A linear RFQ ion trap for the Enriched Xenon
Observatory
B. Flatt a,∗, M. Green a, J. Wodin a, R. DeVoe a, P. Fierlinger a,
G. Gratta a, F. LePort a, M. Montero Dı́ez a, R. Neilson a,
K. O’Sullivan a, A. Pocar a, S. Waldman a,1, E. Baussan b,
M. Breidenbach c, R. Conley c, W. Fairbank Jr. d, J. Farine e
C. Hall c,2, K. Hall d, D. Hallman e, C. Hargrove f , M. Hauger b,
J. Hodgson c, F. Juget b, D.S. Leonard g, D. Mackay c,
Y. Martin b, B. Mong d, A. Odian c, L. Ounalli b, A. Piepke g,
C.Y. Prescott c, P.C. Rowson c, K. Skarpaas c, D. Schenker b,
D. Sinclair f , V. Strickland f , C. Virtue e, J.-L. Vuilleuimier b,
J.-M. Vuilleuimier b, K. Wamba c, P. Weber b
aPhysics Department, Stanford University, Stanford CA, USA
bInstitut de Physique, Université de Neuchatel, Neuchatel, Switzerland
cStanford Linear Accelerator Center, Menlo Park CA, USA
dPhysics Department, Colorado State University, Fort Collins CO, USA
ePhysics Department, Laurentian University, Sudbury ON, Canada
fPhysics Department, Carleton Univerisity, Ottawa ON, Canada
gDept. of Physics and Astronomy, University of Alabama, Tuscaloosa AL, USA
Abstract
The design, construction, and performance of a linear radio-frequency ion trap
(RFQ) intended for use in the Enriched Xenon Observatory (EXO) are described.
EXO aims to detect the neutrinoless double-beta decay of 136Xe to 136Ba. To sup-
press possible backgrounds EXO will complement the measurement of decay energy
and, to some extent, topology of candidate events in a Xe filled detector with the
identification of the daughter nucleus (136Ba). The ion trap described here is capa-
ble of accepting, cooling, and confining individual Ba ions extracted from the site of
the candidate double-beta decay event. A single trapped ion can then be identified,
with a large signal-to-noise ratio, via laser spectroscopy.
Key words: RFQ trap, EXO, fluorescence spectroscopy
PACS: 34.10.+x, 42.62.Fi, 14.60.Pq
Preprint submitted to Elsevier 30 October 2018
1 Introduction
In the last decade, compelling evidence for flavor mixing in the neutrino sec-
tor has clearly shown that neutrinos have finite masses [1]. These experiments
reveal mass differences between single mass eigenstates, but not their absolute
values. The measurement of such masses has become arguably the most im-
portant frontier in neutrino physics, with implications in astrophysics, particle
physics, and cosmology. β-decay endpoint spectroscopy measurements provide
an increasingly sensitive probe of neutrino mass [2]. However, a less direct but
potentially more sensitive technique is the observation and measurement of
the rate of neutrinoless double-beta (0νββ) decay [3]. The discovery of this
exotic nuclear decay mode would provide an absolute scale for neutrino masses
and establish the existence of two-component Majorana particles [4].
Sensitivity to Majorana neutrino masses in the interesting 10 - 100 meV re-
gion is achievable by experiments utilizing a ton-scale 0νββ isotope source [3].
This assumes that backgrounds from natural radioactivity, cosmic rays, and
the standard-model two-neutrino double-beta (2νββ) decay can be sufficiently
reduced and understood. Several proposals exist to perform this daunting
task [3]. The Enriched Xenon Observatory (EXO) is designed to identify the
atomic species (136Ba) produced in the decay process, using high resolution
atomic spectroscopy [5]. This isotope specific “Ba tagging,” working in con-
junction with more conventional measurements of decay energy and crude
event topology, will potentially provide a clean signature of 0νββ decay.
The EXO collaboration is currently pursuing a 0νββ detector R&D program,
focusing on a time projection chamber (TPC) filled with xenon enriched to
80% 136Xe in liquid (LXe) or gaseous (GXe) phase. Many of the detector
parameters and, in particular, the details of the Ba tagging technique would
be different in LXe and GXe. The ion trap described here is designed to accept,
trap, and cool individual Ba ions extracted from a 0νββ detector. While the
technique to efficiently transport ions from their production site is still under
investigation (and is beyond the scope of this article), the ion trap discussed
here is optimized to operate with a LXe detector and a mechanical system
to retrieve and inject the ions. This ion trap is capable of confining ions for
extended periods of time (∼ min) to a small volume (∼ (500 µm)3), essential
for observing single ions via laser spectroscopy with a high signal-to-noise
ratio. These properties are required to drastically suppress candidate decays
∗ Corresponding author. Address: Physics Department, Stanford University, 382
Via Pueblo Mall, Stanford, CA 94305, USA. Tel: +1-650-723-2946; fax: +1-650-
725-6544.
Email address: flatt@stanford.edu (B. Flatt).
1 Now at Caltech, Pasadena CA, USA
2 Now at University of Maryland, College Park MD, USA
that do not create Ba ions in the TPC, while maintaining a high detection
efficiency for Ba-ion-producing events. In addition, this trap can operate in
the presence of some Xe contamination, which is likely in any Ba tagging
system coupled to a Xe filled detector. The ion trap system described here is
designed to be appropriately flexible as an R&D device. Simplifications and
modifications of this system can be adopted for the actual trap to be used in
2 Linear RFQ traps and buffer gas cooling
RF Paul traps confine charged particles in a quadrupole RF field [7]. Spherical
traps have a closed geometry consisting of a ring and two endcap electrodes,
providing trapping in three spatial dimensions. Linear Paul traps generally
consist of four parallel cylindrical electrodes, placed symmetrically about a
central (longitudinal) axis, as shown in fig. 1. An RF field applied across
diagonally opposing electrodes provides transverse (x− y plane in fig. 1) con-
finement of the ion.
...UDC 0V
S1 S4 S5 S16S14S13 S15
-1.2 V -1.2 V
Ba oven
e--gun
-0.1 V 10V-0.2 V
Spectro-
scopy 
lasers
Trapped 
Fig. 1. Schematic of the linear RF trap. Ions are loaded in S3 and stored in S14. The
lower part of the figure shows the DC potential distribution.
Appropriately chosen DC voltages, applied to longitudinally (z axis in fig. 1)
segmented electrodes, provide longitudinal confinement. A single group of four
symmetrically placed electrodes is referred to as a “segment”. Electrodes are
constructed and positioned such that their radius, re, is related to the char-
acteristic radial trap size, r0, by
re = 1.148r0 (1)
where r0 is the distance from the axis of the trap to the innermost edge of an
electrode. This configuration creates the closest approximation to a hyperbolic
potential at the trap center for cylindrically shaped electrodes [6]. The ion’s
orbit in the transverse plane is described by the Mathieu equation [9]. Analysis
of the solutions to this equation reveal stability criteria for the ion’s motion in
the trap. The dimensionless Mathieu stability parameters, a and q, are defined
q = 2
mr20ω
a = 4
mr20ω
where where e and m are the ion’s charge and mass, ωRF is the angular RF
frequency, URF is the RF voltage (amplitude), and UDC is the DC voltage.
