Depth and size dependence of ⁵³Mn activity in chondrites

Abstract

The cosmogenic ⁵³Mn 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 ⁵³Mn 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 ²⁶Al show agreement with observations within 20%.