Status and New Data of the Geochemical Determination of the pp-Neutrino Flux by LOREX

Figure 3

The neutral ²⁰⁵Tl⁰ atom is stable with respect to 𝛽⁻ decay into the continuum since the 𝑄 value for this decay is negative, 𝑄_𝛽−=−𝑄_𝐸𝐶=−50.5keV. However, for bare (²⁰⁵Tl⁸¹⁺) or hydrogen-like ²⁰⁵Tl ions bound-state 𝛽 decay becomes allowed, where the created electron remains bound in the K-shell of the daughter ion ²⁰⁵Pb⁸¹⁺. Due to the “saved” K-shell binding energy, the 𝑄 value for this decay (𝑄_𝛽b (K)) gets positive. Both, the capture of solar pp-neutrinos and the bound-state beta decay populate predominantly the first exited state (1/2⁻) of the ²⁰⁵Pb nucleus at 2.3 keV and share the same nuclear matrix element for this transition. Therefore, the measurement of the bound-state 𝛽⁻ decay probability of bare or hydrogen-like  ²⁰⁵Tl ions provides the unknown capture probability of the solar pp-neutrinos by the ²⁰⁵Tl atoms. Based on the log ft values of comparable 𝛽⁻ decays, the bound-beta half-life 𝑇_1/2 (𝛽_b) of bare ²⁰⁵Tl ions was estimated by Takahashi and Yokoi [19] to about 120 d. Since this extrapolated value has to be regarded as uncertain within (at least) a factor of three, only the measurement of the bound-beta half-life can provide a safe number with a reliable error margin for the capture probability of solar pp-neutrinos by ²⁰⁵Tl.