EXO-200. (Enriched Xenon Observatory) Douglas Leonard University of Seoul

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1 EXO-200 (Enriched Xenon Observatory) Douglas Leonard University of Seoul

2 Motivation 2

3 Discovery of neutrino flavor oscillations showed that neutrinos are massive. Oscillation observed with: Solar and reactor, neutrinos and anti-neutrinos: m 2 21, m 1 < m 2, sin 2 Θ 12 Atmospheric and accelerator neutrinos: m 2 23, sin 2 Θ 23 LSND oscillation evidence not observed in MiniBooNE Small number of parameters describe variety of experiments using different methods and energies. 3

4 Double Beta Decay ββ is a second order weak process: Very rare! T 1/2 > y (much higher for neutrinoless decays) Signal is overwhelmed by standard β decay (if it occurs). Neutrinoless ββ provides information about: -Majorana nature of neutrino. (Is a neutrino it s own anti-particle?) -Absolute mass of neutrinos. (Effective mass actually; oscillation experiments only sensitive to mass differences) -To some extent the mass hierarchy. 4

5 Two types of double beta decay e - ν e - e ν e e - e - ν e ν e 136 Xe 136 Ba Xe 136 Ba ++ L e = 0 standard second order process observed in multiple isotopes lepton number violation ( L e = 2) m ν 0 ν = ν ( Majorana neutrinos ) 5

6 Measure Effective Neutrino Mass Calculate ( ) T 2 0ν 1 2 = G 0ν M 0ν m 1 /2 ν 2 2 m = η U 2 m ν i ei i i CP-phases: ±1 (Can cause cancellations!) Neutrino masses Elements upper row of MNS-matrix 6

7 Mass estimates from oscillations Hierarchical Inverse hierarchical 3 m atm υ e lives here m ν m solar m m ν solar m atm 3 mν driven by solar splitting 5 mev by atmospheric splitting 50 mev 7

8 What can we Really Measure? Heidleberg Moscow Plank +Sloan Projected Inverted Normal WMAP+2DF KATRIN Not Observable! (values may be out of date) 8

9 Double-beta decay: a second-order process only detectable if first order beta decay is energetically forbidden Candidate nuclei with Q>2 MeV Candidate Q Abund. (MeV) (%) 48 Ca 48 Ti Ge 76 Se Se 82 Kr Zr 96 Mo Mo 100 Ru Pd 110 Cd Cd 116 Sn Sn 124 Te Te 130 Xe Xe 136 Ba Atomic number (Z) 150 Nd 150 Sm

10 Decay modes distinguished by measurement of electron sum energy. Left: expected spectra for 200 kg 136 Xe in one year (EXO example). Right: leakage of ββ2ν-events into ββ0ν analysis interval. 5.8 th power! 10

11 The decay rate: R N = τ 0 = N 0 ln 2 T 1/2 To achieve a decay rate of 5 y -1 for a neutrino mass of 30 mev (in middle of inverted hierarchy band): 8300 kg of Xe enriched to 80% in 136Xe is needed! 11

12 Features of good experiments: Lots of decay material. Reduction of intrinsic radioactivity by finding clean materials (VERY DIFICULT). Control cosmogenic activition of materials. Passive shielding of cosmic ray showers. (go underground) Passive shielding of external radioactivity (ex: lead) Active shielding, especially for muons, usually scintillator layers. High resolution Calorimetry Includes ionization, scintillation and bolometers. With low Backgrounds and no other event discrimination, resolution typically needs to be below a couple of percent Spatial tracking: Good single-site discrimination alone can reduce backgrounds significantly. Several techniques ranging from high-resolution wire chambers to coarse segmentation. Residual nucleus identification (EXO) Major Distinctions: Source is Detector? (Improves intrinsic background, but less versatile) Good tracking vs. good calorimetry 12

13 Key Xenon Advantages Reasonable Q-value, ±0.4 kev (M. Redshaw, J.McDaniel, E. Wingfield and E.G. Myers, to be submitted to Phys. Rev. C ) Source is the detector. (Relatively) easy to enrich (8.9% natural abundance even without enriching). Can be purified and re-purified. Reusable. Can be used in both liquid and gas phase with different advantages Has no long-lived isotopes to activate. bb-decay product atom remains charged opens possibility of Ba removal and final state tagging through Ba single ion detection 13

14 Competition 14

15 Heidelberg Evidence: Claimed ββ-peak 4.2 σ -Used 10.9 kg of Ge enriched to 86% in 76Ge. -Full data set Nov May kg*y, no PSD. 15

