1. NEXT: FNAL 20121 NEXT A High-pressure Xenon Gas TPC: How superior energy resolution benefits both 0-  decay in 136 Xe and WIMP searches David Nygren LBNL

2. NEXT: FNAL 20122 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

3. NEXT: FNAL 20123 “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

4. NEXT: FNAL 20124 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 ISU

5. NEXT: FNAL 20125 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

6. NEXT: FNAL 20126 6 Laboratorio Subterraneo de Canfranc Waiting for NEXT!

7. NEXT: FNAL 20127 Double beta decay spectra Only 2- v decays Rate (  electron energy) Q-value Only 0- v decays No backgrounds above Q-value The ideal result: a spectrum of only  events, with a 0- signal present as a narrow peak, well-separated from 2- 0

8. NEXT: FNAL 20128 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 For  <0.55 g/cm 3, ionization energy resolution is “intrinsic” Ionization signal only!

9. NEXT: FNAL 20129 What does a search for 0-  require? Sensitivity and Background Rejection 1.High sensitivity  large mass of candidate isotope NEXT has 100 kg of enriched xenon: ~85% 136 Xe 2.Extremely good background rejection! Shielding, radio-purity, excellent energy resolution, event topology are critical High Q-value of 136 Xe, 2457 keV, places signal above most  -rays NEXT energy resolution:  E/E <0.7 % FWHM expected 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

10. NEXT: FNAL 201210 What does a search for WIMPs require? Sensitivity and Background Rejection 1.High sensitivity  large sensitive mass NEXT has 100 kg of enriched xenon: ~85% 136 Xe A large component of neon can be added for better match to low-mass WIMPs 2.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

11. NEXT: FNAL 201211 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”

12. NEXT: FNAL 201212 NEXT Asymmetric TPC “Separated function” Transparent -HV plane Readout plane B Readout plane A. ions energy & primary scintillation signals recorded here, with PMTs Field cage: reflective teflon (+WLS) EL signal created here Tracking performed here, with “SiPMT” array Fiducial surface Operating pressure: 10 -15 bars

13. NEXT: FNAL 201213 New: World’s best energy resolution for 137 Cs  -rays in xenon! Best results, to show off our approach Tight fiducial volume cut imposed here I will explain... 662 keV, ionization signal only

14. NEXT: FNAL 201214 Full 137 Cs  -ray Spectrum with looser fiducial volume cut low threshold includes fluorescence x-rays no correction applied for known radial dependence of signal

15. NEXT: FNAL 201215 Peak spectral region for 137 Cs  -rays: LBNL-TAMU HPXe TPC, 15 bars pure xenon Note suppressed zero! This spectrum taken with the “normal” fiducial volume, as in last slide

16. NEXT: FNAL 201216 LBNL-TAMU TPC Prototype

17. NEXT: FNAL 201217 TIPP 201117 Field cages/Light cage PTFE with copper stripes Electroluminescence region 10 kV across a 3 mm gap 19 PMTs and PMT bases

18. NEXT: FNAL 201218 TIPP 201118 PMT Array: inside the pressure vessel Quartz window 2.54 cm diameter PMTs

19. NEXT: FNAL 201219 TIPP 201119

20. NEXT: FNAL 201220 TIPP 201120 A typical 137 Cs  waveform (sum of 19 PMTs) ~300,000 detected photoelectrons 10ns/sample 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

21. NEXT: FNAL 201221 Complex topologies are common!

22. NEXT: FNAL 201222 A Diagonal Muon Track! - “reconstructed”; Signal depends on radius in chamber ~ 14 cm

23. NEXT: FNAL 201223 Attenuation of electrons during drift is very low correction for attenuation is modest, and introduces insignificant error to energy

24. NEXT: FNAL 201224 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: 137 Cs  -rays: 662 keV  E/E = 2.35   N /N = 2.35  (F  w/Q) 1/2 FWHM

25. NEXT: FNAL 201225 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

26. NEXT: FNAL 201226 The basic signal For 137 Cs: 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?

27. NEXT: FNAL 201227 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 For  <0.55 g/cm 3, ionization energy resolution is “intrinsic” Ionization signal only!

