EXO barium tagging R{\&}D: identification in solid Xe

To eliminate all backgrounds and reach the 10meV level of neutrino mass sensitivity, it is desired for the next generation EXO experiment to be able to identify the $^{136}$Ba daughter nucleus in coincidence with candidate neutrinoless double beta decays. A few techniques involving laser-induced fluorescence of the Ba daughter are being developed to accomplish the detection. One technique involves freezing the Ba$^{+}$ ion in a small amount of surrounding Xe on a cryogenic probe, and looking for the characteristic fluorescence of Ba$^{+}$ or Ba in solid xenon. Progress on a technique where laser light is delivered by a fiber down the probe and fluorescence photons are collected back up the same fiber will be described. The current sensitivity is around 10$^{4}$ Ba atoms on a sapphire window, and there is good reason to believe that this sensitivity could be improved by 10$^{4}$ using a fiber. Indeed detection at the single molecule level has already been reached with a fiber probe.