Atmospheric ⁸¹Kr as an Integrator of Cosmic‐Ray Flux on the Hundred‐Thousand‐Year Time Scale

Abstract

The atmospheric abundance of 81Kr is a global integrator of cosmic rays. It is insensitive to climate shifts, geographical variations, and short‐term solar cycle activity, making it an ideal standard to test models of cosmic‐ray flux on the time scale of 105 years. Here we present the first calculation of absolute 81Kr production rates in the atmosphere, and a measurement of the atmospheric 81Kr/Kr abundance via the Atom Trap Trace Analysis method. The measurement result significantly deviates from previously reported values. The agreement between measurement and model prediction supports the current understanding of the production mechanisms. Additionally, the calculated 81Kr atmospheric inventory over the past 1.5 Myr provides a more accurate input function for radiokrypton dating.

Stellar s -process neutron capture cross sections on Kr78,80,84,86 determined via activation, atom trap trace analysis, and deca

“…abundance by noble-gas mass spectrometry; see details in [39]. The measurement yielded a value 81 Kr /Kr = 9.3(3) × 10 −13 in modern atmosphere, higher than that measured in 1969 [ 81 Kr /Kr = 6.6(7) × 10 −13 ] by Loosli and Oeschger [36].…”

“…The (γ ,n) corrections are small and discussed in Section IV. The contemporary Kr gas sample was required also to correct for the cosmogenic 81 Kr content of atmospheric Kr [36,39] in both irradiated samples; the cosmogenic abundance of 81 Kr, shielded from fission products by 81 Br, is the same within 1% in pre-and post-nuclear age Kr [39]. The gas samples used and irradiation details are presented in Table I.…”

“…During this time span the sample 9) in good agreement with the intensity ratio 0.186(4) in [49] (see Table II). [39]). Figure adapted from [52], see text.…”

“…For 81 Kr, the reference gas is the 81 Kr isotopic abundance in modern atmospheric Kr. This value was recently redetermined [39] by ATTA measurements on a dilution series of an enriched 81 Kr sample analyzed for 81 Kr Only a fraction of the particle decay energy (687 keV) is released within the counter before they reach its wall. Events with energies greater than 33 keV are recorded as saturated events in the last MCA channels and are included in the 85 Kr activity analysis.…”

Optical Excitation and Trapping of $^{81}$Kr

“…when the sidebands are resonant with the two neighboring transitions. The intense and resonant output of the discharge lamp allows us to trap and detect the extremely rare isotope 81 Kr (9.3×10 −13 isotopic abundance [35]) using optical excitation instead of discharge excitation. Fig.…”