# Imaging of single barium atoms in a second matrix site in solid xenon for
barium tagging in a 136Xe double beta decay experiment

M. Yvaine  Physics Department, Colorado State University, Fort Collins, CO
80523, USA  D. Fairbank  Physics Department, Colorado State University, Fort
Collins, CO 80523, USA  J. Soderstrom  Physics Department, Colorado State
University, Fort Collins, CO 80523, USA  C. Taylor  Physics Department,
Colorado State University, Fort Collins, CO 80523, USA  J. Stanley  Physics
Department, Colorado State University, Fort Collins, CO 80523, USA  T. Walton
Now at: Prism Computational Sciences, Madison, WI 53711, USA  Physics
Department, Colorado State University, Fort Collins, CO 80523, USA  C.
Chambers  Now at: TRIUMF, Vancouver, BC V6T 2A3, Canada  Physics Department,
McGill University, Montréal, QC H3A 2T8, Canada  A. Iverson  Physics
Department, Colorado State University, Fort Collins, CO 80523, USA  W.
Fairbank<EMAIL_ADDRESS>Physics Department, Colorado State
University, Fort Collins, CO 80523, USA  S. Al Kharusi  Now at: Physics
Department, Stanford University, Stanford, CA 94305, USA  Physics Department,
McGill University, Montréal, QC H3A 2T8, Canada  A. Amy  SUBATECH, Nantes
Université, IMT Atlantique, CNRS/IN2P3, Nantes 44307, France  E. Angelico
Physics Department, Stanford University, Stanford, CA 94305, USA  A. Anker
SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA  I. J.
Arnquist  Pacific Northwest National Laboratory, Richland, WA 99352, USA  A.
Atencio  Department of Physics, Drexel University, Philadelphia, PA 19104, USA
J. Bane  Amherst Center for Fundamental Interactions and Physics Department,
University of Massachusetts, Amherst, MA 01003, USA  V. Belov  National
Research Center “Kurchatov Institute”, Moscow, 123182, Russia  E. P. Bernard
Lawrence Livermore National Laboratory, Livermore, CA 94550, USA  T. Bhatta
Department of Physics and Astronomy, University of Kentucky, Lexington, KY
40506, USA  A. Bolotnikov  Brookhaven National Laboratory, Upton, NY 11973,
USA  J. Breslin  Department of Physics, Applied Physics, and Astronomy,
Rensselaer Polytechnic Institute, Troy, NY 12180, USA  P. A. Breur  SLAC
National Accelerator Laboratory, Menlo Park, CA 94025, USA  J. P. Brodsky
Lawrence Livermore National Laboratory, Livermore, CA 94550, USA  E. Brown
Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic
Institute, Troy, NY 12180, USA  T. Brunner  Physics Department, McGill
University, Montréal, QC H3A 2T8, Canada  TRIUMF, Vancouver, BC V6T 2A3,
Canada  E. Caden  SNOLAB, Lively, ON P3Y 1N2, Canada  School of Natural
Sciences, Laurentian University, Sudbury, ON P3E 2C6, Canada  Physics
Department, McGill University, Montréal, QC H3A 2T8, Canada  G. F. Cao  Also
at: University of Chinese Academy of Sciences, Beijing, China  Institute of
High Energy Physics, Chinese Academy of Sciences, Beijing, 100049, China  D.
Cesmecioglu  Amherst Center for Fundamental Interactions and Physics
Department, University of Massachusetts, Amherst, MA 01003, USA  E. Chambers
Department of Physics, Drexel University, Philadelphia, PA 19104, USA  B.
Chana  Now at: Canadian Nuclear Laboratories, Chalk River, ON K0J 1J0, Canada
Department of Physics, Carleton University, Ottawa, ON K1S 5B6, Canada  D.
Chernyak  Department of Physics and Astronomy, University of Alabama,
Tuscaloosa, AL 35405, USA  M. Chiu  Brookhaven National Laboratory, Upton, NY
11973, USA  R. Collister  Department of Physics, Carleton University, Ottawa,
ON K1S 5B6, Canada  M. Cvitan  Also at: McMaster University, Department of
Physics & Astronomy, Hamilton, ON L8S 4L8, Canada  TRIUMF, Vancouver, BC V6T
2A3, Canada  T. Daniels  Department of Physics and Physical Oceanography,
University of North Carolina Wilmington, Wilmington, NC 28403, USA  L. Darroch
Physics Department, McGill University, Montréal, QC H3A 2T8, Canada  R. DeVoe
Physics Department, Stanford University, Stanford, CA 94305, USA  M. L. di
Vacri  Pacific Northwest National Laboratory, Richland, WA 99352, USA  M. J.
Dolinski  Department of Physics, Drexel University, Philadelphia, PA 19104,
USA  B. Eckert  Department of Physics, Drexel University, Philadelphia, PA
19104, USA  M. Elbeltagi  Department of Physics, Carleton University, Ottawa,
ON K1S 5B6, Canada  R. Elmansali  Department of Physics, Carleton University,
Ottawa, ON K1S 5B6, Canada  N. Fatemighomi  SNOLAB, Lively, ON P3Y 1N2, Canada
B. Foust  Pacific Northwest National Laboratory, Richland, WA 99352, USA  Y.
