MEASUREMENTS OF NI-59 IN METEORITES BY ACCELERATOR MASS-SPECTROMETRY

Isotopic abundances of the radionuclide Ni-59 (T1/2 = 76000 yr) were measured by accelerator mass spectrometry with the 14UD Pelletron tandem accelerator at the Australian National University and a detection system solely based on a multi-anode ionization chamber. The sensitivity limit in the measurement of Ni-59 isotopic abundances is 5 x 10(-13), as determined by residual interferences from isobaric Co-59 and isotopic Ni-58 ions. Cosmogenic Ni-59 abundances Ni-59/Ni = (8-20) x 10(-12) were measured in four samples prepared from the metal phase of two meteorites (mesosiderites). The ratio of the Ni-59 abundances to those measured for Ca-41 in the silicate phase of the same samples, is in fair agreement with the ratio of the production rates via thermal-neutron capture on Ni-58 and Ca-40.