Values of a and q between 0 and 0.91, falling within a region defined by
the characteristic numbers of the Mathieu equation, correspond to stable ion
orbits [8].
Transverse confinement is attributed to a pseudopotential
VRF (r) =
r2. (4)
quadratically dependent on the radial distance, r, from the longitudinal axis of
the trap. The DC voltages applied to the longitudinally segmented electrodes
are chosen to create a trapping potential
VDC(r, z) =
where z and z0 are the longitudinal coordinate and length of the segment in
which the ion is trapped (the “trapping segment”). The radial dependence of
the longitudinal potential well arises from Laplace’s equation applied to the
interior region of the ion trap. This radial defocusing reduces the depth of the
transverse pseudopotential well, created by the RF field. The total potential
well used to trap ions is the sum of eqns. 4 and 5.
The open geometry of this type of trap allows for an unobstructed view of
the trapping region, with large optical angular acceptance. In addition, the
longitudinal electrode segmentation allows for multiple configurations of the
longitudinal potentials as required for the injection, trapping, and ejection of
a single ion.
In order to confine an energetic ion of mass m injected from outside the trap,
a mechanism of energy loss must be provided in order to dissipate the ion’s
kinetic energy to below both eVRF and eVDC . Collisions with a ”buffer” gas
of mass mB can provide such an energy-loss mechanism. The phenomenology
of ion-neutral interactions in an RF Paul trap can be divided into three cases.
If mB � m, the ion is cooled via a large number of ion-neutral collisions,
each exchanging a small amount of energy and momentum. In this case, the
cooling process is adiabatic compared to the period of the ion’s motion in
the RF field, and the pseudopotential formulation is valid during the cooling
process. If mB � m, each collision can add or remove substantial momentum
and energy from a trapped ion. This large instantaneous momentum transfer
can alter the ion’s trajectory appreciably, which may result in energy transfer
from the RF-field to the ion (”RF heating”). Under these conditions, the ion
is unstable in the trap, and is rapidly ejected. In the intermediate regime,
mB ≈ m, a form of RF heating also occurs and the amount of time that a
single ion is trapped depends on trap parameters.
3 Simulation of ion cooling and trapping
The DC and RF voltage amplitudes, the longitudinal dimensions of the indi-
vidual segments, and the buffer gas pressure and type are optimized using the
SIMION 7.0 simulation package 3 for single ion stability. Ion-neutral collisions
are implemented using a hard-sphere model with a variable radius, depending
on the buffer gas and trapped ion species. This model, applicable in the case
of a single atomic ion interacting with a noble buffer gas [12], uses a cross sec-
tion dependent on the ion’s velocity, v, and buffer gas polarizability to account
for the dipole moment of the neutral atom induced by the ion. Collisions are
implemented by specifying a mean free path λ, the buffer gas mass mB, and
a buffer gas temperature TB. The probability that a trapped ion collides with
a buffer gas atom in a time interval ∆t is given by
P (∆t) = 1− e−v∆t/λ (6)
Before each time-step of the ion’s trajectory, a random number is chosen.
This number is used to decide if a collision occurs during that time-step.
If a collision occurs, the kinematics of the collision are calculated assuming
that the velocity distribution of the neutral buffer gas atoms follows Maxwell-
Boltzmann statistics with a temperature TB.
The total longitudinal trap length, 604 mm, is chosen as a result of cooling
simulations of an ion at various initial kinetic energies, interacting with a
3 http://www.sisweb.com/simion.htm
range of buffer gases (He, Ar, Kr, and Xe). The trap is split into 16 segments,
to provide sufficient versatility in shaping the longitudinal field for different
phases of the R&D program. The segments are labeled Si, where i runs from
1 to 16 as shown in fig. 1. The segments are chosen to be 40 mm long, except
for a single short 4 mm segment (S14, the “trapping segment”), used to tightly
confine the ion longitudinally, optimizing the single ion fluorescence signal-to-
noise ratio.
The DC potential profile chosen for most operations is UDC = {0V, -0.1V, -
0.2V, ..., -1.2V, -8.0V, -1.2V, +10V}, with the minimum of -8 V at S14. Because
of a limitation in the number of input parameters of SIMION, this profile is
approximated as UDC = {+10V, +10V, +10V, -0.4V, -0.5V, ..., -1.3V, -1.2V,
-8.0V, -1.2V, +10V}, in the simulation. This does not appreciably affect the
ion cooling at the potential minimum.
The values of the trap radius, r0, and electrode diameter, re are chosen to
optimize the external optical access to a trapped ion, as well as the shape of
the RF field. The electrode radius is re = 3 mm, resulting in r0 = 2.61 mm
(see eqn. 1). The RF frequency is ωRF/2π = 1.2 MHz, with an amplitude of
150 V. These parameters correspond to q = 0.52 and a = 0.05 (see eqn. 3),
well within the region of stability of the Mathieu equation.
An example of the simulated cooling process is shown in fig. 2. In this simula-
tion, a single Ba ion is created in S3, with an energy of 10 eV in 1× 10−2 torr
He. The ion’s kinetic energy and z-trajectory are plotted during the initial
cooling (panels a and b), and after the ion is confined in the potential well at
S14 (panels c and d). During the initial cooling, the ion is reflected back and
forth longitudinally in the trap. On average, the ion loses energy with each col-
lision with a He atom. Once the ion is confined to S14, the ion continues to cool
to the minimum, until it comes into thermal equilibrium with the buffer gas.
The same processes are shown in fig. 3 in the case of Ar as a buffer gas. The
ion cools much faster in the presence of Ar; however, the frequency and ampli-
tude of RF heating collisions increase as well. Ar is therefore a more efficient
cooling gas for Ba ions, though the higher rate of RF heating likely decreases
the ion’s stability in the trap. Whereas SIMION is useful for studying these
cooling and heating processes, reliable trajectory simulation is limited to the
timescale of a few seconds. This is due to finite computational resources, as
well as error buildup during trajectory integration. For this reason, single ion
storage times longer than a few seconds, relevant for the study of RF heating
and ion deconfinement, cannot be simulated.
Fig. 2. Simulation of a single 136Ba+ in He (1 · 10−2torr) using SIMION. The ion
in the simulation was started in the center of segment S3 of the trap. Panel (a)
shows the collisional cooling during the first few hundred µs after the start of the
simulation. Panel (b) shows the respective trajectory. Panels (c) and (d) show the
evolution of the same quantities on a longer time scale. The ion is confined to the
trapping segment and it cools further down to the buffer gas temperature.
