26Al and 10Be production in iron meteorites

We have measured by accelerator mass spectrometry the 26Al contents of 20 and the 10Be contents of 14 iron meteorites. The 26Al contents are typically 30% or more lower than values obtained by counting techniques; the 10Be contents are 10-15% lower. The production rates (P) of these nuclides decrease by more than a factor of two as the 4He/21Ne ratio increases with increasing shielding from 200 to 400. For the lighter shielding conditions expected in stony meteorites we estimate P26(Fe) as 3-4 dpm/kg and P10(Fe) as 4-5 dpm/kg. The average P10/P26 activity ratio is close to 1.5. Exposure ages calculated from 21Ne/26Al ratios cannot be calibrated so as to agree with both 40K/K ages and ages based on the shorter-lived nuclides 39Ar and 36Cl. If agreement with the latter is forced, then the disagreement with 40K/K ages may signal a 35% increase in the cosmic-ray intensity during the last 107 a. (orig.)