Direct measurement of neon production rates by (α,n) reactions in minerals

Abstract

The production of nucleogenic neon from alpha particle capture by ^(18)O and ^(19)F offers a potential chronometer sensitive to temperatures higher than the more widely used (U-Th)/He chronometer. The accuracy depends on the cross sections and the calculated stopping power for alpha particles in the mineral being studied. Published ^(18)O(α,n)^(21)Ne production rates are in poor agreement and were calculated from contradictory cross sections, and therefore demand experimental verification. Similarly, the stopping powers for alpha particles are calculated from SRIM (Stopping Range of Ions in Matter software) based on a limited experimental dataset. To address these issues we used a particle accelerator to implant alpha particles at precisely known energies into slabs of synthetic quartz (SiO_2) and barium tungstate (BaWO_4) to measure ^(21)Ne production from capture by ^(18)O. Within experimental uncertainties the observed ^(21)Ne production rates compare favorably to our predictions using published cross sections and stopping powers, indicating that ages calculated using these quantities are accurate at the ∼3% level. In addition, we measured the ^(22)Ne/^(21)Ne ratio and (U-Th)/He and (U-Th)/Ne ages of Durango fluorapatite, which is an important model system for this work because it contains both oxygen and fluorine. Finally, we present ^(21)Ne/^4He production rate ratios for a variety of minerals of geochemical interest along with software for calculating neon production rates and (U-Th)/Ne ages.

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