26 Al: measurement and applications

Abstract

26 Al(r1/2 = 0.7 Ma), like 10 Be (r1/2 = 1.5 Ma), is produced by cosmic rays in the atm osphere, but its production rate is low ( ca 4 x 10 -3 that of 10 Be) and the crustal abundance o f 27 Al is high, resulting in 26 A1:27Al ratios rarely exceeding 10 ~14 . Both isotopes are also produced in surficial rocks by cosmic rays; in quartz the production rates of 26 Al and 10 Be are ca . 70 atoms g -1 a -1 and ca . 10 atoms g -1 a -1 , respectively (at sea level). Because quartz frequently contains 27 Al at less than 100 p.p.m. (by mass), the 26 Al : 27 A1 ratio resulting from this situ production can be as high as 3 x 10 -11 . In extraterrestrial m atter, where the production rates are several hundred times greater, samples containing a few percent o f 27 Al have 26 Al: 27 Al ratios of 10 -11 or larger. We describe an experim ental arrangem ent that was developed at the University of Pennsylvania for measuring 26 A1: 27 Al ratios as low as 10-15. The size requirements for samples are approximately a factor of 100000 smaller than those of decay counting, and measurement times are less than an hour, compared with weeks or months. We discuss several applications of the technique to the measurement of terrestrial as well as extraterrestrial samples. Emphasis is placed on the combined measurement of 10 Be and 26 Al and the various uses to which the ratio o f 26 Al: 10 Be in a sample can be put. In particular, we discuss the very high 26 Al: 10 Be ratios that are found in the outer few millimetres of extraterrestrial bodies, resulting from low-energy protons emitted by solar flares, and the diagnostic value of these ratios in determining the size of objects in space. The low value of the 26 A1: 10 Be ratio resulting from atmospheric production is used to prove the terrestrial origin of tektites. And finally, we discuss the manner in which the variable 26 Al: 10 Be ratio that results from in situ production in objects on the Earth ’s surface can be used to measure rates of weathering and exposure histories.