Diode-laser-based resonance ionization mass spectrometry of the long-lived radionuclide 41Ca with <10-12 sensitivity

Abstract

There are a number of applications requiring measurement of the long-lived radionuclide ⁴¹ Ca at isotopic abundance, relative to ⁴⁰ Ca, in the range of 10^-8 to 10^-16 . The long lifetime and electron-capture decay mode make radio-analytical measurements impractical and conventional mass spectrometric methods can not overcome isobaric interferences such as ⁴¹ K⁺ or ⁴⁰ CaH⁺ at these levels. We approach this problem by combining laser-based resonance ionization spectroscopy with mass spectrometry. Using high-resolution single-mode lasers for resonant excitation and an intense non-resonant laser for ionization of the selectively created excited-state atoms, it is possible to essentially eliminate interference from isobars and to obtain additional optical isotopic selectivity, which can be combined with that of the mass spectrometer. Single-, double-, and triple-resonance excitation schemes have been investigated experimentally and theoretically, and it is found that performance with respect to both selectivity and sensitivity increases dramatically with each additional resonance step. Thus far, triple-resonance measurements have demonstrated minimum detectable isotopic abundance of 2.5·10^-13 with detection limits of ~2·10⁵ atoms.