Investigation of suitable targets for accelerator mass spectrometry of 26Al

Abstract

Accelerator Mass Spectrometry (AMS) has been a frequently used technique for analysis of low-level concentrations of 26Al in environmental, biological and space samples with detection limits down to 0.01 fg. Al2O3 targets have been mostly used in ion sources for the production of aluminium negative ions because of high-temperature stability, non-toxicity, in-air stability, and relatively easy production procedures. It is well known, however, that aluminium does not yield as intense negative ion beams as other elements used in tandem AMS accelerators. An alternative solution could be to use as the target material AlN, which can yield higher Al− currents. On the other hand, the AlN targets are more difficult to synthesize, and they decompose with water in the air to form Al(OH)3 and ammonia. Commercially available compounds of Al2O3 and AlN were mixed with copper, silver and iron high-purity powders and sputtered in a MC-SNICS ion source for studying ionization yields. Since the production of magnesium and nitrogen negative ions is negligible, a production of MgN− molecules has been questionable. Obtained results indicate that the aluminium nitride matrix could be a suitable material for AMS measurements as the production of 27Al− is higher by a factor of 1.8 from the aluminium oxide matrix, while aluminium sulphate and aluminium fluoride showed a very low sputtering efficiency. The formation probability of the MgN− was tested using AlN and Mg3N2 matrices. The AMS measurements of 26Al using AlN− as injected ions from the AlN matrix, similar to AlO− ions from Al2O3 matrix, have shown that further isobar suppression due to a high 26Mg3+ production rate will be needed.