Transcript High-pressure 136Xe TPC: Emerging opportunities in the

Combined
WIMP and 0-  decay searches
with
High-pressure
136Xe
Dave Nygren
LBNL
Gas TPC
Energy partitioning in Xe()
Ionization
signal only
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Energy Partitioning
• The anti-correlations observed in LXe reflect
a partitioning of event energy between:
– Scintillation S
– Ionization I
– Heat
Event visible energy = (a x S) + (b x I)
• Only a fraction of the light can be detected
• But: Highly non-gaussian fluctuations!
• Measurement precision is compromised
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 Strategy:
• Use High pressure xenon gas TPC
– ~20 bars ( = ~0.1 gm/cm3)
– E/E (I) is intrinsic:  =√FN F = 0.15
– E/E = 2.7 x 10-3 FWHM @ 2.48 MeV
• 2 = (F + G + L) x E/W:
(NIM A 581 (2007) 632-642)
– Best energy resolution in practice:
• Gas proportional scintillation: G ~0.2
• Losses: L <0.05
E/E ~4 x 10-3 FWHM @ 2.48 MeV
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Combined Search: Issues
• 0- 
• WIMP
– Energy: 2.5 MeV
– Resolution: Etotal
• E/E <1% FWHM
• Ionization signal only
– Primary Scintillation:
– 5 < Erecoil < ~50 keV
– Resolution: S2/S1
• N/ Discrimination
• I + S gives energy
– Primary Scintillation:
• TPC start signal
• Modest sensitivity
– Background:
• -rays
• TPC start signal
• Maximum sensitivity
– Backgrounds
• -rays
• neutrons
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Scintillation Dynamic range: OK
– S1 WIMP signals are very small
• Optimize detector for maximum S1 detectiion efficiency
• Not a problem if S1 signals from  events saturate PMT
– S2 signals are not very different
• only ~2 mm of  track is present in the PS gap
•  track instantaneous signal is ~1000 e– (25 keV)
• OK, tracks parallel to PS plane will saturate…
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S2/s1 Fluctuations…
– S2/S1 resolution in LXe
• degraded by anomalous large fluctuations in LXe
• dominant effect in LXe
• sensitivity compromised in LXe
– S2/S1 ( < 0.5 g/cm3) in HPXe:
• anomalous fluctuations are surely absent for  particles;
• very little recombination  S2/S1 should be “large”
– S2/S1 for nuclear recoils in HPXe is unmeasured
• Careful, systematic measurements need to be done
N/ discrimination might be (much) better in HPXe
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Now: 7-PMT cell @ TAMU
Uncalibrated, raw,
initial data from
60 keV  source
(similar to GPSC
on board BeppoSAX satellite what is that? )
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Beppo-SAX Xe Gas Proportional Counter
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Beppo-SAX Gas Proportional Counter
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Next: 37-PMT Cell
• 3 PMT rings added around center PMT
– Total number of PMTs: 37
– Diameter: ~18 cm, drift length ~18 cm
• Goal:
– Demonstrate tracking of ~1 MeV  particles
– Determine best practical E/E resolution
– Determine N/ discrimination in system
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Time scales
• 7-PMT TAMU system: now  2008
• 37 PMT system (DUSEL R&D proposal)
– 2008  2010
– Costs: ~150 k$/year, for three years
• 200 kg system
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Proposal in 2009/ reviews 2010
Construction start 2011/12
Costs: $6,782,361.49
1000 kg system also feasible
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1000 kg Xe:  = 225 cm, L =225 cm
 ~ 0.1 g/cm3 (~20 bars)
A.
Sensitive volume
B.
HV cathode plane
C.
GPSC readout planes, optical
gain gap is ~1-2 mm
D.
Flange for gas & electrical
services to readout plane
E.
Filler and neutron absorber,
polyethylene, or liquid
scintillator, or …
F.
Field cages and HV insulator,
(rings are exaggerated here)
likely site for photo-detectors
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Two identical HPXe TPCs
Two distinct physics goals
• “WIMP Detector”
• “ Detector”
– Fill with enriched Xe
mainly 136Xe
– Events include all 
events + backgrounds
– Fill with normal Xe or fill
with “depleted” Xe
– Events include only
backgrounds to 
– Isotopic mix is mainly
even-A
– WIMP events include
more scalar interactions
– Isotopic mix is ~50%
odd-A: 129Xe 131Xe
– WIMP events include
more axial vector
interactions
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Barium daughter tagging
and ion mobilities…
• Ba+ and Xe+ mobilities are quite different!
– The cause is resonant charge exchange
– RCE is macroscopic quantum mechanics
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occurs only for ions in their parent gases
no energy barrier exists for Xe+ in xenon
energy barrier exists for Ba ions in xenon
RCE is a long-range process: R >> ratom
glancing collisions = back-scatter
RCE increases viscosity of majority ions
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Barium daughter tagging
and ion mobilities…
– Ba++ ion survives drift: IP = 10.05 eV
– Ba++ ion arrives at HV plane, well ahead of all other Xe+ ions
in local segment of track
– Ba++ ion liberates at least one electron at cathode surface,
which drifts back to anode
– arriving electron signal serves as “echo” of the Ba++ ion,
providing strong constraint
– Clustering effects are likely to alter this picture!
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A small test chamber
can show whether ion
mobility differences
persist at higher gas
density (no data now).
This could offer an automatic method to tag the
“birth” of barium in the
decay, by sensing an echo
pulse if the barium ion
causes a secondary
emission of one or more
electrons at the cathode.

