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David Nygren LBNL

NEXT A High-pressure Xenon Gas TPC: How superior energy resolution benefits both 0-   decay in 136 Xe and WIMP searches. David Nygren LBNL. Outline. What’s NEXT ? Xenon gas TPC: new R&D results! Both WIMP & 0-   decay searches? Electroluminescence (EL): a neglected tool

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David Nygren LBNL

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  1. NEXTA High-pressure Xenon Gas TPC:How superior energy resolution benefits both 0- decay in 136Xe and WIMP searches David Nygren LBNL NEXT: FNAL 2012

  2. Outline • What’s NEXT? • Xenon gas TPC: new R&D results! • Both WIMP & 0-  decay searches? • Electroluminescence (EL): a neglected tool • The bigger picture: EL with tracking • Intended US role in NEXT NEXT: FNAL 2012

  3. “Neutrino Experiment Xenon TPC” NEXT is an approved & funded search for 0-  decay based on a high-pressure xenon gas (HPXe) TPC NEXT will be constructed in Spain, in the new, improved Canfranc Underground Laboratory. NEXT has been funded by Spanish Funding Agencies at the level of € 6M+ NEXT R&D phase is nearing completion, construction to start in FY2012 NEXT: FNAL 2012

  4.  ISU Spain provides: Most of the collaborators Most secured funding Host Laboratory - LSC Key contributions from international groups Engineering and integration TPC expertise high-pressure gas detectors Xenon supply & enrichment NEXT: FNAL 2012

  5. US groups involved in new DOE proposal (in preparation): • LBNL: Azriel Goldschmidt (NSD), John Joseph (Elec. Eng.), Tom Miller (Mech. Tech.), David Nygren (Physics), Josh Renner (student),Derek Shuman (Mech. Eng.) • Texas A&M: James White (Faculty), Clement Sofka (student) • Iowa State University: John Hauptman (Faculty) + students TBD NEXT: FNAL 2012

  6. Laboratorio Subterraneo de Canfranc Waiting for NEXT! NEXT: FNAL 2012 6

  7. Double beta decay spectra Only 2-vdecays Only 0-v decays Rate No backgrounds above Q-value 0 ( electron energy) Q-value The ideal result: a spectrum of only  events, with a 0- signal present as a narrow peak, well-separated from 2- NEXT: FNAL 2012

  8. Energy resolution in Xenon:Strong dependence on density! Very large fluctuations between light/charge! F ~ 20 WIMPs: S2/S1 suffers! Here, the fluctuations are normal F = 0.15 Unfolded resolution: E/E ~0.6% FWHM Ionization signal only! For  <0.55 g/cm3, ionization energy resolution is “intrinsic” NEXT: FNAL 2012

  9. What does a search for 0-  require? Sensitivity and Background Rejection • High sensitivity  large mass of candidate isotope NEXT has 100 kg of enriched xenon: ~85% 136Xe • Extremely good background rejection! • Shielding, radio-purity, excellent energy resolution, event topology are critical • High Q-value of 136Xe, 2457 keV, places signal above most -rays • NEXT energy resolution: E/E <0.7 % FWHMexpected at E = Q-value • The TPC monolithic fiducial volume presents a fully active surface • Good 3-D tracking in high-pressure xenon gas reveals event topology • Excellent discrimination between 1- and 2- electron events • All charged particles from surfaces will be rejected • Neutrons not an important background NEXT: FNAL 2012

  10. What does a search for WIMPs require? Sensitivity and Background Rejection • High sensitivity  large sensitive mass NEXT has 100 kg of enriched xenon: ~85% 136Xe A large component of neon can be added for better match to low-mass WIMPs • Extremely good background rejection! • NEXT offers superior discrimination between nuclear and electron recoils, Huge S2/S1 fluctuations degrade discrimination in LXe, but not in HPXe • NEXT will exploit the TPC idea to realize a monolithic fully active fiducial volume, Essentially all charged particle background events excluded. • NEXT will possess good 3-D tracking in high-pressure xenon gas Event topology reveals single & mulitple-site interactions, reject gammas & neutrons NEXT: FNAL 2012

  11. The requirements have similarities... • At TAMU, Moscow, and LBNL, near-intrinsic energy resolution has been been shown in HPXe TPCs, using -rays of 60, 122, and 662 keV • Our new result is a world record for Xe-based detectors • An electroluminescent gain stage is the key concept. • We assert: “0-  and direct detection WIMP searches can be made simultaneously in one detector, without compromise to either search, and with superior performance” NEXT: FNAL 2012

  12. NEXT Asymmetric TPC“Separated function” Fiducial surface Transparent -HV plane Readout plane A Readout plane B energy & primary scintillation signals recorded here, with PMTs EL signal created here . Tracking performed here, with “SiPMT” array ions Operating pressure: 10 -15 bars Field cage: reflective teflon (+WLS) NEXT: FNAL 2012

  13. New: World’s best energy resolution for 137Cs -rays in xenon! Best results, to show off our approach Tight fiducial volume cut imposed here I will explain... 662 keV, ionization signal only NEXT: FNAL 2012

