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Excitation and emission spectra at 12 K temperature are presented for the first time for Ba+ in solid xenon. This is only the third such atomic ion studied in a solid noble gas matrix. Two groups of emission peaks, each with similar excitation spectra, photobleaching behavior and temperature dependence, are identified that may be associated with transitions from a common excited state in a particular solid xenon matrix site. Favorable conditions are found for imaging of single Ba+ ions in solid xenon, including reasonably strong emission efficiency, acceptable photobleaching and a method for overcoming background emission from the surface of the cooled sapphire window. A comparison of the new excitation spectrum of Ba+ in solid xenon with unpublished results on spectroscopy of Ba+ in liquid xenon helps confirm the identification of the latter. 

Neutrinoless double beta decay (0νββ) provides a way to probe physics beyond the Standard Model of particle physics. The upcoming nEXO experiment will search for 0νββ decay in 136Xe with a projected half-life sensitivity exceeding 1028 years at the 90% confidence level using a liquid xenon (LXe) Time Projection Chamber (TPC) filled with 5 tonnes of Xe enriched to ∼90% in the ββ-decaying isotope 136Xe. In parallel, a potential future upgrade to nEXO is being investigated with the aim to further suppress radioactive backgrounds and to confirm ββ-decay events. This technique, known as Ba-tagging, comprises extracting and identifying the ββ-decay daughter 136Ba ion. One tagging approach being pursued involves extracting a small volume of LXe in the vicinity of a potential ββ-decay using a capillary tube and facilitating a liquid-to-gas phase transition by heating the capillary exit. The Ba ion is then separated from the accompanying Xe gas using a radio-frequency (RF) carpet and RF funnel, conclusively identifying the ion as 136Ba via laser-fluorescence spectroscopy and mass spectrometry. Simultaneously, an accelerator-driven Ba ion source is being developed to validate and optimize this technique. The motivation for the project, the development of the different aspects, along with the current status and results, are discussed here.