Imaging individual Ba atoms in solid xenon for barium tagging in nEXO

The identification, or "tagging", of the $^{136}$Ba daughter atom that results from double beta decayof $^{136}$Xe provides a promising technique for the elimination of backgrounds in a future upgrade of the nEXO neutrinoless double beta decayexperiment. The tagging scheme being developed in this work utilizes a cryogenic probe to trap the bariumatom in solid xenonand extract it from the time projection chamberfilled with liquid xenon. The bariumatom is then tagged via fluorescence imaging in the solid xenonmatrix. Individual bariumatoms in a particular matrix site have been imaged and counted with high definition by scanning a focused laser across the solid xenonmatrix deposited on a sapphire window. This is the first time single atoms have been imaged in solid noble element and represents significant progress towards a practical bariumtagging technique for nEXO.