††thanks: Now at Weizmann Institute of Science, Israel.††thanks: NEXT
Spokesperson.

# Fluorescence Imaging of Individual Ions and Molecules in Pressurized Noble
Gases for Barium Tagging in 136Xe

N. Byrnes  Department of Physics, University of Texas at Arlington, Arlington,
TX 76019, USA  E. Dey  Department of Physics, University of Texas at
Arlington, Arlington, TX 76019, USA  F.W. Foss  Department of Chemistry and
Biochemistry, University of Texas at Arlington, Arlington, TX 76019, USA
B.J.P. Jones  Department of Physics, University of Texas at Arlington,
Arlington, TX 76019, USA  R. Madigan  Department of Chemistry and
Biochemistry, University of Texas at Arlington, Arlington, TX 76019, USA  A.
McDonald  Department of Physics, University of Texas at Arlington, Arlington,
TX 76019, USA  R.L. Miller  Department of Chemistry and Biochemistry,
University of Texas at Arlington, Arlington, TX 76019, USA  L.R. Norman
Department of Physics, University of Texas at Arlington, Arlington, TX 76019,
USA  K.E. Navarro  Department of Physics, University of Texas at Arlington,
Arlington, TX 76019, USA  D.R. Nygren  Department of Physics, University of
Texas at Arlington, Arlington, TX 76019, USA  C. Adams  Argonne National
Laboratory, Argonne, IL 60439, USA  H. Almazán  Department of Physics and
Astronomy, Manchester University, Manchester. M13 9PL, United Kingdom  V.
Álvarez  Instituto de Instrumentación para Imagen Molecular (I3M), Centro
Mixto CSIC - Universitat Politècnica de València, Camino de Vera s/n,
Valencia, E-46022, Spain  B. Aparicio  Department of Organic Chemistry I,
University of the Basque Country (UPV/EHU), Centro de Innovación en Química
Avanzada (ORFEO-CINQA), San Sebastián / Donostia, E-20018, Spain  A.I.
Aranburu  Donostia International Physics Center, BERC Basque Excellence
Research Centre, Manuel de Lardizabal 4, San Sebastián / Donostia, E-20018,
Spain  L. Arazi  Unit of Nuclear Engineering, Faculty of Engineering Sciences,
Ben-Gurion University of the Negev, P.O.B. 653, Beer-Sheva, 8410501, Israel
I.J. Arnquist  Pacific Northwest National Laboratory (PNNL), Richland, WA
99352, USA  F. Auria-Luna  Department of Organic Chemistry I, University of
the Basque Country (UPV/EHU), Centro de Innovación en Química Avanzada (ORFEO-
CINQA), San Sebastián / Donostia, E-20018, Spain  S. Ayet  Instituto de Física
Corpuscular (IFIC), CSIC & Universitat de València, Calle Catedrático José
Beltrán, 2, Paterna, E-46980, Spain  C.D.R. Azevedo  Institute of
Nanostructures, Nanomodelling and Nanofabrication (i3N), Universidade de
Aveiro, Campus de Santiago, Aveiro, 3810-193, Portugal  J.E. Barcelon  Centro
de Física de Materiales (CFM), CSIC & Universidad del Pais Vasco (UPV/EHU),
Manuel de Lardizabal 5, San Sebastián / Donostia, E-20018, Spain  Donostia
International Physics Center, BERC Basque Excellence Research Centre, Manuel
de Lardizabal 4, San Sebastián / Donostia, E-20018, Spain  K. Bailey  Argonne
National Laboratory, Argonne, IL 60439, USA  F. Ballester  Instituto de
Instrumentación para Imagen Molecular (I3M), Centro Mixto CSIC - Universitat
Politècnica de València, Camino de Vera s/n, Valencia, E-46022, Spain  M. del
Barrio-Torregrosa  Donostia International Physics Center, BERC Basque
Excellence Research Centre, Manuel de Lardizabal 4, San Sebastián / Donostia,
E-20018, Spain  A. Bayo  Laboratorio Subterráneo de Canfranc, Paseo de los
Ayerbe s/n, Canfranc Estación, E-22880, Spain  J.M. Benlloch-Rodríguez
Donostia International Physics Center, BERC Basque Excellence Research Centre,
Manuel de Lardizabal 4, San Sebastián / Donostia, E-20018, Spain  F.I.G.M.
Borges  LIP, Department of Physics, University of Coimbra, Coimbra, 3004-516,
Portugal  A. Brodolin  Donostia International Physics Center, BERC Basque
Excellence Research Centre, Manuel de Lardizabal 4, San Sebastián / Donostia,
E-20018, Spain  Centro de Física de Materiales (CFM), CSIC & Universidad del
Pais Vasco (UPV/EHU), Manuel de Lardizabal 5, San Sebastián / Donostia,
E-20018, Spain  S. Cárcel  Instituto de Física Corpuscular (IFIC), CSIC &
Universitat de València, Calle Catedrático José Beltrán, 2, Paterna, E-46980,
Spain  A. Castillo  Donostia International Physics Center, BERC Basque
Excellence Research Centre, Manuel de Lardizabal 4, San Sebastián / Donostia,
E-20018, Spain  S. Cebrián  Centro de Astropartículas y Física de Altas
Energías (CAPA), Universidad de Zaragoza, Calle Pedro Cerbuna, 12, Zaragoza,
E-50009, Spain  E. Church  Pacific Northwest National Laboratory (PNNL),
Richland, WA 99352, USA  L. Cid  Laboratorio Subterráneo de Canfranc, Paseo de
los Ayerbe s/n, Canfranc Estación, E-22880, Spain  C.A.N. Conde  LIP,
Department of Physics, University of Coimbra, Coimbra, 3004-516, Portugal  T.
Contreras  Department of Physics, Harvard University, Cambridge, MA 02138, USA
F.P. Cossío  Donostia International Physics Center, BERC Basque Excellence
Research Centre, Manuel de Lardizabal 4, San Sebastián / Donostia, E-20018,
Spain  Department of Applied Chemistry, Universidad del Pais Vasco (UPV/EHU),
Manuel de Lardizabal 3, San Sebastián / Donostia, E-20018, Spain  G. Díaz
Instituto Gallego de Física de Altas Energías, Univ. de Santiago de
Compostela, Campus sur, Rúa Xosé María Suárez Núñez, s/n, Santiago de
Compostela, E-15782, Spain  T. Dickel  II. Physikalisches Institut, Justus-
Liebig-Universitat Giessen, Giessen, Germany  C. Echevarria  Donostia
International Physics Center, BERC Basque Excellence Research Centre, Manuel
