Isotope abundance measurement of the half-life of the ββ -decaying nucleus Zr96 from a 2.68 Gyr zircon sample

Abstract

The decay half-life of the $^{96}\mathrm{Zr}$ isotope was measured by applying isotope geochemistry techniques to ancient (0.9 and 2.68 Gyr) ${\mathrm{ZrSiO}}_{4}$ (zircon) samples, with the objective to separate the single and double $\ensuremath{\beta}$-decay branches. The single $\ensuremath{\beta}$ decay provides one of the most direct tests for theoretical models describing neutrinoless $\ensuremath{\beta}\ensuremath{\beta}$ decays. Both the single and the double $\ensuremath{\beta}$-decay branches lead to the same final nucleus $^{96}\mathrm{Mo}$ and generate an isotopic anomaly over geological time scales. After a chemical separation, the Mo isotopic composition was measured by inductively coupled plasma mass spectrometry (ICPMS). The $^{96}\mathrm{Mo}$ isotopic anomaly was determined in two complete replicate analyses to be 107(40) and 88(20) ppm, which translates to a $^{96}\mathrm{Zr}$ half-life of ${T}_{1/2}=\left(2.{03}_{\ensuremath{-}0.31}^{+0.46}\right)\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.28em}{0ex}}\mathrm{yr}$. With the $2\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$ partial decay half-life of the ground-state to ground-state transition known from NEMO-3 to be $2.35\ifmmode\pm\else\textpm\fi{}0.21\ifmmode\times\else\texttimes\fi{}{10}^{19}$ yr and all other partial $\ensuremath{\beta}\ensuremath{\beta}$-decay half-lives expected to be many orders of magnitude longer, a lower limit for the single-$\ensuremath{\beta}$-decay half-life is set at ${T}_{1/2}^{\ensuremath{\beta}}\ensuremath{\ge}6.2\ifmmode\times\else\texttimes\fi{}{10}^{19}$ yr.