Fig. 3. The same simulation as in fig.2 of a single 136Ba+ in Ar at a pressure of
3.7 · 10−3torr. Faster cooling and larger momentum transfer collisions are evident.
4 Trap construction
A single trap segment is made of four stainless steel tubes threaded onto a
center stainless steel rod, as shown in an exploded view in fig. 4. Vespel 4
tube spacers insulate each segment from its neighbors, and from the center
4 Vespel is a trademark of DuPont de Nemours
rod. Special care is taken to insure that all vespel parts are recessed behind
conductors, in order to avoid any insulator charging that could affect the DC
field inside the trap. Details of the RF and DC feed circuitry for two segments
are shown in fig. 5. A DC voltage is applied to all four electrodes in a segment,
using a 16-bit computer controlled DAC. The RF is applied to one diagonal
pair of electrodes in a segment, while the other pair is RF grounded through
a capacitor. The RF signal is supplied by a function generator 5 , which is
amplified by a broadband 50 dB amplifier 6 , internally back-terminated with
50 Ω. The system can deliver the RF voltage required without the use of a
tuned circuit. Each segment has a capacitance of ∼ 18 pF, however the total
capacitance of the trap is closer to 600 pF due to contributions from cables
and vacuum feedthroughs.
Fig. 4. Exploded view of electrodes 13-16, showing the internal support and electrical
insulation.
The whole trap is housed in a custom-made, electropolished stainless steel
UHV tank pumped by a turbomolecular pump 7 backed by a dry scroll pump 8 (fig. 6).
A septum inside the vacuum tank allows for the installation of an aperture,
to be used in a differentially pumped scheme (not used for the work described
here), to maintain different buffer gas pressures in the injection and trapping
regions of the system. The pressure in the tank is read out in the upper (injec-
tion) and lower (trapping) regions of the vacuum system by vacuum gauges 9 .
A gas handling manifold, connected to the trap by a computer-controlled leak
5 HP 8656B
6 ENI A150
7 Pfeiffer TMU521P
8 BOC Edwards XDS5
9 Pfeiffer PKR251
valve 10 , allows for the introduction of individual buffer gas species and binary
mixtures (fig. 7). The leak valve keeps the buffer gas pressure in the vacuum
chamber constant by regulating gas flow into the chamber, based on the vac-
uum gauge measurement closest to S14. The turbo pump runs continuously, so
that the gas pressure can be either increased or decreased at any time. Using
this method, the gas pressure in the vacuum system can be regulated between
3× 10−9 and 1× 10−2 torr, with a stability of ≤ 1 %. The lower range is the
limit of the vacuum gauges, while the upper limit is the maximum allowable
pressure in front of the turbo-pump running at full speed. The upper pressure
limit can be extended if required, with simple modifications to the vacuum
system. Before any ion trapping operations begin, the entire vacuum system
is baked for two days at 135 ◦C in an oven that completely encloses the tank.
After bakeout, the system reaches a base pressure of < 3× 10−9 torr.
Vacuum system
A1 RF Monitor
1 nF 5 pF
100 nF 
100 nF 
1 MΩ 100 nF 
Fig. 5. Electrical schematics of the ion trap. Only two of 16 identical segments are
shown.
Fluorescence from a trapped Ba ion is induced, following the classic “shelving”
scheme [13], by resonant lasers cycling the ion between the 6S1/2 (ground),
6P1/2 (excited), and 5D3/2 (metastable) states. The ion undergoes sponta-
neous emission when in the 6P1/2 state, emitting either a 493 nm or 650 nm
photon. The 493 nm fluorescence photons are the signal collected for this ex-
periment. The 6S1/2 ↔ 6P1/2 transition at 493 nm is excited by a frequency-
10 Pfeiffer EVR116
Fig. 6. Cutaway view of the vacuum system with the linear trap mounted inside.
doubled external-cavity diode laser (ECDL) 11 . The 6P1/2 ↔ 5D3/2 transition
at 650 nm is excited by a ECDL 12 . 20 mW (10 mW) of 493 nm (650 nm) light
is available for spectroscopy, far in excess of that required to observe a single
ion. The blue laser is frequency stabilized to ∼ 20 MHz (relative) using an
11 TOPTICA SHG 100
12 TOPTICA DL 100
V6 V8
Ar He Xe
V3 V2 V1
Getter
   P1
(70 l/s turbo)
(400 l/s turbo)
Linear trap vacuum 
system
Pressure 
control
Pressure feedback loop
P3 (Scroll)
V12 V13
Fig. 7. Schematic view of the gas handling system.
Invar Fabry-Perot reference cavity. Long term absolute frequency stabilization
is achieved by locking both lasers to a hollow-cathode Ba lamp 13 . A schematic
of the laser setup is shown in fig. 8.
The laser systems reside on a vibration isolated, optics table in a dust con-
trolled environment, completely separated from the linear ion trap vacuum
system. Both lasers are fed into one single-mode fiber, which is routed over an
arbitrary distance to the ion trap system. The output beams are coupled into
the ion trap via injection optics, consisting of an aspheric focusing lens, an iris
to reduce beam halo, and a beam-steering mirror. The beams are directed into
the vacuum system through an anti-reflection (AR) coated window 14 along
the longitudinal axis of the trap, from the end of the trap closest to S14. An
additional aperture inside the vacuum system aids in beam alignment and
further halo reduction. Due to the chromaticity of the aspheric lens, the beam
waists are separated by 260 mm, which is on the order of the beams Rayleigh
length. The 493 nm and 650 nm waists at the trapping segment (S14) are
370 µm and 570 µ m, respectively. The laser powers at the injection region are
sampled in real-time by photodiodes. These powers are fed-back to acousto-
optic modulators before the fiber input on the laser table, and used to keep the
injected beam powers stable to ∼ 1 % indefinitely. This configuration is found
to suppress background laser light levels to the level required for observing a
single ion.
13 Perkin-Elmer Ba Lumina HCL model N305-0109
14 “Super-V” AR coating (99.98 % transmission at 493 nm) by OptoSigma
Chopper 
wheel
Ba hollow 
cathode lamp
Lock-in 
amplifier
Laser 
Controller
650 nm ECDL
493 nm ECDL (986 SHG)
Isolator
Isolator
Lock-in 
amplifier
Laser 
Controller
Chopper 
wheel
Ba hollow 
cathode lamp
Wavemeter
Scanning 
Fabry-Perot
AOM 3
EOM 1
Cavity 
controller
Optogalvanic lock power control
Pound-Drever-Hall lock
Optogalvanic lock
Optogalvanic lock
Optogalvanic lock power control
AOM 1
AOM 4
Fiber 
launch to 
Burleigh 
confocal 
cavity
Scope
Red power 
feedback
Blue 
power 
feedback
Fig. 8. Schematic view of the laser setup.