de Lardizabal 4, San Sebastián / Donostia, E-20018, Spain  M. Elorza  Donostia
International Physics Center, BERC Basque Excellence Research Centre, Manuel
de Lardizabal 4, San Sebastián / Donostia, E-20018, Spain  J. Escada  LIP,
Department of Physics, University of Coimbra, Coimbra, 3004-516, Portugal  R.
Esteve  Instituto de Instrumentación para Imagen Molecular (I3M), Centro Mixto
CSIC - Universitat Politècnica de València, Camino de Vera s/n, Valencia,
E-46022, Spain  R. Felkai  Unit of Nuclear Engineering, Faculty of Engineering
Sciences, Ben-Gurion University of the Negev, P.O.B. 653, Beer-Sheva, 8410501,
Israel  L.M.P. Fernandes  LIBPhys, Physics Department, University of Coimbra,
Rua Larga, Coimbra, 3004-516, Portugal  P. Ferrario  Donostia International
Physics Center, BERC Basque Excellence Research Centre, Manuel de Lardizabal
4, San Sebastián / Donostia, E-20018, Spain  Ikerbasque (Basque Foundation for
Science), Bilbao, E-48009, Spain  A.L. Ferreira  Institute of Nanostructures,
Nanomodelling and Nanofabrication (i3N), Universidade de Aveiro, Campus de
Santiago, Aveiro, 3810-193, Portugal  Z. Freixa  Department of Applied
Chemistry, Universidad del Pais Vasco (UPV/EHU), Manuel de Lardizabal 3, San
Sebastián / Donostia, E-20018, Spain  Ikerbasque (Basque Foundation for
Science), Bilbao, E-48009, Spain  J. García-Barrena  Instituto de
Instrumentación para Imagen Molecular (I3M), Centro Mixto CSIC - Universitat
Politècnica de València, Camino de Vera s/n, Valencia, E-46022, Spain  J.J.
Gómez-Cadenas  Donostia International Physics Center, BERC Basque Excellence
Research Centre, Manuel de Lardizabal 4, San Sebastián / Donostia, E-20018,
Spain  Ikerbasque (Basque Foundation for Science), Bilbao, E-48009, Spain  R.
González  Donostia International Physics Center, BERC Basque Excellence
Research Centre, Manuel de Lardizabal 4, San Sebastián / Donostia, E-20018,
Spain  J.W.R. Grocott  Department of Physics and Astronomy, Manchester
University, Manchester. M13 9PL, United Kingdom  R. Guenette  Department of
Physics and Astronomy, Manchester University, Manchester. M13 9PL, United
Kingdom  J. Hauptman  Department of Physics and Astronomy, Iowa State
University, Ames, IA 50011-3160, USA  C.A.O. Henriques  LIBPhys, Physics
Department, University of Coimbra, Rua Larga, Coimbra, 3004-516, Portugal
J.A. Hernando Morata  Instituto Gallego de Física de Altas Energías, Univ. de
Santiago de Compostela, Campus sur, Rúa Xosé María Suárez Núñez, s/n, Santiago
de Compostela, E-15782, Spain  P. Herrero-Gómez  Racah Institute of Physics,
The Hebrew University of Jerusalem, Jerusalem 9190401, Israel  V. Herrero
Instituto de Instrumentación para Imagen Molecular (I3M), Centro Mixto CSIC -
Universitat Politècnica de València, Camino de Vera s/n, Valencia, E-46022,
Spain  C. Hervés Carrete  Instituto Gallego de Física de Altas Energías, Univ.
de Santiago de Compostela, Campus sur, Rúa Xosé María Suárez Núñez, s/n,
Santiago de Compostela, E-15782, Spain  P. Ho  Department of Chemistry and
Biochemistry, University of Texas at Arlington, Arlington, TX 76019, USA  Y.
Ifergan  Unit of Nuclear Engineering, Faculty of Engineering Sciences, Ben-
Gurion University of the Negev, P.O.B. 653, Beer-Sheva, 8410501, Israel  F.
Kellerer  Instituto de Física Corpuscular (IFIC), CSIC & Universitat de
València, Calle Catedrático José Beltrán, 2, Paterna, E-46980, Spain  L.
Larizgoitia  Donostia International Physics Center, BERC Basque Excellence
Research Centre, Manuel de Lardizabal 4, San Sebastián / Donostia, E-20018,
Spain  A. Larumbe  Department of Organic Chemistry I, University of the Basque
Country (UPV/EHU), Centro de Innovación en Química Avanzada (ORFEO-CINQA), San
Sebastián / Donostia, E-20018, Spain  P. Lebrun  Fermi National Accelerator
Laboratory, Batavia, IL 60510, USA  F. Lopez  Donostia International Physics
Center, BERC Basque Excellence Research Centre, Manuel de Lardizabal 4, San
Sebastián / Donostia, E-20018, Spain  N. López-March  Instituto de Física
Corpuscular (IFIC), CSIC & Universitat de València, Calle Catedrático José
Beltrán, 2, Paterna, E-46980, Spain  R.D.P. Mano  LIBPhys, Physics Department,
University of Coimbra, Rua Larga, Coimbra, 3004-516, Portugal  A.P. Marques
LIP, Department of Physics, University of Coimbra, Coimbra, 3004-516, Portugal
J. Martín-Albo  Instituto de Física Corpuscular (IFIC), CSIC & Universitat de
València, Calle Catedrático José Beltrán, 2, Paterna, E-46980, Spain  G.
Martínez-Lema  Unit of Nuclear Engineering, Faculty of Engineering Sciences,
Ben-Gurion University of the Negev, P.O.B. 653, Beer-Sheva, 8410501, Israel
M. Martínez-Vara  Donostia International Physics Center, BERC Basque
Excellence Research Centre, Manuel de Lardizabal 4, San Sebastián / Donostia,
E-20018, Spain  K. Mistry  Department of Physics, University of Texas at
Arlington, Arlington, TX 76019, USA  J. Molina-Canteras  Department of Organic
Chemistry I, University of the Basque Country (UPV/EHU), Centro de Innovación
en Química Avanzada (ORFEO-CINQA), San Sebastián / Donostia, E-20018, Spain
F. Monrabal  Donostia International Physics Center, BERC Basque Excellence
Research Centre, Manuel de Lardizabal 4, San Sebastián / Donostia, E-20018,
Spain  Ikerbasque (Basque Foundation for Science), Bilbao, E-48009, Spain
C.M.B. Monteiro  LIBPhys, Physics Department, University of Coimbra, Rua
Larga, Coimbra, 3004-516, Portugal  F.J. Mora  Instituto de Instrumentación
para Imagen Molecular (I3M), Centro Mixto CSIC - Universitat Politècnica de
