Diode laser absorption measurement of uranium isotope ratios in solid samples using laser ablation

The isotopes U-235 and U-238 were simultaneously measured by absorption in a laser-induced plasma ignited by a Nd:YAG laser. Two separate diode lasers were used which probed almost the same plasma volume. The diode lasers were tuned to the absorption lines 682.6736 nm for U-235 and 682.0768 nm for U-238. The U-235/U-238 isotope ratio was measured on a pulse-to-pulse basis evaluating the transient absorption peaks as analytical signals. Three uranium oxide pellets were used for calibration with natural (0,714 wt.%) and depleted U-235 concentration (0.407 and 0.204 wt.%). The measured U-235/U-238 mass ratio in an uranium containing mineral sample was (0,686+/-0.119) x 10(-2) and found to be in good agreement with the mass ratio of naturally occurring isotopes (0.719 x 10(-2)). The observed accuracy (< 5%) and precision (similar to 17% R.S.D.) are encouraging if it is taken into account that the mineral sample was probed directly without any preceding preparation step. Spatially and temporally resolved emission spectroscopic measurements were per-formed to investigate the kinetics of the laser plasma and to optimize the operating conditions of the laser ablation with regard to sensitivity and selectivity of the U-235 isotope detection. Optimal conditions were obtained at a distance of similar to0.3 cm from the sample surface, an argon pressure of similar to30 mbar and for 7 mJ pulse energy of the Nd:YAG laser. The limit of detection of the U-235 isotope, evaluated on the basis of the 3sigma criteria, was estimated to be 47 mug g(-1) and improved by a factor of two compared with the sequential measurement with a single laser diode used in a previous work. (C) 2002 Elsevier Science B.V. All rights reserved.