16 CUORE/Cuoricino Bolometer TeO 2 Bolometer: Source = Detector Heat sink: Cu structure (8 mk) Thermal coupling: Teflon (G = 4 pw/mk) Thermometer: NTD Ge-thermistor (dr/dt 100 kω/µk) Absorber: TeO 2 crystal (C 2 nj/k 1 MeV / 0.1 mk) For E = 1 MeV: T = E/C 0.1 mk Signal size: 1 mv Time constant: τ = C/G = 0.5 s Amplitude (a.u.) Single pulse example Energy resolution (FWHM): ~ 5-10 kev at 2.5 MeV Time (ms) 17

17 Cuoricino Total detector mass: 40.7 kg kg 130 Te 11 modules, 4 detector each, crystal dimension: 5x5x5 cm 3 crystal mass: 790 g 44 x 0.79 = kg of TeO 2 2 modules x 9 crystals each crystal dimension: 3x3x6 cm 3 crystal mass: 330 g 9 x 2 x 0.33 = 5.94 kg of TeO 2 (2 enriched in 128 (2 enriched in 130 Shielding: Cu box + Roman Pb inside cryostat 20 cm Pb & 10 cm borated polyethylene outside 18

18 Higlighted Results Cuoricino, Kg*years of 130 Te data <m ββ > < ( ev ), range in limit due to nuclear physics. Part of H-M, 71.7 kg*years of 76 Ge data <m ββ > ~ ev EXO-200 aims to reach 0.13 to 0.19 ev in about 2 years (~ 200 to 400 kg*yr). BUT!!... These all measure different isotopes. Multiple measurements are needed check the nuclear physics. We expect 5*σ sensitivity to Ge result even with worst-case Matrix elements. 19

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20 EXO-200 Scientific goals: 1) Measurement of yet unobserved ββ2ν decay of 136 Xe. Important background for full EXO. 2) Test of the Heidelberg evidence for ββ0ν decay. 24

21 EXO Facts EXO-200: 200 kg of Isotopically enriched liquid 136 Xe (80%) - Qββ ±0.4 kev. (M. Redshaw, J.McDaniel, E. Wingfield and E.G. Myers, to be submitted to Phys. Rev. C) -Semi-cryogenic (-115 o C, small liquid range) Tracking (Time Projection Chamber): -LXe drift chamber detects ionization track on x and y wire grid. (~50 e - 3keV/cm ) -Scintillation from recombination (175nm, gives timing start for z axis position reconstruction. - Resolution will be about 1 cm^3. Calorimetry: Ionization + Scintillation give ~ 1.4% resolution at Q ββ. Shielding: Multi-layer layer passive+ plastic scintillator active veto. Full EXO: Mass: tons Identify residual Ba ions! 25

22 Components (from outside to inside): Space to put things (without many muons) An Active Veto System Cleanrooms Lead Shield A Cryostat A Detection Chamber Xenon Everything needs to be made from clean materials. 26

23 25 cm Pb 5 cm Cu cryostat 50 cm cryogenic fluid HFE-7000 EXO-200 Thin walled Cu TPC 27

24 The Xenon 28

25 200 kg 136 Xe test production completed spring 03 (enr. 80%) Largest highly enriched stockpile not related to nuclear industry Largest sample of separated ββ isotope (by ~factor of 10) 29

26 The Place 30

27 Waste Isolation Pilot Plant (WIPP) near Carlsbad NM US Department of Energy facility for storage of transuranic (TRU) waste. 31

28 EXO-200 WIPP SITE 655 m underground (rock + salt), ~1600m.w.e vertical muon flux = s -1 cm -2 sr -1 (Looked like this about 3 years ago) (NIMA 538 (2005) 516) 32

29 Moving to WIPP July , Moving to WIPP 33

30 Adjustable supports 34

31 October 2007: cleanrooms and gowning area installed at WIPP. Staging container for component pre-cleaning installed at WIPP. 35

32 The Cryostat 36

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34 Etching with dilute HNO3 after receipt in

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37 Superinsulation Application of new SI after it was found that original SI was too radioactive (2007). Required design of a large extraction device. 41

