Abstract

Resonance ionization mass spectrometry (RIMS) is being developed to measure ^230/232Th ratios (~5:10⁵), which are used in U-series disequilibrium dating of geological samples of the last 400,000 years. We will discuss the precision and accuracy of the ratio measurements when resonant excitation with a broad-band dye laser and a narrow-band frequency-doubled Ti:sapphire laser is used. The dye laser is evaluated with and without optogalvanic frequency locking onto a Th transition to provide better reproducibility. The relative advantages and disadvantages of the two excitation sources will be summarized, including 1) narrowband excitation yielding greater isotope selectivity versus 2) broad-band laser excitation providing higher sensitivity (simultaneous excitation of all relevant isotopes and all atoms within the Doppler profile).

© 1992 Optical Society of America