Transcript Enriched Xenon Observatory for double beta decay

Enriched Xenon Observatory
for double beta decay
DOE Review June 7, 2006
P.C. Rowson, SLAC
Z.Djurcic, D.Leonard, A.Piepke, University of Alabama
P.Vogel Caltech
A.Bellerive, M.Bowcock, M.Dixit, I.Ekchtout, C.Hargrove, D.Sinclair, V.Strickland
Carleton University, Canada
W.Fairbank Jr., S.Jeng, K.Hall Colorado State University
M.Moe UC Irvine
D.Akimov, A.Burenkov, M.Danilov, A.Dolgolenko, A.Kovalenko, D.Kovalenko, G.Smirnov, V.Stekhanov
ITEP Moscow, Russia
J.Farine, D.Hallman, C.Virtue
Laurentian University, Canada
M.Hauger, F.Juget, L.Ounalli, D.Schenker, J-L.Vuilleumier, J-M.Vuilleumier, P.Weber
University of Neuchatel, Switzerland
M.Breidenbach, C.Hall, D.MacKay, A.Odian,, C.Prescott, P.C.Rowson, K.Skarpaas, K.Wamba
SLAC
R.DeVoe, P. Fierlinger, B.Flatt, G.Gratta, M.Green, F.LePort, R.Neilson, K.O’Sullivan, A.Pocar, J.Wodin
Stanford University
Neutrinos have
mass and
they oscillate.
Still unknown:
• Are neutrinos
Majorana ?
•What are the absolute
masses ?
normal
hierarchy
inverted
hierarchy
•What is the mass
hierarchy ?
Profound implications if Majorana neutrinos exist :
• Lepton number violation.
• Possible explanation of the tiny neutrino mass via the “see-saw mechanism”.
• Possible generation of baryon/antibaryon asymmetry via “leptogenesis”.
How does one best address the unanswered questions of mass scale and the long
standing Dirac/Majorana issue ?
1.
Direct mass determinations via beta decay (tritium) are limited to about
200 meV precision for any planned experiment.
Best limit now is 2.8 eV (Mainz/Troitsk) from H3,
and ~1 eV from Astrophysics (WMAP)
2. As Dirac and Majorana properties converge in the relativistic limit,
neutrino beam experiments cannot distinguish Majorana and Dirac
neutrino fields.
In the limit m → 0, the two are not distinguishable
even in principle.
The best presently available option that addresses the issue of m and its origin is
neutrinoless double beta decay :
Exploiting macroscopic quantities of matter to search for a rare
process to reach ~10 meV mass sensitivity.
Nuclear Double Beta Decay
Process a) occurs in the Standard model. Process b) only proceeds :
• If ’s are their own antiparticles (Majorana)
AND
• If the ’s are massive (a spin flip is required to conserve angular
momentum).
 For 0 decay, the rate ~ <m> 2.
• 0 decay does not conserve lepton number
4-body decay
continuous spectrum
for e energy sum
2-body decay
e energy sum
is at the max. (Q)
Calculating the rates for 2 and 0
2 1
1/ 2
] G
[T
2
Well known
phase space factors
0 1
1/ 2
[T
] G
where …
0
M
2 2
: 2 (typ. >1019 y)
Matrix elements calculated
from nuclear models
M
0 2
m
2
: 0 (>1023 y eV2)
m  m1 U e1  m2 U e 2 ei 21  m3 U e3 ei 31
2
2
2
The effective mass <m> is a complex linear comb. of the 3 generations of mass
eigen-states (and cancellations can occur).
From the present neutrino oscillation data, one can deduce, with some assumptions,
that this effective mass may be in the range from below 1 meV to 100 meV or higher.
(Uncertainties in the nuclear matrix elements typ. ~ factor of ~2)
There is an opportunity to make an important discovery if one
pushes the <m> sensitivity to the ~ 10 meV region
<m> 90% C.L. ranges from all data :
~(1 - 4 meV)normal hierarchy
~(15 - 60 meV)inverted hierarchy
for example Feruglio et al. CERN-TH/2002-13. Elliott and Vogel, Ann.Rev.Nucl.Part.Sci. 52 (2002) 115-151. }
Normal
Hierarchy
large mixing, N=3
Inverted
Hierarchy
Detection of 0 Decay : e energy sum is the primary tool
In this rare decay search, superb E resolution is essential for bkgrd. control,
particularly bkgrd. due to the Standard Model 2 decay.
