AMS measurements of 26Al in quartz to assess the cosmic ray background for the geochemical solar neutrino experiment LOREX

doi:10.1016/j.nimb.2004.04.122

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

Volumes 223–224, August 2004, Pages 660-667

https://doi.org/10.1016/j.nimb.2004.04.122 Get rights and content

Abstract

LORandite EXperiment (LOREX) plans to measure the time integrated solar neutrino flux of the last few million years via the product of the reaction ²⁰⁵Tl(υ_e,e⁻)²⁰⁵Pb in lorandite of the Allchar mine in Macedonia. Utilizing this reaction is only possible if the background of ²⁰⁵Pb produced by the interaction of secondary cosmic ray particles and particles originating from the natural radioactivity within the rock mineral itself is substantially lower than the expected signal from neutrino interactions. Low abundance of cosmic ray induced ²⁰⁵Pb implies good shielding of the mine by the overlying rock. For the assessment of this background fraction it is necessary to acquire information about the past erosion activity at this site.

In the present study the erosion rates have been estimated via cosmogenic ²⁶Al produced in situ in quartz of surface rock materials of the Allchar site. Details of the first determinations of in situ produced ²⁶Al with the AMS method at VERA are described, and a rough estimate of the erosion rates at the Allchar site is given.

Introduction

All of the current solar neutrino experiments measure the present-day solar neutrino flux, but it has been debated for some time to determine the time-integrated flux of the last few million years through the production of long-lived radionuclides in a suitable mineral [1]. In particular, it was suggested [2] to utilize the reaction ²⁰⁵Tl(υ_e,e⁻)²⁰⁵Pb which has the lowest threshold (52 keV) for detection of pp neutrinos. ²⁰⁵Pb has a half-life of (1.53 + 0.07) × 10⁷ years [3] and should accumulate as the result of the above mentioned reaction in a suitable Tl-bearing mineral. LORandite EXperiment (LOREX) is a multidisciplinary research project to utilize this reaction in the mineral lorandite (TlAsS₂) from the Allchar region situated in Macedonia [4], [5]. Fig. 1 shows a map of the Balkan area with the Allchar region marked.

One of the pre-requisites for the feasibility of the LOREX experiment is a low abundance of non-neutrino induced ²⁰⁵Pb [2]. Depending on the depth of the mineral deposit, the ²⁰⁵Pb background is produced by the interaction of secondary cosmic ray particles (mainly stopped negative muons and fast muons), and of particles originating from the natural radioactivity (spontaneous fission of ²³⁸U, (α,p) and (α,n) reactions) within the rock minerals itself. The effect of fast muons was studied by Neumaier et al. [6]. Finally, ²⁰⁵Pb mobilized from the surrounding environment may also contribute to the background [7].

Cosmic ray induced ²⁰⁵Pb background depends on the shielding by the overlying rock and therefore on the erosion rate. In principle, erosion rates can be obtained from in situ produced cosmogenic ¹⁰Be and ²⁶Al in quartz from surface rock samples [8]. From estimates about the expected concentration of these radionuclides in surface rocks from the Allchar region and the detection sensitivity of AMS at VERA, we found out that a ¹⁰Be detection would be marginal, whereas ²⁶Al should be well detectable.

As outlined in [8], [9], mean erosion rates of a geological formation can be calculated from the content of a single cosmogenic in situ produced nuclide in surface rock if the exposure time of the rock is known.

Section snippets

Sample description

Six rock samples were collected in the vicinity of the Allchar mine. The geographical positions of the sample localities are shown in Fig. 2. Crven Dol (“red valley”), the area of highly concentrated Tl mineralisation, e.g. Lorandite, is indicated on the map. After the removal of 10 cm of the surface rock about 1000 cm³ of rock material was collected at each side. A petrological investigation with thin sections under the light microscope revealed that the rock material consisted of either

Isolation of quartz

An overview of the isolation and purification procedure of quartz is given in the flow scheme shown in Fig. 3. In a first step, material enriched in silica (>90%) was produced from the rock samples by standard geological methods: (i) crushing and grinding, (ii) sieving to select grain sizes ⩽0.5 mm, (iii) shaking and heavy liquid separation to concentrate quartz. From this material purified quartz was isolated by etching repeatedly (3–5 times) with H₂SiF₆. The H₂SiF₆ treatment was performed at

AMS measurements of ²⁶Al

The ²⁶Al/²⁷Al ratio of the Al₂O₃ produced from each sample was measured using the AMS system at VERA. The high energy beam line was recently upgraded with an electrostatic analyser and the position of the individual filters used so far has been re-arranged. A detailed description of the upgraded AMS system at VERA is given by Vockenhuber et al. [12].