The fluorescence from a trapped ion is detected by an electron-multiplied
CCD camera (EMCCD), sensitive to single photons 15 . The fluorescence is
imaged onto the EMCCD by a 64 mm working distance microscope 16 , with
a numerical aperture of 0.195. The outer lens of the microscope objective is
placed close to S14, outside a re-entrant vacuum window (see fig. 6). A spherical
mirror inside the vacuum tank, directly behind S14, reflects fluorescence light
back through the trap that would otherwise be lost. This roughly doubles
the fluorescence light collection efficiency. A filter in front of the EMCCD
attenuates the 650 nm light by more than 99 % while transmitting ∼ 85% of
the 493 nm light. The light collection efficiency of the system is estimated to
be 10−2, including the 90 % quantum efficiency of the EMCCD.
15 Andor iXon EM+
16 Infinity InFocus KC with IF4 objective
5 Trap operation and results
After the initial pump-down and bakeout, the trap is loaded with ions by
ionizing neutral Ba in the central region of S3 (see fig. 1). Barium is chemically
produced, after the system has been pumped to good vacuum, by heating a
“barium dispenser” 17 loaded with BaAl4-Ni, and depositing Ba on a Ta foil.
The foil can be resistively heated repeatedly, producing a Ba vapor in S3,
which is ionized by a 500 eV electron beam from an electron gun 18 . A 32-
gauge thermocouple on the Ta foil is used to control the temperature of the
foil, regulating the amount of Ba emitted. Before loading ions into the trap,
a buffer gas is introduced into the system. To load small numbers of ions
(< 10), the oven is operated at 100 ◦C and the e-gun pulsed for 10 s. Ions
are cooled by the buffer gas, and trapped at S14 as discussed earlier. The trap
can also be loaded by turning on one of the cold cathode vacuum gauges.
This effect is explained by the possible emission of electrons and ions from the
gauge, which is presumably coated with Ba from the initial Ba source creation
process. This should not be considered a background for Ba tagging in EXO,
as the trap used in EXO will not be heavily contaminated with Ba, nor will
vacuum gauges be operated during the tagging process.
Fig. 9 shows a grayscale image of the fluorescence from three ions in the trap,
imaged by the EMCCD over 20 s. The DC potentials are set as shown in
fig. 1. The brightness of each pixel is proportional to the number of collected
photons. The cloud of white pixels (encompassed by the black square) is flu-
orescence from three ions trapped in S14. The cloud has two lobes, due to
a slight misalignment of the spherical mirror behind S14. The white vertical
bands on either side of the cloud are due to laser light scattering off the elec-
trodes, which is at the same wavelength as the fluorescence photons. Precise
alignment of the laser beams is required to minimize scattered laser light and
optimize the signal to noise in the region of interest, in order to observe the
fluorescence from a single ion in the trap.
A time-series of the Ba ion fluorescence signal is shown in fig. 10 [10], starting
with four ions loaded into the trap at 4.4 × 10−3 torr He. The x-axis is in
seconds, and the y-axis pepresents the fluorescence in arbitrary units of EM-
CCD counts. Each data point in this series is the sum of the pixels the the
square box in fig. 9, after 5 s of integration by the EMCCD. The y-axis is zero-
suppressed due to the large EMCCD pedestal. Over time, ions spontaneously
eject from the trap due to RF heating, and possibly ion-ion Coulomb interac-
tions. As individual ions eject, the fluorescence signal decreases in quantized
steps. The difference in the fluorescence signals for the single steps is constant
17 SAES: http://www.saesgetters.com/default.aspx?idpage=460
18 Kimball Physics FRA-2X1-2016
http://www.saesgetters.com/default.aspx?idpage=460
r (mm)
-1.5 -1 -0.5 0 0.5 1 1.5
Fig. 9. Greyscale picture of three ions contained in segment 14 of the trap. The
image was taken at 5 · 10−4 torr He. The signal in the region of interest (black box)
was integrated over 20 seconds with the EMCCD.
within 4%, clearly establishing the capability of the system in detecting single
ions.
A high signal-to-noise ratio of the fluorescence from a single trapped ion will
be required to confirm a 0νββ decay. The signal-to-noise ratio for this purpose
is defined as
S/N =
〈RI〉 − 〈RB〉√
where 〈RI〉 and 〈RS〉 are the average single-ion fluorescence and background
rates, σI and σB are the Gaussian widths of the single-ion fluorescence and
background rates, tI and tB are the total single-ion fluorescence and back-
ground rate observation times, and ∆t is the integration time of single mea-
surement (hence tI/∆t and tB/∆t are the number of measurements for the
signal and the background, respectively). This metric assumes that both the
single ion fluorescence and background rates follow Gaussian statistics. If the
Time [s]
0 200 400 600 800 1000 1200 1400 1600 1800
4 ions
3 ions
2 ions
1 ion
Background level
Fig. 10. Time series of the 493 nm ion fluorescence rate in the trap at 4.4·10−3 torr
He. Ions unload, causing clear quantized drops in the fluorescence rate. Each point
represents 5 s of integration with the EMCCD [10].
two integration times are equal (tI = tB = t), eqn. 7 becomes
S/N =
〈RI〉 − 〈RB〉√
σ2I + σ
t (8)
where the signal and background rates have been absorbed into the constant k,
in units of Hz−1/2. The signal-to-noise ratio of the fluorescence from an individ-
ual ion increases with the square root of the measurement time, or equivalently
number of measurements. For the single ion fluorescence and background rates
in fig. 10, k = 2.75 Hz−1/2, so that S/N = 18 for a 60s measurement time.
Similar values are found in the cases of Ar, and He/Xe mixtures as buffer
gases. Drifts in the laser beam position at S14 may lead to fluctuations in the
background rate, RB, that are not accounted for by the assumptions of Gaus-
sian statistics made here. Such drifts are caused primarily by temperature
variations in the lab, affecting the alignment of the trap injection optics. In
the setup used for the data presented here, no provision is made for the tem-
perature stabilization of such optics that drift by as much as ±2◦C on a daily
cycle. Even under these non-ideal conditions, the system is stable enough to
apply the S/N description of eqn. 7 for periods of minutes, much longer than
required for non-ambiguous single Ba ion identification. Temperature stabi-
lization of the injection optics capable of ±0.1◦C would be straightforward
to implement and would reduce non-Gaussian background fluctuations to a
negligible level.
The lifetimes of single Ba ions in the trap at different He pressures have been
measured and reported elsewhere [10]. In the same paper, the destabilizing
effects of Xe contaminations are also studied and modeled. It is found that
a sufficient partial pressure of He in the trap can counter the effects of Xe,
and provide lifetimes that are sufficient for single-ion detection with very high
significance.
6 Summary
A linear RFQ ion trap, designed and built within the R&D program towards
the EXO experiment, is described. The trap is capable of confining individual
Ba ions for observation by laser spectroscopy, in the presence of light buffer
gases and low Xe concentrations. Single trapped Ba ions are observed, with a
high signal-to-noise ratio. A similar trap will be used to identify single Ba ions
produced in the 0νββ decay of 136Xe in the EXO experiment. The successful
operation of this trap, as described here, is one of the cornerstones of a full
Ba tagging system for EXO, which will lead to a new method of background
suppression in low-background experiments. In parallel, several systems to
capture single Ba ions in LXe, and transfer them into the ion trap are under
development.