S. Fu  Also at: University of Chinese Academy of Sciences, Beijing, China
Institute of High Energy Physics, Chinese Academy of Sciences, Beijing,
100049, China  D. Gallacher  Physics Department, McGill University, Montréal,
QC H3A 2T8, Canada  N. Gallice  Brookhaven National Laboratory, Upton, NY
11973, USA  G. Giacomini  Brookhaven National Laboratory, Upton, NY 11973, USA
W. Gillis  Now at: Bates College, Lewiston, ME 04240, USA  Amherst Center for
Fundamental Interactions and Physics Department, University of Massachusetts,
Amherst, MA 01003, USA  C. Gingras  Physics Department, McGill University,
Montréal, QC H3A 2T8, Canada  R. Gornea  Department of Physics, Carleton
University, Ottawa, ON K1S 5B6, Canada  G. Gratta  Physics Department,
Stanford University, Stanford, CA 94305, USA  C. A. Hardy  Physics Department,
Stanford University, Stanford, CA 94305, USA  S. Hedges  Lawrence Livermore
National Laboratory, Livermore, CA 94550, USA  E. Hein  Skyline College, San
Bruno, CA 94066, USA  J. D. Holt  TRIUMF, Vancouver, BC V6T 2A3, Canada
Physics Department, McGill University, Montréal, QC H3A 2T8, Canada  E. W.
Hoppe  Pacific Northwest National Laboratory, Richland, WA 99352, USA  A.
Karelin  National Research Center “Kurchatov Institute”, Moscow, 123182,
Russia  D. Keblbeck  Department of Physics, Colorado School of Mines, Golden,
CO 80401, USA  I. Kotov  Brookhaven National Laboratory, Upton, NY 11973, USA
A. Kuchenkov  National Research Center “Kurchatov Institute”, Moscow, 123182,
Russia  K. S. Kumar  Amherst Center for Fundamental Interactions and Physics
Department, University of Massachusetts, Amherst, MA 01003, USA  A. A.
Kwiatkowski  Also at: University of Victoria, Department of Physics and
Astronomy, Victoria, BC V8P 5C2, Canada  TRIUMF, Vancouver, BC V6T 2A3, Canada
A. Larson  Department of Physics, University of South Dakota, Vermillion, SD
57069, USA  M. B. Latif  Also at: Center for Energy Research and Development,
Obafemi Awolowo University, Ile-Ife, 220005 Nigeria  Department of Physics,
Drexel University, Philadelphia, PA 19104, USA  K. G. Leach  Also at: Facility
for Rare Isotope Beams, Michigan State University, East Lansing, MI 48824, USA
Department of Physics, Colorado School of Mines, Golden, CO 80401, USA  A.
Lennarz  Also at: McMaster University, Department of Physics & Astronomy,
Hamilton, ON L8S 4L8, Canada  TRIUMF, Vancouver, BC V6T 2A3, Canada  D. S.
Leonard  IBS Center for Underground Physics, Daejeon, 34126, Korea  H. Lewis
TRIUMF, Vancouver, BC V6T 2A3, Canada  G. Li  Institute of High Energy
Physics, Chinese Academy of Sciences, Beijing, 100049, China  Z. Li  Physics
Department, University of California San Diego, La Jolla, CA 92093, USA  C.
Licciardi  Department of Physics, University of Windsor, Windsor, ON N9B 3P4,
Canada  R. Lindsay  Department of Physics and Astronomy, University of the
Western Cape, P/B X17 Bellville 7535, South Africa  R. MacLellan  Department
of Physics and Astronomy, University of Kentucky, Lexington, KY 40506, USA  S.
Majidi  Physics Department, McGill University, Montréal, QC H3A 2T8, Canada
C. Malbrunot  TRIUMF, Vancouver, BC V6T 2A3, Canada  Physics Department,
McGill University, Montréal, QC H3A 2T8, Canada  J. Masbou  SUBATECH, Nantes
Université, IMT Atlantique, CNRS/IN2P3, Nantes 44307, France  K. McMichael
Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic
Institute, Troy, NY 12180, USA  M. Medina Peregrina  Physics Department,
University of California San Diego, La Jolla, CA 92093, USA  M. Moe
Department of Physics and Astronomy, University of California, Irvine, Irvine,
CA 92697, USA  B. Mong  SLAC National Accelerator Laboratory, Menlo Park, CA
94025, USA  D. C. Moore  Wright Laboratory, Department of Physics, Yale
University, New Haven, CT 06511, USA  C. R. Natzke  Department of Physics,
Colorado School of Mines, Golden, CO 80401, USA  X. E. Ngwadla  Department of
Physics and Astronomy, University of the Western Cape, P/B X17 Bellville 7535,
South Africa  K. Ni  Physics Department, University of California San Diego,
La Jolla, CA 92093, USA  A. Nolan  Amherst Center for Fundamental Interactions
and Physics Department, University of Massachusetts, Amherst, MA 01003, USA
S. C. Nowicki  Physics Department, McGill University, Montréal, QC H3A 2T8,
Canada  J. C. Nzobadila Ondze  Department of Physics and Astronomy, University
of the Western Cape, P/B X17 Bellville 7535, South Africa  A. Odian  SLAC