València, Camino de Vera s/n, Valencia, E-46022, Spain  P. Novella  Instituto
de Física Corpuscular (IFIC), CSIC & Universitat de València, Calle
Catedrático José Beltrán, 2, Paterna, E-46980, Spain  A. Nuñez  Laboratorio
Subterráneo de Canfranc, Paseo de los Ayerbe s/n, Canfranc Estación, E-22880,
Spain  E. Oblak  Donostia International Physics Center, BERC Basque Excellence
Research Centre, Manuel de Lardizabal 4, San Sebastián / Donostia, E-20018,
Spain  J. Palacio  Laboratorio Subterráneo de Canfranc, Paseo de los Ayerbe
s/n, Canfranc Estación, E-22880, Spain  B. Palmeiro  Department of Physics and
Astronomy, Manchester University, Manchester. M13 9PL, United Kingdom  A. Para
Fermi National Accelerator Laboratory, Batavia, IL 60510, USA  I. Parmaksiz
Department of Physics, University of Texas at Arlington, Arlington, TX 76019,
USA  A. Pazos  Department of Applied Chemistry, Universidad del Pais Vasco
(UPV/EHU), Manuel de Lardizabal 3, San Sebastián / Donostia, E-20018, Spain
J. Pelegrin  Donostia International Physics Center, BERC Basque Excellence
Research Centre, Manuel de Lardizabal 4, San Sebastián / Donostia, E-20018,
Spain  M. Pérez Maneiro  Instituto Gallego de Física de Altas Energías, Univ.
de Santiago de Compostela, Campus sur, Rúa Xosé María Suárez Núñez, s/n,
Santiago de Compostela, E-15782, Spain  M. Querol  Instituto de Física
Corpuscular (IFIC), CSIC & Universitat de València, Calle Catedrático José
Beltrán, 2, Paterna, E-46980, Spain  A.B. Redwine  Unit of Nuclear
Engineering, Faculty of Engineering Sciences, Ben-Gurion University of the
Negev, P.O.B. 653, Beer-Sheva, 8410501, Israel  J. Renner  Instituto Gallego
de Física de Altas Energías, Univ. de Santiago de Compostela, Campus sur, Rúa
Xosé María Suárez Núñez, s/n, Santiago de Compostela, E-15782, Spain  I.
Rivilla  Donostia International Physics Center, BERC Basque Excellence
Research Centre, Manuel de Lardizabal 4, San Sebastián / Donostia, E-20018,
Spain  Ikerbasque (Basque Foundation for Science), Bilbao, E-48009, Spain  C.
Rogero  Centro de Física de Materiales (CFM), CSIC & Universidad del Pais
Vasco (UPV/EHU), Manuel de Lardizabal 5, San Sebastián / Donostia, E-20018,
Spain  L. Rogers  Argonne National Laboratory, Argonne, IL 60439, USA  B.
Romeo  Donostia International Physics Center, BERC Basque Excellence Research
Centre, Manuel de Lardizabal 4, San Sebastián / Donostia, E-20018, Spain  C.
Romo-Luque  Instituto de Física Corpuscular (IFIC), CSIC & Universitat de
València, Calle Catedrático José Beltrán, 2, Paterna, E-46980, Spain  F.P.
Santos  LIP, Department of Physics, University of Coimbra, Coimbra, 3004-516,
Portugal  J.M.F. dos Santos  LIBPhys, Physics Department, University of
Coimbra, Rua Larga, Coimbra, 3004-516, Portugal  M. Seemann  Donostia
International Physics Center, BERC Basque Excellence Research Centre, Manuel
de Lardizabal 4, San Sebastián / Donostia, E-20018, Spain  I. Shomroni  Racah
Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 9190401,
Israel  P.A.O.C. Silva  LIBPhys, Physics Department, University of Coimbra,
Rua Larga, Coimbra, 3004-516, Portugal  A. Simón  Donostia International
Physics Center, BERC Basque Excellence Research Centre, Manuel de Lardizabal
4, San Sebastián / Donostia, E-20018, Spain  S.R. Soleti  Donostia
International Physics Center, BERC Basque Excellence Research Centre, Manuel
de Lardizabal 4, San Sebastián / Donostia, E-20018, Spain  M. Sorel  Instituto
de Física Corpuscular (IFIC), CSIC & Universitat de València, Calle
Catedrático José Beltrán, 2, Paterna, E-46980, Spain  J. Soto-Oton  Instituto
de Física Corpuscular (IFIC), CSIC & Universitat de València, Calle
Catedrático José Beltrán, 2, Paterna, E-46980, Spain  J.M.R. Teixeira
LIBPhys, Physics Department, University of Coimbra, Rua Larga, Coimbra,
3004-516, Portugal  S. Teruel-Pardo  Instituto de Física Corpuscular (IFIC),
CSIC & Universitat de València, Calle Catedrático José Beltrán, 2, Paterna,
E-46980, Spain  J.F. Toledo  Instituto de Instrumentación para Imagen
Molecular (I3M), Centro Mixto CSIC - Universitat Politècnica de València,
Camino de Vera s/n, Valencia, E-46022, Spain  C. Tonnelé  Donostia
International Physics Center, BERC Basque Excellence Research Centre, Manuel
de Lardizabal 4, San Sebastián / Donostia, E-20018, Spain  J. Torrent
Donostia International Physics Center, BERC Basque Excellence Research Centre,
Manuel de Lardizabal 4, San Sebastián / Donostia, E-20018, Spain  Escola
Politècnica Superior, Universitat de Girona, Av. Montilivi, s/n, Girona,
E-17071, Spain  A. Trettin  Department of Physics and Astronomy, Manchester
University, Manchester. M13 9PL, United Kingdom  A. Usón  Instituto de Física
Corpuscular (IFIC), CSIC & Universitat de València, Calle Catedrático José
Beltrán, 2, Paterna, E-46980, Spain  P.R.G. Valle  Donostia International
Physics Center, BERC Basque Excellence Research Centre, Manuel de Lardizabal
4, San Sebastián / Donostia, E-20018, Spain  Department of Applied Chemistry,
Universidad del Pais Vasco (UPV/EHU), Manuel de Lardizabal 3, San Sebastián /
Donostia, E-20018, Spain  J.F.C.A. Veloso  Institute of Nanostructures,
Nanomodelling and Nanofabrication (i3N), Universidade de Aveiro, Campus de
Santiago, Aveiro, 3810-193, Portugal  J. Waiton  Department of Physics and
Astronomy, Manchester University, Manchester. M13 9PL, United Kingdom  A.
Yubero-Navarro  Donostia International Physics Center, BERC Basque Excellence
Research Centre, Manuel de Lardizabal 4, San Sebastián / Donostia, E-20018,
Spain