Acknowledgements
This work was supported, in part, by DoE grant FG03-90ER40569-A019 and
by private funding from Stanford University. We also gratefully acknowledge
a substantial equipment donation from the IBM corporation, as well as very
informative discussions with Guy Savard.
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(2000) 12.
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http://arxiv.org/abs/hep-ex/0109033
http://arxiv.org/abs/physics/0702122
	Introduction
	Linear RFQ traps and buffer gas cooling
	Simulation of ion cooling and trapping
	Trap construction
	Trap operation and results
	Summary
	References

  The design, construction, and performance of a linear radio-frequency ion
trap (RFQ) intended for use in the Enriched Xenon Observatory (EXO) are
described. EXO aims to detect the neutrinoless double-beta decay of $^{136}$Xe
to $^{136}$Ba. To suppress possible backgrounds EXO will complement the
measurement of decay energy and, to some extent, topology of candidate events
in a Xe filled detector with the identification of the daughter nucleus
($^{136}$Ba). The ion trap described here is capable of accepting, cooling, and
confining individual Ba ions extracted from the site of the candidate
double-beta decay event. A single trapped ion can then be identified, with a
large signal-to-noise ratio, via laser spectroscopy.

Introduction
In the last decade, compelling evidence for flavor mixing in the neutrino sec-
tor has clearly shown that neutrinos have finite masses [1]. These experiments
reveal mass differences between single mass eigenstates, but not their absolute
values. The measurement of such masses has become arguably the most im-
portant frontier in neutrino physics, with implications in astrophysics, particle
physics, and cosmology. β-decay endpoint spectroscopy measurements provide
an increasingly sensitive probe of neutrino mass [2]. However, a less direct but
potentially more sensitive technique is the observation and measurement of
the rate of neutrinoless double-beta (0νββ) decay [3]. The discovery of this
exotic nuclear decay mode would provide an absolute scale for neutrino masses
and establish the existence of two-component Majorana particles [4].
Sensitivity to Majorana neutrino masses in the interesting 10 - 100 meV re-
gion is achievable by experiments utilizing a ton-scale 0νββ isotope source [3].
This assumes that backgrounds from natural radioactivity, cosmic rays, and
the standard-model two-neutrino double-beta (2νββ) decay can be sufficiently
reduced and understood. Several proposals exist to perform this daunting
task [3]. The Enriched Xenon Observatory (EXO) is designed to identify the
atomic species (136Ba) produced in the decay process, using high resolution
atomic spectroscopy [5]. This isotope specific “Ba tagging,” working in con-
junction with more conventional measurements of decay energy and crude
event topology, will potentially provide a clean signature of 0νββ decay.
The EXO collaboration is currently pursuing a 0νββ detector R&D program,
focusing on a time projection chamber (TPC) filled with xenon enriched to
80% 136Xe in liquid (LXe) or gaseous (GXe) phase. Many of the detector
parameters and, in particular, the details of the Ba tagging technique would
be different in LXe and GXe. The ion trap described here is designed to accept,
trap, and cool individual Ba ions extracted from a 0νββ detector. While the
technique to efficiently transport ions from their production site is still under
investigation (and is beyond the scope of this article), the ion trap discussed
here is optimized to operate with a LXe detector and a mechanical system
to retrieve and inject the ions. This ion trap is capable of confining ions for
extended periods of time (∼ min) to a small volume (∼ (500 µm)3), essential
for observing single ions via laser spectroscopy with a high signal-to-noise
ratio. These properties are required to drastically suppress candidate decays
∗ Corresponding author. Address: Physics Department, Stanford University, 382
Via Pueblo Mall, Stanford, CA 94305, USA. Tel: +1-650-723-2946; fax: +1-650-
725-6544.
Email address: flatt@stanford.edu (B. Flatt).
1 Now at Caltech, Pasadena CA, USA
2 Now at University of Maryland, College Park MD, USA
that do not create Ba ions in the TPC, while maintaining a high detection
efficiency for Ba-ion-producing events. In addition, this trap can operate in
the presence of some Xe contamination, which is likely in any Ba tagging
system coupled to a Xe filled detector. The ion trap system described here is
designed to be appropriately flexible as an R&D device. Simplifications and
modifications of this system can be adopted for the actual trap to be used in
2 Linear RFQ traps and buffer gas cooling
RF Paul traps confine charged particles in a quadrupole RF field [7]. Spherical
traps have a closed geometry consisting of a ring and two endcap electrodes,
providing trapping in three spatial dimensions. Linear Paul traps generally
consist of four parallel cylindrical electrodes, placed symmetrically about a
central (longitudinal) axis, as shown in fig. 1. An RF field applied across
diagonally opposing electrodes provides transverse (x− y plane in fig. 1) con-
finement of the ion.
...UDC 0V
S1 S4 S5 S16S14S13 S15
-1.2 V -1.2 V
Ba oven
e--gun
-0.1 V 10V-0.2 V
Spectro-
scopy 
lasers
Trapped 
Fig. 1. Schematic of the linear RF trap. Ions are loaded in S3 and stored in S14. The
lower part of the figure shows the DC potential distribution.
Appropriately chosen DC voltages, applied to longitudinally (z axis in fig. 1)
segmented electrodes, provide longitudinal confinement. A single group of four
symmetrically placed electrodes is referred to as a “segment”. Electrodes are
constructed and positioned such that their radius, re, is related to the char-
acteristic radial trap size, r0, by
re = 1.148r0 (1)
where r0 is the distance from the axis of the trap to the innermost edge of an
electrode. This configuration creates the closest approximation to a hyperbolic
potential at the trap center for cylindrically shaped electrodes [6]. The ion’s
orbit in the transverse plane is described by the Mathieu equation [9]. Analysis
of the solutions to this equation reveal stability criteria for the ion’s motion in
the trap. The dimensionless Mathieu stability parameters, a and q, are defined
q = 2
mr20ω
a = 4
mr20ω
where where e and m are the ion’s charge and mass, ωRF is the angular RF
frequency, URF is the RF voltage (amplitude), and UDC is the DC voltage.
Values of a and q between 0 and 0.91, falling within a region defined by
the characteristic numbers of the Mathieu equation, correspond to stable ion
orbits [8].
Transverse confinement is attributed to a pseudopotential
VRF (r) =
r2. (4)
quadratically dependent on the radial distance, r, from the longitudinal axis of
the trap. The DC voltages applied to the longitudinally segmented electrodes
are chosen to create a trapping potential
VDC(r, z) =
where z and z0 are the longitudinal coordinate and length of the segment in
which the ion is trapped (the “trapping segment”). The radial dependence of
the longitudinal potential well arises from Laplace’s equation applied to the
interior region of the ion trap. This radial defocusing reduces the depth of the
transverse pseudopotential well, created by the RF field. The total potential
well used to trap ions is the sum of eqns. 4 and 5.
