STUDIES OF THE PRODUCTION-RATE OF COSMIC-RAY PRODUCED C-14 IN ROCK SURFACES

We have developed a method of extracting C-14 in rock samples which is produced by the direct spallation of oxygen in the rock. The low levels of C-14 produced in rocks, approximately 10(6) atoms/g of rock at an altitude of 2000 m, are detectable only by accelerator mass spectrometry (AMS). This radioisotope can be extracted from the rock by melting in an rf furnace in a flow of oxygen, and earlier work has discussed this in some detail. Several improvements have been made to this system, allowing for processing of up to 100 g of rock sample at one time, and reduction of the blank. Our measurements from Tabernacle Hill, Utah, an intermediate-altitude basalt with well-constrained conventional C-14 ages, yield an estimated cosmogenic C-14 production rate of about 50 C-14 atoms/g/yr. When corrected to sea level we obtain a value of 20 +/- 2 C-14/g/yr for high-latitude samples.