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

LOREX (LORandite EXperiment) is a geochemical experiment addressing the solar (pp) neutrino flux for the period of 4.3 Ma from the reaction 205Tl + ve → 205Pb + e- with an unprecedentedly low threshold (52 keV) for solar pp-neutrino capture. A decisive step for this purpose is getting the precise, background-corrected ratio of 205Pb/205Tl in lorandite (TlAsS2). This report presents the status of major challenges being addressed, in particular the determination of the paleo-depth of lorandite, including the eroded layer over 4.3 Ma, as well as the choice of appropriate techniques for extraction, separation and quantitative determination of the ultra-low 205Pb concentration in the extracted lorandite samples.


Introduction
The goal of LOREX [1] is the determination of the long-time average (over ~ 4 MY) of the solar ppneutrino flux Φν via the neutrino-capture reaction [2]: (TlAsS2) at the mine of Allchar. This geochemical experiment was proposed originally by Freedman [2]. The average neutrino flux ν over the exposure time a (age of lorandite since its mineralization) follows from the common activation equation: with N, the total number of 205 Tl atoms, T, the total number of 205 Pb atoms, B, the background-induced number of 205 Pb atoms [mainly from 205 Tl (μp, n) 205 Pb], σ, the neutrino capture cross section, ε, the overall detection efficiency, λ = 4.00 10 -8 /y the decay constant of 205 Pb, and a = 4.3 10 6 y, the age of lorandite. This renders finally the mean solar pp-neutrino flux, i.e. the mean luminosity of the sun during the last 4.3 million years, the geological age a of lorandite.
The central problem of LOREX is the quantitative determination of 205 Pb atoms in lorandite. For this purpose three problems must be reliably addressed and solved:

Background, erosion and paleo-depth:
The background of 205 Pb atoms produced by cosmic radiation and by natural radioactivity must be determined quantitatively. In this context, the knowledge of the erosion rate of the overburden rock during the existence of lorandite is of utmost importance.

Extraction, separation and detection of 205 Pb trace concentration:
How can the expected ultra-low abundance of 205 Pb be reliably measured?

Background, erosion rate and paleo-depth
About 10 tons of lorandite have been extracted from ore body Crven Dol (Figures 1a, 1b and 1c). The separation of lorandite has been performed by crushing, hand-picking and cleaning of lorandite crystals, obtaining finally about 1 kg of 98 % pure lorandite grains that were controlled and quality-checked by means of SEM-EDX and ICP-MS methods.
With the paleo-depth known, the amount of cosmic-ray (mainly muons) induced 205 Pb atoms could be calculated as function of the depth of the lorandite location (see Table 1). For the natural radioactivity the following values (in ppm) were figured out: U = 0.102(10), Th = 0.096(75) Bi = 0.008(2), Hg = 231(92). Finally, for the total lead concentration in lorandite a value 1.5(5) has been found.

Extraction, separation and detection of ultra-low amounts of 205 Pb in lorandite
Decisive steps of LOREX already started with the prospection and separation of lorandite from the Allchar mine (Figure 1), the extraction of thallium and lead (the mean concentration of lead in lorandite amounts to 1.5 ppm) and the quantitative determination of the ratio 205 Pb / 205 Tl sc. 205 Pb / Pb.  After the last step of chemical separation, a lead matrix will be obtained, where the 205 Pb/Pb ratio is expected to range from 10 -14 to 5 10 -13 . Supposing a value of 146 SNU for the solar neutrino capture rate (this number is based on the presently best theoretical value), the geological age a since the Tlmineralization as a = 4.3 10 6 y, the decay probability λ for the electron-capture decay of 205 Pb back to 205 Tl as λ = 4.00 10 -8 y -1 and a molar mass M of lorandite as M = 343 g / Mol, one gets for the expected time-integrated number of solar pp-neutrino induced 205 Pb atoms the value of 22(7) atoms of 205 Pb/g lorandite.
Identification of the 205 Pb nuclei in the lead sample extracted from the lorandite mineral requires from 10 -10 to 10 -11 overall detection sensitivity for 205 Pb/Pb and comparable suppression of the 205 Tl isobar [5]. 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 in principle already largely achieved 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 control 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 205 Tl (p,n) 205 Pb reaction in the energy-loss and ion-stripping steps in the accelerator and the subsequent BigRIPS/Mass-Ring. In this case, the whole approach then involves 4 steps: i) Establishing beam tuning and control for a trace beam with an 1% 205 Tl sample; ii) using the guide beam to confirm 205 Tl beam control, for the 1% and a ~10 -10 Tl sample; iii) extension to a calibration sample with known 205 Pb concentration at the 10 -13 level; iv) measurement of the 205 Pb neutrino sample. Test experiments to verify the various aspects of the proposed approach at the RIBF are under development.

Solar pp-neutrino capture probability into the keV state of 205 Pb
The ratio 205 Pb/ 205 Tl provides only the product of solar neutrino flux and neutrino capture probability into the different nuclear states of 205 Pb. The capture of neutrinos should populate predominantly the first excited state at E* = 2.3 keV [4]. Its probability can be determined from the bound-state β decay probability (βb) according to 205 Tl 81+ → 205 Pb* 81+ (E* = 2.3 keV) + eb + νbar, since this decay shares the same nuclear transition matrix element with the neutrino capture (cf. Figure 2) The proposal for the measurement at the Experimental Storage Ring of GSI of the βb decay of bare 205 Tl81+ to 205 Pb* 81+´h as been already approved by the international Program Advisory Committee. However, due to a long break of the GSI accelerators in course of the construction of the new FAIR facility, this experiment cannot be addressed before the year 2018.

Present status of LOREX and conclusions
Taking into account the present-day state-of-the-art of all the techniques needed to solve the main challenges of LOREX, and acknowledging the achievements of hard working during the last years concerning the determination of the erosion rate and of the background, as well as the development of a probably feasible scenario for the detection of the ultra-low 205 Pb concentration, we conclude that it seems realistic to expect the first result for the solar pp-neutrino flux averaged over the last 4.3 million years in the foreseeable future.
However, this assessment supposes that the βb experiment -irrevocable for the precise knowledge of the solar pp-neutrino capture probability-could be (successfully) performed. The final number for the background-corrected amount of solar neutrino-induced 205 Pb atoms will have probably still a large error margin in the order of 30% (68% CL) or even more. We expect, however, that this accuracy could be improved with time, and that it might finally reach a level below 30%.