High efficiency resonance ionization of thorium

Multi-step resonance ionization spectroscopy of thorium has been performed with Ti:Sapphire lasers and a hotcavity laser ion source. Resonance ionization schemes involving three-color, three-photon ionization processes are evaluated and a new three-photon scheme is demonstrated with measured overall ionization efficiency of 38.6(5)%. This high efficiency makes it possible to determine the thorium impurities in materials used for ultralow background neutrino detectors and could also benefit ultra-trace thorium analysis using resonance ionization mass spectrometry for various radiochemical tracer applications.