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WIMPS, 0-   decay & xenon High-pressure 136 Xe Gas TPC: An emerging opportunity

WIMPS, 0-   decay & xenon High-pressure 136 Xe Gas TPC: An emerging opportunity. David Nygren Physics Division - LBNL. Outline. 0-   decay & WIMPs Conclusion Background rejection Energy resolution TPC options - “due diligence” Scaling to the 1000 kg era Issues Perspective.

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WIMPS, 0-   decay & xenon High-pressure 136 Xe Gas TPC: An emerging opportunity

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  1. WIMPS, 0- decay & xenonHigh-pressure 136Xe Gas TPC:An emerging opportunity David Nygren Physics Division - LBNL

  2. Outline • 0-  decay & WIMPs • Conclusion • Background rejection • Energy resolution • TPC options - “due diligence” • Scaling to the 1000 kg era • Issues • Perspective Stockholm October 2007

  3. WIMPs • Dark matter: ~23% of universe mass • visible ordinary matter: ≤1% • atoms, ordinary matter ~4% • WIMPs - lightest supersymmetric particle? • Mass range: ~100 - 1000 GeV/c2 • Interaction strength not pinned down • Recoil energy: ~ 0 - 100 keV • For a positive claim: Very robust evidence shall be required! Stockholm October 2007

  4. Z Two Types of Double Beta Decay A known standard model process and an important calibration tool 2  If this process is observed: Neutrino mass ≠ 0 Neutrino = Anti-neutrino! Lepton number is not conserved! 0  Neutrino effective mass Neutrinoless double beta decay lifetime Stockholm October 2007

  5. Conclusions: • An attempt to optimize a detector for a 0-  decay search in 136Xe has led me to the following three conclusions: • High-pressure xenon gas (HPXe ~20 bar) will offer much better resolution than liquid xenon • Optimal energy resolution is achieved with a proportionalscintillation readout plane • Result: An optimum WIMP detector too!  Two identical TPCs, filled with different isotopes… Stockholm October 2007

  6. “ Detector” Fill with enriched Xe mainly 136Xe Isotopic mix is mainly even-A Events include all  events + background WIMP events include more scalar interactions “WIMP Detector” Fill with normal Xe or fill with “depleted” Xe Isotopic mix is ~50% odd-A: 129Xe 131Xe Events include only backgrounds to  WIMP events include more axial vector interactions Two Identical HPXe TPCs Stockholm October 2007

  7. Double beta decay Only 2-vdecays! Only 0-v decays! Rate No backgrounds above Q-value! 0 Energy Q-value The robust result we seek is a spectrum of all  events, with negligible or very small backgrounds. Stockholm October 2007

  8. Perils of backgrounds • Sensitivity to active mass deteriorates if backgrounds begin to appear: • m ~ {1/MT}1/2 • m ~ {(bE)/MT}1/4 • b = number of background events/unit energy • E = energy resolution of detector system • M = active mass • T = run time of experiment Ouch! Stockholm October 2007

  9. Current Status • Present status (partial list): • Heidelberg-Moscow(76Ge) result: mv= 440 +14-20 meV - disputed! • Cuoricino (130Te): taking data, but - background limited... • EXO (136Xe): installation stage, but - E/E shape/resolution issue... • GERDA (76Ge): under construction at LNGS • Majorana (76Ge): proposal & R&D stage • NEMO  Super-NEMO (foils): proposal & R&D stage • Global synthesis:mi < 170 meV(95% CL)*  100’s to 1000 kg active mass likely to be necessary! • Rejection of internal/external backgrounds in ~1027 atoms! • Excellent energy resolution: E/E <10 x 10-3 FWHM - at least! • Target (from oscillations): m ~50 meV *Mohapatra & Smirnov 2006 Ann. Rev. Nucl. Sci.56 - (other analyses give higher values) Stockholm October 2007

  10.  - Background rejection 1. Charged particles • With a TPC, a fiducial volume surface can be defined with all the needed characteristics: • fully closed • Precise spatial definition • 100% active - no partially sensitive surfaces • deadtime-less operation • surface is variable ex post facto • Charged particles cannot penetrate fiducial surface unseen; • 100.000% rejection Stockholm October 2007

  11.  - Background rejection 2. Neutral Particles • -rays(most serious source, and needs real work) • Radon daughters (208Tl, 214Bi) plated out on surfaces • Photo-conversion fluorescence: 85% ( ~1 cm @ 20 bar) • Multiple Compton events more likely at large scale • Cannot get adequate self-shielding for MeV -rays • Neutrons can activate detector components, • Excitation of xenon may be useful calibration signal • Neutral particles need serious study - high priority Stockholm October 2007

  12. HPXe, LXe TPC Fiducial volume -HV plane Readout plane Readout plane Fiducial volume surface . * electrons ions Real event Backgrounds Stockholm October 2007

