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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.

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


  1. Tanabashi, M. et al. (Particle Data Group) Review of particle physics. Phys. Rev. D 98, 030001 (2018).

    Article  ADS  Google Scholar 

  2. Albert, J. et al. (EXO-200 Collaboration) An improved measurement of the 2νββ half-life of 136Xe with EXO-200. Phys. Rev. C 89, 015502 (2014).

    Article  ADS  Google Scholar 

  3. Albert, J. et al. (EXO-200 Collaboration) Search for neutrinoless double-beta decay with the upgraded EXO-200 detector. Phys. Rev. Lett. 120, 072701 (2018).

    Article  CAS  ADS  Google Scholar 

  4. Moe, M. Detection of neutrinoless double-beta decay. Phys. Rev. C 44, R931 (1991).

    Article  CAS  ADS  Google Scholar 

  5. Al Kharusi, S. et al. (nEXO Collaboration) nEXO pre-conceptual design report. (2018).

  6. Albert, J. et al. (nEXO Collaboration) Sensitivity and discovery potential of nEXO to neutrinoless double beta decay. Phys. Rev. C 97, 065503 (2018).

    Article  CAS  ADS  Google Scholar 

  7. Mong, B. et al. Spectroscopy of Ba and Ba+ deposits in solid xenon for barium tagging in nEXO. Phys. Rev. A 91, 022505 (2015).

    Article  ADS  Google Scholar 

  8. Twelker, K. et al. An apparatus to manipulate and identify individual Ba ions from bulk liquid Xe. Rev. Sci. Instrum. 85, 095114 (2014).

    Article  CAS  ADS  Google Scholar 

  9. Brunner, T. et al. An RF-only ion-funnel for extraction from high-pressure gases. Int. J. Mass Spectrom. 379, 110–120 (2015).

    Article  CAS  Google Scholar 

  10. McDonald, A. D. et al. (NEXT Collaboration). Demonstration of single-barium-ion sensitivity for neutrinoless double-beta decay using single-molecule fluorescence imaging. Phys. Rev. Lett. 120, 132504 (2018).

    Article  CAS  ADS  Google Scholar 

  11. Wamba, K. Aspects of the R&D for the Enriched Xenon Observatory for Double Beta Decay. PhD Thesis, Stanford Univ. (2006).

  12. Rowson, P.C. Cryogenic ion capture probe R&D at SLAC. SLAC-PUB-17110 (2017).

  13. Albert, J. et al. (EXO-200 Collaboration). Measurements of the ion fraction and mobility of alpha and beta decay products in liquid xenon using EXO-200. Phys. Rev. C 92, 045504 (2015).

    Article  ADS  Google Scholar 

  14. Davis, B. M. & McCaffrey, J. G. Absorption spectroscopy of heavy alkaline earth metals Ba and Sr in rare gas matrices—CCSD(T) calculations and atomic site occupancies. J. Chem. Phys. 144, 044308 (2016).

    Article  ADS  Google Scholar 

  15. Davis, B. M., Gervais, B. & McCaffrey, J. G. An investigation of the sites occupied by atomic barium in solid xenon—a 2D-EE luminescence spectroscopy and molecular dynamics study. J. Chem. Phys. 148, 124308 (2018).

    Article  ADS  Google Scholar 

  16. Sepiol, J. et al. Detection and spectroscopy of single molecules in rare gas matrices: dibenzanthanthrene in krypton and xenon. Chem. Phys. Lett. 311, 29–35 (1999).

    Article  CAS  ADS  Google Scholar 

  17. Starukhin, A. et al. Single molecule spectroscopy of Mg-tetrazaporphyrin in xenon matrix. Heavy atom effect. Chem. Phys. 285, 121–126 (2002).

    Article  CAS  Google Scholar 

  18. Tam, S. & Fajardo, M. E. Matrix isolation spectroscopy of metal atoms generated by laser ablation. III. The Na/Ar, Na/Kr, and Na/Xe systems. J. Chem. Phys. 99, 854 (1993).

    Article  CAS  ADS  Google Scholar 

  19. Silverman, D. C. & Fajardo, M. E. Matrix isolation spectroscopy of Na atoms deposited as ions. J. Chem. Phys. 106, 8964 (1997).

    Article  CAS  ADS  Google Scholar 

  20. Vaskonen, K., Eloranta, J. & Kunttu, H. Trapping of laser-vaporized alkali metal atoms in rare-gas matrices. Chem. Phys. Lett. 310, 245 (1999).

    Article  CAS  ADS  Google Scholar 

  21. Ahokas, J., Kiljunen, T., Eloranta, J. & Kunttu, H. Theoretical analysis of alkali metal trapping sites in rare gas matrices. J. Chem. Phys. 112, 2420 (2000).

    Article  CAS  ADS  Google Scholar 

  22. Buchachenko, A. A. & Viehland, L. A. Interaction potentials and transport properties of Ba, Ba+, and Ba2+ in rare gases from He to Xe. J. Chem. Phys. 148, 154304 (2018).

    Article  ADS  Google Scholar 

  23. Granfors, P. R., Macrander, A. T. & Simmons, R. O. Crystalline xenon: lattice parameters, thermal expansion, thermal vacancies, and equation of state. Phys. Rev. B 24, 4753 (1981).

    Article  CAS  ADS  Google Scholar 

  24. Ziegler, J. F., Ziegler, M. D. & Biersack, J. P. SRIM—the stopping and range of ions in matter. Nucl. Instr. Meth. Phys. Res. B 268, 1818–1823 (2010).

    Article  CAS  ADS  Google Scholar 

  25. Murad, E. Thermochemical properties of the gaseous alkaline earth monohydroxides. J. Chem. Phys. 75, 4080 (1981).

    Article  CAS  ADS  Google Scholar 

  26. Charton, M. & Gaydon, A. G. Rotational analysis of the B2Σ+-X2Σ+ transition of BaOH and BaOD. Proc. Phys. Soc. A 69, 520 (1956).

    Article  ADS  Google Scholar 

  27. Sakurai, K., Johnson, S. E. & Broida, H. P. Laser-induced fluorescence of BaO. J. Chem. Phys. 52, 1625 (1970).

    Article  CAS  ADS  Google Scholar 

  28. Johnson, S. E. Measured radiative lifetimes and electronic quenching cross sections of BaO(A1Σ). J. Chem. Phys. 56, 149 (1972).

    Article  CAS  ADS  Google Scholar 

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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.

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Authors and Affiliations



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. Imaging individual barium atoms in solid xenon for barium tagging in nEXO. Nature 569, 203–207 (2019).

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