For remote isotope analysis of low‐decontaminated trans‐uranium (TRU) fuel, absorption spectroscopy has been applied to a laser‐ablated plume of lanthanide elements. To improve isotopic selectivity and detection sensitivity of the ablated species, various experimental conditions were optimized. Isotope‐selective absorption spectra were measured by observing the slow component of the plume produced under low‐pressure rare‐gas ambient. The measured minimum line width of about 0.9 GHz was close to the Doppler width of the Gd atomic transition at room temperature. The relaxation rate of high‐lying metastable state was found to be higher than that of the ground state, which suggests that higher analytical sensitivity can be obtained using low‐lying state transition. Under helium gas environment, Doppler splitting was caused from particle motion. This effect was considered for optimization for isotope selection and analysis. Some analytical performances of this method were determined under optimum conditions and were discussed.

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