Z.Djurcic, D.Leonard, A.Piepke, University of Alabama

1
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.OSullivan, A.Pocar, J.Wodin Stanford
University
2

• Neutrinos have mass and
• they oscillate.
• Still unknown
• Are neutrinos Majorana ?
• What are the absolute masses ?
• 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 ?

3

• 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.
4
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

5
Calculating the rates for 2? and 0???
The effective mass ltmgt 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 ltmgt sensitivity to
the 10 meV region
6
ltmgt 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.
7
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.
8
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.
9
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.)
10
There are many planned experiments, using a
variety of techniques and a variety of isotopes,
and several of these are ambitious tonne-scale
programs
Experiment Nucleus Detector T0? (y) ltm?gt eV
CUORE 130Te .77 t of TeO2 bolometers (nat) 7 x 1026 .014-.091
EXO 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 gt 1026 .053-.24
COBRA 116Cd , 130Te 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 Gd2SiO5Ce scint in liq scint 2 x 1026 .038-.172
Xmass 136Xe 1 t of liquid Xe 3 x 1026 .086-.252
I will now turn to a novel approach to the
experimental problems of 0??? searches that is
being pursued by the EXO collaboration
11
A claim for 0?ßß 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
The upcoming prototype experiments plan to
confirm or disprove the claim within the next
typ. 3 years, including EXO
12
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 eg.)
• 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

13
EXO RD
The 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
lt1ppb 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
14
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.
15
Xenon purification system at SLAC
hot Zr gas purifier (SAES)
purity monitor (drift cell with laser P.E.
cathode)
Best observed electron lifetimes 10 ms
distillation bottles
Continuous circulation of xenon gas through
purifier works well. This capability will be
available in our first experimental
prototype. (outgassing is likely to be an issue
and purity may have to be maintained continually).
recirculation bellows pump
16
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
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
17
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.
18
Laser fluorescence barium identification
A well-studied technique pioneered by atomic
physicists in the 1980s 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
neutral 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
19
Linear Paul ion trap RD at Stanford
TMP/cryo pump
probe unloading
ion trap/laser tag
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. RD underway for ion trap/probe
interface at SLAC/Stanford.
20
Linear ion trap vacuum enclosure Integration
with a cold-probe ion delivery system being
planned, as well as alternative probe schemes.
21
Barium ion extraction RD at SLAC preliminary
studies
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).
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.
22
probe down position
3-position pneumatic actuator
probe up position
Probe test cell Demonstrate ion capture/release
in LXe with electrostatic probe
Xenon cell
23
Second EXO publication (NIM. A)
Probe Test Cell used to measure ion
mobility Observed mobility of 0.240.02 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).
24
Ion Capture Cryo Probe prototype Enabling ion
release
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). 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
25
Ungoing RD for ion capture/tagging Proceeding
on several fronts in parallel
Additional engineering/tech support will be
needed to continue RD at SLAC
26
40K ? 1461 keV ?s, a background for 2??? only
238U chain
214Bi ? 2448 keV ?
222Rn produces (214Bi) ? bkgrd, daughter ?, a
emitters can move throughout the apparatus with
deadly results.
Radioactive Backgrounds - primary isotopes
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.
232Th chain
208Tl ? 2615 keV ?
27
Qualification of low background materials (U.of
Ala., Neuchatel, INMS, Laurentian)
Material requirements for ßß2?-bkg lt 10
events/day and ßß0?-bkg lt 3 events/year (no
tracking cuts)
Assay _at_ U. of A. Ge detector following neutron
activation _at_ the MIT research reactor, Neuchatel
Ge counter and INMS (Canada) mass spec. (ICPMS
and GDMS methods). Radon counting (Laurentian).
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).
28
EXO Performance Projections

• 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 2.479 MeV endpoint
• 4) Use for 2?ßß T1/2gt11022yr (Bernabei et al.
  measurement)

Case Mass (ton) Eff. () Run Time (yr) sE/E _at_ 2.5MeV () 2?ßß Background (events) T1/20? (yr, 90CL) Majorana mass (meV) QRPA (NSM) Majorana mass (meV) QRPA (NSM)
Conservative 1 70 5 1.6 0.5 (use 1) 21027 50 (68)
Aggressive 10 70 10 1 0.7 (use 1) 4.11028 11 (15)
s(E)/E 1.4 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 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
29
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.

30
EXO200 cryostat and shielding Schematic only
inner outer copper vessels
xenon plumbing
TPC vessel
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.
31
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
359 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
32
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)
LAAPDs from API (Advanced Photonics) immersed in
LXe. High radiopurity, High QE (gt100) in
VUV Operated at -1.4kV, Gain 100. Noise for
gang of 7 2000 electrons isacceptable.
33
EXO200 cleanrooms in End Station III at HEPL
(Stanford)
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 cryostat is presently in module 1, along with
some Pb shielding. Xenon plumbing is going in
in module 2, and module 3 contains 3 high-power
refrigerators.
crane rails
34
EXO200 cleanroom activity June 06
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 assembled and tested along with
the control systems.
Refrigerators
In module 3 (cryosystems)
35
EXO200 Projections

• s(E)/E 1.6
• Low but finite radioactive background 20
  events/year in the 2s interval centered around
  the 2.479 MeV endpoint
• 3) Negligible background from 2?ßß (T1/2gt11022yr
  R.Bernabei et al. measurement)

Case Mass (ton) Eff. () Run Time (yr) sE/E _at_ 2.5MeV () Radioactive Background (events) T1/20? (yr, 90CL) Majorana mass (eV) QRPA (NSM) Majorana mass (eV) QRPA (NSM)
Prototype 0.2 70 2 1.6 40 6.41025 0.27 (0.38)
Testing the Klapdor et al. claim 3s range
1.23-0.5 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
11.2s effect the best case. QRPA/lower limit
19 signal events (and 40 bkg), a 2.6s effect
the worst case.
36
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
37
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 RD
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 RD
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
year-old 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.
38
Candidate Q Abund. Isotope
(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
?? 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.

• 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.

39
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 lt 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,?)Xe137
Xe137 production by cosmics (Xe?Cs?Ba via ?)
40
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.

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 10s), trigger rate would
be lt 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/?t 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 lt 1 minute a reasonable
target. Conclusion Fetch lt1/hour, fetch time
lt1 min. ? lt1/60 or lt1.7 deadtime