Status and new data of the geochemical determination of the pp-neutrino flux by LOREX

LOREX (LORandite EXperiment) addresses the determination of the solar (pp) neutrino flux during the last four million years by exploiting the reaction 205 Tl + e → 205 Pb + e - with an incomparably low-energy threshold of 50 keV for the capture of solar neutrinos. The ratio of ²⁰⁵Pb/ ²⁰⁵Tl atoms in the Tl-bearing mineral lorandite provides, if corrected for the cosmic-ray induced background, the product of the flux of solar neutrinos and their capture probability by ²⁰⁵Tl, averaged over the age of lorandite. To get the mean solar neutrino flux itself, four problems have to be addressed: (1) the geological age of lorandite, (2) the amount of background cosmic-ray-induced ²⁰⁵Pb atoms which strongly depends on the erosion rate of the lorandite-bearing rocks, (3) the capture probability of solar neutrinos by ²⁰⁵Tl and (4) the extraction of lorandite and the appropriate technique to "count" the small number of ²⁰⁵Pb atoms in relation to the number of ²⁰⁵Tl atoms. This paper summarizes the status of items 1 (age) and 3 (neutrino capture probability) and presents in detail the progress achieved most recently concerning the items 2 (background/erosion) and 4 ("counting" of ²⁰⁵Pb atoms in lorandite).