National Accelerator Laboratory, Menlo Park, CA 94025, USA  J. L. Orrell
Pacific Northwest National Laboratory, Richland, WA 99352, USA  G. S. Ortega
Pacific Northwest National Laboratory, Richland, WA 99352, USA  C. T. Overman
Pacific Northwest National Laboratory, Richland, WA 99352, USA  L. Pagani
Pacific Northwest National Laboratory, Richland, WA 99352, USA  H. Peltz
Smalley  Amherst Center for Fundamental Interactions and Physics Department,
University of Massachusetts, Amherst, MA 01003, USA  A. Perna  Department of
Physics, Carleton University, Ottawa, ON K1S 5B6, Canada  A. Pocar  Amherst
Center for Fundamental Interactions and Physics Department, University of
Massachusetts, Amherst, MA 01003, USA  V. Radeka  Brookhaven National
Laboratory, Upton, NY 11973, USA  E. Raguzin  Brookhaven National Laboratory,
Upton, NY 11973, USA  H. Rasiwala  Physics Department, McGill University,
Montréal, QC H3A 2T8, Canada  D. Ray  Physics Department, McGill University,
Montréal, QC H3A 2T8, Canada  TRIUMF, Vancouver, BC V6T 2A3, Canada  S. Rescia
Brookhaven National Laboratory, Upton, NY 11973, USA  G. Richardson  Wright
Laboratory, Department of Physics, Yale University, New Haven, CT 06511, USA
R. Ross  Physics Department, McGill University, Montréal, QC H3A 2T8, Canada
P. C. Rowson  SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA
R. Saldanha  Pacific Northwest National Laboratory, Richland, WA 99352, USA
S. Sangiorgio  Lawrence Livermore National Laboratory, Livermore, CA 94550,
USA  S. Schwartz  Lawrence Livermore National Laboratory, Livermore, CA 94550,
USA  S. Sekula  Department of Physics, Queen’s University, Kingston, ON K7L
3N6, Canada  SNOLAB, Lively, ON P3Y 1N2, Canada  L. Si  Physics Department,
Stanford University, Stanford, CA 94305, USA  A. K. Soma  Now at: Mirion
Technologies, Inc., 800 Research Pkwy, Meriden, CT 06450, USA  Department of
Physics, Drexel University, Philadelphia, PA 19104, USA  F. Spadoni  Pacific
Northwest National Laboratory, Richland, WA 99352, USA  V. Stekhanov  National
Research Center “Kurchatov Institute”, Moscow, 123182, Russia  X. L. Sun
Institute of High Energy Physics, Chinese Academy of Sciences, Beijing,
100049, China  S. Thibado  Amherst Center for Fundamental Interactions and
Physics Department, University of Massachusetts, Amherst, MA 01003, USA  A.
Tidball  Department of Physics, Applied Physics, and Astronomy, Rensselaer
Polytechnic Institute, Troy, NY 12180, USA  T. Totev  Physics Department,
McGill University, Montréal, QC H3A 2T8, Canada  S. Triambak  Department of
Physics and Astronomy, University of the Western Cape, P/B X17 Bellville 7535,
South Africa  T. Tsang  Brookhaven National Laboratory, Upton, NY 11973, USA
O. A. Tyuka  Department of Physics and Astronomy, University of the Western
Cape, P/B X17 Bellville 7535, South Africa  E. van Bruggen  Amherst Center for
Fundamental Interactions and Physics Department, University of Massachusetts,
Amherst, MA 01003, USA  M. Vidal  Physics Department, Stanford University,
Stanford, CA 94305, USA  M. Walent  School of Natural Sciences, Laurentian
University, Sudbury, ON P3E 2C6, Canada  K. Wamba  SLAC National Accelerator
Laboratory, Menlo Park, CA 94025, USA  H. W. Wang  Institute of High Energy
Physics, Chinese Academy of Sciences, Beijing, 100049, China  Q. D. Wang
Institute of Microelectronics, Chinese Academy of Sciences, Beijing, 100029,
China  W. Wang  Department of Physics and Astronomy, University of Alabama,
Tuscaloosa, AL 35405, USA  Y. G. Wang  Institute of High Energy Physics,
Chinese Academy of Sciences, Beijing, 100049, China  M. Watts  Wright
Laboratory, Department of Physics, Yale University, New Haven, CT 06511, USA
M. Wehrfritz  Skyline College, San Bruno, CA 94066, USA  L. J. Wen  Institute
of High Energy Physics, Chinese Academy of Sciences, Beijing, 100049, China
U. Wichoski  School of Natural Sciences, Laurentian University, Sudbury, ON
P3E 2C6, Canada  Department of Physics, Carleton University, Ottawa, ON K1S
5B6, Canada  S. Wilde  Wright Laboratory, Department of Physics, Yale
University, New Haven, CT 06511, USA  M. Worcester  Brookhaven National
Laboratory, Upton, NY 11973, USA  H. Xu  Physics Department, University of
California San Diego, La Jolla, CA 92093, USA  L. Yang  Physics Department,
University of California San Diego, La Jolla, CA 92093, USA  M. Yu  SLAC
National Accelerator Laboratory, Menlo Park, CA 94025, USA  O. Zeldovich
National Research Center “Kurchatov Institute”, Moscow, 123182, Russia