  14. Full 137Cs -ray Spectrum with looser fiducial volumecut low threshold includes fluorescence x-rays no correction applied for known radial dependence of signal NEXT: FNAL 2012

  15. Peak spectral region for 137Cs -rays:LBNL-TAMU HPXe TPC, 15 bars pure xenon This spectrum taken with the “normal” fiducial volume, as in last slide Note suppressed zero! NEXT: FNAL 2012

  16. LBNL-TAMU TPC Prototype NEXT: FNAL 2012 16

  17. Field cages/Light cagePTFE with copper stripes 19 PMTs and PMT bases Electroluminescence region10 kV across a 3 mm gap NEXT: FNAL 2012 TIPP 2011 17

  18. PMT Array: inside the pressure vesselQuartz window 2.54 cm diameter PMTs NEXT: FNAL 2012 TIPP 2011 18

  19. NEXT: FNAL 2012 TIPP 2011 19

  20. A typical 137Cs  waveform (sum of 19 PMTs)~300,000 detected photoelectrons Primary Scintillation (S1) T0 of event Electroluminescence (S2) Structure suggests topology due to Compton scatters Drift Time:z-position (~0.01mm/sample) Drift velocity ~1 mm/ms NEXT: FNAL 2012 TIPP 2011 20 10ns/sample

  21. Complex topologies are common! NEXT: FNAL 2012

  22. A Diagonal Muon Track! - “reconstructed”; Signal depends on radius in chamber ~ 14 cm NEXT: FNAL 2012

  23. Attenuation of electrons during drift is very low correction for attenuation is modest, and introduces insignificant error to energy NEXT: FNAL 2012

  24. What is the Intrinsic Energy Resolution? N = √FN = √FQ/w F  Fano factor: F = 0.15 (HPXe) (LXe: F ~20 !!) w  Average energy per ion pair: w ~ 25 eV Q  Energy deposited in xenon: 137Cs -rays: 662 keV E/E = 2.35N /N = 2.35 (Fw/Q)1/2 FWHM NEXT: FNAL 2012

  25. The Intrinsic Energy Resolution @ 662 keV E/E = 2.35 (Fw/Q)1/2 E/E = 0.56% FWHM (HPXe) We are about a factor of ~2 from this value NEXT: FNAL 2012

  26. The basic signal For 137Cs: N = Q/W ~26,500 primary electrons N =(FN)1/2~63 electrons rms! This is a very small number! How can this signal be detected with minimal degradation? What are the main degrading factors? NEXT: FNAL 2012

  27. Energy resolution in Xenon:Strong dependence on density! Very large fluctuations between light/charge! F ~ 20 WIMPs: S2/S1 suffers! Here, the fluctuations are normal F = 0.15 Unfolded resolution: E/E ~0.6% FWHM Ionization signal only! For  <0.55 g/cm3, ionization energy resolution is “intrinsic” NEXT: FNAL 2012

  28. Energy Partitioning in LXe Anomalously large fluctuations in energy partition between ionization and scintillation generate the large Fano factor in LXe The large fluctuations in LXe are caused by delta-rays, zones of very high ionization density, but few in number, and with “Landau” fluctuations Within zones of both high ionization and atomic density, nearly full recombination leads to light creation at the expense of ionization. The recombination process amplifies the non-Poisson statistics of the energy loss process of electrons in LXe... But not for xenon gas! NEXT: FNAL 2012

  29. Strong anti-correlations in LXe! 1 kV/cm Bi-207 source ~570 keV EXO data NEXT: FNAL 2012

  30. Xenon10 data Gamma events (e - R) Why do  events show large S2/S1 fluctuations at all energies, not improving with energy? Log10 S2/S1 Neutron events (N - R) NEXT: FNAL 2012

  31. Anti-correlation of Q & L • For fixed energy, such as Q = 2457 keV, energy resolution can be restored, in principle, by measuring both Q & L and forming the right linear combination. • In practice, this doesn’t work very well because only a few % of the light is detected; statistical precision is poor. • EXO predicted energy resolution @ Q (with light signal): • 3.4 % FWHM • EXO measured energy resolution (ionization signal only) • 10.6% FWHM @ 2615 keV NEXT: FNAL 2012

  32. Double beta decay spectra and 136Xe Only 2-vdecays Q = 2457 keV for 136Xe Rate 0 ( electron energy) Q-value The ideal result: a spectrum of only  events, with a 0- signal present as a peak, width dictated by resolution NEXT: FNAL 2012

  33. Energy resolution at Q E/E = 2.35  (FW/Q)1/2 • F  Fano factor (HPXe) : F = 0.15 • W  Average energy per ion pair: W ~ 25 eV • Q  Energy deposited from 136Xe --> 136Ba:2457 keV E/E = 0.28% FWHM intrinsic! N = Q/W ~100,000 primary electrons N =(FN)1/2~124 electrons rms! NEXT: FNAL 2012

  34. Energy resolution in Xenon gas:Gain & noise Impose a requirement on gain stage: (noise + fluctuations)  N Simple charge detection can’t meet this goal  Need gain with very low noise/fluctuations!  Electroluminescence (EL) is the key! NEXT: FNAL 2012