28. NEXT: FNAL 201228 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!

29. NEXT: FNAL 201229 1 kV/cm Strong anti-correlations in LXe! ~570 keV Bi-207 source EXO data

30. NEXT: FNAL 201230 Gamma events (e - R) Neutron events (N - R) Why do  events show large S 2 /S 1 fluctuations at all energies, not improving with energy? Log 10 S2/S1 Xenon10 data

31. NEXT: FNAL 201231 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

32. NEXT: FNAL 201232 Double beta decay spectra and 136 Xe Only 2- v decays Rate (  electron energy) Q-value Q = 2457 keV for 136 Xe The ideal result: a spectrum of only  events, with a 0- signal present as a peak, width dictated by resolution 0

33. NEXT: FNAL 201233 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 136 Xe --> 136 Ba: 2457 keV  E/E = 0.28% FWHM intrinsic! N = Q/W ~100,000 primary electrons  N = (F  N) 1/2 ~124 electrons rms!

34. NEXT: FNAL 201234 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!

35. NEXT: FNAL 201235 Electro-Luminescence (EL) (aka: Gas Proportional 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  N UV = J CP  N 1/2 (Poisson: J CP = 1) Optimal EL conditions: J CP = 0.01

36. NEXT: FNAL 201236 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, and tracking functions are accomplished with optical detectors

37. NEXT: FNAL 201237 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 = J CP /N UV + (1 +  2 PMT ) 2 /N pe N pe = number of photo-electrons per primary electron  2 PMT  2 (due to after-pulsing!) G  3/N pe  N pe > 20 per electron so that G ≤ F = 0.15  E/E = 0.9% FWHM ( 137 Cs: 662 keV)

38. NEXT: FNAL 201238 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 n e is less than 20. –Accurate radial correction requires real tracking. 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

39. NEXT: FNAL 201239 Operating pressure: 10 - 15 bars  decay: “spaghetti with two meatballs”

40. NEXT: FNAL 201240 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 mm 2 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

41. NEXT: FNAL 201241 Silicon Photomultiplier “SiPM” SiPM from Hamamatsu, “MPPC”

42. NEXT: FNAL 201242 SiPM photoelectron spectrum

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44. NEXT: FNAL 201244 N b <4 x 10 -4 counts/keV  kg  y If backgrounds are as low as we calculate, then NEXT will be more than competitive! Backgrounds are the limiting factor!

45. NEXT: FNAL 201245 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

46. NEXT: FNAL 201246 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  decay WIMP 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

47. NEXT: FNAL 201247 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,...

48. NEXT: FNAL 201248 Predecessor: 7-PMT, 20 bar TAMU HPXe TPC 1 inch R7378A J. White, TPC08, (D. Nygren, H-G Wang)

49. NEXT: FNAL 201249 Nr Discrimination in HPXe with TAMU 7-PMT TPC neutrons gammas

50. NEXT: FNAL 201250 Beppo-SAX satellite: a HPXe TPC in space!

51. NEXT: FNAL 201251 Electroluminescence in 4.5 bar of Xenon 2.2% FWHM resolution corresponds to  E/E = 5 x 10 -3 FWHM -- if naively extrapolated to Q  of 2.5 MeV

52. NEXT: FNAL 201252 R&D Summary The energy resolution of the HPXe EL TPC has been demonstrated. Direct WIMP detection with excellent discrimination appears possible. Primary FY2012 HPXe TPC R&D Goals –Tracking plane for event topology learn to do the radial correction properly Reconstruct gamma-ray events. –Nuclear/electron recoil discrimination add neon expose chamber to neutrons

53. NEXT: FNAL 201253 NEXT construction summary The US groups propose construction contributions for NEXT: –Energy Plane mechanics - LBNL –Tracking Plane electronics - LBNL –Engineering, design, and integration - LBNL –TPC structures - TAMU –Energy resolution/calibration - ISU –Additional US Collaborators desirable

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57. NEXT: FNAL 201257 Perspective Both 0-  and WIMP searches can be done - WIMP sensitivity comes “free”, but WIMP performance needs demonstration. Optical detection efficiency for S1 has to be maximized to capitalize on the superb intrinsic resolution - WLS research Molecular additives such as tri-methyl amine (TMA) might offer much lower Fano factor, with WLS properties to 300 nm range A future ~1000 kg detector for simultaneous 0-  and WIMP searches could be located at SURF...

58. NEXT: FNAL 201258