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DUSEL Workshop 2007 3
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
DUSEL Workshop 2007 4
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
DUSEL Workshop 2007 5
Combined Search: Issues
• 0- – Energy: 2.5 MeV– Resolution: Etotal
E/E <1% FWHM• Ionization signal only
– Primary Scintillation:• TPC start signal• Modest sensitivity
– Background: -rays
• WIMP– 5 < Erecoil < ~50 keV– Resolution: S2/S1
• N/ Discrimination• I + S gives energy
– Primary Scintillation:• TPC start signal• Maximum sensitivity
– Backgrounds -rays• neutrons
DUSEL Workshop 2007 6
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…
DUSEL Workshop 2007 7
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
DUSEL Workshop 2007 8
Now: 7-PMT cell @ TAMU
Uncalibrated, raw,
initial data from
60 keV source
(similar to GPSC on board Beppo-SAX satellite - what is that? )
DUSEL Workshop 2007 12
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
DUSEL Workshop 2007 13
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– Proposal in 2009/ reviews 2010– Construction start 2011/12– Costs: $6,782,361.49 – 1000 kg system also feasible
DUSEL Workshop 2007 14
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
DUSEL Workshop 2007 15
Two identical HPXe TPCsTwo distinct physics goals
• “ Detector”– Fill with enriched Xe
mainly 136Xe– Events include all
events + backgrounds
– Isotopic mix is mainly even-A
– WIMP events include more scalar interactions
• “WIMP Detector”– Fill with normal Xe or fill
with “depleted” Xe– Events include only
backgrounds to
– Isotopic mix is ~50% odd-A: 129Xe 131Xe
– WIMP events include more axial vector interactions
DUSEL Workshop 2007 17
Barium daughter tagging and ion mobilities…
• Ba+ and Xe+ mobilities are quite different!– The cause is resonant charge exchange– RCE is macroscopic quantum mechanics
• 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
DUSEL Workshop 2007 18
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!
DUSEL Workshop 2007 19
A small test chamber can show whether ion mobility differences persist at higher gas density (no data now).
This could offer an auto-matic 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.
DUSEL Workshop 2007 24
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!
DUSEL Workshop 2007 25
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Gamma events ()
Neutron events (NR)
Latest Xenon-10 results look better, but nuclear recoil acceptance still needs restriction
L og 1
0 S
2/S
1
DUSEL Workshop 2007 26
“Intrinsic” energy resolutionfor HPXe ( < 0.55 g/cm3)
Q-value of 136Xe = 2480 KeVW = E per ion/electron pair = 22 eV (depends on E-field)N = number of ion pairs = Q/WN 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 !
DUSEL Workshop 2007 27
Pioneer HPXe TPC detector for 0- decay search
• “Gotthard tunnel TPC”– 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
DUSEL Workshop 2007 28
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
DUSEL Workshop 2007 29
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*
DUSEL Workshop 2007 30
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)
particles
DUSEL Workshop 2007 33
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
DUSEL Workshop 2007 35
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?)