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QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
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Molecular physics of xenon
– Ionization process creates regions of high
ionization density in a very non-uniform way
– As density of xenon increases, aggregates form,
with a localized quasi-conduction band
– Recombination is ~ complete in these regions
• Complex multi-step processes exist:
• Xe+ + e–  Xe* (or direct excitation)
• Excimer formation: Xe*+ Xe  Xe2*  h + Xe
– Also: Xe*+ Xe*  Xe**  Xe++ e- + heat
• Two excimers are consumed to make one photon!
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Gamma events ()
Latest Xenon-10
QuickTime™ and a
results look better,
TIFF (LZW) decompressor
are needed to see this picture.
but nuclear recoil
acceptance still
needs restriction
Neutron events (NR)
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“Intrinsic” energy resolution
for HPXe ( < 0.55 g/cm3)
Q-value of 136Xe = 2480 KeV
W = E per ion/electron pair = 22 eV (depends on E-field)
N = number of ion pairs = Q/W
N  2.48 x 106 eV/22 eV = 113,000
N2 = FN
(F = Fano factor)
F = 0.13 - 0.17 for xenon gas

N = (FN)1/2 ~ 130 electrons rms
E/E = 2.7 x 10-3 FWHM @ 2.48 MeV (intrinsic fluctuations only)
Compare: Ge diodes (energy per pair) √3/22 = 0.37
Compare: LXe/HPXe Fano factors: √20/.15 = 11.5 !
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Pioneer HPXe TPC detector
for 0-  decay search
• “Gotthard tunnel TPC”
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5 bars, enriched 136Xe (3.3 kg)
MWPC readout plane, wires ganged for energy
No scintillation detection!  no TPC start signal
E/E ~ 80 x 10-3 FWHM (1592 keV)
 66 x 10-3 FWHM (2480 keV)
Reasons for this less-than-optimum resolution are not clear
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Energy resolution issues
in traditional gas detectors
• Main factors affecting ionization:
– Intrinsic fluctuations in ionization yield
• Fano factor (partition of energy)
– Loss of signal
• Recombination, impurities, grids, quenching,
– Avalanche gain fluctuations
• Bad, but wires not as bad as one might imagine...
– Head-tail +ion effects corrupt the wire avalanche gain
• Long tracks may really suffer from this in MWPC
– Electronic noise, signal processing, calibration
• Extended tracks  extended signals
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Loss of signal
Fluctuations in collection efficiency  introduce another factor: L
L = 1 -  similar to Fano factor (assume uncorrelated errors)
N 2 = (F + L)N
– Loss on grids is small: Lgrid < F seems reasonable
• If Lgrid = 5%, then E/E = ~3 x 10-3 FWHM
– Other sources of L include:
• Electronegative impurities that capture electrons (bad correlations)
• Escape to edges
• Quenching - of both ionization and scintillation!
Xe* + M  Xe + M*  Xe + M + heat (similarly for Xe2*, Xe**, Xe2*+… )
Xe+ + e–(hot) + M  Xe+ + e–(cold) + M*  Xe+ + e–(cold) + M + heat 
e–(cold) + Xe+  Xe*
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 particles
A surprising result: adding a
tiny amount of simple
molecules - (CH4, N2, H2 )
quenches both ionization
and scintillation ( particle)
 dE/dx is very high; does this
effect depend on  too? (yes...)
Impact for atomic recoils?…
Gotthard TPC: 4% CH4
how much ionization for 
particles was lost?
K. N. Pushkin et al, IEEE
Nuclear Science Symposium
proceedings 2004
(~25 bars)
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H. E. Palmer & L. A. Braby
Nucl. Inst. & Meth. 116 (1974) 587-589

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From this spectrum: G ~0.19
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Fluctuations in PS
• G for PS contains three terms:
• Fluctuations in nuv (UV photons per e):
uv = 1/√nuv
– nuv ~ HV/E = 6600/10 eV ~ 660
• Fluctuations in npe (detected photons/e): pe = 1/√npe
– npe ~ solid angle x QE x nuv x 0.5 = 0.1 x 0.25 x 660 x 0.5 ~ 8
• Fluctuations in PMT single PE response: pmt ~ 0.6
G = 2 = 1/(nuv) + (1 + 2pmt)/npe) ~ 0.17
Assume G + L = F, then
Ideal energy resolution (2 = (F + G + L) x E/W):
E/E ~4 x 10-3 FWHM
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Some Issues:
– Operation of PS at 20 bar? should work...
– Maintain E/P, but surface fields larger, gaps smaller…
– Large diffusion in pure xenon
–   tracking good enough? (I think so… needs study)
– Integration of signal over area? (… needs study)
– are pixels on the track edge adding signal or noise?
– Role of additives such as: H2 N2 CH4 CF4 Ne?
– molecular additives reduce diffusion, increase mobility, but
– Do molecular additives quench  signals? (atomic recoils?)
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Surface/volume
• For given mass M, HPXe detector has
~9 x more surface area than LXe…
 backgrounds 9x larger ?
– Will background rejection be >>9x?
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