2 spectrum
(normalized to 1)
0 peak (5% FWHM)
(normalized to 10-6)
Important issue : 2 rate
must be determined.
(A smaller 2 : 0 rate
ratio is experimentally
favorable.)
0 peak (5% FWHM) The most sensitive experiments
(normalized to 10-2) to date (using 76Ge) have relied
on superb energy resolution
(0.2% at the 2.0 MeV endpoint)
This strategy is planned for the
future 76Ge & 130Te programs.
Summed electron energy in units of the kinematic endpoint (Q)
Advantages of a LXe TPC
Energy resolution is poorer than the crystalline devices (~factor 10), but
Xenon isotopic enrichment is easier. Xe is already a gas & Xe136 is the heaviest isotope.
Xenon is “reusable”. Can be repurified & recycled into new detector (no crystal growth).
Monolithic detector. LXe is self shielding, surface contamination minimized.
Minimal cosmogenic activation. No long lived radioactive isotopes of Xe.
Energy resolution in LXe can be improved. Scintillation light/ionization correlation.
… admits a novel coincidence technique. Background reduction by Ba daughter tagging.
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
Observation of a  from an excited
daughter ion, but the rates compared
to ground state decays are generally very small
(best chance might be 150Nd, but E is only 30keV.)
X  (Y++)* + e– + e–
Y++ + 
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.
There are many planned experiments, using a variety of
techniques and a variety of isotopes, and several
of these are ambitious tonne-scale programs
T0ν (y)
<mν> eV
.77 t of TeO2 bolometers (nat)
7 x 1026
.014-.091
136Xe
10 t Xe TPC + Ba tagging
1 x 1028
.013-.037
GERDA
76Ge
0.5 t Ge diodes in LN/LAr phase 3
order 1028
order(0.01)
Majorana
76Ge
0.5 t Ge diodes
4 x 1027
.021-.070
MOON
100Mo
34 t nat.Mo sheets/plastic sc.
1 x 1027
.014-.057
DCBA
150Nd
20 kg Nd-tracking
2 x 1025
.035-.055
CAMEO
116Cd
1 t CdWO4 in liquid scintillator
> 1026
.053-.24
COBRA
116Cd ,
10 kg of CdTe semiconductors
1 x 1024
.5-2.
Candles
48Ca
Tons of CaF2 in liq. scint.
1 x 1026
.15-.26
GSO
116Cd
2 t Gd2SiO5:Ce scint in liq scint
2 x 1026
.038-.172
Xmass
136Xe
1 t of liquid Xe
3 x 1026
.086-.252
Experiment
Nucleus
Detector
CUORE
130Te
EXO
130Te
I will now turn to a novel approach to the experimental problems
of 0 searches that is being pursued by the EXO collaboration
A claim for 0νββ decay discovery in Ge76
Phys. Lett. B 586 (2004) 198-212
Updated best value is T1/2 = 1.2 · 1025 yr  mass of 0.44 eV
(a subset of the Heidelberg-Moscow collaboration)
There are many skeptics … a very controversial claim
spectrum after PSD. Fit done after
recognized peaks (not all seen) removed
The
upcoming prototype
experiments plan
to confirm or disprove
the claim within the
next typ. ~3 years,
including EXO …
Liquid Xenon TPC conceptual design
• 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
Compact and scalable
probe eg.)
3
(3 m for 10 tons).
• Deliver the Barium to a laser
system for Ba136 identification.
Issues to be addressed (R&D
progress where indicated) :
 Ba+ lifetimes in LXe
(expected to be long)
 Ba ion drift velocities
(should be a few mm/sec)
 Ba capture and release –
various probe designs
 Ba transport to the laser
spectroscopy station
The EXO R&D
program has addressed the following issues
Xenon procurement and isotopic enrichment.