###### Abstract

The imaging of individual Ba2+ ions in high pressure xenon gas is one possible
way to attain background-free sensitivity to neutrinoless double beta decay
and hence establish the Majorana nature of the neutrino. In this paper we
demonstrate selective single Ba2+ ion imaging inside a high-pressure xenon gas
environment. Ba2+ ions chelated with molecular chemosensors are resolved at
the gas-solid interface using a diffraction-limited imaging system with scan
area of 1$\times$1 cm2 located inside 10 bar of xenon gas. This new form of
microscopy represents an important enabling step in the development of barium
tagging for neutrinoless double beta decay searches in 136Xe, as well as a new
tool for studying the photophysics of fluorescent molecules and chemosensors
at the solid-gas interface.

The only known sensitive way to establish the Majorana nature of the neutrino
is via direct observation of neutrinoless double beta decay
($0\nu\beta\beta$). Its detection would provide a revolutionary insight into
the nature of neutrino mass, likely the only observed manifestation of physics
above the electroweak scale. It would also be compelling as a potential window
into the mechanism leading to the dominance of matter over antimatter in the
Universe [1].

All existing techniques to search for $0\nu\beta\beta$ have been limited by
backgrounds from radiogenic activity in detector materials [2]. To achieve
sensitivity sufficient to discover $0\nu\beta\beta$ if the neutrino mass
ordering is inverted and a light Majorana mediates the decay, detectors with
$\geq$ 1 Ton of the double beta decay isotope and background indices of order
$b<0.1(\mathrm{ct\,ton\,ROI})^{-1}$ are required [3]. Factors between 20 and
2000 beyond existing demonstrated technologies are required to meet these
goals for the immediately forthcoming generation of experiments. Beyond this
“ton-scale” phase [4, 5, 6, 7], the task of extending sensitivities into the
normal mass ordering band of parameter space appear truly formidable. The
relevant experiments would employ far larger quantities of active isotope [8]
and require background indices below the ton-scale demands. A distinct but
related question is how to confirm a signal of $0\nu\beta\beta$ following a
suggestive but inconclusive hint from the coming generation of still-
background-limited experiments. New and possibly radically new ultra-low
background technologies will be required to meet these challenges.

One especially promising technical approach to reaching the ultra-low
background limit is “barium tagging”— identification of the daughter ion
produced in the double beta decay of 136Xe [9]. An efficient barium ion tag
with low background could reduce contamination from radiogenic backgrounds to
near zero. Demonstration of a method of capture and imaging of barium ions
from one to several tons of xenon requires significant advances in
instrumentation. Much progress has been made on promising methods for single
barium ion or atom identification in liquid and gaseous xenon [10, 11, 12, 13,
14, 15, 16, 17, 18, 19, 20, 21, 22], but many technological challenges remain.

Most approaches to barium tagging apply some form of fluorescence imaging,
exciting transitions in either atoms, molecules, or materials that are caused
by presence of a barium atom or ion. In xenon gas experiments, the target
charge state for barium tagging is the dication $\mathrm{Ba}^{2+}$, due to
absence of recombination from thermalized electrons around the decay [23].
This ion has no low-lying atomic fluorescence lines to access with visible
lasers. However, when chelated within a suitable organic molecule, its
appearance can in principle be observed via single molecule fluorescence
imaging (SMFI). The proposal to use SMFI to tag the daughter ion in
$0\nu\beta\beta$ was first outlined in Refs. [24, 17]. Commercial dyes
developed for $\mathrm{Ca^{2+}}$ sensing in biological applications were
demonstrated as sensitive barium tagging agents in solution phase. Soon
thereafter, single ion sensitivity was achieved [15]. The model system in that
work, liquid droplets within a polymer matrix, is not well representative of
conditions within a xenon gas time projection chamber. Several aspects of the
chemistry of binding and fluorescence of the commercial dyes were found to be
inadequate for dry barium sensing, instigating a program of novel organic
fluorophore development, which has culminated in multiple candidates for
single $\mathrm{Ba}^{2+}$ sensing based on crown-ether derivatives [25, 19,
26, 27, 16, 22, 28]. These molecules have been used to demonstrate single
molecule sensitivity to barium in dry and solvent-less conditions [18, 28] and
shown via electron microscopy to react with neutral barium perchlorate in
vacuum [22]. An active ongoing R&D program is underway within the NEXT
collaboration to bring these techniques to fruition in barium tagging sensors
for a future ton- to multi-ton neutrinoless double beta decay experiment.

Experiments searching for $0\nu\beta\beta$ in 136Xe typically employ time
projection chambers filled with purified, pressurized or liquified xenon at
part-per-billion purity in oxygen and water to avoid electron attachment. To
augment such a detector with a barium tagging system, two distinct problems
must be overcome: first, how to bring the ion to a sensor [20, 21] or sensor
to an ion [29]; and second, how to sense the arrival of Ba2+ with single-ion
precision within a large, pure volume of xenon. This is a complex environment
in which to realize single ion microscopy, with no commercial devices or past
proofs-of-principle available.

##  I Fluorescence imaging of single ions in high pressure xenon

We present here novel results from NEXT barium tagging program that show a
sensing scheme meeting many of the requirements of single ion detection for
barium tagging in 136Xe gas. In addition to this motivating application, this
technology also opens a new field of single molecule / single ion imaging at
the gas-solid interface.

Figure 1: Top: the head of the high pressure microscope that sits inside the
pressure chamber.r Bottom: Optical paths and components in the high pressure
single molecule microscope. Figure 2: Large scale raw data image of BODIPY
molecules drip-coated onto a slide surface. The image is resolved with point-
spread function close to the Abbe Diffraction Limit. Bright points consistent
with single molecule fluorescence are observed over a scan region of 1 mm2.
Figure 3: Image of a slide spin-coated in IPG-1 chemosensor, showing the
activity with (right) and without (left) added Ba2+.  Figure 4: Left: Pixel
intensity distributions for images taken using IPG vs IPG+Ba in 10 bar xenon
gas. Right: reconstructed single molecule candidate brightnesses and step
times for IPG vs IPG+Ba in 10 bar xenon gas. The plotted points are obtained
over seven exposure regions on a single slide, and the trend is found to be
repeatable over multiple slides.

