Imaging of Individual Barium Atoms in Solid Xenon*

Images of individual barium atoms in solid xenon with high definition have been obtained by scanning a focused laser across a solid xenon matrix on a cold sapphire window. A few pulses of a mass-selected Ba⁺ beam are deposited as the solid xenon matrix growns. Some neutralization to Ba occurs. When the laser is fixed on a single Ba atom peak, the fluorescence suddenly drops to background level after times of as short as 30 s. The sudden drop to background is a clear confirmation of single atoms. 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. The identification, or “tagging” of the Ba-136 daughter atom that results from double beta decay of Xe-136 could eliminate all false radioactive backgrounds in nEXO that do not produce a Ba-136 daughter. 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 fluorescence image 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. Observation of neutrinoless double beta decay is of fundamental importance, as it would show that neutrinos and anti-neutrinos are the same particle.