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

To improve our understanding of present-day cosmogenic production systematics of Be-10, H-3 and He-3, 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 He-3 contributions from decay of "inherited" tritium initially present in the water. After analyses of He-3 and Be-10, both the summit and tunnel H-3-He-3 tanks were redegassed and stored underground for an additional year. The stored summit tanks were then analyzed to determine cosmogenic H-3 levels by the He-3 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 He-3 and Be-10 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 H-3:He-3 and 20.2 +/- 1.5 for (H-3+He-3): Be-10. Our Be-10 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 H-3:He-3 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)). (C) 2000 Elsevier Science B.V. All rights reserved.