Expected atmospheric concentration of 42Ar

Recent estimates [1,2] of the atmospheric 42Ar concentration are likely to be low due to neglect of cosmic-ray production via the reaction 40Ar(α,2p) in the upper atmosphere. The calculations in this letter lead to an expected ratio of 42Ar40Ar of roughly 10−20, compared with existing experimental [2,3] and theoretical upper limits of 10−18 and 1.3 × 10−23, respectively.