Xe136 natural abundance of 9% - increase to ~80%
Xenon purification.
long electron lifetimes  electronegative impurities <1ppb
Sufficient energy resolution in Xenon, particularly in LXe.
incl. studies of scintillation light/ionization correlation
Single ion Barium spectroscopy in vacuum and in Xe gas.
conversion of Ba++ to Ba+ or neutral Ba, line broadening
Barium ion “capture and release” in Xenon (LXe)
Barium ion lifetimes, mobility, charge state in LXe, ion-acquisition technologies
Testing and procurement of low background materials
neutron activation, mass spectroscopy, radon counting and Ge detector assay
Isotopic enrichment of gaseous Xe by ultracentrifugation
136Xe,
being the heaviest Xe isotope, is
particularly easy to separate.
The separation step that rejects the light
fraction is also very effective in removing
85Kr (T =10.7 yr) that is constantly
1/2
produced by fission in nuclear reactors.
Large facilities exist
In Russia – 500,000
centrifuges per plant.
We have received
200 kg of 80% enriched Xe
to be used in prototype.
Xenon purification system at SLAC
hot Zr gas purifier (SAES)
purity monitor
(drift cell with
laser P.E.
cathode)
distillation bottles
Best observed electron
lifetimes ~ 10 ms
Continuous circulation of
xenon gas through purifier
works well. This capability
will be available in our first
experimental prototype.
recirculation bellows pump
(outgassing is likely to be an
issue and purity may have to
be maintained continually).
Stanford 1 liter LXe chamber to study energy resolution.
Preliminary results using ionization only reproduced the best resolutions seen.
Can scintillation light detection (175 nm in LXe) improve resolution ?
[J.Seguinot et al. NIM A 354 (1995) 280]
1 kV/cm
~570 keV
Observed (noise
subtracted) resolution
the at 570 keV Bi peak
corresponds to 2% at
the 2.5 MeV endpoint.
PMT resolution is not as
good, but a
clear anticorrelation is
seen :
A linear comb. of
ionization and
scintillation will
optimize resolution
First EXO publication
(Phys.Rev. B) …
Compilation of resolution
data in LXe
this work (ionization)
Improved resolution incl.
scintillation is state of the art in
LXe.
Resolution is optimized by a ~(10-15)O “mixing angle”.
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.
Laser Spectroscopy Lab at Stanford
red laser
Ba++ lines in the UV – convert ion to Ba+ or
neutral Ba.
blue laser
reference cavities
“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
Stable and reliable laser system
Linear Paul ion trap R&D at Stanford
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
Trapping
region
Linear trap confinement :
radially by RF quads, axially by DC fields
There is considerable experience among
nuclear/atomic physicists with ion transport in
linear traps (eg. ISOLDE heavy ion traps).
A linear ion trap has been built at Stanford and has
demonstrated ion capture - testing continues.
R&D underway for ion trap/probe interface at
SLAC/Stanford.
Ion trajectory
Ba oven
e- gun
Loading
region
Barium cloud in trap
Ion delivery system prototype to
be attached here
Linear ion trap vacuum enclosure
Integration with a cold-probe ion delivery
system being planned, as well as alternative
probe schemes.
Single Ba ions
in trap
Barium ion extraction R&D at SLAC : preliminary studies
Pa produced in a cyclotron
230Th + p  230Pa + 3n
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).
230Pa
(17.4d)
8.4% 
230U (20.8d)

226Th
5.99MeV
(30.5min)

6.45MeV
222Ra
(38s)
3-steps of
 decay
In the earliest experiments the prototype electrostatic probes were W tipped.
Ions are not released by reversing HV in these cases (due to image charge the
required E field too high). As you shall see, subsequent prototypes addressed
this issue.
230U
source α spectrum
as delivered by LLNL
(measured in vacuum)
probe down position
α spectrum from
whatever is grabbed
by the tip
(in Xe atmosphere)
3-position
pneumatic actuator
probe up position
Probe test cell
Demonstrate ion capture/release in
LXe with electrostatic probe
Xenon cell
outer vac. vessel
Second EXO publication
(NIM. A) …
Probe Test Cell used to measure ion mobility
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).
