Transcript Barium Ion Tagging : Ion Acquisition in LXe &

Barium Ion Tagging :
Ion Acquisition in LXe &
Laser Fluorescence Identification
As has been outlined in the plenary talk, the tonne-scale
EXO experiment can reach the 10 meV mass scale
exploiting the dramatic background reduction provided by
coincidence tagging of the barium daughter
of xenon double beta decay.
The ongoing R&D program will be summarized here.
P.C. Rowson
SLAC
Background reduction by
coincidence measurement
It was recognized early on that coincident detection
of the two decay electrons and the daughter decay species
can dramatically reduce bkgrd.
One possibility would be the
X  (Y++)* + e– + e–
Observation of a  from an excited
daughter ion, but the rates compared
Y++ + 
to ground state decays are generally very small
(best chance might be 150Nd, but E is only 30 keV.)
A more promising approach :
Barium detection from 136Xe decay
136Xe
 136Ba++ + e– + e–
Identify event-by-event
Described in 1991 by M. Moe (PRC, 44, R931,(1991)).
The method exploits the well-studied spectroscopy of Ba
and the demonstrated sensitivity to a single Ba+ ion in an
ion trap.
Barium Tagging R&D
Our decision to proceed with a LXe TPC (as opposed to gXe)
led us to investigate ion retrieval, or “ion to laser”, schemes
for barium tagging.
So far, this work has proceeded in parallel
mainly at Stanford and SLAC :
• Laser ion trap program - trap design & operation
• Ion capture program - electrostatic probe designs
• Interface program* - ion-to-trap transfer
* (recently begun)
In addition, at CSU, W. Fairbank, is investigating :
• “in situ” tagging - laser tagging in LXe
Comment on Barium backgrounds
Barium atoms hypothetically present in the xenon would not
normally constitute a background, as we only collect barium ions.
Barium ions from 2 decay are produced in the xenon at a rate
not yet determined, but limited to ~300,000/tonne-year, or roughly
1 per 100 seconds per tonne. These are continually swept out of
the liquid by the TPC E-field in < 30 seconds for our nominal
~3 kV/cm field strength.
(The ion mobility is known - more on this later).
... some preliminary studies …
Correlated sources of barium ions have been investigated
and appear to be negligible. Rates are low and in addition,
event topologies should be distinctive.
Detailed MC simulation has
not yet been deemed high priority but will be done.
Xe136(p,n)Cs136 : Cs136 production by cosmics (Cs→Ba via  decay)
Xe136(,)Cs136 : Cs136 production by solar neutrinos
Xe136(n,γ)Xe137 : Xe137 production by cosmics (Xe→Cs→Ba via )
Liquid Xenon TPC conceptual design
Compact and scalable
(3 m3 for 10 tons).
The basic concept, shown here for a LXe option, is :
• Use ionization and scintillation light in the TPC to determine
the event location, and to do precise calorimetry.
• Extract the Barium ion from the event location (electrostatic
probe eg.)
• Deliver the Barium to a laser system for Ba136 identification.

Ion capture in LXe TPC
Basic electrostatic capture procedure
Probe motion (3 d.o.f.) triggered by
event E threshold.
electrostatic probe
The probe moves above the TPC, and then
vertically down to the event location.
The Ba+ is collected electrostatically
(doesn’t move far from the event location),
and the probe is withdrawn.
LXe level
175 nm scintillation
cathode
eTPC charge & UV detection
Issues to be addressed (R&D progress where indicated) :
 Ba+ lifetimes in LXe (expected to be long - data exists)
 Ba ion drift velocities (should be a few mm/sec - confirmed)
 Ba capture and release – various probe designs
Ba transport to the laser spectroscopy station
Laser fluorescence barium identification
A well-studied technique
pioneered by atomic
physicists in the 1980’s
for the detection of single
atoms and ions, in particular,
alkali and alkaline-earth
metals.
Ba++ lines in the UV – convert ion to Ba+ or Ba.
“Intermodulation”
“Shelving” into metastable D state allows for modulation of 650nm light
to induce modulated 493nm emission out of synch. with excitation
(493nm) light – improves S/N
Laser Spectroscopy Lab at Stanford
red laser
reference cavities
blue laser
Stable and reliable laser system
RF applied
Ba oven
Ion trap : hyperbolic Paul type
laser, Ba ionizer
and detection
line-of-sight
through these
gaps
Glare from
electrodes
Single Ba+
signal
The trap is loaded with multiple ions:
We observe the signal intensity as ions
are dropped one by one…
The effects of buffer gas on trap performance
The operating environment of the EXO ion trap will likely
include some level of background xenon gas, and the effects
of this “buffer gas” have been studied.
It has been found that the addition of helium can
improve trapping times (which are essentially indefinite
for UHV conditions for modest xenon pressures.