  13. Installation now, at WIPP: 200kg of 80% enriched 136Xe Liquid xenon TPC Localization of the event in x,y,z (using scintillation for T0 ) Avalanche Photo-Diodes (APD) used to detect scintillation Strong anti-correlation of scintillation & Ionization components Expect E/E= 3.3% FWHM for 0nbb For a subsequent phase of experiment: Barium daughter tagging byoptical spectroscopy Electrostatic capture & mechanical transfer of ion to trap - Can this ambitious idea really work? Liquid or gas under consideration EXO-200: LXe TPC Stockholm October 2007

  14. EXO Experimental Design Stockholm October 2007

  15. EXO: • Anti-correlation observed between scintillation and ionization signals • Anti-correlation also observed in other LXe data • E/E= 3.3% FWHM for 0nbb expected resolution (2480 KeV) • What about the tails? Stockholm October 2007

  16. Energy resolution in xenon shows complexity: ionizationscintillation Ionization signal only For  >0.55 g/cm3, energy resolution deteriorates rapidly! Stockholm October 2007

  17. Molecular physics of xenon • Complex multi-step processes exist: *... • Ionization process creates regions of high ionization density in very non-uniform way • As density of xenon increases, aggregates form, with a localized quasi-conduction band • Recombination is ~ complete in these regions • Excimer formation Xe*+ Xe  Xe2* h + Xe • Also: Xe*+ Xe*  Xe**  e- + Xe++ heat • Density dependence of signals is strong! * not completely understood - most likely, not by anyone Stockholm October 2007

  18. Impact for WIMP search? But...wait a second! WIMP searches in LXe use the ratio  = Primary ionization Primary scintillation to discriminate nuclear from electron recoils nuclear < electron Stockholm October 2007

  19. Stockholm October 2007

  20. Impact for WIMP Search... The anomalous fluctuations in LXe must degrade the nuclear recoil discriminant S2/S1 =  ... So: Maybe HPXe would be better! ...but Do the fluctuations depend on • atomic density? • ionization density? • particle type? • all of the above? Many -induced events reach down to nuclear band in plots of S2/S1 versus energy Stockholm October 2007

  21. Impact for WIMP Search... The anomalous fluctuations in LXe must degrade the nuclear recoil discriminant S2/S1 =  ... So: Maybe HPXe would be better! ...but Do the fluctuations depend on • atomic density? • ionization density? • particle type? • all of the above? - YES, in LXe Many -induced events reach down to nuclear band in plots of S2/S1 versus energy Stockholm October 2007

  22. Gamma events () Latest Xenon-10 results look better, but nuclear recoil acceptance still needs restriction Log10 S2/S1 Neutron events (NR) Stockholm October 2007

  23. Fluctuations • Let R = [log10 {S2/S1()}] [log10 {S2/S1(N)}] • R for LXe: ~101/2(from previous slide) • R for HPXe: unmeasured • Let D = discrimination power vs density D = R/T T =[2 {log10S2/S1()} + 2 {log10S2/S1(N)}]1/2 Stockholm October 2007

  24. Fluctuations… • S2/S1(, HPXe) ( < 0.5 g/cm3) is roughly known • anomalous fluctuations are surely absent; •  (Q/S ()) is normal in HPXe for sure! But: • S2/S1([N, , , energy,E-field], HPXe) unmeasured • At least, haven’t found any relevant data • R could be smaller/larger in HPXe, but: D could/should be much larger! Stockholm October 2007

  25. Stockholm October 2007

  26. What about “nuclear” recoil? • v/c  0.001 for 50 keV recoiling xenon • v/c  0.003 for a thermal electron  atomic shells stay with nucleus… • recoiling atom, not just nucleus • Xe-Xe collisions should be different from , • “resonant charge exchange” isolates electrons • fast ion moves away quickly • isolated electrons are easily swept up by field Stockholm October 2007

  27. “Due Diligence” Evaluation of TPC operating modes Emphasis placed on: energy resolution for  decays Stability of operation One unlikely mode stands out Stockholm October 2007

  28. Pioneer HPXe TPC detector for 0-  decay search • “Gotthard tunnel TPC” • 5 bars, enriched 136Xe (3.3 kg) • 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 poor resolution are not clear... What is the intrinsic resolution? Can resolution near the intrinsic value be reached? Stockholm October 2007

  29. “Intrinsic” energy resolutionfor 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(intrinsic fluctuations only) Compare: Ge diodes (energy for pair) √3/22 = 0.37 Compare: LXe/HPXe Fano factors: √20/.15 = 11.5 !! Stockholm October 2007

  30. 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... • Electronic noise, signal processing, calibration • Extended tracks  extended signals Stockholm October 2007

  31. Loss of signal Fluctuations in collection efficiency  introduce another factor: L L = 1 -  similar to Fano factor (assuming no correlations) 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: • Recombination - ionization density, track topology • 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* Stockholm October 2007

  32.  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) Stockholm October 2007

  33. Avalanche gain • Gain fluctuations: another factor, “G” • N = ((F + G)N)1/20.6 < G < 0.8 * • N = ((0.15 + 0.8)N) 1/2 = 328 • E/E = 7.0 x 10-3 FWHM- not bad, but: • No more benefit from a small Fano factor • Very sensitive to density (temperature) • Monitoring and calibration a big effort • Space charge effects change gain dynamically • Micromegas, GEM, may offer smaller G *Alkhazov G D 1970 Nucl. Inst. & Meth.89 (for cylindrical proportional counters) Stockholm October 2007

