We present results of first measurements of underground production of cosmogenic in situ 14C in deep underground vein quartz from Macraes Flat, East Otago (New Zealand). The in situ 14C was extracted from quartz using a low temperature wet extraction method [1], in samples up to depths of 400 g cm-2 below sea-level, to study the transition in underground production from energetic neutrons to low energy negative and fast muons. The cosmic ray exposure history of the Macraes Flat has been well characterized by studies of 10Be and 26Al in quartz samples up to depths of 49,000 g cm-2 below sea-level [2]. The surface exposure age of the site was determined to be 25.0 ± 3.3 Kyr, which leads to the expectation that the in situ 14C production in Macraes Flat should be in secular equilibrium.
The measured 14C concentrations were found to range between (0.8-3.0) x 105 14C atoms/ g quartz, considerably in excess over the theoretically expected values for fast neutron production at depths < 150 g cm-2 below sea-level, but in agreement with the expected value for 14C production by muons [3] at depths (150-400) g cm-2. This led us to explore the possibility of production of 14C by capture of thermal neutrons in N2 present in the fluid inclusions in the quartz. The nitrogen concentrations in quartz were determined by measuring the yield of alpha particles from the 14N(d,α) reaction [4] using a 1.4 MeV deuteron beam. The measured nitrogen concentrations in the 4 samples (22-400 g cm-2 depth) ranged between (60-350) ppm N (g N/g SiO2). Note that these are not the values measured in aliquots of quartz from the samples analyzed for 14C, and therefore the actual N concentrations may be quite different in the quartz samples analyzed. Nevertheless, the corresponding expected 14C production rates in N, based on expected thermal neutron fluxes [5,6] seem to explain fairly well the observed excess 14C concentrations in quartz samples at < 150 g cm-2 below sea-level.
[1] Lal and Jull, NIM B92 (1994) 291-296; [2] Kim and Englert, EPSL 223 (2004) 113-126; [3] Heisinger et al., EPSL 200(2002)357-369; [4] Vickridge et al., NIM B118 (1996) 608-612; [5] Phillips et al., 175 (2001) 689-701; [6] Lal and Peters, Handbuck der physik XLVI/2 (1967).
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