Imaging individual barium atoms in solid xenon for barium tagging in nEXO


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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Fig. 1: Experimental setup for Ba/Ba+ imaging in solid xenon.
Fig. 2: Barium atom fluorescence.
Fig. 3: CCD images showing successive steps of a raster scan of a deposit of barium in solid xenon.
Fig. 4: Composite images of a sequence of laser scans.
Fig. 5: Time evolution of the fluorescence signal from a single barium peak.
Fig. 6: Spectra of deposits from three different sources in SXe.

Data availability

Source Data for Figs. 26 are provided with the online version of the paper.


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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.

Reviewer information

Nature thanks Mark Chen, John McCaffrey and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Author information




The eight authors listed first (C.C., T.W., D.F., A.C., D.R.Y., J.T., A.I. and W.F.) contributed to the design, construction and operation of this experiment, the data acquisition, and the data analysis and interpretation. The remaining authors listed in alphabetical order are nEXO Collaboration members who have contributed to the formulation of the problem and the general application of the results.

Corresponding author

Correspondence to C. Chambers.

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Extended data figures and tables

Extended Data Fig. 1 Excitation spectra for emission at 619 nm and 670 nm.

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

Extended Data Fig. 2 Time-resolved photon counting of 619-nm fluorescence.

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

Extended Data Fig. 3 Example CCD image of a Ba+ deposit in SXe.

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

Extended Data Fig. 4 Scan image of background emission after bleaching.

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.

Source data

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nEXO Collaboration., Chambers, C., Walton, T. et al. Imaging individual barium atoms in solid xenon for barium tagging in nEXO. Nature 569, 203–207 (2019).

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