R. Middleton, J. Klein

G.M. Raisbeck, F. Yiou Laboratoire Rene Bernas Orsay France

Nuclear Instruments and Methods 218 (1983) 430-438

The cosmogenic radioisotopes 10Be, 14C and 10Cl are routinely being measured in several laboratories but the potentialty important 26Al (tau 1 2 = 7.2x lO5 y) has received scant attention - largely because it is difficult to produce negative-ion beams of 1 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 mu A of 27Al- ions from small samples ( - 4 rag) of Al2O3 with an ionitation efficiency of about 0.25%. Modifications have been made to the University of Pennsylvania FN tandem accelerator for quantitative measurement of 26Al: 27Al ratios as low as 5 x 10^-15 and some of the problems encountered are contrasted with those met during 10Be measurements. Although measurement of 26Al: 27Al ratios in terrestrial 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 meteorite and several lunar samples.