102Rhm: Could it be a cosmic-ray chronometer?

Looking for a possible cosmic-ray chronometer, we have studied the decay modes of 102Rhm,g. Its ground state has a half-life of 207 days, and decays to 102Ru and 102Pd via β++e (80%) and β− (20%) respectively. The meta-stable state has a 2.9-year half-life, and decays to 102Ru and 102Rhg via e decay (≈100%) and a highly converted isomeric transition (0.23%) respectively, with no indication of β+ or β− decays. In the absence of the atomic electrons, the isomeric state will only decay either via β+ or an M4 γ decay of 90 keV, both not observed. Measurements of the most probable β+ branch were performed with a germanium multi-detector system, through sum coincidence method, and allow setting an upper limit of 2.5×10−6 per decay, with 95% confidence level. From this upper limit, a partial half-life greater than 1.2×106 years is inferred. Investigation of the isomeric transition is under way.