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)).
|Original language||English (US)|
|Number of pages||11|
|Journal||Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms|
|State||Published - Dec 1 2000|
Bibliographical noteFunding Information:
We thank J. Lestringuez and D. Deboffle for their expertise in AMS measurements, and particularly acknowledge J. Lestringuez for assistance in the field operations. This research was supported by the French INSU program “Dynamique et Bilan de la Terre”. Tandetron operation is supported by CNRS, IN2P3 and CEA. Trull thanks DITAC and DIST, Australia, for travel grants. Thoughtful and detailed comments from David Fink enabled us to improve the manuscript.
- Cosmogenic nuclides
- Exposure ages
- Production rates