High precision uranium, thorium and radium isotope ratio measurements by high dynamic range thermal ionisation mass spectrometry

Mass spectrometric methods are presented for the precise determination of the isotopic compositions of uranium, thorium and radium. M+ ions were produced by thermal ionisation. Analysis involved either a dynamic routine using the Daly detector or a static routine employing both the Daly detector and off-axis Faraday detectors. Instrument performance was assessed using a set of uranium standards, CBNM IRM 072. The response of the Daly detector was linear up to ratios of at least 5 x 10(4) and when operated dynamically it is ideally suited to the measurement of ion beams in the range 5-1 x 10(5) cps. The static mode of analysis was employed successfully for the measurement of isotope ratios up to 5 x 10(5)