Accelerator mass spectrometry with 26Al

Abstract

The cosmogenic radioisotopes 10Be, 14C and 36Cl are routinely being measured in several laboratories but the potentially important 26 Al (τ 1 2 =7.2×10 5 y) has received scant attention - largely because it is difficult to produce negative-ion beams of aluminum (particularly from the oxide) and the high abundance of 27Al results in small 26Al: 27Al ratios (typically ≲10 −14 terrestrial and 10 −10 extraterrestrial). A new ion source is described which typically generates 1 to 2 μA of 27Al − ions from small samples (≈4mg) of Al 2O 3 with an ionization efficiency of about 0.25%. Modifications have been made to the University of Pennsylvania's FN-tandem accelerator for quantitative measurement of 26Al: 27Al ratios as low as 5×10 −15 and some of the problems encountered are contrasted with those met during 10Be measurements. Although measurement of 26Al: 27Al ratios in terresrial samples are often close to the limit of our sensitivity, we have made measurements on a variety of terrestrial samples including stratospheric air filters, manganese nodules, tektites, and various impact glasses and on extraterrestrial samples including cosmic spherules, a meteoric and several lunar samples.