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 ap plication of a high power ultraviolet (UV) argon ion laser for the sec ond step in a two-step (1+1) (photon-to-resonance plus photon-to-ioniz ation) ionization scheme. Over 90 thorium transitions are identified f or use in a (1+1) continuous-wave resonance ionization mass spectromet ry (cw-RIMS) ionization scheme. The excitation cross-section of severa l strong transitions was determined to be 10(-13)cm2. The optimum cw e xcitation scheme was with the resonant laser tuned to the 384.08 nm (2 6 036 cm-1) transition when using the multi-line UV argon ion laser fo r signal enhancement. For thorium, the increase in ionization efficien cy was documented to be a minimum of one order-of-magnitude improvemen t over that achieved by conventional thermal ionization mass spectrome try (TIMS). The measured total ionization efficiency (detected ion sig nal/sample atoms loaded) was as high as 0.41%, which easily provided s ignal levels for efficiency measurements on sample sizes down to 25 ng , and should provide sufficient signal for isotopic analysis of volcan ic-like samples as small as 1-5 ng of thorium. Based on geometric over lap considerations, the cw-RIMS ionization efficiency within the laser focal volume approaches approximately 100%. This cw-RIMS ionization e fficiency promises to provide ample signal for the Th-230/Th-232 isoto pe ratio analysis of nanogram volcanic-like samples. The ability to de termine accurately and precisely the Th-230/Th-232 isotopic ratios for nanogram samples represents an improvement over the TIMS technique, a nd is anticipated to have a significant effect on uranium-series diseq uilibrium measurements important in geochemistry and geochronology.