EFFECTIVE ATTENUATION LENGTHS OF COSMIC-RAYS PRODUCING BE-10 AND AL-26 IN QUARTZ - IMPLICATIONS FOR EXPOSURE AGE DATING

We have measured cosmic ray produced Be-10 (t1/2 = 1.5 My) and Al-26 (t1/2 = 0.72 My) as functions of depth in a core of quartz sandstone bedrock collected in South Victoria Land, Antarctica. These data were used to place limits on the exposure age and erosion rate of the exposed surface, and to calculate the effective attenuation lengths of cosmic rays producing these nuclides. These latter results indicate that the production attenuation length for Al-26 (156 +13/-12 g cm-2) is comparable to that for Be10 (145 +8/-6 g cm2 This suggests that the production ratio Al-26:Be-10 does not vary substantially with depth in a rock, and that its value at the surface of exposed rock is not strongly affected by erosional losses, allowing wider use of the ratio in determining exposure ages.