Combined laser ablation-tuneable diode laser absorption spectroscopy and laser-induced breakdown spectroscopy for rapid isotopic analysis of uranium

Laser-induced breakdown spectroscopy (LIBS) can provide immediate in situ elemental characterisation of a sample from a stand-off distance with no sample preparation for bulk, minor and trace species. These attributes suggest the technique could be applied to many processes in the nuclear industry. However, at present, LIBS cannot quickly and easily be used to determine the isotopic composition of a sample, which limits its potential. We report the development of a system comprising laser ablation-tuneable diode laser absorption spectroscopy (LA-TDLAS) and LIBS which can maintain the benefits of traditional LIBS and enable isotopic analysis of certain elements, including lithium and uranium. In this feasibility study, we used this approach to analyse a uranium metal wire sample under ambient conditions of pressure and temperature. Simultaneously, from the same laser produced plasma, the 235U isotope fraction was determined with LA-TDLAS while the plasma emission was used to obtain the electron density of the plasma using Stark broadening of uranium and sodium emission lines. The accuracy of the isotopic analysis performed with LA-TDLAS was limited by the noise of the absorption spectra and low concentration of the minor 235U isotope (0.7%). The limit of detection was estimated to be 1.5 wt%. Using the Doppler broadening of absorption lineshapes, the plasma temperature was calculated to drop from 25,000 K to 2000 K during the plasma lifetime. The electron density rapidly decreased from 6 × 1016 cm−3 immediately after laser ablation to around 3 × 1016 cm−3 after a few μs. The combination of LIBS and LA-TDLAS opens the possibility of rapid elemental and isotopic analysis which would be useful across the nuclear fuel cycle. Furthermore, the LA-TDLAS technique could be further developed for a portable measuring instrument for nuclear forensics applications.