In the dismantling of nuclear power plants, a simple and highly sensitive analysis of radionuclide calcium-41 (⁴¹Ca) to verify clearance levels is crucial for the rapid disposal of large amounts of concrete waste. ⁴¹Ca has a long half-life of ~100,000 years and the X-ray energy emitted during decay is low, making it a difficult-to-measure radionuclide in terms of radiation measurement. Inductively coupled plasma mass spectrometry, a widely used technique for mass analysis, does not have sufficient analytical sensitivity for samples with the ⁴¹Ca/⁴⁰Ca ratio of ≤10⁻⁷, which corresponds to the clearance level of ⁴¹Ca. This is attributed to spectral interference from adjacent ⁴⁰Ca and argon-40 (a carrier gas component).

In this study, laser resonance ionization is employed, which selectively ionizes ⁴¹Ca atoms using lasers 1 and 2 at resonant wavelengths specific to ⁴¹Ca (Fig. 1). This approach suppresses spectral interference from other isotopes and isobars. Utilizing Rydberg levels, the approach exploits the specific energy shift of odd isotopes to easily separate ⁴¹Ca from ⁴⁰Ca. As shown in Fig. 2, a stable Ca isotope sample was heated in a graphite furnace to generate a Ca atomic beam. Fig. 3 shows the Ca⁺ ion spectrum measured by scanning the frequency of laser 2. This spectrum confirmed that the resonance frequency of the odd isotope ⁴³Ca shifted to higher frequencies. A similar trend is expected for ⁴¹Ca. By combining laser resonance ionization with conventional mass spectrometry, the study anticipates achieving an analytical sensitivity of ≤10⁻¹⁰ for the ⁴¹Ca/⁴⁰Ca ratio, taking into account the isotope selectivity of lasers 1 and 2 and the mass resolution of the mass spectrometer.

This enhanced sensitivity can be useful for the rapid analysis of concrete waste.

This work was partially supported by JSPS KAKENHI Grant Number JP19H02646.

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