Ion Capture “Cryo Probe” prototype : Enabling ion release
2.4mm
Vacuum
jacket
TC
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
sublimation of the Xe ice.
Joule-Thompson cooling is used
for cooling (argon gas).
J-T
nozzle
X-ray image
of new cryoprobe
An additional benefit : the Ba+
charge state may be stable in solid
and liquid xenon.
With U230 sources installed, xenon has
been liquified in the cell, ion capture and
release from Xe ice has been demonstrated,
ion mobilities have been measured.
Issue for cold probe method - Xe gas release
buffer gas effects ion trap stability
Ungoing R&D for ion capture/tagging : Proceeding on several fronts in parallel
• Surface ionization “hot probe” – in principle simpler than cryoprobe - SLAC
It is well known that heated metal surfaces can release captured metal atoms in
both the neutral or ionized state : “impact ionization”/Saha-Langmuir effect.
• Continuing R&D on cryo-probe – minimize Xe ice load, & integrate with trap - Stanford
• R&D on sharp (STM tip) probes for high E-field ion release - Stanford
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 .
• Resonant Ionization Spectroscopy (RIS) – Stanford/CSU
Tip is a ~200μm fiber with a semitransparent metallization at end :
- Ba ion is attracted to metallization and neutralized
- A desorption laser pulse evaporates the Ba in the trap
- A second pulse (2 specific wavelengths) resonantly ionizes the Ba when it is
~100μm from the fiber tip
• In situ tag : “laser beam to ion” in LXe studies – Issues are S/N, line broadening - CSU
Additional engineering/tech support will be needed to continue R&D at SLAC
238U
→ 1461 keV γ’s, a
background for 2 only
40K
chain
214Bi
→ 2448 keV γ
produces (214Bi) γ
bkgrd, & daughter , α
emitters can move
throughout the apparatus
with deadly results.
222Rn
Radioactive Backgrounds - primary isotopes
232Th
chain
208Tl
→ 2615 keV γ
Dozens of materials have been
tested to date including various
Pb, Cu, bronze alloys, plastics
(teflon,polycarbonate), thermal
fluids, LAAPDs with many more
tests still in the queue – ALL
materials will be tested.
Cosmogenic activation (of Cu)
must be avoided → shielding
needed.
Qualification of low background materials (U.of Ala., Neuchatel, INMS, Laurentian)
Assay @ U. of A. Ge
detector following
neutron activation @ the
MIT research reactor,
Neuchatel Ge counter
and INMS (Canada)
mass spec.
(ICPMS and GDMS
methods). Radon
counting (Laurentian).
Material requirements for ββ2ν-bkg < 10 events/day and
ββ0ν-bkg < 3 events/year (no tracking cuts)
Material
Mass
[kg]
K
[ppt]
Th
[ppt]
U
[ppt]
Xenon
200
30 / na
0.1 / 0.04
0.02 / 0.0008
Teflon
SG
100
790 / na
0.6 / 0.6
0.2 / 0.2
HFE
4681
520 / na /
0.4 / 0.03
0.2 / 0.02
Copper
2956
37000 / na /
35 / 1
13 / 2.5
NAA/Ge counter measurements are in some cases only limits – the required purities can
exceed our sensitivity.
This is true in the case of copper (Norddeutsche Affinerie upper bound of ~0.8 ppt U,Th but for copper cosmogenics contribute) & the heat transfer fluid (HFE-7000)
where limits are the best seen (less than 1 ppt), but still above the target. “liquid organics”, if
handled carefully, can be very pure, based on experience with KamLAND scintillator.
For the xenon purity we rely on the enrichment and purification procedures (no NAA
measurement possible).
