Double-β-decay involves the simultaneous conversion of two neutrons into two protons, and the emission of two electrons and two neutrinos; the neutrinoless process, although not yet observed, is thought to involve the emission of the two electrons but no neutrinos. The search for neutrinoless-double-β-decay probes fundamental properties of neutrinos, including whether or not the neutrino and antineutrino are distinct particles. Double-β-decay detectors are large and expensive, so it is essential to achieve the highest possible sensitivity with each study, and removing spurious contributions (‘background’) from detected signals is crucial. In the nEXO neutrinoless-double-β-decay experiment, the identification, or ‘tagging’, of the 136Ba daughter atom resulting from the double-β decay of 136Xe provides a technique for discriminating background. The tagging scheme studied here uses a cryogenic probe to trap the barium atom in a solid xenon matrix, where the barium atom is tagged through fluorescence imaging. Here we demonstrate the imaging and counting of individual barium atoms in solid xenon by scanning a focused laser across a solid xenon matrix deposited on a sapphire window. When the laser irradiates an individual atom, the fluorescence persists for about 30 seconds before dropping abruptly to the background level—a clear confirmation of one-atom imaging. Following evaporation of a barium deposit, the residual barium fluorescence is 0.16 per cent or less. Our technique achieves the imaging of single atoms in a solid noble element, establishing the basic principle of barium tagging for nEXO.
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We thank Picoquant for the loan of time-resolved-photon-counting equipment. Discussions with J. G. McCaffrey, B. Gervais and A. van Orden are appreciated. This material is based upon work supported by the National Science Foundation under grant number PHY-1649324 and the US Department of Energy, Office of Science, Office of High Energy Physics under award number DE-FG02-03ER41255.
Nature thanks Mark Chen, John McCaffrey and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
These spectra were obtained with deposits of high barium density and with low laser intensity in order to avoid bleaching effects. The low wavelength portion (below 567 nm) was excited using rhodamine-100 laser dye, and the high wavelength portion (above 567 nm) was excited using rhodamine-6G laser dye. The two portions were normalized, because the laser intensity and barium densities were different for the two experiments. The shapes of the spectra are similar and do not exhibit resolved Jahn–Teller splitting. The peak locations differ, indicating that the emissions do not originate from a shared upper state. Source data
Histograms showing the decay of the 619-nm fluorescence for barium in SXe (blue), SXe-only (green) and the cryoprobe tube (red). The decay lifetime of the barium fluorescence is 7.0 ± 0.3 ns. The SXe-only and cryoprobe emissions have shorter decay lifetimes of approximately 3 ns and 1.5 ns respectively. Source data
The barium atoms were excited by a focused 570-nm laser, using a 620-nm fluorescence band-pass filter. The bright spot at the top of the image is the front surface of the window on which the Ba+ ions were deposited. The broad spot at the bottom of the image is the surface fluorescence of the back surface of the window. This spot is broadened because of the laser focus as well as the collection optics being optimized for the front surface. The faint line between the surfaces is the faint fluorescence of Cr3+ impurities in the bulk of the sapphire that extends into the wavelength region of the filter. Source data
We used a 532-nm laser to bleach the sapphire surface background in a 14 × 14 grid pattern with 8-μm steps. A roughly 30× reduction of the background is observed in the low area where the bleaching laser was scanned.