A detailed description of the instrument is provided in the Supplementary
Information. Briefly, a fluorescence microscope based on nanometer precision
vacuum stages is mounted on the inside of a high pressure chamber connected to
a recirculating xenon gas system. Images are recorded by an electron
multiplying CCD camera, following laser excitation of barium-induced
fluorescence in Ba2+ selective organic chemosensors. The system has been
characterized using emission from BODIPY fluorophores [30] excited at 488 nm.
The configuration for this characterization study is shown in Fig. 1. A raw
data image of sparsely distributed molecules, produced by rastering in 33 $\mu
m$ steps over a 1 mm2 surface area and auto-focusing on the single molecule
candidates at each point is shown in Fig. 2. The capability for single
molecule imaging over this large surface area is clearly demonstrated, with
resolution near the Abbe diffraction limit. Further studies of the system
imaging resolution can be found in the SI.

To demonstrate single Ba2+ ion imaging in high pressure gas, slides were
coated with nanomolar concentrations of IPG-1 fluorophore reported in Ref
[28]. Solutions were prepared at 10-8M concentration, with and without barium
ions supplied via 100 mM barium perchlorate, and activity compared between
Ba2+ chelated and unchelated samples. The optical system for this test was
configured with a 510 nm laser with 10 mW incoming laser power and a filter
set suitable for this dye, as described in the SI. The vessel was then sealed,
evacuated and filled with cleaned, pressurized xenon. Figure 3 shows images
taken under purified 10 bar xenon gas for slides prepared with and without
added Ba2+. In this image each frame has been Fourier transformed and
spatially filtered. A low pass filter is used to remove the broad background
from residual glass fluorescence, and a high pass filter to remove speckles
associated with thermal CCD noise. The resultant images display clear bright
spots consistent with single molecule emission.

A clear increase in activity is observed upon addition of Ba2+. Some visible
streaks in the distribution of single molecule candidates emanate in a radial
direction and are a result of the spin-coating protocol. Single molecules
remain resolvable within the streaks, though our analysis methods
preferentially select those that are isolated single spots. The raw pixel
histogram indicates a dramatic increase in activity in bright pixels, as shown
in, Fig. 4, left. A small number of very weak emitters are present in the
Ba2+-free samples, though the bright spots associated with Ba2+-bound IPG
molecules are unambiguously identified as being present only in the
Ba2+-chelated sample.

Figure 5: Photo-bleaching time distribution for Ba2+ candidates chelated in
IPG dyes in xenon and air. The X axis is shared between the top and bottom
histogram.

As a control experiment, blank coverslips with no fluorophore deposited were
also scanned. The characteristic diffuse glow from background fluorescence in
the slide was observed in all locations. However, no localized bright emitters
could be brought into focus at any point on the slides. This suggests that the
background due to accidental fluorescent molecules in the environment is
extremely low, and has proven to be unquantifiably so in the present system.
Because no emitters could be found, the microscope could not be focused and so
no images from the blank slide surfaces can be reported.

The hallmark of single molecule fluorescent imaging is bright points that are
characterized by their sharp photobleaching transitions. A set of analysis
tools for single molecule fluorescence imaging and analysis have been
developed and are applied to the images captured by the high pressure
microscope. To produce a time sequence, the first ten images in a sequence are
summed, and a list of local fluorescent maxima ordered by their intensity is
used to determine initial points of interest. Each frame is Fourier
transformed and convolved with a bipolar filter kernel consisting of one
normalized narrow Gaussian ($\pm 4$ pixels) with positive amplitude and one
normalized broad Gaussian ($\pm 8$ pixels) with negative amplitude. This
filter serves to subtract local background while integrating fluorescence
within the characteristic width of the fluorescent emission area. A cut on the
maximal identified step height in each trace is used to identify single
molecule photo-bleaching event candidates.

Such candidates are identifiable in all conditions tested, including ambient
air, vacuum and and pressurized argon and xenon, in Ba2+ spiked samples. The
time and intensity distributions of the reconstructed steps in samples with
and without Ba2+ are provided in Fig. 4, right. In these plots, the $y$ axis
corresponds to the fluorescence integral up to the photobleaching transition
divided by the total time to the step, providing a measure of average
brightness of the emitter. The $x$ axis corresponds to the step time. Only
very small steps are observed in the Ba2+-free samples, whereas the
Ba2+-enriched ones contain large steps associated with single barium ion
candidates.

Fig. 6, left shows the time traces for some single Ba2+ ions identified via
single-step photo-bleaching 10 bar xenon gas. To the right of each trace are
shown the fluorescence intensity maps corresponding to each, representing the
activity recorded on a small subset of the CCD pixels. Each surface plot shows
the integral of the activity between in the last 20 frames before the photo-
bleaching step. The characteristic photo-bleaching and in some cases
photoblinking behaviour associated with single molecule fluorescence imaging
is clearly observed in each time series.

Studying the photo-bleaching time distribution illuminates a striking
difference in the bleaching dynamics of Ba2+-chelated IPG dyes in noble gas vs
air environments, with substantially faster bleaching behaviour observed in
air. The mean photo-bleaching time in 10 bar xenon gas is extracted as 241.5
s, whereas in air it is much faster at 22.3 s, as shown in Fig. 5. This
observation supports the hypothesis that has been discussed in association
with solution-based studies (for example, Refs. [31, 32]), that reactions with
oxygen are likely primarily responsible for the bleaching process.
Nevertheless, even absent ambient oxygen some photo-bleaching mechanism
appears to be present, albeit at a far slower rate. While large datasets of
IPG+Ba2+ images were not collected in vacuum or argon conditions for this
work, visual inspection of the photo-bleaching behaviour while imaging in
those conditions showed a time profile far more similar to the the xenon data
than to air, as may be expected under the oxygen-mediated hypothesis.

##  II Conclusions and outlook

We have demonstrated a novel, diffraction-limited, high pressure fluorescence
imaging system that is capable of single ion identification in high pressure
gases at the gas-solid interface. Single fluorescent molecules are resolved
over large surface areas, and we have demonstrated a sweep over $1\times 1$
mm2 via 2D raster scan. An autofocus algorithm that reliably brings emitters
as weak as a single molecule into focus is used to provide a map of the focal
plane that can be extrapolated for rapid and repeatable imaging. The total
possible scan region is in excess of $1\times 1$ cm2, with effective focusing
possible over the full area.

Single Ba2+ ions have been imaged in high pressure xenon gas using turn-on
fluorophores, representing the first demonstration of imaging of individual
Ba2+ ions within a candidate active medium of a time projection chamber for
$0\nu\beta\beta$.

