Bound-State Beta Decay of Tl 205 81 + Ions and the LOREX Project

Stable Tl 205 ions have the lowest known energy threshold for capturing electron neutrinos ( ν e ) of E ν e ≥ 50.6 keV . The Lorandite Experiment (LOREX), proposed in the 1980s, aims at obtaining the longtime averaged solar neutrino flux by utilizing natural deposits of Tl-bearing lorandite ores. To determine the ν e capture cross section, it is required to know the strength of the weak transition connecting the ground state of Tl 205 and the 2.3 keV first excited state in Pb 205 . The only way to experimentally address this transition is to measure the bound-state beta decay ( β b ) of fully ionized Tl 81 + 205 ions. After three decades of meticulous preparation, the half-life of the β b decay of Tl 81 + 205 has been measured to be 291 − 27 + 33 days using the Experimental Storage Ring (ESR) at GSI, Darmstadt. The longer measured half-life compared to theoretical estimates reduces the expected signal-to-noise ratio in the LOREX, thus challenging its feasibility.