Determination of cosmogenic production rates of ¹⁰Be, ³He and ³H in water

To improve our understanding of present-day cosmogenic production systematics of ¹⁰Be, ³H and ³He, we exposed three sets of targets of purified water at altitudes of 620, 3810 and 4745 m in the Mont Blanc Massif of the French Alps. In addition, tanks were stored 1780 m underground to quantify ³He contributions from decay of "inherited" tritium initially present in the water. After analyses of ³He and ¹⁰Be, both the summit and tunnel ³H-³He tanks were redegassed and stored underground for an additional year. The stored summit tanks were then analyzed to determine cosmogenic ³H levels by the ³He ingrowth method, and the tunnel tanks used to re-determine inherited tritium. Production rates (in atoms per g H₂O per year) for direct production of ³He and ¹⁰Be were 1824 ± 52 and 112 ± 9; 1013 ± 16 and 70 ± 5; and 134 ± 58 and 5.9 ± 0.7 at the three elevations, respectively. We determined production ratios of 0.32 ± 0.08 for ³H:³He and 20.2 ± 1.5 for (³H+³He):¹⁰Be. Our ¹⁰Be production rates, when normalized for interlaboratory calibration and for differences in geomagnetic latitude of exposure, are somewhat lower than results of a similar experiment undertaken by Nishiizumi et al. (1996). Our ³H:³He ratio is consistent with theoretical and meteorite estimates (Kruger and D. Heymann, 1968), but considerably lower than values assumed in many exposure age studies of igneous rocks (e.g., (Kurz, 1986; Trull et al., 1995)).