EXO at SNOLAB

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EXO at SNOLAB David Sinclair For the EXO
COllaboration
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• Z.Djurcic, D.Leonard, A.Piepke
• Physics Dept, University of Alabama, Tuscaloosa
  AL
• P.Vogel
• Physics Dept Caltech, Pasadena CA
• A. Bellerive, M. Dixit, C. Hargrove, D. Sinclair
• Carleton University, Ottawa, Canada
• W.Fairbank Jr., S.Jeng, K.Hall
• Colorado State University, Fort Collins CO
• M.Moe
• Physics Dept UC Irvine, Irvine CA
• 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
• Physics Dept University of Neuchatel, Neuchatel
  Switzerland
• M.Breidenbach, R.Conley, C.Hall, A.Odian,
  C.Prescott, M.Pyle, P.Rowson, J.Sevilla,
  K.Skarpaas, K.Wamba, T.Wizansky
• SLAC, Menlo Park CA
• E.Conti, R.deVoe, G.Gratta, M.Green, T.Koffas,
  R.Leon, R.Neilson, Y.Uchida, S.Waldman, J.Wodin

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Main challenge in 0?ßß decay
1) Very large fiducial mass (tons) need
large-scale isotopic enrichment 2) Reduce and
control backgrounds in qualitatively new
ways existing experiments are already
background limited, unlikely to gain big
factors without new techniques
For no background
For a background scaling like Nt
Need 2) to fully utilize 1) and make a worthwhile
experiment
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Xe is ideal for a large experiment

• No need to grow crystals
• Can be re-purified during the experiment
• No long lived Xe isotopes to activate
• Can be easily transferred from one detector to
• another if new technologies become
  available
• Noble gas easy(er) to purify
• 136Xe enrichment easier and safer
• - noble gas (no chemistry involved)
• - centrifuge feed rate in gram/s, all
  mass useful
• - centrifuge efficiency ?m. For Xe
  4.7 amu
• 129Xe is a hyperpolarizable nucleus recently FDA
• approved for lung NMR tomography
• a joint enrichment program ?

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EXO Prototype Detector

• Purpose
• to demonstrate that adequate energy resolution
  can be achieved in large Lq Xe detector
• To measure the 2 neutrino rate in Xe
• The detector
• 200 kg enriched Xe
• Ionization tracking and scintillation

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EXO Prototype Detector
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3D TPC model for signal calculations
Small red circles are pads, black circles are
APDs, green lines are wire grids, and the large
red circle is the cathode Cathode is midplane at
HV. There are two anode planes. Each anode has
an array of 356 APDs ganged in groups of
7 Three ionization measurement schemes have
been studied. Leading contender is crossed wires.
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Ionization detection channel count
Three possible TPC designs Individual Pad
readout number of pads is greater than twice the
number of APDs, (356 APDs per half detector, so gt
712 per pads per half detector) Ganged pad
readout Used by Gotthard TPC and MUNU TPC. 40
channels of u view , 40 channels of v view, for
80 channels per half detector. Crossed wires
Used by ICARUS and LXeGRIT. Number of channels
can be adjusted to taste. Example One cm pitch
gives 44 x channels, 44 y channels, for 88
channels per half detector
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APD plane below crossed wire array
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Cryostat Cross Section
Outer Door
Condenser
FC-87
Xenon Chamber
Inner Door
Xenon Chamber Support
Xenon Heater should be on this area
FC-87
1 thick Thermal Insulation (MLI-vacuum),
not shown to scale
Inner Copper Vessel
Outer Copper Vessel
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Full detector view
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Pancake shaped 1liter LXe ionization chamber to
test energy resolution Good acceptance to
scintillation light AND ionization
Electron/gamma source 208Bi needs to be very
small to avoid self shadowing (20µm plated wire)
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Found a clear (anti)correlation between
ionization and scintillation
1 kV/cm
570 keV
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The correlation is present at all fields
The best resolution is obtained by a linear
combination of the scintillation and ionization
signals
Ionization only
E.Conti et al. Phys Rev B 68 (2003) 054201
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Have demonstrated that we can get sufficient
energy resolution in LXe to separate the 2? from
the 0? modes
Resolutions at 570 keV
Now we turn on our new correlation technique
3.3_at_570keV or 1.6_at_2.5MeV
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Status of 2? mode in 136Xe
2?ßß decay has never been observed in 136Xe.
Some of the lower limits on its half life are
close to (and in one case below) the theoretical
expectation.
The 200kg EXO prototype should definitely resolve
this issue
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Xe offers a qualitatively new tool against
background 136Xe 136Ba e- e- final
state can be identified using optical
spectroscopy (M.Moe PRC44 (1991) 931)
Ba system best studied (Neuhauser,
Hohenstatt, Toshek, Dehmelt 1980) Very specific
signature shelving Single ions can be
detected from a photon rate of 107/s
2P1/2
650nm
493nm
metastable 47s
4D3/2