###### Abstract

Neutrinoless double beta decay is one of the most sensitive probes for new
physics beyond the Standard Model of particle physics. One of the isotopes
under investigation is 136Xe, which would double beta decay into 136Ba.
Detecting the single 136Ba daughter provides a sort of ultimate tool in the
discrimination against backgrounds. Previous work demonstrated the ability to
perform single atom imaging of Ba atoms in a single-vacancy site of a solid
xenon matrix. In this paper, the effort to identify signal from individual
barium atoms is extended to Ba atoms in a hexa-vacancy site in the matrix and
is achieved despite increased photobleaching in this site. Abrupt fluorescence
turn-off of a single Ba atom is also observed. Significant recovery of
fluorescence signal lost through photobleaching is demonstrated upon annealing
of Ba deposits in the Xe ice. Following annealing, it is observed that Ba
atoms in the hexa-vacancy site exhibit antibleaching while Ba atoms in the
tetra-vacancy site exhibit bleaching. This may be evidence for a matrix site
transfer upon laser excitation. Our findings offer a path of continued
research toward tagging of Ba daughters in all significant sites in solid
xenon.

††preprint: APS/123-QED

##  I Introduction

Although neutrinos were discovered several decades ago, some of their most
fundamental properties remain unknown. Neutrinoless double beta decay (0) is a
sensitive probe for neutrino properties and new physics beyond the Standard
Model of particle physics. If this hypothesized decay were observed, it would
demonstrate a process that violates lepton number conservation and confirm
that neutrinos are Majorana particles (that is, their own antiparticle). It
would also determine the absolute neutrino mass-scale, albeit with
uncertainties of a theoretical nature [1]. The EXO-200 experiment, utilizing a
liquid xenon (LXe) time-projection chamber (TPC), made the first observation
of two-neutrino double beta decay (2) in 136Xe and precisely measured its
half-life at 2.165 ± 0.016 (stat) ± 0.059 (sys) x 1021 yr [2, 3]. It also set
a limit on the 0 process in 136Xe of $T_{1/2}^{0\nu}$ $>$ 3.5 x 1025 yr at a
90% confidence level (CL) [4]. A next-generation LXe experiment, nEXO, with 5
tonnes of enriched Xe, is expected to reach a half-life sensitivity to 0 decay
of 1.35 x 1028 yr in a 10 year run [5].

When a double beta decay event occurs in a 136Xe TPC, a 136Ba daughter, that
could potentially be identified, is formed at the decay site. The detection of
the daughter, referred to as barium tagging, could improve 0 decay sensitivity
by eliminating all backgrounds except those from 2 [6]. Implementation of
barium tagging as a potential future upgrade to nEXO is being investigated by
the collaboration. The elimination of all non- decay backgrounds in nEXO could
increase the sensitivity by about a factor of 2-3 [5].

One Ba tagging effort in nEXO focuses on extraction from LXe and detection in
solid xenon (SXe) [7, 8], and another on extraction to gaseous Xe and
detection in vacuum [9, 10]. Capture of Ba++ in a fluorescent molecule is also
being investigated by the NEXT collaboration for barium tagging for a next-
generation gaseous 136Xe double beta decay experiment [11, 12, 13].

The principles of the method of Ba extraction in SXe and subsequent tagging
have been described previously [7, 8]. Briefly, a candidate 0 decay event is
identified by ionization signal characteristics such as total electron energy
and multiplicity, and its location in the TPC is determined. A cryoprobe is
inserted into the TPC, the tip with a sapphire window is cooled below the Xe
freezing point, and the potential 136Ba daughter is captured from the decay
site in a small volume of solid xenon (SXe) on the window. The probe is
extracted from the TPC volume and the LXe to a separate region where the SXe
deposit can be cooled to a lower temperature and scanned for the presence of
one or zero Ba atoms. Coincident observation of a Ba atom extracted from a
candidate 0 event site provides a positive confirmation of ββ decay.

In LXe, the Ba++ daughter from the ββ decay is expected to quickly recombine
to form Ba+ and mostly remain as such, as the band gap for LXe is less than
the ionization potential for Ba+ [6]. The best present analogue for estimating
the final ion fraction for the Ba daughters in LXe is the 76(6)% measured for
214Bi+ daughters of 214Pb beta decay [14]. When Ba+ is captured into SXe from
LXe at thermal energy, it is unclear what fraction will remain as Ba+ and what
fraction will neutralize to Ba by capturing a stray electron. What is known is
that both Ba atoms and Ba+ ions result from the deposition of 2000 eV Ba+ ions
into SXe in vacuum [7].

In previous work, the spectroscopy of Ba atoms and Ba+ ions in SXe [7, 15, 16,
17] and first results on imaging of single Ba atoms in SXe [8] have been
presented. Theoretical work has identified the geometries of the matrix sites
in SXe that give rise to some portions of the observed Ba absorption and
emission spectra [17, 18]. The 591 nm “blue site” emission (so named in Ref.
[17] by the order of absorption band wavelength) is associated with Ba atoms
in tetra-vacancy (TV) site. The 577 nm emission peak has overlapping
contributions from Ba atoms in two distinct sites. The 578 nm “violet site”
emission is assigned to Ba atoms in asymmetric 5- or 7-atom vacancy (5/7V)
sites [17, 18]. The 577 nm “green site” emission, with an excitation peak
between those of the violet site, is associated with Ba atoms in HV sites [17,
16, 18]. The SXe matrix site of the weak 570 nm Ba emission peak is not yet
identified. The site for the 619 nm emission is not addressed in published
theoretical work, but is attributed to a single-vacancy (SV) site [8, 19]. One
possible formation mechanism for this less energetically favorable site in our
work is the capture of a Ba+ ion in a SV site and its later neutralization by
recombination with a free electron. The predominant emission lines of Ba+ in
SXe have also been found experimentally [7], but to date no theoretical
results on Ba+ in SXe have been published.