  34. “Conventional” TPC • Pure xenon (maybe a small molecular admixture is OK...) • Modest avalanche gain is possible • Diffusion is large, up to ~1 cm @ 1 meter • drift velocity very slow: ~1 mm/s • Add losses, other effects,… N = ((F + G + L)N)1/2 E/E ~ 7 x 10-3 FWHM Avalanche gain may be OK, but can it be avoided? Stockholm October 2007

  35. “Ionization Imaging” TPC 1. No avalanche gainanalog readout (F + L) • dn/dx ~ 1.5 fC/cm:  ~9,000 (electron/ion)/cm • gridless “naked” pixel plane (~5 mm pads) • very high operational stability • E/E = 3 x 10-3 FWHM(F + L) only • but, electronic noise must be added! • 50 pixels/event @ 40 e– rms  N ~280 e– rms • E/E ~7 x 10-3 FWHM • Complex signal formation • Need waveform capture too Ionization imaging: Very good, but need experience Stockholm October 2007

  36. “Negative Ion” TPC “Counting mode” = digital readout, (F + L) • Electron capture on electronegative molecule • Very slow drift to readout plane; • Strip electron in high field, generate avalanche • Count each “ion” as a separate pulse: • Ion diffusion much smaller than electron diffusion • Avalanche fluctuations don’t matter, and • Electronic noise does not enter directly, either • Pileup and other losses: L~ 0.04 ? • E/E = ~3 x 10-3 FWHM • Appealing, but no experience in HPXe... Stockholm October 2007

  37. Proportional Scintillation (PS): “HPXe PS TPC” • Electrons drift (m) to high field region (5 mm) • Electrons gain energy, excite xenon, make UV • Photon generation up to ~1000/e, but no ionization • Multi-anode PMT readout for tracking, energy • PMTs see primary and secondary light • “New” HPXe territory, but real incentives exist • Sensitivity to densitysmaller than avalanche • Losses should be very low: L < F • Maybe: G ~ F ? Stockholm October 2007

  38. H. E. Palmer & L. A. Braby Nucl. Inst. & Meth. 116 (1974) 587-589  Stockholm October 2007

  39. From this spectrum: G ~0.19 Stockholm October 2007

  40. Fluctuations in Proportional Scintillation • 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 xnuvx 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 Stockholm October 2007

  41. HPXe PS TPC is the “Optimum”  detector • Readout plane is high-field layer with PMT array • Transparent grids admit both electrons and UV scintillation • Multi-anode PMT array “pixels” match tracking needs • Two-stage gain: optical + PMT: Noise < 1 electron! • No secondary positive ion production • Much better stability than avalanche gain • Major calibration effort needed, but should be stable • Beppo-SAX: 7-PMT HPXe (5 bar) GPSCin space! Stockholm October 2007

  42. Stockholm October 2007

  43. Stockholm October 2007

  44. Some Gas Issues: • Operation of PS at 20 bar? should work... • Maintain E/P, but surface fields are larger… • 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? • molecular additives reduce diffusion, increase mobility, but • Do molecular additives quench  signals? (atomic recoils?) Stockholm October 2007

  45. More gas issues: Neon • Multiple roles of neon(must add a lot) • to diminish Bremmstrahlung? • to increase drift velocity? • to diminish multiple scattering? • to add Penning effect, thereby reducing F? • to facilitate topology recognition by B field? • to add higher energy WIMP-nuclear recoil signals? • Operation of PS with neon? should work... • neon excitation energy transfers to xenon • May be “invisible” Stockholm October 2007

  46. More issues Detection of xenon primary UV scintillation (175 nm - ~7.3 eV) • nelectrons nphotons (depends on E-field, particle, etc) • 0-  decays (Q = 2480 keV): No problem! • WIMP signals (5 - 100 keV): Need very high efficiency! • Design augmentation: Must maximize primary UV detection! Add reflective surfaces along sides - for sure... Add optimized dielectric mirror to cathode plane (corner reflectors?) Stockholm October 2007

  47. Even more issues • For given mass M, HPXe detector has ~9 x more surface area than LXe…  backgrounds larger !? • Background rejection may be so much better that overall performance is better. • Scintillator shell along sides may help… Stockholm October 2007

  48. Scaling to 1000 kg • Minimum S/ V for cylindrical TPC: L = 2r • Fixed mass M & track size l: • l/L  -2/3 • higher density  less leakage • High voltage L M1/3(fixed ) • Number of pixels involved in event: • Diffusion  L 1/2so: very little change Scaling to large M is fairly graceful Stockholm October 2007

  49. 1000 kg Xe:  = 225 cm, L =225 cm  ~ 0.1 g/cm3 (~20 bars) Sensitive volume B. HV cathode plane GPSC readout planes, optical gain gap is ~1-2 mm Flange for gas & electrical services to readout plane Filler and neutron absorber, polyethylene, or liquid scintillator, or … Field cages and HV insulator, (rings are exaggerated here) possible site for scintillators Stockholm October 2007

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