Spectroscopic study of thorium using continuous-wave resonance ionization mass spectrometry with ultraviolet ionization

This paper presents recent results on an improved method of ionization for the cw-RIMS process for thorium (Th). This method involves the application of a high power ultraviolet (UV) argon ion laser for the second step in a two-step (1+1) (photon-to-resonance plus photon-to-ionization) ionization scheme. Over 90 thorium transitions are identified for use in a (1+1) continuous-wave resonance ionization mass spectrometry (cw-RIMS) ionization scheme. The excitation cross-section of several strong transitions was determined to be 10^-13cm². The optimum cw excitation scheme was with the resonant laser tuned to the 384.08 nm (26 036 cm^-1) transition when using the multi-line UV argon ion laser for signal enhancement. For thorium, the increase in ionization efficiency was documented to be a minimum of one order-of-magnitude improvement over that achieved by conventional thermal ionization mass spectrometry (TIMS). The measured total ionization efficiency (detected ion signal/sample atoms loaded) was as high as 0.41%, which easily provided signal levels for efficiency measurements on sample sizes down to 25 ng, and should provide sufficient signal for isotopic analysis of volcanic-like samples as small as 1-5 ng of thorium. Based on geometric overlap considerations, the cw-RIMS ionization efficiency within the laser focal volume approaches ~100%. This cw-RIMS ionization efficiency promises to provide ample signal for the ²³⁰Th/²³²Th isotope ratio analysis of nanogram volcanic-like samples. The ability to determine accurately and precisely the ²³⁰Th/²³²Th isotopic ratios for nanogram samples represents an improvement over the TIMS technique, and is anticipated to have a significant effect on uranium-series disequilibrium measurements important in geochemistry and geochronology.