COSMOGENIC PRODUCTION OF BE-7 AND BE-10 IN WATER TARGETS

We have measured Be-10 (t(1/2)=1.5x10(6) years) and Be-7 (t(1/2)=53.28 days) concentrations in water targets exposed for 1 to 2 years at Ech o Lake, Colorado (elevation=3246 m) and at La Jolla, California (140 m ). Neutron monitor data were used to normalize the measured concentrat ions in order to calculate production rates equivalent to the cosmic r ay flux averaged over four solar cycles (43 years). The Be-7 productio n rates thus obtained correspond to 6.03+/-0.07x10(-6) atom g(-1). O s (-1) at Echo Lake and 5.06+/-0.20x10(-7) atom g(-1). O s(-1) at La Jol la. The Be-10 production rates correspond to 3.14+/-0.18x10(-6) atom g (-1). Os-1 at Echo Lake and 2.68+/-0.47x10(-7) atom g(-1). Os-1 at La Jolla. When compared with Be-10 production rates determined in Be-10-s aturated rocks from the Antarctic and with theoretical calculations ba sed on meteorite and lunar sample data, we find that the million-year average production rate is about 14-17% greater than the present produ ction rate averaged over the last four solar cycles. Comparison with p roduction rates determined by measuring glacially polished rocks from the Sierra Nevada in California indicates that average production (bas ed on a revised 13,000-year deglaciation age and a geographic latitude correction) is about 11% greater than the average over the last four solar cycles. The measured B-10/Be-7 production ratio in oxygen is 0.5 2+/-0.03 at Echo Lake and 0.55+/-0.07 at La Jolla.