The open geometry of this type of trap allows for an unobstructed view of
the trapping region, with large optical angular acceptance. In addition, the
longitudinal electrode segmentation allows for multiple configurations of the
longitudinal potentials as required for the injection, trapping, and ejection of
a single ion.
In order to confine an energetic ion of mass m injected from outside the trap,
a mechanism of energy loss must be provided in order to dissipate the ion’s
kinetic energy to below both eVRF and eVDC . Collisions with a ”buffer” gas
of mass mB can provide such an energy-loss mechanism. The phenomenology
of ion-neutral interactions in an RF Paul trap can be divided into three cases.
If mB � m, the ion is cooled via a large number of ion-neutral collisions,
each exchanging a small amount of energy and momentum. In this case, the
cooling process is adiabatic compared to the period of the ion’s motion in
the RF field, and the pseudopotential formulation is valid during the cooling
process. If mB � m, each collision can add or remove substantial momentum
and energy from a trapped ion. This large instantaneous momentum transfer
can alter the ion’s trajectory appreciably, which may result in energy transfer
from the RF-field to the ion (”RF heating”). Under these conditions, the ion
is unstable in the trap, and is rapidly ejected. In the intermediate regime,
mB ≈ m, a form of RF heating also occurs and the amount of time that a
single ion is trapped depends on trap parameters.
3 Simulation of ion cooling and trapping
The DC and RF voltage amplitudes, the longitudinal dimensions of the indi-
vidual segments, and the buffer gas pressure and type are optimized using the
SIMION 7.0 simulation package 3 for single ion stability. Ion-neutral collisions
are implemented using a hard-sphere model with a variable radius, depending
on the buffer gas and trapped ion species. This model, applicable in the case
of a single atomic ion interacting with a noble buffer gas [12], uses a cross sec-
tion dependent on the ion’s velocity, v, and buffer gas polarizability to account
for the dipole moment of the neutral atom induced by the ion. Collisions are
implemented by specifying a mean free path λ, the buffer gas mass mB, and
a buffer gas temperature TB. The probability that a trapped ion collides with
a buffer gas atom in a time interval ∆t is given by
P (∆t) = 1− e−v∆t/λ (6)
Before each time-step of the ion’s trajectory, a random number is chosen.
This number is used to decide if a collision occurs during that time-step.
If a collision occurs, the kinematics of the collision are calculated assuming
that the velocity distribution of the neutral buffer gas atoms follows Maxwell-
Boltzmann statistics with a temperature TB.
The total longitudinal trap length, 604 mm, is chosen as a result of cooling
simulations of an ion at various initial kinetic energies, interacting with a
3 http://www.sisweb.com/simion.htm
range of buffer gases (He, Ar, Kr, and Xe). The trap is split into 16 segments,
to provide sufficient versatility in shaping the longitudinal field for different
phases of the R&D program. The segments are labeled Si, where i runs from
1 to 16 as shown in fig. 1. The segments are chosen to be 40 mm long, except
for a single short 4 mm segment (S14, the “trapping segment”), used to tightly
confine the ion longitudinally, optimizing the single ion fluorescence signal-to-
noise ratio.
The DC potential profile chosen for most operations is UDC = {0V, -0.1V, -
0.2V, ..., -1.2V, -8.0V, -1.2V, +10V}, with the minimum of -8 V at S14. Because
of a limitation in the number of input parameters of SIMION, this profile is
approximated as UDC = {+10V, +10V, +10V, -0.4V, -0.5V, ..., -1.3V, -1.2V,
-8.0V, -1.2V, +10V}, in the simulation. This does not appreciably affect the
ion cooling at the potential minimum.
The values of the trap radius, r0, and electrode diameter, re are chosen to
optimize the external optical access to a trapped ion, as well as the shape of
the RF field. The electrode radius is re = 3 mm, resulting in r0 = 2.61 mm
(see eqn. 1). The RF frequency is ωRF/2π = 1.2 MHz, with an amplitude of
150 V. These parameters correspond to q = 0.52 and a = 0.05 (see eqn. 3),
well within the region of stability of the Mathieu equation.
An example of the simulated cooling process is shown in fig. 2. In this simula-
tion, a single Ba ion is created in S3, with an energy of 10 eV in 1× 10−2 torr
He. The ion’s kinetic energy and z-trajectory are plotted during the initial
cooling (panels a and b), and after the ion is confined in the potential well at
S14 (panels c and d). During the initial cooling, the ion is reflected back and
forth longitudinally in the trap. On average, the ion loses energy with each col-
lision with a He atom. Once the ion is confined to S14, the ion continues to cool
to the minimum, until it comes into thermal equilibrium with the buffer gas.
The same processes are shown in fig. 3 in the case of Ar as a buffer gas. The
ion cools much faster in the presence of Ar; however, the frequency and ampli-
tude of RF heating collisions increase as well. Ar is therefore a more efficient
cooling gas for Ba ions, though the higher rate of RF heating likely decreases
the ion’s stability in the trap. Whereas SIMION is useful for studying these
cooling and heating processes, reliable trajectory simulation is limited to the
timescale of a few seconds. This is due to finite computational resources, as
well as error buildup during trajectory integration. For this reason, single ion
storage times longer than a few seconds, relevant for the study of RF heating
and ion deconfinement, cannot be simulated.
Fig. 2. Simulation of a single 136Ba+ in He (1 · 10−2torr) using SIMION. The ion
in the simulation was started in the center of segment S3 of the trap. Panel (a)
shows the collisional cooling during the first few hundred µs after the start of the
simulation. Panel (b) shows the respective trajectory. Panels (c) and (d) show the
evolution of the same quantities on a longer time scale. The ion is confined to the
trapping segment and it cools further down to the buffer gas temperature.
Fig. 3. The same simulation as in fig.2 of a single 136Ba+ in Ar at a pressure of
3.7 · 10−3torr. Faster cooling and larger momentum transfer collisions are evident.
4 Trap construction
A single trap segment is made of four stainless steel tubes threaded onto a
center stainless steel rod, as shown in an exploded view in fig. 4. Vespel 4
tube spacers insulate each segment from its neighbors, and from the center
4 Vespel is a trademark of DuPont de Nemours
rod. Special care is taken to insure that all vespel parts are recessed behind
conductors, in order to avoid any insulator charging that could affect the DC
field inside the trap. Details of the RF and DC feed circuitry for two segments
are shown in fig. 5. A DC voltage is applied to all four electrodes in a segment,
using a 16-bit computer controlled DAC. The RF is applied to one diagonal
pair of electrodes in a segment, while the other pair is RF grounded through
a capacitor. The RF signal is supplied by a function generator 5 , which is
amplified by a broadband 50 dB amplifier 6 , internally back-terminated with
50 Ω. The system can deliver the RF voltage required without the use of a
tuned circuit. Each segment has a capacitance of ∼ 18 pF, however the total
capacitance of the trap is closer to 600 pF due to contributions from cables
and vacuum feedthroughs.