Figure 6: Single barium ions chelated with IPG-1 turn-on chemosensor imaged in
10 bar xenon gas. The left plots show time traces of fluorescence for the
identified emitter, with discrete photobleaching and photoblinking steps
indicative of single molecule origin. The intensity is taken from the central
pixel after applying a double-Gaussian filter as described in the text, and
thus represents the baseline-subtracted integral over the diffraction-limited
fluorescent spot. The right figures show the 2D spatial map of fluorescence
around each emitter, integrated for 20 frames before photo-bleaching, showing
a well localized peak in each case.

The system can be implemented within the pressure vessel of a time projection
chamber with only small modifications, which we now briefly discuss. First, in
this work, rastering was performed by moving a slide past a fixed objective
lens, due to volume limitations within the vessel. In the final device it
should be the converse, with rastering of the objective across a fixed imaging
region. Since the image rays are brought out of the vessel in infinity space,
this will add little in the way of complexity but requires some mechanical
adjustments. Second, continuous and lossless operation at this resolution
generates a tremendous amount of data (15 GB uncompressed, for the image in
Fig. 2). A rational zero suppression algorithm and a form of online trigger
for frames of interest is will be advantageous for real-time application, with
factors of over 1000 in data reduction to be reasonably expected. These issues
require attention, but appear manageable. Finally, the molecular layer used
here for Ba2+ sensing is produced by spin coating a sparse group of molecules
from solution; ultimately a fully sensitive Ba2+ tagging layer must use
densely packet fluorophores. Detailed investigations into the behaviour of
densely packed fluorophore layers on surfaces [22] and into self-assembled
monolayer growth [26], as well as studies of the efficiency of barium ion
capture at these surfaces using Ba2+ ion beams [33] are currently underway
within the NEXT collaboration, and represent the next crucial frontier in
chemical Ba2+ ion sensor development. While important work must be done to
realize the ultimate sensing layers, an optical system like the one described
in this paper will be capable of imaging ions arriving at their surfaces in a
future xenon gas barium tagging detector.

Alongside ongoing development of molecular synthesis [25, 19, 26, 27, 16, 22,
28], ion transport [21] and detector readout modalities [34, 35, 36, 37] that
can enable barium tagging at the cathode, the new technology demonstrated in
this paper represents an important step toward a large, barium tagging xenon
gas detector. Such a device holds great promise as a concept for a truly
background free, ton-to-multi-ton scale neutrinoless double beta decay
experiment in 136Xe. It also represents a first step into a new field of high
precision, single molecule photo-physical fluorescence analysis at the gas-
solid interface.

## Acknowledgements

This program of single barium ion sensing is a part of a collaborative project
to employ SMFI chemosensors functional at the solid-gas interface, as R&D
toward barium tagging for the NEXT experiment. We acknowledge support from the
US Department of Energy under awards DE-SC0019054 and DE-SC0019223, and US
National Science Foundation under award number NSF CHE 2004111 and the Robert
A Welch Foundation under award number Y-2031-20200401 (University of Texas
Arlington).

The NEXT Collaboration acknowledges support from the following agencies and
institutions: the European Research Council (ERC) under Grant Agreement No.
951281-BOLD; the European Union’s Framework Programme for Research and
Innovation Horizon 2020 (2014–2020) under Grant Agreement No. 957202-HIDDEN;
the MCIN/AEI of Spain and ERDF A way of making Europe under grants
RTI2018-095979 and PID2021-125475NB , the Severo Ochoa Program grant
CEX2018-000867-S and the Ramón y Cajal program grant RYC-2015-18820; the
Generalitat Valenciana of Spain under grants PROMETEO/2021/087 and
CIDEGENT/2019/049; the Department of Education of the Basque Government of
Spain under the predoctoral training program non-doctoral research personnel;
the Portuguese FCT under project UID/FIS/04559/2020 to fund the activities of
LIBPhys-UC; the Pazy Foundation (Israel) under grants 877040 and 877041; the
US Department of Energy under contracts number DE-AC02-06CH11357 (Argonne
National Laboratory), DE-AC02-07CH11359 (Fermi National Accelerator
Laboratory). Finally, we are grateful to the Laboratorio Subterráneo de
Canfranc for hosting and supporting the NEXT experiment.

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  * Note [6] Thorlabs FELH0500.
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  * Note [8] Chroma 49023.
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  * Note [21] Omega FM1800.

##  III Methods

To obtain high quality images, any fluorescence microscope must have carefully
arranged optical paths for both illumination and image collection. We begin by
describing these optical elements in Sec. III.1 and III.2, respectively, and
then describe the mechanical construction of the device in Sec. III.4. Section
III.3 describes the alignment and focusing protocol and quantifies some of the
imaging metrics of the system.

###  III.1 Excitation

Past work on single Ba2+ imaging for NEXT has shown that single ion candidates
can be observed with at little as 0.2 mW of laser power in the field of view
(FOV), corresponding to around 1 W cm-2 [15], given molecules with
sufficiently high quantum efficiencies. Laser power above a 50 W cm-2, on the
other hand, risks rapid photobleaching, preventing the prolonged observation
needed to tag a single emitting molecule, for all of the molecular species we
have studied. As such we have optimized the current system to provide few-mW
levels of excitation to enable clear single molecule resolution, while
offering suitably long (few second) observation times to establish presence of
an individual ion. We have also opted for an epi-fluorescence configuration.
While through-objective total internal reflection fluorescence (TIRF) imaging
was used in our earliest studies of fluorescent single barium ion complexes in
order to suppress deeper backgrounds arising in thick samples [15], more
recent work demonstrated that for sufficiently thin fluorescent samples,
single molecule resolution can also be obtained in an epi-fluorescence mode
[18]. The latter approach is more straightforward to realize with remote
illumination, and forms the basis of our illumination scheme for this device.
For this form of microscopy, light is focused on axis on the back-focal plane
of the objective by an external lens, leading to parallel and uniform passage
of light through the focal plane, which is aligned to the surface supporting
the fluorescent emitters to be imaged

We use a series of solid state lasers as excitation sources, each suitable for
a different set of fluorescent dyes. For the characterizations of the optical
system performance, we have used a 488 nm laser with power controlled by an
adjustable DC power supply. This wavelength choice is sufficiently long to
escape the tail of fluorescence from glass that compromises single molecule
imaging with shorter wavelength excitation (at 450 nm and below, the
autoflourescence from the objective and substrate proved prohibitive for
single molecule imaging), and is well matched to the absorption peak of the
BODIPY [30] molecule used for our optical system characterizations. For Ba2+
sensing, a 510 nm laser is used to excite the IPG-1 species studied in Ref.
[28]. Due to their long excitation wavelength, the IPG class of molecular
probes have have been found to provide excellent signal-to-background ratio
for single molecule microscopy, in studies undertaken in preparation for this
work [28]. An illustrative diagram showing the system configured with the 488
nm laser is shown in Fig. 1, top left.