• Important additional
• constraint
• Huge background
• reduction

2S1/2
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Ba spectroscopy in high pressure noble gases
RF quadrupole trap loaded in UHV from a Ba
dispenser and e-beam ionizer Xe can be injected
while observing the ions
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First in vacuum photo of a Ba ion
(Hz/pixel)
(Hz/pixel)
850 µm
Background is obtained by turning off the red
light
S/N100 even with a CW measurement !
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Millikan experiment with ions in vacuum
End

• Start with a
• large signal
• Use the endcap
• bias voltage to
• make the trap
• unstable and
• drop ions
• Keep unloading till
• trap is empty

Start
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The signal amplitudeis proportionalto integers
Millikan experiment with ions in vacuum
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Spectra in He and
Xe
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Ba tagging in gas conclusions to date

• Ba very stable in noble gases (He, Ar and Xe
  tried)
• Ions observed for days in 0.01 torr of Xe
• Purity requirements modest, have not used
  purifier for
• these measurements
• Spectra limited by trap dynamics, under
  investigation
• Need to cross the Paschen minimum (0.1-1 torr)
• without breakdown
• Started upgrading the system
• for turn-key operations

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New probe tip is supercooled by Joule-Thompson
effect to form a layer of Xe ice. Ice is
evaporated to release Ba
Release basically works but need to improve
the consistency of the ice layer
Also under test a Pt coated probe that by heating
should emit directly Ba
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EXO neutrino effective mass sensitivity

• Assumptions
• 80 enrichment in 136
• Intrinsic low background Ba tagging eliminate
  all radioactive background
• Energy res only used to separate the 0? from 2?
  modes
• Select 0? events in a 2s interval centered
  around the 2.481MeV endpoint
• 4) Use for 2?ßß T1/2gt11022yr (Bernabei et al.
  measurement)

s(E)/E 1.6 obtained in EXO RD, Conti et al
Phys Rev B 68 (2003) 054201 s(E)/E 1.0
considered as an aggressive but realistic guess
with large light collection
area QRPA A.Staudt et al. Europhys. Lett.13
(1990) 31 Phys. Lett. B268 (1991) 312 NSM
E.Caurier et al. Phys Rev Lett 77 (1996) 1954
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EXO 200kg prototype mass sensitivity

• Assumptions
• 200kg of Xe enriched to 80 in 136
• s(E)/E 1.6 obtained in EXO RD, Conti et al
  Phys Rev B 68 (2003) 054201
• Low but finite radioactive background
• 20 events/year in the 2s interval
  centered around the 2.481MeV endpoint
• 4) Negligible background from 2?ßß (T1/2gt11022yr
  R.Bernabei et al. measurement)

What if Klapdors observation is correct ?
Central value m0.44 eV, 3s range (0.24eV
0.58eV) (Phys. Lett. B 586 (2004) 198-212) In
200kg EXO, 2yr would observe 57 events (QRPA) on
top of 40 events bkgd Using lower bound (0.24
eV) would have 17.3 signal events (and 40
bkgd), a 2.3 s effect
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Isotopic enrichment for a gaseous substance like
Xe is most economically achieved by
ultracentrifugation
Russia has enough production capacity to process
100 ton Xe and extract up to 10 ton 136Xe in a
finite time
This separation step that rejects the light
fraction is also very effective in removing 85Kr
(T1/210.7 yr) that is present in the atmosphere
from spent fuel reprocessing
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A LXe detector more elegant BUT technology needs
testing ? prototype detector

• Very small detector (3m3 for 10tons)
• Need good E resolution
• Position info but blobs not resolved
• Readout Xe scintillation
• Can extract Ba from hi-density Xe
• Spectroscopy at low pressure
• 136Ba (7.8 natl) different signature from
  natural Ba (71.7 138Ba)
• No quencher needed, neutralization done outside
  the Xe

• High Pressure gas TPC backup technology
• 20 atm, 35 m3 modules, 4.2 ton/module, 2 modules
• Xe enclosed in a non-structural bag
• b range 5cm can resolve 2 blobs
• 2.5m e-drift at 250kV
• Readout Xe scintillation with WLSB (T0)
• Additive gas quenching and Ba Ba
  neutralization
• Steer lasers or drift Ba-ion to detection region

RD mainly at Neuchatel ITEP
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Project Schedule

• Prototype being built at SLAC this year
• Will move to WIPP for about 2 years
• Ba tagging development in parallel
• Proposal for full scale detector should be ready
  2-3 years hence

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Materials Studies

• Radioactivity in all detector materials is
  studied.
• Currently, at SNO, investigating U,Th in lead,
  contamination in getters, activity in LAAPDs.
  Critical need to re-establish low level gamma
  counting

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Full Scale EXO Detector in SNOLAB

• Size depends on shielding if lead is OK then
  detector is quite small
• Total Xe volume is comparable with amounts of LN2
  being used at present at SNO. Fail safe concepts
  being developed for total power failure
• Location of the project will be driven by science

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Conclusions

• EXO offers a way to approach the normal hierarchy
  mass expectations in double beta decay
• A vigorous RD programme is in place to address
  the outstanding technical issues
• A wonderful opportunity for SNOLAB