Fig. 4. Exploded view of electrodes 13-16, showing the internal support and electrical
insulation.
The whole trap is housed in a custom-made, electropolished stainless steel
UHV tank pumped by a turbomolecular pump 7 backed by a dry scroll pump 8 (fig. 6).
A septum inside the vacuum tank allows for the installation of an aperture,
to be used in a differentially pumped scheme (not used for the work described
here), to maintain different buffer gas pressures in the injection and trapping
regions of the system. The pressure in the tank is read out in the upper (injec-
tion) and lower (trapping) regions of the vacuum system by vacuum gauges 9 .
A gas handling manifold, connected to the trap by a computer-controlled leak
5 HP 8656B
6 ENI A150
7 Pfeiffer TMU521P
8 BOC Edwards XDS5
9 Pfeiffer PKR251
valve 10 , allows for the introduction of individual buffer gas species and binary
mixtures (fig. 7). The leak valve keeps the buffer gas pressure in the vacuum
chamber constant by regulating gas flow into the chamber, based on the vac-
uum gauge measurement closest to S14. The turbo pump runs continuously, so
that the gas pressure can be either increased or decreased at any time. Using
this method, the gas pressure in the vacuum system can be regulated between
3× 10−9 and 1× 10−2 torr, with a stability of ≤ 1 %. The lower range is the
limit of the vacuum gauges, while the upper limit is the maximum allowable
pressure in front of the turbo-pump running at full speed. The upper pressure
limit can be extended if required, with simple modifications to the vacuum
system. Before any ion trapping operations begin, the entire vacuum system
is baked for two days at 135 ◦C in an oven that completely encloses the tank.
After bakeout, the system reaches a base pressure of < 3× 10−9 torr.
Vacuum system
A1 RF Monitor
1 nF 5 pF
100 nF 
100 nF 
1 MΩ 100 nF 
Fig. 5. Electrical schematics of the ion trap. Only two of 16 identical segments are
shown.
Fluorescence from a trapped Ba ion is induced, following the classic “shelving”
scheme [13], by resonant lasers cycling the ion between the 6S1/2 (ground),
6P1/2 (excited), and 5D3/2 (metastable) states. The ion undergoes sponta-
neous emission when in the 6P1/2 state, emitting either a 493 nm or 650 nm
photon. The 493 nm fluorescence photons are the signal collected for this ex-
periment. The 6S1/2 ↔ 6P1/2 transition at 493 nm is excited by a frequency-
10 Pfeiffer EVR116
Fig. 6. Cutaway view of the vacuum system with the linear trap mounted inside.
doubled external-cavity diode laser (ECDL) 11 . The 6P1/2 ↔ 5D3/2 transition
at 650 nm is excited by a ECDL 12 . 20 mW (10 mW) of 493 nm (650 nm) light
is available for spectroscopy, far in excess of that required to observe a single
ion. The blue laser is frequency stabilized to ∼ 20 MHz (relative) using an
11 TOPTICA SHG 100
12 TOPTICA DL 100
V6 V8
Ar He Xe
V3 V2 V1
Getter
   P1
(70 l/s turbo)
(400 l/s turbo)
Linear trap vacuum 
system
Pressure 
control
Pressure feedback loop
P3 (Scroll)
V12 V13
Fig. 7. Schematic view of the gas handling system.
Invar Fabry-Perot reference cavity. Long term absolute frequency stabilization
is achieved by locking both lasers to a hollow-cathode Ba lamp 13 . A schematic
of the laser setup is shown in fig. 8.
The laser systems reside on a vibration isolated, optics table in a dust con-
trolled environment, completely separated from the linear ion trap vacuum
system. Both lasers are fed into one single-mode fiber, which is routed over an
arbitrary distance to the ion trap system. The output beams are coupled into
the ion trap via injection optics, consisting of an aspheric focusing lens, an iris
to reduce beam halo, and a beam-steering mirror. The beams are directed into
the vacuum system through an anti-reflection (AR) coated window 14 along
the longitudinal axis of the trap, from the end of the trap closest to S14. An
additional aperture inside the vacuum system aids in beam alignment and
further halo reduction. Due to the chromaticity of the aspheric lens, the beam
waists are separated by 260 mm, which is on the order of the beams Rayleigh
length. The 493 nm and 650 nm waists at the trapping segment (S14) are
370 µm and 570 µ m, respectively. The laser powers at the injection region are
sampled in real-time by photodiodes. These powers are fed-back to acousto-
optic modulators before the fiber input on the laser table, and used to keep the
injected beam powers stable to ∼ 1 % indefinitely. This configuration is found
to suppress background laser light levels to the level required for observing a
single ion.
13 Perkin-Elmer Ba Lumina HCL model N305-0109
14 “Super-V” AR coating (99.98 % transmission at 493 nm) by OptoSigma
Chopper 
wheel
Ba hollow 
cathode lamp
Lock-in 
amplifier
Laser 
Controller
650 nm ECDL
493 nm ECDL (986 SHG)
Isolator
Isolator
Lock-in 
amplifier
Laser 
Controller
Chopper 
wheel
Ba hollow 
cathode lamp
Wavemeter
Scanning 
Fabry-Perot
AOM 3
EOM 1
Cavity 
controller
Optogalvanic lock power control
Pound-Drever-Hall lock
Optogalvanic lock
Optogalvanic lock
Optogalvanic lock power control
AOM 1
AOM 4
Fiber 
launch to 
Burleigh 
confocal 
cavity
Scope
Red power 
feedback
Blue 
power 
feedback
Fig. 8. Schematic view of the laser setup.
The fluorescence from a trapped ion is detected by an electron-multiplied
CCD camera (EMCCD), sensitive to single photons 15 . The fluorescence is
imaged onto the EMCCD by a 64 mm working distance microscope 16 , with
a numerical aperture of 0.195. The outer lens of the microscope objective is
placed close to S14, outside a re-entrant vacuum window (see fig. 6). A spherical
mirror inside the vacuum tank, directly behind S14, reflects fluorescence light
back through the trap that would otherwise be lost. This roughly doubles
the fluorescence light collection efficiency. A filter in front of the EMCCD
attenuates the 650 nm light by more than 99 % while transmitting ∼ 85% of
the 493 nm light. The light collection efficiency of the system is estimated to
be 10−2, including the 90 % quantum efficiency of the EMCCD.
15 Andor iXon EM+
16 Infinity InFocus KC with IF4 objective
5 Trap operation and results
After the initial pump-down and bakeout, the trap is loaded with ions by
ionizing neutral Ba in the central region of S3 (see fig. 1). Barium is chemically
produced, after the system has been pumped to good vacuum, by heating a
“barium dispenser” 17 loaded with BaAl4-Ni, and depositing Ba on a Ta foil.