EXO Performance Projections
Assumptions:
1)
2)
3)
4)
80% enrichment in 136
Intrinsic low background + Ba tagging eliminate all radioactive
background
Energy res only used to separate the 0ν from 2ν modes:
Select 0ν events in a ±2σ interval centered around the 2.479 MeV
endpoint
Use for 2νββ T1/2>1·1022yr (Bernabei et al. measurement)
Case Mass Eff. Run σE/E @
2νββ
T1/20ν
Majorana
mass
(ton) (%) Time 2.5MeV Backgroun
(yr,
(yr)
(%)
d
90%CL)
(meV)
(events)
QRPA‡ (NSM)#
Conser
vative
Aggres
sive
*
1
70
5
1.6*
10
70
10
1†
0.5 (use 1)
2*1027
0.7 (use 1) 4.1*1028
50
(68)
11
(15)
s(E)/E = 1.4% in EXO R&D, Conti et al Phys Rev B 68 (2003) 054201
† s(E)/E = 1.0% considered as an aggressive but realistic guess with
larger light collection area
‡ QRPA: Rodin et al Phys Rev C 68 (2003) 044302
# NSM: E.Caurier et al. Phys Rev Lett 77 (1996) 1954
The main effort in the EXO collaboration at present is the
construction of a prototype experiment of significant scale :
“EXO200”
This prototype will use the 200 kg of 80% enriched Xe136.
This first device will not employ barium ion tagging.
The goals of the prototype program are :
•
•
Test the LXe TPC operation : energy and spatial
resolution, chemical purity issues, mechanical design
and all backgrounds due to radioactivity and cosmic
radiation.
Observe 2 decay in Xe136 for the first time and
measure the rate of this important 0 background.
•
Confirm or refute the claim of Klapdor et al.
EXO200 cryostat and shielding – Schematic only
xenon plumbing
TPC vessel
inner & outer
copper vessels
vessel support
thermal insulating
vacuum space
inner vessel is filled with
heat transfer fluid (HFE-7000)
for cooling and shielding.
inner & outer doors
~ 1.6 meters
Shown here is the cryostat with the doors open
and the TPC vessel withdrawn. Also visible
is the high-radiopurity Pb shielding that
completely covers the cryostat.
EXO200 LXe TPC
reusable weld
copper pressure vessel
x wires collect drifted electrons,
induced signal on y wires for
transverse localization
( ~ 1 cm), 38 x/y ch. per end.
175 nm scintillation light is
collected with LAAPDs (21mm)
Total of 359 per end,
and is used for timing (z) and to
enhance energy resolution.
175 nm scintillation
e-
cathode plane
(75 kV max.)
Ultra-low activity Cu e-beam
welded pressure vessel
(1.5 mm thick wall, ~20kg)
~40 cm
detection planes :
y wires, x wires
and the APDs
field shaping rings
A Liquid Xenon TPC with Avalanche Photodiodes
Detail of detector plane showing ganged (7)
APD “plaquettes”, and 60º crossing x/y
wires ( in this design, etched parts used
instead of ordinary 100 micron wires)
prototype “gang of 7” APDs
LAAPDs from API (Advanced Photonics) immersed
in LXe. High radiopurity, High QE (>100%) in VUV
Operated at -1.4kV, Gain ~ 100. Noise for gang of 7
~2000 electrons isacceptable.
EXO200 cleanrooms – in End Station III at HEPL (Stanford)
1
2
3
cleanroom #1 detector/cryostat
cleanroom #2 xenon handling systems
cleanroom #3 houses the refrigerators
Assembled cleanrooms at Stanford :
total of 6 rooms, innermost class 1000
for cryostat – support equipment and
work areas (incl. LAAPD test stand).
7ft thick concrete roof
EXO-200 cleanrooms
The cryostatcrane
is presently
in
rails
module 1, along with some Pb
shielding. Xenon plumbing
is going in in module 2, and
module 3 contains 3 highpower refrigerators.
Pre-assembly clean room
Lead cradle
HFE storage dewar
EXO200 cleanroom activity June ‘06
Crane for assembling
lead arches
In module 2 (xenon systems)
(shown here in SLAC cleanroom)
In module 1 (detector)
Xenon purification
system
At this time, the cryostat is in module 1,
along with a portion of the lead shielding.
Plumbing work will start ASAP.
In module 2, the xenon system is being
Refrigerators
assembled and tested along with the
control systems.