38 Lead Shield 42

39 Lead Shielding All Lead bricks interlock to avoid line-ofsight gaps in shielding. Lead selection was very strict

40 May 2008: installation of the barrel section of the lead, at WIPP. 44

41 Chamber and Instrumentation 45

42 Found a clear (anti)correlation between ionization and scintillation 1 kv/cm ~570 kev Ionization alone: σ(e)/e = 570 kev or Q ββ Ionization & Scintillation: σ(e)/e = 570 kev or Q ββ E.Conti et al. Phys. Rev. B (68) EXO-200 will collect 3-4 times as much scintillation 46

43 EXO-200 Detector -1.4kV Amp X-Y collection grids Ionization e- eēe- e- e- e- e- e- e- e- e- e- e- 200 kg Liquid 136 Xe Ground Scintillation -75kV 209 APDs per side 47

44 Charge Detection Double-ended TPC chamber with ~20 cm drift regions Mid-plane cathode biased up to -70 kv but bias optimization will be studied. 48 Inductive Y wires per side at -4 kv, 100% charge transparent. 48 X wires at virtual ground to collect the charge. All wires are 100 um with 3mm spacing and readout in groups of three. LXE electron mobility ~2000 cm 2 /(Vs) Saturation velocity ~ 0.28*cm/µs Electron lifetime goal of 3ms => 2.4% loss at 20 cm. 48

45 Light Detection mm APD s (Avalanche Photo Diodes) custom made with clean material stocks. Gangs of seven APD s Yield enhanced by reflective Teflon coatings in the TPC. 125pf each => 1000 pf per gang of seven. Low gain (compared to PMT s), roughly 200 electrons/photon..=> bad for dark matter. Clean materials, mostly refined silicon. QE measured at 129% at 175nm with 300V by comparison with NIST standard. Connections made by contact springs for easy maintenance. 49

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47 APD delivery status All APD s delivered from API and tested: QE > 0.7, noise < 3000 electrons). Some were rejected but we have many working spares. APD s were tested by combination of ICPMS for metalic coatings and NAA for silicon wafers. Raw production materials were selected to insure low radioactivity. These APD s probably cannot be replaced 51

48 U = 1409 V σ = U 20 V σ QE QE rel = 0.96 = 0.11 N = 1215 e All tests done at liquid σ = N 413 e Xe temperature (-108 C). 52

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51 Chamber leg construction 55

52 Field Ring Resistor Installation Painted sapphire reistors, home- made for low radioactivity. 56

53 TPC internals with reflector and cathode plane. 57

54 APD installation 58

55 Cable Insertion Readout cables are etched from copper on 40um Kapton (Polyimide). Small material amount minimizes radioactive contamination Pitch is easy(~.1mm), but cable length is incompatible with typical commercial equipment Custom etching process, monitoring at every step, and cleaning etches to insure radio-purity. Price is high Process is slow and requires in-house man-hours and expertise. Difficult to reproduce Cables are fragile and must be handled with extreme care. Some of the Kapton was cut by hand where needed. 59

56 Installation at WIPP, last Fall. 60

57 Electronics are built 61

58 Data Handling System Under Heavy Development and now Testing. Control and real-time viewing from SLAC, California. Storage, event reconstruction, and analysis at SLAC. Truck full of redundant hard drives can have high bandwidth and reliability. First Data Challenge in progress, with MC data 62

59 Background control Target: < 40 events per year in 0-ν and < 4000 in 2-ν 63

60 EXO-200 Background Studies Impacts background sources have been studied by Monte Carlo simulation with realistic cuts: 232 Th in salt walls including leakage through realistic. construction joints 40 K, 232 Th and 238 U in all detector and shielding parts (nuts, bolts, o-rings, welds, residues, etc). 222 Rn in shielding joints, coolant system, Xenon. 210 Pb in lead shield and TPC walls Cosmogenic isotopes in copper cryostat. Muon bremsstrahlung We have now analyzed over 300 construction materials and parts via direct gamma counting, neutron activation, alpha counting, and mass spectroscopy. D. S. Leonard et al. doi: /j.nima

61 Techniques: Pros and Cons Gamma Counting: Above and underground low background counting at UA, Laurentian and especially Neuchatel/Bern). Best Suited for cryostat level or small parts. Sensitivity (Th/U): few ppb at UA above ground ~100 ppt at Neuchatel underground Advantage: direct test for background causing isotopes, no assumptions on chain equilibrium needed. Works in principle for essentially any material. Disadvantage: compartively limited sensitivity, long analysis time, large samples. ICPMS and GDMS by the INMS (Canada). Sensitivity (Th/U): 10 ppt with GDMS 1 ppt ICPMS Advantage: high sensitivity, fast analysis, small samples. Disadvantage: acid digestable materials (ICPMS) or conductive (GDMS), needs assumption of chain equilibrium for background impact. 65

