P.C. Rowson, SLAC

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The EXO Double Beta Decay Program
P.C. Rowson, SLAC
An outline the physics goals of the project,
recent progress in the preparation of the first
stage experiment, w/o Ba tagging,
EXO-200, including a short mention of the RD
work presently underway towards full EXO with
coincidence Ba ion tagging.
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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 ltmgt 2.

• 0??? decay does not conserve lepton number

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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.
Important issue 2??? rate must be
determined. (A smaller 2??? 0??? rate ratio is
experimentally favorable.)
The most sensitive experiments 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.
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Some 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. We have already demonstrated the
advantages of Xe136 enrichment, and have shown
how E resolution can be improved. We will test
LXe TPC operation and the effectiveness of our
careful radioactive background controls in a
prototype experiment starting soon. We have made
progress towards coincidence Ba daughter nucleus
identification.
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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 (T1/210.7 yr) that is
constantly produced by fission in nuclear
reactors.
We have received 200 kg of 80 enriched Xe to be
used in prototype.
Mass spectra from RGA 2001 test sample enriched
to 89.5 in Xe136.
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Improved E resolution incl. 175 nm scintillation
is state of the art in LXe. (Demonstrated in
LXe test cell)
Compilation of resolution data in LXe
First EXO publication (Phys.Rev. B)
this work (ionization)
Our results recently confirmed elsewhere
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Procurement/Qualification of Low Backgroud
Materials A substantial effort is necessary for
EXO a leading design constraint
40K ? 1461 keV ?s, a background for 2??? only
222Rn produces (214Bi) ? bkgrd, daughter ?, a
emitters can move throughout the apparatus.
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 within Pb shielding will be tested down
to the tiniest screw, incl. radon emanation in
external plumbing. Cosmogenic activation (of Cu)
must be avoided ? shielding needed during
storage. So far, our accounting shows we are on
target to meet out spec.
Radioactive Backgrounds primary contributing
isotopes
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Backgrounds in a LXe TPC A GEANT4 monte carlo
simulating backgrounds, the physics signals, 175
nm light propagation and reflection, ionization
transport in the TPC, electronics response and
noise, has been used to study backgrounds in the
2? and 0? channels Low energy backgrounds
These effect the 2? channels (due to the low
400 keV E cut), arise from K40, Pb210
eg. reduced by fiducial cuts (distance to TPC
vessel wall) use TPC position resolution. High
energy backgrounds These arise mainly from U,
Th, Rn. The Bi214 gamma in particular is an
issue for 0?, along with the Tl208 gamma (2.6
MeV) which Compton scatters to lower
energy. reduced by better E resolution and
single cluster cut (use TPC position
resolution to remove multi cluster events
signal events are contained within a few mm in
LXe). Includes delayed coincidence in Bi214
decay to Po214 a decay (160 µs) leading to drift
direction separation) and Alpha ID separates
a decays from ß, ? due to ionization quenching
light/charge ratio is significantly larger for
the densely-ionizing charged nuclei
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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.)
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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
• probe can attract or repell ion)
• Deliver the Barium to a laser system for Ba136
  identification.

Compact and scalable (3 m3 for 10 tons).

• Issues to be addressed (RD 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

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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.

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The SLAC EXO Group (fulltime,
part time, retired) Physicists Breidenbach,
Odian, Prescott, Rowson Postdoc Physicists
Yang, Wodin Graduate Students Ackerman, Herrin,
MacKay Visitors (2008) Farine,
Stekhanov Mechanical Engineering Skarpaas,
Campell, Craddock, Hodgson, Electrical
Engineering Freytag, Haller, Herbst Mechanical
Technicians Conley, Swift Electrician
Zalog WIPP Electrical Power Jones WIPP Safety
Support Pierson In recent times, SLAC EXO
received substantial contributions from
Cryogenic Engineering/Mechanical SLAC cryo
group
July 7th 2008
SLAC Annual Program Review
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EXO Financial and Manpower data
Note that only DOE funded manpower is included.
For example, we have 3 fulltime graduate
students at SLAC
July 7th 2008
SLAC Annual Program Review
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• The SLAC EXO Group Contributions
• The SLAC group has been the EXO collaborating
  group primarily responsible for a large fraction
  of the EXO200 effort
• RD, design, construction of the xenon
  gas/liquid delivery, purification
• purity monitoring systems
• RD, design, construction of the cryogenic
  refrigeration systems
• Design and construction of the HFE delivery and
  control system
• RD, design, test assembly, of the TPC
  (mechanical, electrode structure
• signal/HV cabling initial testing of LAAPDs
  for UV light)
• Design, construction, initial testing of the TPC
  analog digital electronics
• and DAQ system.
• RD, design and construction of the slow
  controls system software
• Complete detector monte carlo
• WIPP operations (SLAC postdoc one of two
  postdocs in charge)
• WIPP safety/utilities
• The SLAC group also led initial RD work on ion
  mobility and ion capture in LXe in particular the
  presently favored approach (cryo probe) and
  continues to study alternative methods (heated
  probe).

