Accelerator mass spectrometry measurement of cosmogenic ²⁶Al in terrestrial and extraterrestrial matter

Abstract

Cosmogenic ²⁶Al (half life = 7.2 × 10⁵ yr) and ¹⁰Be (1.5 × 10⁶ yr) are produced continually in the atmosphere by cosmic rays. They are fixed on aerosol particles and, after a relatively short time (∼1 yr) precipitate to the Earth's surface, and from there into various geophysical reservoirs. The possibility of using this pair of isotopes as a dating tool, independent of cosmic ray intensity variations, was discussed some time ago¹. However, except for several rare exceptions^2,3 the very low activities of these isotopes, especially ²⁶Al, have dissuaded workers from such studies by classical radioactive counting techniques. The development of accelerator mass spectrometry offers to change this situation dramatically. Cosmogenically produced ¹⁰Be has already been measured by several groups using accelerator mass spectrometry⁴. While the technique has also been used to detect artificially produced^5–7 and extraterrestrial⁸, ²⁶Al the ²⁶Al/²⁷Al ratio in each case was several orders of magnitude greater than that expected in natural terrestrial samples. We report here the first accelerator mass spectrometry measurements of ²⁶Al in natural terrestrial samples, and give a value for the atmospheric production ratio of ²⁶Al/¹⁰Be, an essential parameter for use of this pair as an absolute dating tool. To demonstrate the potential of our technique for measuring ²⁶Al in small quantities of extraterrestrial material, we have also measured this isotope in milligramme-size samples of lunar soil. Finally, by measuring the absolute ²⁶Al/²⁷Al ratio of two samples whose radioactive disintegration rate was known, we have obtained an independent check on the ²⁶Al half life.