62 NAA with Ge counting utilizing MIT reactor. Sensitivity (Th/U): >0.3 ppt counting 0.02 ppt with pre-concentration Advantage: high sensitivity, small sample, little pre-analysis treatment needed. Disadvantage: Interferences can be high. Not well suited for most metals, long analysis time, equilibrium assumption needed. Alpha counting for 210Pb analysis of shielding lead (via 210Po) Sensitivity: 5 Bq/kg Relatively inexpensive and easy, but very special purpose. 66

63 Recent example: quantify background impact of flat cables used in quantity to bias APDs and transfer the signals. Optimize production process until impacts of measured contamination was found acceptable. 72

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72 8.2 tons high purity Sea transport in concrete shielded Cu freshly produced container to suppress activation. in Germany 6/1/2006. Limited By FLUKA cosmics Monte exposure Carlo to using avoid realistic activation. cosmic n spectrum and reaction 63 Cu(n,α) 60 Co for which cross section is known suppression factor: 3.7 Shielding bunker at DESY stored 6/2/

73 The Cosmic Ray Muon Veto 84

74 31 large plastic scintillator panels, left over from the concluded KAREMEN neutrino oscillation experiment, have been acquired. They were refurbished, tested, and calibrated at UA in Includes gain matching of about 300 PMTs. 86

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77 Single PE peaks at various voltages 93

78 Experimental Setup Position determined by a muon telescope Cumbersome clamp stands are now replaced with a wooden box for easier telescope placement. 95

79 Panel end spectrum (Gang of 4 PMT s) with telescope coincidence requirement Muon Peak 96

80 Panel S23 98

81 EXO-200 Veto Monte Carlo Geometrical placement is optimized by Monte Carlo. About 98% muon background rejection is achievable. 90% required. 99

82 Veto layout and Support 100

83 Muon Veto Panels at WIPP 102

84 Xenon Purity Studies A typcial RGA (Residual Gas Analyzer) mass spectrometer has, Dynamic range of ~ 1 * 10 6 From highest to lowest signal. The highest signal in xenon is xenon This limits sensitivity to 1 ppm. To improve this one must remove xenon, while keeping the impurities. In other words, concentrate the impurity. 103

85 Xenon Purity Important for: 1) Electron drift, 2) Radioactivity, 3) Calibration sources?? 104

86 Xenon Purity Studies Xe purifier Gas flow direction P ~ 1 atm leak valve to mass spec for analysis P ~ 10-5 torr Xe + 1ppm of impurities recovery bottle LN cold trap to remove Xe 105

87 Cold Trap at Maryland Cold trap RGA Leak Valve Purifier 106

88 Cold-Trap RGA analysis of Xenon Purification 107

89 RGA/Cold Trap response for Methane Relative 15 u (meth hane) Partial Pressure Methane/He Methane/Ar Methane Concentration (g/g) 108

90 EXO-200 Majorana mass sensitivity Assumptions: 1) 200kg of Xe enriched to 80% in 136 2) σ(e)/e = 1.4% obtained in EXO R&D, Conti et al Phys Rev B 68 (2003) ) Low but finite radioactive background: 20 events/year in the ±2σ interval centered around the 2.46MeV endpoint 4) Negligible background from 2νββ (T 1/2 >1 10 yr R.Bernabei et al. measurement) Case Mass Eff. Run (ton) (%) Time (yr) σ E 2.5MeV (%) Radioactive Background (events) T 1/2 0ν (yr, 90%CL) Majorana mass (mev) QRPA 1 NSM 2 EXO * ) Rodin, et. al., Nucl. Phys. A 793 (2007) ) Caurier, et. al., arxiv: v2 109

91 What can we Really Measure? Heidleberg Moscow Plank +Sloan Projected Inverted Normal WMAP+2DF KATRIN Not Observable! (values may be out of date) 110

92 Status and Conclusions EXO-200 construction is complete. Need final systems testing. First technical data (aside from existing muon veto data) is expected soon. EXO-200 will have the largest active ββ mass by a significant factor, of any existing experiment to date. Extensive background control will allow us to test present mass limits with high confidence levels. The Ba atom tagging technique is under active development for a larger scale EXO in the future. Stay tuned! First data will come soon. 111

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