To obtain smooth excitation profiles over the image plane, the excitation
laser beam is first spatially cleaned. For the 488 nm laser, it is first
launched from the laser into a single mode optical fiber 111Thorlabs
P1-405B-FC-2 via a 20X microscope objective 222Olympus PLN20X with 0.4 NA. At
the other end of the fiber a 10X objective 333Olympus PLN10X produces a
parallel Gaussian beam of around 2 mm diameter. Longitudinal positioning of
this second objective on a micrometer stage allows for fine-tuning of the size
of the illumination site on the sensing plane by controlling the beam
divergence from the objective. For the 510 nm laser, the beam is instead
launched into a 5x beam expander, with adjustable divergence, and then
spatially filtered through a pinhole.

Immediately down-beam, a flip-mirror can be used to redirect the laser over a
4 m path to quantify its divergence. Optimal illumination performance was
found to correspond to a very slightly divergent beam from this mirror. The
excitation light is then passed through an adjustable iris to remove halo. The
488 nm laser configuration then includes a 488 nm laser line filter
444Thorlabs FL488-10; whereas for the 510 nm configuration, no subsequent
excitation filtering proved to be necessary external to the vessel. The
reflected ray from the line filter contains the long and short wavelength
tails of the laser spectrum and is absorbed on an external beam dump.

The spatially and spectrally cleaned laser light is reflected from two
external adjustable mirrors in a periscope arrangement to allow for fine
steering of the path into the vacuum / pressure chamber. The beam next passes
through a 1.6 mm thick sapphire pressure window into the vacuum or gas volume.
Just in front of the sapphire window, a final 40 cm spherical planoconvex lens
is placed at approximately its focal length from the main microscope
objective, focusing the nearly parallel beam onto the back-focal plane.

Inside the chamber the beam arrives at a fine-adjustment mirror with two
degrees of freedom, which is set before the chamber is closed and left aligned
for the duration of the experiment. This mirror directs the beam into an
internal filter cube, which has emission and excitation filters and a long-
pass dichroic beam splitter. For the 488 nm configuration, these are 500 nm
short pass (SP) excitation 555Thorlabs FEHH0500 and 500 nm long pass (LP)
emission 666Thorlabs FELH0500 filters, and a 490 nm dichroic mirror, also LP
777Thorlabs DMLP490R. For 510 nm, a fluorsescence microscopy filter set
888Chroma 49023 consisting of excitation filter: band pass 500$\pm$ 20 nm,
emission filter: band pass 560$\pm$ 25 nm, and dichroic mirror: long pass 525
nm. On the forth side of the filter cube a high optical density neutral
density filter serves as a shallow-depth internal beam dump.

The excitation beam is guided to the back face of a vented 100X, high NA
microscope objective 999Customized PLFLN100X; PLAN FLUOR 100X DRY OBJ, NA
0.95, WD 0.2 which we have customized for operation in high pressure
environments. Earlier experiments with commercial objectives resulted in
various bursting and internal misalignment failures due to the pressurization
and depressurization process. The beam is focused by the objective onto the
front face of a 160 $\mu$m thick glass coverslip 101010Ted Pella Schott D263M
22x30 mm Glass Coverslips in transmission mode, and the fluorescence emitted
from the sample is collected back through the same high numerical aperture
objective.

###  III.2 Imaging

The longer wavelength fluorescence light emitted from the sample plane
transmits through the dichroic mirror and exits through a second 2-inch
diameter sapphire viewport 111111CeramTec 18617-01-CF. The infinity-space
optical path on the fluorescence side is completely enclosed in black optical
piping to prevent ambient background from laboratory lighting. The light
isolation in the image path is found to be sufficiently effective that single
molecule resolution is comfortably accomplished with laboratory lights on. The
image is reflected twice from a pair of mirrors in a periscope arrangement
that allow for external adjustment of the region of the objective in the
camera FOV. A second, external 500 nm LP filter 121212Thorlabs FELH0500
removes any residual short-wavelength light, and then a tube lens focuses the
image from infinity space onto the camera. Adjusting the tube lens position
and focal length allows for the system to be run in modes with different
levels of magnification. For this paper we have used a medium level of
magnification, with approximately 40 $\mu$m field of view per frame achieved
using a 15 cm focal length tube lens.

The image acquisition device is an electron multiplying CCD camera
131313Hamamatsu ImagEM2 EMCCD with 90% quantum efficiency and a 512$\times$512
array of $16~{}\mathrm{\mu m}\times 16~{}\mathrm{\mu m}$ pixels (pix),
operating at 500 ms exposure time. The camera is connected via a demountable
tube coupling with an external shutter to protect the camera from being
exposed to high light environments. It is read out over IEEE 1394 conduits to
a PC running a bespoke data acquisition software suite that we have developed
specifically for this device that interfaces to the internal micrometer stages
and camera readout.

###  III.3 Alignment and Focusing

Figure 7: Illustration of the single molecule autofocus procedure. An
autofocus metric defined in terms of image brightness and kurtosis is applied
that maximizes for an in focus image. The left plot shows the Z dependence of
the autofocus metric, with images at different focal depths provided on the
right plot. A small signal is seen on the back face of the slide (A). No
features are present when totally out of focus (B). Within around 2 microns of
focus some activity is seen (C), with sharp images only at the focal plane
(D). The inset shows that the depth of the in-focus region is around $1\mu m$.
Figure 8: Left: Measured point-spread function in X and Y directions at five
re-focused locations using single molecules in 7 bar argon gas. Right: average
X and Y PSF compared to the Abbe diffraction limit. The horizontal line on the
right plot represents the baseline, as measured away from the bright spot.

At each $X$ and $Y$ position there is an approximately $1~{}\mu$m deep range
of $Z$ values where the fluorescence plane is in focus. Finding this plane at
each position requires identification of a weak single molecule fluorescence
signal among the range of plausible focal points, which vary by around 70
$\mu$m in absolute terms given repeatability afforded by our slide
installation protocol. Few $\mu$m variations of the focal plane depth with
temperature are observed, consistent with expectations based on thermal
expansion of structural materials and the range of temperatures recorded.
Furthermore, when changing between gaseous and vacuum conditions we observe a
slow drift of the focal plane position on few-hour timescales, consistent with
the expected effect of swelling of plastics in the structure through gas
absorption and degassing [51]. These effects taken together imply that a
dynamical method of establishing and tracking the focal plane is required.