First images of single Ba atoms in SXe were achieved with SV site Ba atoms
emitting at 619 nm [8]. For high efficiency Ba tagging, it is important to
extend single Ba imaging from the SV site to all Ba matrix sites in SXe and to
also demonstrate single Ba+ imaging in SXe. In this paper, the first step in
this extension is presented, imaging of single Ba atoms in a second site, the
HV matrix site in SXe using the 577 nm emission peak. Because a Ba atom in
this matrix site photobleaches more rapidly than in the SV site [7], single
atom imaging is more challenging.

In parallel work, imaging of single rubidium atoms in a solid neon matrix by
laser induced fluorescence was reported recently [20]. A unique aspect of that
work is the ability to optically pump and read out the spin state of the atom.
Single atom imaging of 25Mg in solid neon has also been proposed for
measurement of an extremely low nuclear reaction cross section of
astrophysical importance [21].

##  II Experimental Setup

Figure 1: A schematic diagram of the sample apparatus. The excitation laser is
opposite the Ba+ ion beam, and the fluorescence is collected in a
perpendicular direction. Xenon gas is directed toward the window by a tube
that enters the vacuum behind the plane of the diagram.

The apparatus used in this work is similar to that used for imaging single Ba
atoms in SV sites of SXe [8]. A schematic diagram of the sample region is
shown in Fig. 1. Matrix samples for the imaging experiments are created on a
sapphire window of 0.75 inch diameter [22] that is mounted on the cold finger
of a cryostat at a temperature of 50 K. Near the window an inlet tube provides
gaseous xenon flow that freezes on the window to create the matrix sample. In
Ba+ deposits, pulses of Ba+ ions are directed towards the center of the window
during the deposition of xenon. The ion pulses begin about 5 seconds after the
start of xenon flow and are followed by ~5 s of xenon deposit. During
implantation, some of the Ba+ ions recombine with the ~ 20 free electrons that
result from ionization of the SXe matrix by each 2000 eV Ba+ ion. Thus, a
sparse layer of Ba atoms and Ba+ ions remains in the middle of a SXe layer of
thickness ~0.5 µm. After deposition, the window and sample are cooled to 12 K
for imaging experiments. The window plane is tilted at an angle of about 45°
relative to the ion beam so that sample creation, excitation, and fluorescence
collection can be done without adjusting the position of the window and the
optics.

For the imaging of single Ba atoms, a Coherent 599 dye laser is used to
illuminate the sample from a direction opposite the ion beam. A laser
wavelength of 565.0(1) nm is chosen for near-optimal excitation of HV site Ba
atoms and negligible excitation of those in the 5/7V site [23, 17]. The
typical laser power is 0.4 µW, about two orders of magnitude less than that
used for imaging Ba atoms in the SV site, to accommodate the faster
fluorescence bleaching of HV site Ba atoms. The laser is focused at the window
front surface (toward the ion beam) using an aspheric lens of 7.9 cm focal
length to a radius, w, of $\sim 3$ µm. An angled optical flat is used for
compensation of the astigmatism of the tilted window.

The resulting fluorescence is collected and collimated by a f/1.4 50 mm focal
length NIKKOR camera lens mounted directly above the window. The light is
filtered by a Semrock sharp-edged 582 nm bandpass filter with 21 nm FWHM
bandpass and an additional tilted 568nm long pass filter for greater blocking
of stray laser light. The combined bandpass transmits the 577 nm peak with
high efficiency and some of the 590 nm peak. Because the latter peak has
greater bleaching at excitation wavelength 565 nm [7], the contribution of TV
Ba atoms to the fluorescence signal is small. The collected fluorescence is
steered by two alignment mirrors into a 200 mm focal length camera lens, which
focuses the light into an Andor iXON 3 CCD camera. This model features EM
(electron multiplying) gain for single photon counting and is held at -100 °C.
For the experiments reported in this paper, CCD settings of EM gain 300 and
preamp gain 1 were used. The conversion factor from counts to detected photons
is 12.8 counts/photoelectron, determined by experimental scaling from factory
calibration with no EM gain. This value is used in this work. An alternate
confirmation of this calibration from the measured slope of the exponential
distribution of single photoelectron pulse heights is ~15
counts/photoelectron.

Movements of the sample window with an amplitude of ~13 µm occur during pulses
of pressurized helium in the cryostat, so the laser is shuttered with ~40%
duty cycle to minimize this effect. The residual blurring of the laser
position relative to single Ba atoms was determined from analysis of images of
single Ba atoms in SV sites to be equivalent to effective laser radii of wx=
5.73 ± 0.26 µm and wy= 6.62 ± 0.40 µm [24].

The ion source is a commercial Colutron ion gun with E$\times$B mass filter
for selecting singly ionized Barium ions at 2000 eV [25]. A series of
deflection plates and Einzel lenses provides steering and focusing to deposit
the Ba+ ions in the center of the window. Typical currents measured by an
extractable Faraday cup close to the window were on the order of 5-10 nA. To
deposit a small number of ions in a matrix sample, a pair of pulsing plates
are set at ±200 V to deflect the beam away from the window. These plates are
set to 0 V during pulse periods of ~1.5 µs to create ion bunches. The
induction signal in a set of three disks through which the ions pass provides
a monitor of the pulse size during deposits. The effective area of the ion
beam was determined to be 6.7 mm2 prior to the experiments by replacing the
window with a Faraday cup. The uncertainty on ion beam density at the window
is +20% and -30% [24].

