High resolution ablation fluorescence spectroscopy for remote isotopic analysis

Miyabe, Masabumi   ; Iwata, Yoshihiro   ; Hasegawa, Shuichi*

Laser ablation has been studied for application in many fields, including materials science and the nuclear industry. In the decommissioning of TEPCO's Fukushima Daiichi Nuclear Power Plant, the use of laser-induced breakdown spectroscopy (LIBS) for remote analysis of highly radioactive fuel debris is desired, but there is concern that LIBS may not provide sufficient isotopic selectivity when isotopic composition is required. In this study, we attempted high-resolution spectroscopy by irradiating laser plasmas cooled in a gas with lights from two external cavity diode lasers in opposite directions and observing fluorescence from the 2nd-step excited states produced by two-step resonant excitation of atoms; whereas the Doppler broadened linewidth of Ca and Sr is about 2 GHz, the linewidth of the high-resolution fluorescence resonance was found to be reduced to about 70 MHz. As a result, it was confirmed that the resonance of 42Ca with a natural abundance of 0.647% and that of 84Sr with a natural abundance of 0.56% can be observed separately from the other major isotope resonances. This demonstrated that remote isotope analysis is possible even for the nuclides having smaller isotope-shifts, such as Ca and Sr, which have been difficult to measure with conventional Doppler-limited fluorescence spectroscopy.