부산대학교 도서관

The search for neutrinoless double beta decay probes the fundamentalproperties of neutrinos, including whether or not the neutrino and antineutrinoare distinct. Double beta detectors are large and expensive, so backgroundreduction is essential for extracting the highest sensitivity. Theidentification, or 'tagging', of the $^{136}$Ba daughter atom from double betadecay of $^{136}$Xe provides a technique for eliminating backgrounds in thenEXO neutrinoless double beta decay experiment. The tagging scheme studied inthis work utilizes a cryogenic probe to trap the barium atom in solid xenon,where the barium atom is tagged via fluorescence imaging in the solid xenonmatrix. Here we demonstrate imaging and counting of individual atoms of bariumin solid xenon by scanning a focused laser across a solid xenon matrixdeposited on a sapphire window. When the laser sits on an individual atom, thefluorescence persists for $\sim$30~s before dropping abruptly to the backgroundlevel, a clear confirmation of one-atom imaging. No barium fluorescencepersists following evaporation of a barium deposit to a limit of $\leq$0.16\%.This is the first time that single atoms have been imaged in solid nobleelement. It establishes the basic principle of a barium tagging technique fornEXO.