To find the focal plane, a single-molecule-sensitive autofocus mechanism has
been developed. The focal metric is the ratio of the pixel-wise kurtosis over
the mean pixel intensity, which is maximized at each $X$,$Y$ to establish the
focal plane location in stage travel coordinates. This metric favors images
with a small number of bright pixels among a field of dimmer ones, which is
the characteristic of an in-focus image. Occasionally a cosmic ray muon or
radioactive event passes through the EMCCD leading to one or a few extremely
bright pixels, presenting to the autofocus metric as an anomalously in-focus
image. We exclude such anomalous events by rejecting images with up to 5
bright pixels among a field of otherwise dim ones. This signature can also be
distinguished from that associated with an extremely bright fluorescence
emitter by its lack of persistence when stationary in the fluorescence plane.

Figure 7 illustrates the performance of the autofocus metric using single
BODIPY molecules. Far from the focal plane there are no discernible features,
either in the images or in the metric. A set of very dim features are visible
on the back of the slide. Since there is no fluorescent layer coated onto this
back surface, we interpret these features as being associated with small
amount of out-of-focus fluorescence light scattering from optical
imperfections on the reverse of the glass. Inside the cover-slip volume no
visible features are present. On the far side of the coverslip where we
approach the desired focal plane, a sharp spike in autofocus metric is
apparent. A higher resolution scan in this region (inset in Fig 7, left) shows
that the depth of focus is around 1 $\mu m$. Maximizing the autofocus metric
is reliably found to obtain focal plane to $\pm 0.3$ $\mu$m precision.

After focusing, the images of single point-like emitters exhibit consistent
point-spread function (PSF) in air, vacuum and pressurized gases. Fig. 8 shows
the $X$ and $Y$ projections of the measured point-spread function obtained
using single molecules within 7 bar of argon gas. These PSF projections are
obtained by averaging over the the $X$ and $Y$ directions around the brightest
20 fluorescent emitters, re-focusing in 5 distinct locations (Fig. 8, left).
The width of the PSF depends slightly upon how perfectly the focal plane has
been obtained, but in all cases is very similar to the expectation from Abbe’s
theory of diffracting optics [52]. The averaged PSFs in the $X$ and $Y$
directions are compared directly to the Abbe limit in Fig. 8, right. These
results suggests that the microscope essentially saturates the theoretical
limit of optical resolution, even when running within a pressurized noble gas
environment. We note that, due to its single molecule sensitivity, super-
resolution techniques can also be used in this system to advance beyond the
diffraction limit, though this is of limited value for the barium tagging
application.

Once a slide is installed, the standard procedure is to find the focal $Z$
position at 5–10 points on the face of the slide. These data are then used to
generate a 3D map of the focal plane by extrapolating a 2D surface through the
data points. This fitted focal plane can then be used to find the focus at any
other point for subsequent imaging. The focal plane map continues to
accumulate data points as in-focus images are found, which serves to
continuously refine its precision for extrapolation to new points. This
approach allows the focus to be found rapidly at new imaging locations, and
thus allows for construction of larger, rastered images by scanning over X-Y
positions.

###  III.4 Mechanical design and gas handling

The high pressure microscope system is built into a 16 inch long, 6 inch
diameter custom-manufactured pressure vessel with 8-inch ConFlat flanges on
both ends. The microscope front end-cap is fixed down to an air-levitated
optical table with a thick aluminum bracket. Vibration isolation was found to
be crucial to achieving optimal resolution, and careful positioning of the
various vacuum pumps and circulation pumps around the optical table proved to
be instrumental in achieving sharp images. The pressure vessel pipe and back
end cap slide on a set of two parallel rails to open and close the vessel,
leaving the microscope head fixed in place in order to maintain rough
alignment when opening and closing the large CF flanges.

Inside the vessel and cantilevered from the front end-cap, a machined aluminum
frame supports a small HDPE bracket that holds the internal microscope
objective in a fixed position. A 3-axis vacuum stage system 141414PI Q545.140
is used to maneuver and monitor a microscope slide in three dimensions in
front of the objective. The stage is rated for ultra-high vacuum and has
nanometer precision along all three axes, with an active feedback loop inside
the device. Our specifications demand only few hundred nanometer precision,
below which diffraction limits the point spread function, and the stage
comfortably meets these requirements. Careful frequency tuning of the feedback
mechanism within the stages had to be made in order to avoid exciting resonant
mechanical normal modes of the cantilevered system that inhibited stable
positioning. The full range of motion of the stages is $\pm 6.5$ mm in each
direction, though our control software restricts the $X$ and $Y$ (in-plane)
motion to a region of 10$\times$10 mm and the range of $Z$ (focusing) motion
to within 1.5 mm in front of the microscope slide, to avoid damage from
scratching the objective.

The system is evacuated using a turbo pumping station 151515Pfeiffer HiCube 80
Eco which can be decoupled using an isolation valve 161616Carten HB-51 when
pressurized. Despite the use of some plastics in the system, the vacuum
quality as monitored by a hot filament ion gauge 171717Kurt Lesker KJLC 354
routinely reaches $10^{-6}$ Torr prior to filling with gas, which is an
adequate vacuum quality for subsequent fill and operation of time projection
chamber detectors. Pressurized xenon gas is supplied to the system by a
specially constructed gas handling system. The majority of the gas handling
system is formed from 1/4 inch stainless steel piping with Swagelok fittings
mounted to a gas control panel. The same system was used to supply purified
xenon to the NEXT-CRAB0 detector in Ref. [37].

Noble gases are supplied from cylinders with 99.999% purity and circulated
through hot 181818SAES PS4-MT3-R-1 and cold 191919SAES MicroTorr HP190-902F
zeolite getters to clean at $\sim$3 slpm for several hours to remove oxygen,
water and nitrogen contamination. Pumping action is provided by a hermetic,
piston-driven gas pump with neoprene buffers 202020PumpWorks PW2070 with pump-
speed controlled by a variac on the power line. Experience with devices on the
same gas system show that this is sufficient to achieve part-per-billion
levels of oxygen and water impurity. Gas pressure is monitored by several
analog pressure gauges on the gas panel and over-pressurization is prevented
by a 400 psig relief on the panel, 250 psig relief on the vessel, and a 15
psig burst disk on the vacuum line. The temperature inside the vessel is
monitored by an internal thermocouple, and flow both into and out of the
vessel in standard liters per minute is monitored using mass flow meters
212121Omega FM1800.

After running with argon, the gas is typically vented to the room through a
vent line, whereas due to its much higher cost xenon is recaptured into
bottles by cryogenic condensation with liquid nitrogen. A few psi of xenon is
typically lost with each fill cycle due to incompleteness of this capture
process. A manifold-like gas mixing arrangement allows for use of multiple
gases or mixtures, if needed.