The foil can be resistively heated repeatedly, producing a Ba vapor in S3,
which is ionized by a 500 eV electron beam from an electron gun 18 . A 32-
gauge thermocouple on the Ta foil is used to control the temperature of the
foil, regulating the amount of Ba emitted. Before loading ions into the trap,
a buffer gas is introduced into the system. To load small numbers of ions
(< 10), the oven is operated at 100 ◦C and the e-gun pulsed for 10 s. Ions
are cooled by the buffer gas, and trapped at S14 as discussed earlier. The trap
can also be loaded by turning on one of the cold cathode vacuum gauges.
This effect is explained by the possible emission of electrons and ions from the
gauge, which is presumably coated with Ba from the initial Ba source creation
process. This should not be considered a background for Ba tagging in EXO,
as the trap used in EXO will not be heavily contaminated with Ba, nor will
vacuum gauges be operated during the tagging process.
Fig. 9 shows a grayscale image of the fluorescence from three ions in the trap,
imaged by the EMCCD over 20 s. The DC potentials are set as shown in
fig. 1. The brightness of each pixel is proportional to the number of collected
photons. The cloud of white pixels (encompassed by the black square) is flu-
orescence from three ions trapped in S14. The cloud has two lobes, due to
a slight misalignment of the spherical mirror behind S14. The white vertical
bands on either side of the cloud are due to laser light scattering off the elec-
trodes, which is at the same wavelength as the fluorescence photons. Precise
alignment of the laser beams is required to minimize scattered laser light and
optimize the signal to noise in the region of interest, in order to observe the
fluorescence from a single ion in the trap.
A time-series of the Ba ion fluorescence signal is shown in fig. 10 [10], starting
with four ions loaded into the trap at 4.4 × 10−3 torr He. The x-axis is in
seconds, and the y-axis pepresents the fluorescence in arbitrary units of EM-
CCD counts. Each data point in this series is the sum of the pixels the the
square box in fig. 9, after 5 s of integration by the EMCCD. The y-axis is zero-
suppressed due to the large EMCCD pedestal. Over time, ions spontaneously
eject from the trap due to RF heating, and possibly ion-ion Coulomb interac-
tions. As individual ions eject, the fluorescence signal decreases in quantized
steps. The difference in the fluorescence signals for the single steps is constant
17 SAES: http://www.saesgetters.com/default.aspx?idpage=460
18 Kimball Physics FRA-2X1-2016
http://www.saesgetters.com/default.aspx?idpage=460
r (mm)
-1.5 -1 -0.5 0 0.5 1 1.5
Fig. 9. Greyscale picture of three ions contained in segment 14 of the trap. The
image was taken at 5 · 10−4 torr He. The signal in the region of interest (black box)
was integrated over 20 seconds with the EMCCD.
within 4%, clearly establishing the capability of the system in detecting single
ions.
A high signal-to-noise ratio of the fluorescence from a single trapped ion will
be required to confirm a 0νββ decay. The signal-to-noise ratio for this purpose
is defined as
S/N =
〈RI〉 − 〈RB〉√
where 〈RI〉 and 〈RS〉 are the average single-ion fluorescence and background
rates, σI and σB are the Gaussian widths of the single-ion fluorescence and
background rates, tI and tB are the total single-ion fluorescence and back-
ground rate observation times, and ∆t is the integration time of single mea-
surement (hence tI/∆t and tB/∆t are the number of measurements for the
signal and the background, respectively). This metric assumes that both the
single ion fluorescence and background rates follow Gaussian statistics. If the
Time [s]
0 200 400 600 800 1000 1200 1400 1600 1800
4 ions
3 ions
2 ions
1 ion
Background level
Fig. 10. Time series of the 493 nm ion fluorescence rate in the trap at 4.4·10−3 torr
He. Ions unload, causing clear quantized drops in the fluorescence rate. Each point
represents 5 s of integration with the EMCCD [10].
two integration times are equal (tI = tB = t), eqn. 7 becomes
S/N =
〈RI〉 − 〈RB〉√
σ2I + σ
t (8)
where the signal and background rates have been absorbed into the constant k,
in units of Hz−1/2. The signal-to-noise ratio of the fluorescence from an individ-
ual ion increases with the square root of the measurement time, or equivalently
number of measurements. For the single ion fluorescence and background rates
in fig. 10, k = 2.75 Hz−1/2, so that S/N = 18 for a 60s measurement time.
Similar values are found in the cases of Ar, and He/Xe mixtures as buffer
gases. Drifts in the laser beam position at S14 may lead to fluctuations in the
background rate, RB, that are not accounted for by the assumptions of Gaus-
sian statistics made here. Such drifts are caused primarily by temperature
variations in the lab, affecting the alignment of the trap injection optics. In
the setup used for the data presented here, no provision is made for the tem-
perature stabilization of such optics that drift by as much as ±2◦C on a daily
cycle. Even under these non-ideal conditions, the system is stable enough to
apply the S/N description of eqn. 7 for periods of minutes, much longer than
required for non-ambiguous single Ba ion identification. Temperature stabi-
lization of the injection optics capable of ±0.1◦C would be straightforward
to implement and would reduce non-Gaussian background fluctuations to a
negligible level.
The lifetimes of single Ba ions in the trap at different He pressures have been
measured and reported elsewhere [10]. In the same paper, the destabilizing
effects of Xe contaminations are also studied and modeled. It is found that
a sufficient partial pressure of He in the trap can counter the effects of Xe,
and provide lifetimes that are sufficient for single-ion detection with very high
significance.
6 Summary
A linear RFQ ion trap, designed and built within the R&D program towards
the EXO experiment, is described. The trap is capable of confining individual
Ba ions for observation by laser spectroscopy, in the presence of light buffer
gases and low Xe concentrations. Single trapped Ba ions are observed, with a
high signal-to-noise ratio. A similar trap will be used to identify single Ba ions
produced in the 0νββ decay of 136Xe in the EXO experiment. The successful
operation of this trap, as described here, is one of the cornerstones of a full
Ba tagging system for EXO, which will lead to a new method of background
suppression in low-background experiments. In parallel, several systems to
capture single Ba ions in LXe, and transfer them into the ion trap are under
development.
Acknowledgements
This work was supported, in part, by DoE grant FG03-90ER40569-A019 and
by private funding from Stanford University. We also gratefully acknowledge
a substantial equipment donation from the IBM corporation, as well as very
informative discussions with Guy Savard.
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http://arxiv.org/abs/hep-ex/0109033
http://arxiv.org/abs/physics/0702122
	Introduction
	Linear RFQ traps and buffer gas cooling
	Simulation of ion cooling and trapping
	Trap construction
	Trap operation and results
	Summary
	References
