Accelerator mass spectrometry of ⁵⁹Ni in extraterrestrial matter

Cosmic-ray produced ⁵⁹Ni (t _{1 2} = 76000 years) was detected in meteoritic and lunar material by AMS with fully stripped ions. At 641 MeV beam energy, a clean separation of ⁵⁹Ni²⁸⁺ ions from ⁵⁹Co²⁷⁺ background was achieved with a magnetic spectrograph. A background limit of ⁵⁹Ni/Ni < 7 × 10^-14 was determined with a blank Ni sample. A ⁵⁹Ni/Ni ratio of (2.3 ± 0.4) × 10^-11 was measured in a sample from the Admire stony-iron (pallasite) meteorite, corresponding to a specific activity of 280±50dpm/kg meteorite. In a surface layer of Apollo 16 lunar rock 68815, a ⁵⁹Ni/Ni ratio of (8.8 ± 3.3) × 10^-13 was measured (diluted with Ni carrier), corresponding to a specific activity of 4.1 ± 1.5 dpm/kg rock. This result compares well with values calculated by Reedy from the ⁵⁶Fe(α, n)⁵⁹Ni reaction induced by solar cosmic ray alpha particles. Methodological aspects of the full-stripping technique and future possibilities of ⁵⁹Ni measurements in lunar materials are discussed.