The vacuum was provided by a small turbo pump on a CF2.75” port attached near
the cryostat and two larger turbo pumps on CF8” ports attached to the ion beam
system. Residual gas levels monitored with a SRS residual gas analyzer are
typically around 1x10-7 Torr before the cryostat is turned on. Flow of
research grade xenon gas (99.995% purity) is controlled with a needle valve
for a desired SXe growth rate that has been calibrated by interference
fringes. No further purification is used due to the small flow rates used for
sample creation.

As in our previous work, to image single Ba atoms we raster scan the focused
laser over the sample in steps of $4~{}\mathrm{\mu m}$ (transverse to the
laser and ion beams) and identify spatially resolved Ba peaks through their
fluorescence response. For each laser scan step, the fluorescence signal in
counts is determined by summing the counts in a 5$\times$5 pixel region
centered at the laser position and subtracting a digitizing offset (pedestal)
in the CCD readout electronics of 100 counts/pixel. A plot of net counts vs.
laser x,y position is called a composite image and shows where single Ba atoms
are located.

In some of our experiments, the output coupler of the dye laser was adjusted
between deposits. This resulted in a shift in the starting y position of some
scans of up to 6 laser steps relative to previous deposits. There were no x
position shifts.

##  III Results

Figure 2: A set of lego plots of integrated fluorescence counts vs. xy laser
position in successive scans from a) to d): a) SXe-only deposit; b) new SXe
with barium deposit, with one distinct peak at laser position (4,2) of the
scan; c) rescan of the SXe with barium deposit in which the prominent feature
has photobleached; d) new SXe-only after the previous deposit was evaporated.
The axes in these plots are rotated 90° clockwise compared to plots shown
later in this manuscript for better visibility. Figure 3: A set of lego plots
of successive $15\times 15$ pixel ccd images from the scan depicted in Fig. 2
(b). The laser position in scan steps is given in parentheses at the top right
of each image. The CCD counts are pedestal subtracted. In the first four
images, as the laser passes over the single atom, the fluorescent signal
increases, but remains in the same position as the laser moves across. This is
an indicator that the fluorescence is coming from a single location in the
scan region, rather than being a feature of the moving laser.

Results for one set of $6\times 6$ step scans with 0.4 µW of laser power for
10 s (25 $s\times 40$% duty cycle) and peak exposure 67 nWs/µm2 per position
are shown in Fig. 2. Summed fluorescence counts vs. laser x,y position are
shown for each scan. In (a), a scan over a fresh solid xenon deposit exhibits
little signal. In the first two laser positions (1,7) and (2,7), extra
background light from the filament of the residual gas analyzer was present,
which was turned off by the third position. The average background for the
other 34 positions is 281 counts with a standard deviation of 102 counts.

After the Xe-only deposit was evaporated at 100 K, a new Xe deposit with
57${}_{-17}^{+12}$ Ba+ ions per scan area (576 µm2) was deposited at 50 K. In
the composite image (b) a single Ba peak stands out clearly above the xenon
background with a peak of ~2600 counts in the frame with the largest signal.
The raw images of the laser positions in the second row, (6,2) through (1,2),
are displayed in Fig. 3. As the laser moves over the Ba atom, the signal
dramatically increases, until vanishing as the laser moves away from that
position.

In a second scan of this sample, Fig. 2 (c), fluorescence of the single Ba
atom is absent, indicating a bleaching of this atom’s fluorescence during the
exposure of the first scan. It is not clear whether or not the higher signal
around laser position (1,6) in scan (b) and rescan (c) of Fig. 2 is from weak
excitation of another Ba atom that was just outside the scan area. After the
second scan, the deposit was evaporated and a new xenon-only deposit was made
and scanned. No significant fluorescence peaks were observed in this SXe-only
scan, Fig. 2 (d).

Figure 4: Lego plots of a successive set of deposits and composite scans with
lower exposure time: a) SXe-only deposit and scan; b) SXe deposit with barium
which has a few higher features compared to (a); c) rescan of the same deposit
in which the strongest feature in (b) persists; d) another rescan of the same
deposit as in (b) and (c) after exposure for 50 frames on the feature of
interest at position (6,10); e) SXe-only deposit with a similar average signal
level as in (a). A laser alignment between scans (d) and (e) resulted in a -4
step y-position shift.

Other experiments were done with the same laser power, 0.4 µW, but reduced
laser exposure time, 2.5 s per position, to reduce the amount of
photobleaching of the candidate peaks during a scan. One purpose of these
experiments was the possible observation of the fluorescence turnoff of one Ba
atom.

The composite $12\times 12$ step scan images of a Xe-only deposit and a
subsequent set of scans of a Ba deposit in SXe are shown in Fig. 4. Lower
exposure resulted in fewer photons detected, 6 on average per position in the
Xe-only scan (a), resulting in higher relative statistical fluctuations. After
the scan, the Xe-only deposit was evaporated and another deposit of xenon with
326${}_{-98}^{+65}$ Ba+ ions in the scan area (2304 µm2) was made, scanned and
then rescanned. Results are shown in (b) and (c). A potential single Ba atom
candidate is present in both the Ba scan (b) and rescan (c) at position (6,10)
with a peak of ~ 360 counts in (b) and ~ 400 counts in (c).