  35. Electro-Luminescence (EL) (aka: GasProportional Scintillation) Physics process generates ionization signal Electrons drift in low electric field region Electrons enter a high electric field region Electrons gain energy, excite xenon: 8.32 eV Xenon radiates VUV (175 nm, 7.5 eV) Electron starts over, gaining energy again Linear growth of signal with voltage Photon generation up to >1000/e, but no ionization Sequential gain; no exponential growth  fluctuations are very small NUV = JCP  N1/2 (Poisson: JCP = 1) Optimal EL conditions: JCP = 0.01 NEXT: FNAL 2012

  36. Virtues of Electro-Luminescence in HPXe Linearity of gain versus pressure, HV Immunity to microphonics Tolerant of losses due to impurities Absence of positive ion space charge Absence of ageing, quenching of signal Isotropic signal dispersion in space Trigger, energy, andtracking functions are accomplished with optical detectors NEXT: FNAL 2012

  37. Gain noise & resolution F  Fano constraint due to fixed energy deposit = 0.15 Let “G” represent noise/fluctuations in EL gain Uncorrelated fluctuations can add in quadrature: n = ((F + G)N)1/2 EL: G = JCP/NUV + (1 + 2PMT)2/Npe Npe = number of photo-electrons per primary electron 2PMT  2 (due to after-pulsing!) G  3/Npe Npe > 20 per electron so that G ≤ F = 0.15 E/E = 0.9% FWHM (137Cs: 662 keV) NEXT: FNAL 2012

  38. 1.04% FWHM  0.9% FWHM? • The primary reasons we have not reached E/E = 0.9% FWHM with our prototype are that: • Our photoelectron yield ne is less than 20. • Accurate radial correction requires realtracking. • Addition of a tracking plane will make possible an accurate radial correction, and increase efficiency • Tracking with EL is a primary R&D goal in FY 12 NEXT: FNAL 2012

  39.  decay: “spaghetti with two meatballs” Operating pressure: 10 - 15 bars NEXT: FNAL 2012

  40. Tracking plane • Previous HPXe TPC (Gotthard Tunnel) showed that a factor of >30 reduction in background is possible with event topology. • A larger factor may be possible, under study... • To reveal topology, a new tracking plane for our HPXe TPC is needed • The tracking plane can be installed without major surgery to our HPXe TPC • Tracking plane will be an x-y grid with MPPCs spaced at ~1 cm pitch • Hamamatsu 1 mm2 SiPM: MPPC s10362-11-100P • Electronics for the tracking plane is a joint development with UPV • Simple low-power circuitry to shape, digitize, and time-stamp waveforms NEXT: FNAL 2012

  41. Silicon Photomultiplier “SiPM” SiPM from Hamamatsu, “MPPC” NEXT: FNAL 2012

  42. SiPM photoelectron spectrum NEXT: FNAL 2012

  43. NEXT: FNAL 2012

  44. Backgrounds are the limiting factor! Nb <4 x 10-4 counts/keV kgy If backgrounds are as low as we calculate, then NEXT will be more than competitive! NEXT: FNAL 2012

  45. Summary: 0-  search • HPXe electroluminescent TPC concept was developed at LBNL • HPXe EL TPC offers superb energy resolution: 0.5% FWHM? • Event topology provides background rejection: >30 • HPXe EL TPC has been embraced by NEXT. • 6M€+ funds provided by Spain to NEXT project • US makes vital contributions to NEXT, plus move toward 1 ton NEXT: FNAL 2012

  46. Direct Dark Matter Search • Neon nuclear mass 20 is a very good match to alleged low-mass WIMPs (consonance with DAMA-LIBRA et al.?). • Lots of neon can be added to HPXe without adverse effects. • Simultaneous 0-v decayWIMP searches appear possible. • The xenon gas still provides shielding for low energy -rays; • High energy -rays typically have multiple substantial Compton scatters • A WIMP search in NEXT has not yet been thoroughly simulated. • R&D goal in FY 12: neon and neutrons in our TPC NEXT: FNAL 2012

  47. WIMPS: Discrimination between electronic and nuclear recoils with S2(charge)/S1(light) • In LXe, large energy partitioning fluctuations between L and Q • Intrinsic to LXe, absent in HPXe • These huge fluctuations enter directly in the ratio S2/S1, • electron and nuclear recoil event discrimination compromised • In HPXe, S2/S1 discrimination is expected to be hugely better • This potential needs to be demonstrated in our setup • The highest optical detection efficiency is desired to capture S1. • Wavelength shifters: Nitrogen ?, plastic bars ?, TMA,... NEXT: FNAL 2012

  48. 1 inch R7378A Predecessor: 7-PMT, 20 barTAMU HPXe TPC NEXT: FNAL 2012 J. White, TPC08, (D. Nygren, H-G Wang)

  49. Nr Discrimination in HPXe with TAMU 7-PMT TPC gammas neutrons NEXT: FNAL 2012

  50. Beppo-SAX satellite: a HPXe TPC in space! NEXT: FNAL 2012

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