Highly accurate measurements of the spontaneous fission half-life of240,242Pu

Abstract

Fast spectrum neutron-induced fission cross-section data for transuranic isotopes are of special demand from the nuclear data community. In particular highly accurate data are needed for the new generation IV nuclear applications. The aim is to obtain precise neutron-induced fission cross sections for ${}^{240}$Pu and ${}^{242}$Pu. To do so, accurate data on spontaneous fission half-lives must be available. Also, minimizing uncertainties in the detector efficiency is a key point. We studied both isotopes by means of a twin Frisch-grid ionization chamber with the goal of improving the present data on the neutron-induced fission cross section. For the two plutonium isotopes the high $\ensuremath{\alpha}$-particle decay rates pose a particular problem to experiments due to piling-up events in the counting gas. Argon methane and methane were employed as counting gases, the latter showed considerable improvement in signal generation due to its higher drift velocity. The detection efficiency for both samples was determined, and improved spontaneous fission half-lives were obtained with very low statistical uncertainty (0.13% for ${}^{240}$Pu and 0.04% for ${}^{242}$Pu): for ${}^{240}$Pu, ${T}_{1/2,SF}=1.165\ifmmode\times\else\texttimes\fi{}{10}^{11}$ yr $(1.1%)$, and for ${}^{242}$Pu, ${T}_{1/2,SF}=6.74\ifmmode\times\else\texttimes\fi{}{10}^{10}$ yr $(1.3%)$. Systematic uncertainties are due to sample mass (0.4% for ${}^{240}$Pu and 0.9% for ${}^{242}$Pu) and efficiency (1%).