A new method to tag the barium daughter in the double-beta decay of ${}^{136}\mathrm{Xe}$ is reported. Using the technique of single molecule fluorescent imaging (SMFI), individual barium dication (${\mathrm{Ba}}^{++}$) resolution at a transparent scanning surface is demonstrated. A single-step photobleach confirms the single ion interpretation. Individual ions are localized with superresolution ($\sim 2\mathrm{nm}$), and detected with a statistical significance of $12.9\sigma$ over backgrounds. This lays the foundation for a new and potentially background-free neutrinoless double-beta decay technology, based on SMFI coupled to high pressure xenon gas time projection chambers.

Options for optimizing the energy and spatial resolution of gamma-ray imaging detectors based on thick, monolithic crystals shaped like flat-topped pyramids were studied. Monte Carlo simulations were made of the scintillation light transport for evaluating the effect of four parameters on the energy resolution, the spatial resolutions, and the depth of interaction (DOI) resolution of the gamma-ray imaging detector. These four parameters are: the reflectivity of the surface coating; the scatter mean free path; the absorption mean free path of the scintillation light; and the angle that defines the inclination of the sides of the pyramidal frustum. In real detectors, the values for the mean free paths for optical photons are normally not known. We estimated these by comparing MC simulations of detector resolutions to measurements for three gamma-ray imaging detectors with LYSO and LSO from different suppliers and with different surface coatings and geometries. The gamma-ray imaging detector measures the energy, centroids, and depth of interaction of the gamma-ray. DOI enhanced charge dividing readouts were used to measure the depth of interaction.