Differential pumping can/will
be used to maintain a low ion trap buffer gas pressure.
Concept for Ba+ tagging in the Liquid
in a LXe Double Beta Decay Experiment :
“laser-to-ion” schemes.
Laser
ßß Decay
then Ba++
Ba+
Fluorescence
CCD/APD
Filters
Slit
Focus
Ba+ cloud image in liquid xenon
At CSU, fluorescence
data in LXe has taken, and
studies are continuing. The
issues here are :
Liquid
surface
8 mm
• Line broadening/loss of
specificity.
Grid
• S/N improvement for in situ
ion detection.
CCD camera image of
Ba+ fluorescence in LXe
Barium ion extraction R&D at SLAC
Pa produced in a cyclotron
230Th + p  230Pa + 3n
230Pa
Ion capture test simulates
Ba ions by using a 230U
source to recoil 222Ra into
the Xenon – Ba and Ra are
chemically similar
(ionization potentials 5.2 eV
and 5.3 eV respectively).
(17.4d)
8.4% 
230U (20.8d)

226Th
5.99MeV
(30.5min)
 6.45MeV
222Ra
(38s)
3-steps of
 decay
1st Prototype electrostatic
probe – W tipped.
Variations have been
tried (diamond coated),
but ions not released by
reversed HV in these cases
(required E field too high)
Probe test cell
Xenon cell
Probe lowered
for ion collection (1)
Electrode (source)
PMT
3-position
pneumatic actuator
probe up position
for release (3).
 detector flange
counting (2) station
Xenon cell
outer vac. vessel
Ion extraction from Xe and LXe
230U
source α spectrum
as delivered by LLNL
(measured in vacuum)
α spectrum from
whatever is grabbed
by the tip
(in Xe atmosphere)
An additional
signature from
the observed Th
and Ra lifetimes.
Ion mobility studies in LXe
We use the probe test cell to measure ion drift speed
forward bias
LXe level
“Paddle” probe
U230 source
electrode
reverse bias
Modulate the electrode voltage,
and measure ion collection rate.
Data taken for various separation
distances and voltage differences.
Observed mobility of 0.24±0.02 cm2/kVs for Thorium ions compares
with result for Thallium ions 0.133 cm2/kVs. (A.J. Walters et al. J.
Phys. D: Appl. Phys.) and with Fairbank etal. for EXO (Ba,Sr,Ca,Mg).
Our work submitted to Phys. Rev. B.
Ion Capture “Cryo Probe” prototype
In order to release a captured
ion, the electrostatic probe can
be cooled such that Xe ice coats
the tip. The captured ion can
then be released by thawing.
Probe tip detail
gas return
(outer tube)
Joule-Thompson cooling is used
for cooling (argon gas).
incoming gas
(inner tube)
An additional benefit : the Ba+
charge state may be stable in
solid and liquid xenon.
small aperture
at tube end
Argon
Remarkably, surgical
cryoprobes seem to be
ideally suited to our
application. We
have adapted 2.4 mm
diameter probes for use
in our probe test cell.
Expected gas cooling from calculated J-T coefficient
and our data with cryoprobe.
Testing the ion extraction probe
U230 sources were installed, xenon was liquefied in the cell, ion
capture and release from Xe ice has been demonstrated.
First cryo-probe was not equipped for acceptably “graceful”
Xe ice release. New version is under test.
Refinement of ion release procedure (rapid ice sublimation is best).
2.4mm
Vacuum
jacket
TC
J-T
nozzle
X-ray image
of new cryoprobe
test version of “thaw” heater
Issue for cold probe method - Xe gas release
Surface ionization or “hot probe” R&D
5.2eV 10eV
~5.9eV
It is well known that heated
metal surfaces can release
captured metal atoms in both
the neutral or ionized state :
“impact ionization”
First (eV)
Ba
Pt (111)
Tl
Th
Ra
work function
Second (eV)
6.1
20.4
6.1
11.5
5.3
10.1
ionization E
Saha-Langmuir effect :
n
 e(  W ) 
Ion emission from heated
 2 exp 
n
 kT 
high-work function surfaces
(shown here for alkaline earth metals)
known from ion beam experiments
May be possible to release Ba+ ions by heating Pt probe.
This procedure would be simpler than the cold probe.
Method requires the Pt surface is heated to a high enough
temperature to efficiently liberate barium, but not so high
that neutral atoms become a significant fraction w.r.t. ions.
Test apparatus for thermal ion release experiments
Th228 (1.9 yr) source produces
Ra224 (3.6 d) daughters
• Source can be forward or backward
biased (±500 V typ.)
• Pt foil (@ ground) receives
ions recoiled from source.
• Foil can be moved in front of detector,
and down to the stopper plate.
