Bound-State Beta Decay of $\mathbf{\mathrm{^{205}{Tl}^{81+}}}$ Ions and the LOREX Project

TL;DR

This paper reports the measurement of the bound-state beta decay half-life of fully ionized $^{205}$Tl ions, providing crucial data for solar neutrino detection experiments and challenging the feasibility of the LOREX project.

Contribution

First experimental measurement of the bound-state beta decay half-life of fully ionized $^{205}$Tl ions, informing neutrino capture cross sections for the LOREX project.

Findings

Measured half-life of $^{205}$Tl$^{81+}$ decay is approximately 291 days.

The measured half-life exceeds theoretical estimates, impacting the expected signal in neutrino detection.

Results pose challenges to the feasibility of the LOREX solar neutrino experiment.

Abstract

Stable $^{205}$Tl ions have the lowest known energy threshold for capturing electron neutrinos ($\nu_e$) of ${ E}_{\nu_e}\ge50.6$\,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 $\nu_e$ capture cross section, it is required to know the strength of the weak transition connecting the ground state of $^{205}$Tl and the 2.3 keV first excited state in $^{205}$Pb. The only way to experimentally address this transition is to measure the bound-state beta decay ($\beta_{b}$) of fully ionized $\mathrm{^{205}Tl^{81+}}$ ions. After three decades of meticulous preparation, the half-life of the $\beta_{b}$ decay of $\mathrm{^{205}Tl^{81+}}$ 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.