Search for electron capture in Lu176 with a lutetium yttrium oxyorthosilicate scintillator

The nuclide $^{176}\mathrm{Lu}$ is one of the few naturally occurring isotopes that are potentially unstable with respect to electron capture (EC). Although experimental evidence for $^{176}\mathrm{Lu}$ EC decay is still missing, this isotope is instead well known to ${\ensuremath{\beta}}^{\ensuremath{-}}$ decay into $^{176}\mathrm{Hf}$ with a half-life of about 38 Gyr. The precise investigation of all possible decay modes for $^{176}\mathrm{Lu}$ is interesting because the Lu/Hf ratio is adopted as an isotopic clock. Previous searches for the $^{176}\mathrm{Lu}$ EC decay were performed by using a passive lutetium source coupled with a high-purity germanium (HPGe) spectrometer. Our approach uses a lutetium yttrium oxyorthosilicate (LYSO) crystal both as a lutetium source and as an active detector. Scintillation light from the LYSO crystal is acquired together with the signals from the HPGe detector, and this allows a powerful suppression of the background sourcing from the well-known ${\ensuremath{\beta}}^{\ensuremath{-}}$-decay branch. This approach led to an improvement on the $^{176}\mathrm{Lu}$ EC branching ratio limits by a factor of 3 to 30, depending on the considered EC channel.