The Al₂O₃ targets from the samples were mounted into the 40 position sample wheel of the SNICS negative ion source. Also mounted in the sample

Results and discussion

The results of the ²⁶Al/²⁷Al measurements for the Allchar quartz samples (weighted mean values from three measurements normalised to the ²⁶Al standards) together with the values of the two different blank materials and of metal aluminium are listed in Table 1. Since the ²⁶Al/²⁷Al ratios of the chemistry blank from the whole Al separation process and the blank from the final hydroxide precipitation agree, it can be concluded, that no significant contamination occurred during the chemical

Conclusion

This paper presents the determination of erosion rates in the Allchar region of Macedonia utilizing the cosmogenic radionuclide ²⁶Al. The high erosion rates are very encouraging with respect to the cosmic ray shielding effect. The results for sample 5 and 6 are about 50% higher than the geomorphological estimates. This suggests that at the deepest location of the lorandite mineralization at Crven Dol (∼300 m shielding including the erosion in the last 4.2 million years) the cosmogenic

Acknowledgements

We thank Hartmut Heinrichs from the University of Göttingen for preliminary ICP-AES measurements. Support through a CEEPUS mobility scholarship of the Austrian Federal Ministry of Education, Science and Culture, the International Relations Office of the University of Vienna, and the Ministry of Science and Technology of Serbia is gratefully acknowledged. We also thank Walter Henning and Fritz Bosch from the GSI Darmstadt for financial support and continuous interest in the LOREX project.

References (21)

S. Rab
Nucl. Data Sheets
(1993)
D. Lal
Earth Planet. Sci. Lett.
(1991)
K. Nishiizumi et al.
Earth Planet. Sci. Lett.
(1991)
C. Vockenhuber et al.
Int. J. Mass Spectrom.
(2003)
A. Wallner et al.
Nucl. Instr. and Meth. B
(2000)
T.A. Kirsten
Rev. Mod. Phys.
(1999)
M.S. Freedman et al.
Science
(1976)
Proc. Int. Conf. on Solar Neutrino Detection with ²⁰⁵Tl and Related Topics, Dubrovnik, Yugoslavia 1986
Nucl. Instr. and Meth. A
(1988)
Proc. Int. Symp. on Solar Neutrino Detection with 205-Tl, Dubrovnik, Yugoslavia 1990
N. Jb. Miner. Abh.
(1994)
S. Neumaier et al.
N. Jb. Miner. Abh.
(1994)

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Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

Abstract

Introduction

Section snippets

Sample description

Isolation of quartz

AMS measurements of ²⁶Al

Results and discussion

Conclusion

Acknowledgements

References (21)

Nucl. Data Sheets

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Int. J. Mass Spectrom.

Nucl. Instr. and Meth. B

Rev. Mod. Phys.

Science

Proc. Int. Conf. on Solar Neutrino Detection with ²⁰⁵Tl and Related Topics, Dubrovnik, Yugoslavia 1986

Nucl. Instr. and Meth. A

Proc. Int. Symp. on Solar Neutrino Detection with 205-Tl, Dubrovnik, Yugoslavia 1990

N. Jb. Miner. Abh.

N. Jb. Miner. Abh.

Cited by (8)

Applications of accelerator mass spectrometry

Cosmogenic <sup>26</sup>Al in the atmosphere and the prospect of a <sup>26</sup>Al/<sup>10</sup>Be chronometer to date old ice

Measurement of <sup>26</sup>Al for atmospheric and climate research and the potential of <sup>26</sup>Al/<sup>10</sup>Be ratios

Progress in isotope analysis at ultra-trace level by AMS

Erosion rate study at the Allchar deposit (Macedonia) based on radioactive and stable cosmogenic nuclides (<sup>26</sup>Al, <sup>36</sup>Cl, <sup>3</sup>He, and <sup>21</sup>Ne)

State of geoscientific research within the lorandite experiment (LOREX)

AMS measurements of 26Al in quartz to assess the cosmic ray background for the geochemical solar neutrino experiment LOREX

Abstract

Introduction

Section snippets

Sample description

Isolation of quartz