Search for electron capture in 176Lu with a lutetium yttrium oxyorthosilicate scintillator

The nuclide 176Lu is one of the few naturally occurring isotopes that are potentially unstable with respect to electron capture (EC). Although experimental evidence for 176Lu EC decay is still missing, this isotope is instead well known to 0- decay into 176Hf with a half-life of about 38 Gyr. The precise investigation of all possible decay modes for 176Lu is interesting because the Lu/Hf ratio is adopted as an isotopic clock. Previous searches for the 176Lu 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 0--decay branch. This approach led to an improvement on the 176Lu EC branching ratio limits by a factor of 3 to 30, depending on the considered EC channel.