New data for the geochemical determination of the solar pp-neutrino flux by means of lorandite mineral

LOREX, the acronym of LORandite EXperiment, is the only long-time solar neutrino experiment still actively pursued. It addresses the long-time detection of the solar neutrino flux with the thallium-bearing mineral lorandite, TlAsS2 at the mine of Allchar, FYRMacedonia, via the neutrino-capture reaction 205Tl+νe→205Pb+e−. The final step of LOREX would be the extraction of lorandite samples and the quantitative determination of the ratio of 205Pb/205Tl atoms providing the product of solar neutrino flux and neutrino-capture cross-section, integrated over the age of lorandite (about 4.3×106 y). There is an unprecedented low threshold of only 50 keV for solar pp-neutrinos, to be compared with 232 keV of the GALLEX and SAGE experiments. Moreover, LOREX would be unique in view of providing the mean luminosity of the sun over the last 4.3 million years. This paper presents new data providing an accurate geological age of mineralization at Allchar, and in particular a lower limit of 36 m/106 y for the erosion rate, based on Accelerator Mass Spectrometry of 26Al and 53Mn samples taken from the mine of Allchar. This data allow first realistic conclusions on the feasibility of the LOREX project. The amount of 205Pb due to fast cosmic rays (e.g. muons), strongly depends on the depth at which the mineral existed from the time of its formation to the present day. For the mineral mined at the present-day depth this contribution then depends upon the average erosion rate at this particular location. A reliable determination of the erosion rate is, therefore, indispensable for a proper estimate of the background of 205Pb induced by cosmic radiation. Provided that this lower limit of 36 m/106 y can be corroborated by the mandatory measurement of additional probes, it is expected to reach a signal-to-background ratio equal or better than one. Finally, it is discussed how to get the still unknown capture probability of solar pp-neutrinos from 205Tl into 205Pb, in particular into its first excited state at E*=2.3 keV.