Barium Daughter Tagging for the nEXO Double Beta Decay Experiment

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This project aims to develop a laser-based measurement technique so precise that it will be able to detect a single atom of barium in a tank of five tons of liquid xenon. Doing so will enable nuclear physicists to efficiently carry out an ongoing experiment that would otherwise require a device that is several times larger. The nuclear physicists are searching for an extremely rare radioactive decay in nature that has been theorized but has not yet been found. The discovery of this decay, which changes one xenon atom into one barium atom, would prove for the first time that one of the smallest and weakly interacting particles in nature, neutrinos, are the same as their anti-particles, the antineutrinos. The proof that a component of matter is the same as its antimatter counterpart could contribute to an understanding of why our universe is composed of matter only, with negligible amounts of antimatter, whereas Big Bang theories of the universe predict initial formation of equal parts of matter and antimatter. In this project a laser of the right color is scanned across a sample containing a few barium atoms or ions captured in xenon that is frozen solid, and each time the laser hits an atom or ion, a characteristic flash of light is recorded.The scientists doing this research have recently demonstrated for the first time that individual atoms trapped in a solid noble element can be imaged and counted with high selectivity against background. This project extends the imaging and counting of individual barium atoms in two particular sites, or configuration of neighbors, in a solid xenon matrix to both barium atoms and ions in all the significant matrix sites. In a xenon neutrinoless double beta decay detector, it will be necessary to grab and count individual daughter barium atoms or ions in solid xenon from the liquid xenon volume. A second focus of the work is to demonstrate and perfect this barium capture and counting capability in a small liquid xenon test chamber. The proposed research is fundamentally interdisciplinary and promises advances of broad scientific interest. It involves applying single atom detection techniques originally developed within the framework of Atomic, Molecular and Optical physics to atoms in a condensed noble gas environment, while offering a transformational advance in nuclear physics. The project has an educational and outreach component to connect high school teachers to state-of-the-art research. The project is jointly funded by three NSF Programs (Experimental Atomic, Molecular, and Optical Physics; Experimental Nuclear Physics, both in the Physics Division; and Chemical Measurement and Imaging, in the Chemistry Division).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.