ISOBAR DISCRIMINATION IN ACCELERATOR MASS-SPECTROMETRY BY DETECTING CHARACTERISTIC PROJECTILE X-RAYS

The measurement of long-lived radioisotopes by accelerator mass spectrometry (AMS) is often limited by interferences of isobars, which cannot be separated easily (e.g. Cl-36/S-36, Ni-59/Co-59, Fe-60/Ni-60, ....). Standard isobar separation techniques using ionisation chambers, gas-filled magnets and complete ion stripping can be applied only at sufficiently high energies. It is not possible to achieve enough discrimination for heavier isobars with small AMS accelerators. Recently, the detection of characteristic projectile X-rays during the stopping of ions in matter has been proposed by H. Artigalas et al. [H. Artigalas et al., Nucl. Instr. and Meth. B 79 (1993) 617.] as another isobar separation technique. In this case the discrimination does not depend on the ion energy. A high X-ray yield can be obtained with targets having a nuclear charge close to that of the projectile because inner shell vacancies are produced by molecular orbital formation. We present first results performed at the ETH/PSI 6 MV EN-tandem accelerator applying this technique in the detection of Ni-59 and Fe-60. Efficiency, suppression factors and detection limits are discussed.