In module 3 (cryosystems)
EXO200 Projections
1)
2)
3)
σ(E)/E = 1.6%
Low but finite radioactive background: 20 events/year in the ±2σ interval centered around the
2.479 MeV endpoint
Negligible background from 2νββ (T1/2>1·1022yr R.Bernabei et al. measurement)
Case Mass Eff. Run σE/E @ Radioactiv T1/20ν
Majorana
e
mass
(ton) (%) Time 2.5MeV
(yr,
(yr)
(%) Backgroun 90%CL)
(eV)
d
QRPA (NSM)
(events)
*
Prototype 0.2
70
2
1.6
40
6.4*1025 0.27 (0.38)
Testing the Klapdor et al. claim ±3σ range : 1.2+3-0.5 × 1025 years
(Phys. Lett. B 586 (2004) 198-212)
→ (0.37 ev - 1.45 eV) effective mass
use Nucl.Phys.A, 766 (2006) 107-131, taking the lifetime and matrix element (QRPA vs. NSM) extremes
In 200kg EXO/2yr would observe :
NSM/upper limit : 159 events (and 40 bkgd), a 11.2σ effect
… the best case.
QRPA/lower limit : 19 signal events (and 40 bkg), a 2.6σ effect
… the worst case.
WIPP Schematic Overall View
Waste Isolation Pilot Plant
Carlsbad, NM
Excavated in underground salt –
lower U/Th activity.
~2,000 m.w.e. depth
EXO
An alcove at WIPP is ready to receive the EXO
cleanrooms and the anxillary equipment.
Our target date is end of 2006
EXO: near term plans and the future
…EXO200 operation
Design & construction of 200 kg prototype – test at Stanford,
disassemble the cleanroom modules, and ship to WIPP.
Operating prototype ~1 years from now, w/o Ba tagging.
…continuing R&D
Demonstrate viable laser system, incl. possible Xe buffer gas.
S/N optimized, trap design suitable for ion delivery
Demonstrate viable ion capture system
High efficiency, suitable design for large scale detector
Pending R&D results, design/build large detector
Complete system, electronics design.
Continuing xenon acquisition
For the first time, it appears that there are prospects for a laboratory resolution of the 70 yearold question concerning the Majorana nature of neutrinos. In addition, a worldwide program
of 0 experiments may tell as much about the absolute scale of neutrino
masses, and will complement ongoing neutrino oscillation measurements.
The experimental techniques are numerous, sometimes exotic, always challenging, and very
interesting in their own right. There is much to do and much to learn.
Candidate
Isotope
 always a second order process: only detectable
if first order  decay is energetically forbidden
This favorable situation occurs for a few dozen
isotopes, and if large Q is required to increase
rate and observability, there remain about 11.
If the isotope also serves as the detection
medium, useful experimentally, few remain.
136Xe is one.
Q
Abund.
(MeV)
(%)
48Ca→48Ti
4.271
0.187
76Ge→76Se
2.040
7.8
82Se→82Kr
2.995
9.2
96Zr→96Mo
3.350
2.8
100Mo→100Ru
3.034
9.6
110Pd→110Cd
2.013
11.8
116Cd→116Sn
2.802
7.5
124Sn→124Te
2.228
5.64
130Te→130Xe
2.533
34.5
136Xe→136Ba
2.479
8.9
150Nd→150Sm
3.367
5.6
• Natural abundance,
• Ease of purification
• Q value (11th power dependence for 2
mode, 5th power for 0)
• Ease of isotopic enrichment radioactivity
(incl. cosmogenesis)
• Experimental ease of use.
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 small. Rates are low and in addition,
event topologies are distinctive. Further study of these
processes for ~tonne scale EXO will be needed.
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 )
Issues for Trigger rates
1.
2.
3.
4.
5.
6.
7.
event energy & space location from TPC
“ion fetch” triggered by energy threshold & ~veto
TPC field switched off (prior ion drift very small).
move probe tip to (just above) ion location.
capture ion electrostatically with ~1 cm radius.
withdraw probe - TPC field back on - detector live
deliver ion to laser for identification.
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).
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.
Conclusion : Fetch <1/hour, fetch time <1 min.  <1/60 or <1.7% deadtime