Speaker
Description
Naturally occurring $^{176}$Lu decays by β- decay to $^{176}$Hf with a half-life of 37.8 Gyr.
This radioactive decay provides an important isotopic clock (Lu/Hf) to date meteorites and minerals, furthermore $^{176}$Lu/$^{176}$Hf can be used as an s-process thermometer in studies of stellar nucleosynthesis.
It has been suggested that some discrepancies involving Lu/Hf age comparisons in different samples could be reconciled if $^{176}$Lu also underwent significant electron capture (EC) decay.
In particular, besides the well known β- decay to $^{176}$Hf, the $^{176}$Lu is also expected to be unstable with respect to electron capture decay to $^{176}$Yb. The Q$_{EC}$ for decay to the $^{176}$Yb ground state is 106.2 keV. Thus, EC decays to both the J$^p$ =0$^+$ ground state and the J$^p$ =2$^+$ 82 keV first excited state of $^{176}$Yb are both possible. These EC decay branches would be 7$^{th}$ and 5$^{th}$ forbidden transitions, respectively, and thus are expected to be negligibly small.
Previous searches of the $^{176}$Lu EC decay were performed by using a passive Lutetium sources and looking for the $^{176}$Yb* 82 keV gamma or the characteristic Yb X-rays in a HP-Ge detector.
Our new approach uses a LYSO crystal scintillator coupled to a PMT as an active Lutetium source, acquired in coincidence with an HP-Ge; this allows a powerful reduction of the background provided by the known $^{176}$Lu β- decay branch.
The preliminary results of the measurement on a detector prototype arranged in the INFN-TIFPA laboratory will be summarized, the upper limits to the EC branching ratio of $^{176}$Lu decay has been improved by a factor 3-20 (depending on the considered EC channel) with respect to previous measurements.
