Imaging Individual Barium Atoms in Solid Xenon for the nEXO Neutrinoless Double Beta Decay Experiment

Abstract:  Neutrinoless double beta decay is a rare and yet unobserved process that violates conservation of lepton number. Observation of neutrinoless double beta decay would represent the discovery of new physics beyond the Standard Model of elementary particles. This would include confirmation of the Majorana character for neutrinos, i.e., that the neutrino is its own antiparticle. It also could contribute to an understanding of the mystery of the missing anti-matter in our universe from the Big Bang. The proposed nEXO experiment utilizes the isotope 136Xe in a liquid xenon time-projection chamber and aims to achieve a half-life sensitivity of about 1x1028 years. As a potential add-on to this experiment, we are developing a technique to identify or "tag" the 136Ba daughter atom that results from double beta decay of one 136Xe atom. This extra decay signature could be used to eliminate all false radioactive backgrounds in nEXO that do not produce a 136Ba daughter. This would enhance the sensitivity of nEXO, and the extra confirmation would be valuable if a positive signal is found. The proposed Ba tagging scheme utilizes a cryogenic probe to trap the barium daughter atom in solid xenon and extract it from the time projection chamber. The observation of a single barium atom in the laser scan of the solid xenon matrix on the widow at the end of the probe would be a positive confirmation of a true double beta decay event. Individual barium atoms in solid xenon have been detected and imaged with high signal-to background fluctuation ratio by scanning a focused laser across the solid xenon matrix deposited on a cold sapphire window. By fixing the laser position on a single Ba atom, it is found that the fluorescence suddenly drops to background level. This is a clear signature of a single atom. A remarkable result is that heating the matrix to 100 K “erases” all signal from a previous Ba deposit. To our knowledge, this is the first time that single atoms have been imaged in solid noble gas and represents significant progress towards a practical barium tagging technique for the proposed nEXO neutrinoless double beta decay experiment.