement of the half-life of this level was conducted. The half-life of the 391.6 keV ${}^{239}\text{Pu}$ level resulted $190.2\pm 0.2$ ns, thus increasing the precision by about a factor of 20 over previous measurements.

Accepted for publication in PRC