Since the fluorescence signal of this potential single Ba atom candidate
persisted on rescan, the laser was then moved to position (6,10). 50 CCD
frames were taken in this location at the same level of exposure as in the
scan. The integrated signal for each of the 50 frames is shown in Fig. 5. For
the first 19 exposures there was an average of 215±18 counts detected per
frame; then the signal abruptly dropped to an average of 26±5 counts detected
per frame. Such a turn-off to background level is a strong indicator of
fluorescence from a single Ba atom. In a subsequent rescan of the area, Fig. 4
(d), the Ba atom signal was absent at position (6,10), as expected. After this
deposit was evaporated, another Xe-only deposit was made and imaged in Fig. 4
(e). The average signal had around 7 detected photons per position, only
slightly higher than in the first Xe only deposit of Fig. 4 (a). Both Xe-only
scans are visibly lower in signal and variations than the three scans of the
deposit with barium.

Figure 5: Signal vs. time from 50 frames with the laser at position (6,10) of
the Ba feature that persisted in Fig. 4 (b) and (c). Each frame had 2.5 s of
0.4 µW laser power.

Figure 6: Lego plots of a) the average of the first 19 frames of Fig. 5 at
position (6,10) (fluorescence ”on”) and b) the average of the remaining frames
(fluorescence ”off”); 2D intensity plots show laser position: c) first 19
frames and d) remaining frames rescaled 4x.

The apparent discrepancy of ~ 2 photons/frame “off” background level in Fig. 5
with 6-7 photons/frame background in the Xe-only scans of Fig. 4 (a) and (e)
is also found in other experiments. For example, in four Xe-only scans and
seven “spot sits” with fixed laser position on this day and the next day, the
spot-sit “off” signals were consistently smaller and ~ 40% of the Xe-only
averages. A possible mechanism is partial photobleaching of the background
that is not observed in individual frames due to low photon statistics.

Lego plots of averages of the raw pedestal-subtracted CCD images of first 19
frames (“on”) of the run in Fig. 5 and of the latter 31 frames (“off”) are
shown in Fig. 6 (a) and Fig. 6 (b), respectively. A clear image of a single Ba
atom is seen in (a). To confirm that the turnoff of atom fluorescence is not
as a result of laser movement, two-dimensional plots of these averages are
shown in Fig. 6 (c) and (d), with a 4x magnified intensity scale in (d). The
small background signal from surface fluorescence in the latter 31 frames lies
at the lower end of a weak line of fluorescence from a very low concentration
of Cr3+ ions in the bulk of the sapphire window [26]. The surface background
is at the same position within 0.5 pixel as the laser fluorescence signal of
the former 19 frames (marked by a circle). This confirms the absence of xy-
motion of the laser. Technically, xy-motion of the window in its plane that
carries the Ba atom out of the laser beam, cannot be excluded by this method.
However, the sudden turnoff and the absence of an atom signal at a shifted
position in the rescan strongly suggest that the window did not move.

##  IV Discussion

A comparison of the images obtained of SV site Ba atoms [8] and those in HV
sites in this work illustrates the need for characterizing, understanding and
mitigating photobleaching. Theoretical understanding is difficult in part
because many absorption-emission cycles (103 to >107) occur before some
bleaching mechanism terminates the process. In addition, in multivacancy sites
in SXe, the smaller excited 1P state Ba atom likely wanders around the larger
SXe hole in the few ns before it emits, as was found in simulations of excited
Na atoms in TV sites in solid krypton [27].

Reversible fluorescence wavelength change due to transfer between thermally
stable matrix configurations has been observed for Yb in SAr [28]. Site
transfer has not yet been identified as a major bleaching mechanism for Ba in
the HV site, although first evidence for transfer from the TV site to the HV
site is presented below. Because recovery in dark from bleaching has not been
observed, even over hundreds of seconds, optical pumping to metastable D or
3P0,2 states, as would occur in vacuum, is not a bleaching mechanism. In a
non-symmetric diatomic BaXe environment, metastable state lifetimes are
significantly reduced because the parity selection rule of atoms is modified
by Stark mixing [29]. Chemical reactions with rare impurities in the matrix
are minimized by forming the matrix at 50 K, which is above the condensation
temperature of N2, O2 and H2 in vacuum. Recovery by modest annealing, as
discussed below, rules out this as the dominant bleaching mechanism.

Figure 7: Normalized 577 nm Ba fluorescence vs. exposure for a large Ba
deposit. Recovery of fluorescence during two subsequent sample anneals to 40 K
in the dark can be seen. The inset shows an expanded view of initial bleaching
of this Ba deposit.

Results of a bleaching experiment with a large Ba deposit, ~ 5 s of DC Ba+
beam, and a defocused laser beam at 565 nm with wx=258 µm and wy=315 µm from a
continuous-wave optical parametric oscillator (OPO) are shown in Fig. 7.
Fluorescence was passed through a spectrometer to produce successive Ba
emission spectra. The combination of a narrow spectrometer slit compared to wx
and a narrow y integration range compared to wy assures uniform laser
intensity in the region sampled. As shown in the inset, in the first 100
frames, with 0.09 nW/µm2 laser intensity and 2 s exposure time (18 nWs/µm2), a
slight antibleaching occurs in the 577 nm fluorescence peak of HV site Ba
atoms. During this period, the 590 nm fluorescence peak for TV site Ba atoms
bleaches by about a factor of 2. The next 300 frames were taken with 1 s
exposure time and a laser intensity 5.1 nW/µm2 that is close to the central
intensity of the focused laser beam in the experiments of Figs. 2 \- 6, 6.7
nW/µm2. The fluorescence in these frames decays non-exponentially with laser
exposure but can be described by a power law with exponent -0.6. At the
additional exposure per frame of the scans in Figs. 2 \- 3, 67 nWs/µm2, about
1/4 of the Ba fluorescence remains in Fig. 7. If this curve can be interpreted
as individual Ba atoms turning off as in Fig. 5 with varying times, then, on
average, 1/4 of the fluorescing HV Ba atoms in the center of the focused laser
beam could contribute a full fluorescence signal for one scan frame at this
exposure. For a frame of the lower exposure scans in Fig. 4 (17 nWs/µm2) more
than 1/2 of the fluorescing HV Ba atoms could fluoresce for a full frame’s
maximum exposure. Apparently, the Ba atom in Figs. 5 and 6 whose fluorescence
persisted through scan, rescan and 19 frames with total exposure ~ 350 nWs/µm2
was a particularly long fluorescing atom with about 10% probability.

