Geochemical Determination of the pp-Neutrino Flux with 205Tl-LOREX: A Progress Report

Intensive research from the 1970's to 1990's towards geochemical detection of solar neutrinos using 205 Tl, 98 Mo and 81 Br has left 205 Tl the only viable candidate. LOREX (LORandite EXperiment ) is based on determining the solar (pp) neutrino flux for the period of 4.3 Ma from the reaction 205 Tl+v e → 205 Pb+e - , the lowest threshold (52 keV) for neutrino capture. The ratio of 205 Pb/ 205 Tl in lorandite (TlAsS 2 ) depends also on contributions from fast muon cosmic radiation. The latter is a function of paleo - depth, including the eroded layer over 4.3 Ma, determined using cosmogenic nuclides ( 26 Al, 36 Cl, 3 He and 21 Ne) in the ore body Crven Dol (Tl - As - Sb - S) in Allchar (Macedonia). The results give a l ower limit of paleo - depth of 490mwe and an upper limit of 2300mwe. From this the ratios 205 Pb ve / 205 Pb tot are estimated as 2:8 and 6:4, respectively. Identification of the 205 Pb nuclei in the lead sample extracted from the lorandite mineral requires 10 - 10 to 10 - 11 overall detection sensitivity for 205 Pb/Pb and comparable suppression of the 205 Tl isobar. This is proposed by full stripping of 205 Pb at high energy (345MeV/u) at the RIKEN - RIBF ion - beam facility. 205 Tl isobar separation is already largely achiev ed by chemical Pb - Tl separation by the overall sample preparation. Samples with a higher concentration ( 205 Tl/natPb=1%) are necessary for a guide - beam and initial accelerator tuning. A sample with a considerably lower level of about 10 - 8 is needed for cont rol of the beam analysis system with 205 Tl ions, in the presence ultimately of a lighter guide beam, to limit the in - beam production of 205 Pb by the (p,n) reaction on 205 Tl in the energy - loss and ion - stripping steps in the accelerator and the subsequent Bi gRIPS/Mass - Ring experimental apparatus. The relative cross sections between full stripping of 205 Pb and (p,n) on 205 Tl is estimated as 10 5 . Test experiments to verify the various aspects of the proposed approach at the RIBF are under preparation.