New investigation of half-lives for the decay modes of ${}^{50}\mathrm{V}$

A new search for the decay modes of the fourfold forbidden nonunique decay of ${}^{50}\mathrm{V}$ has been performed at the Gran Sasso Underground Laboratory (LNGS). In total, an exposure of 197 kg d has been accumulated. The half-life for the electron capture into the first excited state of ${}^{50}\mathrm{Ti}$ has been measured with the highest precision to date as $2.{67}_{-0.18}^{+0.16}\times {10}^{17}$ yr (68% C.I.) in which systematics uncertainties dominate. The search for the $\beta$ decay into the first excited state of ${}^{50}\mathrm{Cr}$ resulted in a lower limit of $1.9\times {10}^{19}$ yr (90% C.I.), which is an improvement of almost one order of magnitude compared to existing results. The sensitivity of the new measurement is now in the region of theoretical predictions.

Figure 1

Decay scheme of ${}^{50}\mathrm{V}$. Two excited states can be populated: one via EC to ${}^{50}\mathrm{Ti}$ under emission of a 1553.77 keV $\gamma$ ray and the other via the $\beta$ decay into the first excited state of ${}^{50}\mathrm{Cr}$ resulting in a 783.29 keV $\gamma$ ray.

Figure 2

Two vanadium discs after multifold electron beam melting (EBM).

Figure 3

Arrangement of the ten purified vanadium discs inside a Marinelli beaker ready to be measured.

Figure 4

Spectrum of the vanadium sample (240.6 d) in red and a corresponding background measurement (70.3 d) in black. Both spectra are scaled to live time. The two regions of interest and prominent background peaks are highlighted.

Figure 5

Spectral fit of the ${}^{50}\mathrm{V}$ ${\beta }^{-}$ decay $\gamma$ line at 783.29 keV and the background $\gamma$ lines. Shown is the best fit function in blue and the signal process set to the obtained 90% C.I. limit in red.

Figure 6

Spectral fit of the ${}^{50}\mathrm{V}$ EC $\gamma$ line at 1553.77 keV and the background $\gamma$ lines. Shown is the best fit function in blue and the signal process set to the $\pm 1\sigma$ fit uncertainties in red.