July 7th 2008
SLAC Annual Program Review
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EXO200 cleanrooms in End Station III at HEPL
(Stanford) prior to move to underground location
(WIPP)
cleanroom 1 detector/cryostat cleanroom 2 xenon
handling systems cleanroom 3 houses the
refrigerators
3
2
1
Assembled cleanrooms at Stanford total of 6
rooms, innermost class 1000 for cryostat
support equipment and work areas (incl. LAAPD
test stand).
The End Station at Stanford was used to assemble
and test the entire cleanroom setup including all
cryogenic and xenon handling systems. Tests were
completed in April 07 prior to shipment to WIPP.
crane rails
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EXO200 cleanrooms about 2 years ago
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. In module
2, the xenon system was built, and in module 3,
there are now 4 refrigerators and transfer lines.
Commissioning tested all this our control
systems.
Refrigerators
In module 3 (cryosystems)
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the EXO200 cleanrooms are now crowded.
Shown here is the cryostat in module 1, which was
tested with temporary SS doors and without Pb
shielding in place while at Stanford.
Temporary SS doors with SS test vessel
attached. Inner Cu vessel is visible (as is
plastic door cover protecting the seal). The
8.5 liter test vessel was later filled with LXe.
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High purity Pb shielding
Cryostat interior filled with HFE7000 heat
transfer fluid
Cu TPC vessel
Cu Cryostat
Layout of the EXO200 cryostat and the LXe TPC
A very recent photo of the high-purity copper TPC
xenon vessel fabricated at Stanford with e-beam
welding done by outside firm. Now nearly complete.
interior view of vessel showing TPC detection
planes, central cathode, teflon reflection walls
shaping rings.
reinforced endplates, and TIG-welded rim
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EXO200 LXe TPC
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
259 per end, and is used for timing (z) and to
enhance energy resolution. Ultra-low activity
Cu e-beam welded pressure vessel (1.5 mm thick
wall, 20kg)
40 cm
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EXO wireless technology
wire triplets are symmetric endto-end, 1 cm
wide, with 125 µm wire size on 3 mm pitch.
Support ring (Cu, with acrylic wire frame
parts). X and Y planes are actually at 60 degrees
The EXO TPC electrode design is in most
respects fairly conventional. But the structure
fabrication method is not. There are no wires.
Instead, both wire planes are made from etched
phosphor-bronze, as are the various TPC assembly
fixtures. The wire triplets (one channel) are
self tensioning and are clipped into position
onto acrylic supports. All materials are highly
radiopure.
Support for APDs (1/8 copper). One per end, 259
APDs each.
Detail of 7 APD cluster (backside) showing
etched phosphor-bronze retainer and interconnect
spider.
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TPC electrode structure (mock up of ½
chamber) Copper, etched phosphor-bronze, teflon
and acrylic parts are being fabricated and will
be ready for assembly this summer.
Kapton/copper readout cables being made now.
Actual copper TPC xenon vessel (Shown here at
e-beam welding shop. The legs for plumbing and
signal cables were welded on last week - Al
fixturing is visible here).
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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 drift (E-300) at WIPP has received the EXO
cleanrooms and the anxillary equipment. Photo
shows this area before our arrival.
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Underground at WIPP
The cleanrooms are installed on adjustable stands
on the salt floor. Assembly work is underway
inside and outside the cleanrooms. All xenon
handling cryosystems will be recommissioned
in the coming months.
Since the time that this photos were
taken, additional support and preassembly
structures have been added in preparation for
the delivery of the TPC. (All modular
rooms, containers etc. are sized to fit into the
WIPP shaft)
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At WIPP inside modules 1 and 2 The cryostat
shielding Pb nearly complete, and a more recent
photo of the xenon test vessel
installation. Below part of the gXe system
incl. the SAES purifiers, is shown.
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• EXO200 Schedule
• At present, cleanrooms, support facilities (small
  machine shops, UPS power, clean pre-assembly
  areas, storage, terminals/communications room)
  are underground at WIPP.
• At WIPP Cryo/xenon system/controls tests
  Aug. Sept.
• In ESIII cleanroom (Stanford) TPC assembly,
  installation
• in the Cu xenon vessel, mechanical/electronics
  testing
• 
  July thru October.
• Detector shipment to WIPP late
  October or late January
• 
  (due to WIPP shaft lift refurbishing)
  
• Cooldown
  early 2009
• Engineering Run (natural xenon), Physics Runs.
  2009

July 7th 2008
SLAC Annual Program Review
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