Figure 8: Selected spectra in the first 100 frames after the first anneal
showing anti-bleaching of the 577 nm peak and concurrent bleaching of the 590
nm peak.

Recovery from excitation-induced transfer from one matrix site to another by
annealing has been reported for Yb and K in SAr and Ag in SXe [28, 30, 31].
Such a process is also observed for Ba in SXe in Fig. 7. Following the
bleaching run in the first 1559 nWs/µm2 of Fig. 7, the sample was annealed in
the dark to 40 K and returned to 12 K over 10 minutes. An immediate recovery
to about 40% of the original 577 nm signal was observed. Over 100 low
intensity frames, antibleaching of 577 nm increased the recovery to 65% while
the 590 nm fluorescence bleached significantly. Sample spectra from this run
are shown in Fig. 8. In the runs to this point the x- and y-positions of the
OPO laser were steady as monitored by the y-position and intensity of the 619
nm peak for SV site Ba atoms that bleach little under these exposures. Thus
for these cases the recovery is not due to movement of the laser to less
bleached Ba atoms. After further bleaching, as much as half of the original
signal returned on a second annealing cycle while the sample was not exposed.
During the second bleach, a little flattening of the bleaching curve is
observed at the bottom due to y-movement of the laser beam at that time of
magnitude 0.4 times the laser radius w. Otherwise, laser motion did not
significantly change the exposure in these runs. Further investigation is
planned to see if annealing could be utilized to extend fluorescence duration
and increase the number of detected photons per atom. This could enhance the
efficiency and validation for imaging of single Ba atoms in HV sites.

##  V Conclusions

A key technology in a Ba tagging scheme for 136Xe neutrinoless double beta
decay searches is the ability to identify and count single atoms in solid
xenon. While single Ba imaging was previously demonstrated for barium atoms in
SV sites in solid xenon, the ability to do this for Ba atoms in other matrix
sites that experience greater photobleaching is critical. The first such
images of single Ba atoms in the HV site are presented in this work. Sudden
fluorescence turnoff of one Ba atom with particularly long fluorescence
duration was also observed.

Thus far, the percentage of Ba+ ions deposited into SXe in vacuum and imaged
as single Ba atoms is in the few percent range for both SV and HV sites. This
ratio includes an unknown fraction of Ba+ ions neutralized to Ba in present
experimental conditions. In the former case, a limiting factor may be low
population of the energetically disfavored SV site. For Ba atoms in the HV
site a method to overcome photobleaching would be helpful. The observed 2/3
recovery of fluorescence for the 577 nm line by annealing is a promising step
in this direction. The conversion of Ba atoms from the faster bleaching TV
site of SXe to the HV site, in which single Ba imaging has been demonstrated
here, is a hopeful avenue to explore further toward counting of the Ba atoms
that are initially in TV sites. The SV, TV, HV and 5/7V sites constitute the
major configurations of Ba in SXe observed in our work. Research is also
underway in our laboratory on single Ba+ imaging in solid xenon. In liquid
xenon, it is not known into which of these sites the daughter Ba atoms and Ba+
ions from double beta decay will be captured when frozen into SXe. Initial
studies are ongoing in our group. Measurement of the Ba-ion fraction following
radioactive beta decay of Cs, for example, 136Cs to 136Ba, 138Cs to 138Ba or
139Cs to 139Ba, would provide additional insights into the expected Ba/Ba+
fraction following a double beta decay. Ultimately, efficient single Ba and
Ba+ imaging in all sites is desirable to investigate. In this regard, a
significant step forward has been presented in this work.

##  VI Acknowledgements

The authors are grateful to Jaret Stickrod and Eli Bayarsaikhan for assistance
with experiments and to John McCaffrey, Alexei Buchachenko and Benoit Gervais
for helpful discussions of the theory of Ba atoms in noble gas matrix. This
material is based upon work supported by the National Science Foundation under
Grants No. 2011948 and 1649324. The authors gratefully acknowledge support for
nEXO from the Office of Nuclear Physics within DOE’s Office of Science, and
NSF in the United States; from NSERC, CFI, FRQNT, NRC, and the McDonald
Institute (CFREF) in Canada; from IBS in Korea; and from CAS and NSFC in
China. This work was supported in part by Laboratory Directed Research and
Development (LDRD) programs at Brookhaven National Laboratory (BNL), Lawrence
Livermore National Laboratory (LLNL), Oak Ridge National Laboratory (ORNL),
Pacific Northwest National Laboratory (PNNL), and SLAC National Accelerator
Laboratory.

##  VII References

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