Depth and size dependence of 53Mn activity in chondrites

Abstract

The cosmogenic 53 Mn activity was measured in samples from cores of four chondrites, Madhipura, Udaipur, Bansur and St. Severin. A simultaneous study of cosmic ray tracks established the pre-atmospheric size of these meteorites and the shielding depth of the samples. The four chondrites are found to have had effective radii, R E , of 6.5, 9, 15 and 25 cm, respectively, in space. The experimental depth profiles of the 53 Mn activity indicate that, (1) the production rate, at any given depth, increases with size of the meteorite up to R E ∼ 25cm; (2) the activity profile for R E ⩽ 15cm is nearly flat for shielding depths greater than 3 cm, the increase being less than 10%; and (3) for R E ∼ 25cm the increase from the near-surface region to the centre is about 40% indicating that the cascade of secondaries becomes very important for R E > 15cm. These profiles are used to determine the spectral hardness parameter, α, as a function of depth in the meteoroid which, in turn, can be used to calculate the production profiles of other low-energy nuclides. Calculations for 26 Al show agreement with observations within 20%.