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.