• Foil heated >1000K, see if Ra released
as neutral or charged.
(if the observed post-heating signal is modulated
by the HV on the plates, ions were released)
heater PS
Movable Pt Foil
source collimator
plate
source (Th228)
HV
Alpha counter
Vessel is filled with 1 atm Xe.
This limits the diffusion of the
ions. The α’s range out in ~5 cm
stopper
plate
HV
At top, the Th228 source
Below, the Si SB α detector
Pt foil
(power leads visible
as is the mounted TC)
Test apparatus : Source collimator not installed for this photo
E field calculation for collimator. Ra224 deposited near foil center
Po-212
Bi-212
Rn-220
Ra-224
Po-216
Red histo : alpha spectrum from foil prepared with reversed
biased source → Ra ions do not reach foil.
Black histo : … and when source at + potential → foil plated.
Experiments are underway in out lab to test the performance
of a Pt foil.
If promising, we will proceed to design a hot probe,
and experiment with different metal tips
(iridium is a possibility - higher m.p. than platinum),
and perhaps high-work-function dielectrics.
Recently, a third probe option is under study at Stanford High field emission from “STM” tip,
or “sharp probe” R&D
Published data suggests that barium will desorb from tungsten needle tips
as a Ba+ ion at electric fields of ~150 MV/cm. These high fields can be
reached with very sharp STM needle tips (radius of curvature of ~10 nm)
at moderate (10 kV) voltages .
SEM image
of W needle
Electric field calculations for ion capture are underway.
One of the issues here will be the robustness of these delicate sharp tips
Issues for Trigger rates
1.
event energy & space location from TPC
2.
“ion fetch” triggered by energy threshold & ~veto
3.
TPC field switched off (prior ion drift very small).
4.
move probe tip to (just above) ion location.
5.
capture ion electrostatically with ~1 cm radius.
6.
withdraw probe - TPC field back on - detector live
7.
deliver ion to laser for identification.
Acceptable deadtime/Δt for steps 2-6 sets maximum “ion fetch” rate.
Our measurements of the mobility of ions (Th and Ba) in LXe indicate a
drift speed of ~2 mm/s in a 1 kV/cm E field. For a 1 mm radius
probe tip, this translates into a 0.8 s collection time from 5 mm, 3.8 s
from 10 mm. The deadtime will be dominated by probe motion and/or
high voltage ramping, if necessary - < 1 minute a reasonable target.
Backgrounds/trigger threshold sets “ion fetch” trigger rate.
While it is difficult to extrapolate from our prototype simulations to a large
multi-tonne detector, we can guess by scaling our bkgrd. simulations by
a factor of 10 tonnes/200 kg = 50. For a low energy trigger threshold of
2.250 MeV (for an E resolution of 1%, this corresponds to 10σ),
trigger rate would be < 1/hour. This is a plausible “ion fetch” rate.
(2 events not as important for the large detector - these and other low energy
phenomena can be acquired using a scaled trigger).
The probe-to-ion trap interface
We have decided to focus our initial R&D efforts on
an interface between an electrostatic probe and a linear
ion trap, including cryo- and differential pumping.
We have made progress studying electrostatic probes.
A number of issues remain …
The ion-release procedure for the designs considered to
date will have different challenges (assuming the basic
concepts are fully demonstrated in R&D).
• The cold probe will deliver a larger Xe load –
Is effective pumping possible ?
• The hot probe may release Ba if it is present in the probe
surface material –
Sensitive tests needed during R&D.
…and a bit further down the road …
• Significant engineering problems will need our attention –
R&D for probe “robot” and interface to the TPC.
Linear Paul ion trap R&D
release ions to trap,
detect and measure
efficiency
probe unloading
capture ions on
probe tip
TMP/cryo pump
ion trap/laser tag
conventional Ba+
loading
detect and count
trapped ions
Linear trap confinement : radially by RF quads, axially by DC fields
6 mm
linear trap RF quadrupoles
segmented (15 sections)
to grade DC axial field.
linear trap vacuum chamber
(excluding probe interface section)
There is considerable experience among nuclear/atomic
physicists with ion transport in linear traps.
Parts are on order for a linear ion trap to be built at Stanford.
R&D continues on trap/probe interface at SLAC/Stanford.
Progress to date
• We have developed the atomic physics and spectroscopy
techniques to achieve good quality tagging in presence of some
Xe gas.
• Gained experience with grabbing on Xe-ice and on metal tips
Continuing R&D
• Building a linear trap that should be very close to final device
and can be used to test loading efficiency.
• Ion release needs more R&D work, field emission from STMtip, “impact” ionization and “cryoprobe”
all under development in parallel.
Highest present priority/risk
Ba tagging R&D must continue in parallel
with the construction of the 200 kg
experiment in order to move EXO
towards the 10 meV regime.