Observation of 136 Xe 2νββ with the Enriched Xenon Observatory (EXO)

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1 Observation of 136 Xe 2νββ with the Enriched Xenon Observatory (EXO) Phil Barbeau for the EXO collaboration Stanford University TAUP 2011, Munich 1

2 136 Xe is an Excellent Candidate for Double Beta Decay Search Xenon is reusable. Can be re-purified & recycled into new detector (no crystal growth). Monolithic detector. LXe is self shielding, surface contamination minimized. Minimal cosmogenic activation. No long lived radioactive isotopes of Xe. 2

3 The EXO-200 Detector clean room Xe Vessel HFE (Heat transfer fluid) Vacuum insulation Copper cryostat 25cm enclosure of low-activity lead TPC 46 cm cryostat 3

4 EXO-200 Time Projection Chamber (TPC) Basics U and V grids ionization drift trajectories V-wires (shielding grid) v-wires (shielding grid) U-wires (energy grid) u-wires (energy grid) TPC Schematics Simulation of Charge Drift Two TPC modules with common cathode in the middle. APD array observes prompt scintillation for drift time measurement. V-position given by induction signal on shielding grid. U-position and energy given by charge collection grid. 4

5 APDs EXO-200 TPC Field shaping rings Cathode Signal Cables charge detection 5

6 EXO-200 Installation Site: WIPP EXO-200 cran e rails EXO-200 is installed at WIPP (Waste Isolation Pilot Plant), in Carlsbad, NM 1600 mwe flat overburden (2150 feet, 650 m) U.S. DOE salt mine for radioactive waste storage Salt rock low activity relative to hard-rock mine 6

7 EXO-200 Enriched Low-Background Run in 2011 EXO-200 was filled with Enriched Xe; data taking began in spring 2011 The data collected were used for immediate measurement of the 2νββ T 1/2 of 136 Xe and to begin energy resolution studies - Rn enclosure not yet in operation - No Rn trap in Xe system - Still missing part of front lead enclosure 7

8 Rn Content in Xenon Scintillation Ionization α-decay α: strong light signal, weak charge signal β: weak light signal, strong charge signal β-decay 214 Bi 214 Po correlations in the EXO-200 detector Using the Bi-Po (Rn daughter) coincidence technique, we can estimate the Rn content in our detector. The 214 Bi decay rate is consistent with measurements from alpha-spectroscopy and the expectation before the Rn trap is commissioned. 8

9 Rn Content in Xenon β T 1/2 = 3.8 d 4.5 μbq kg -1 ~1 per hour α: strong light signal, weak charge signal β: weak light signal, strong charge signal 214 Bi 214 Po correlations in the EXO-200 detector Using the Bi-Po (Rn daughter) coincidence technique, we can estimate the Rn content in our detector. The 214 Bi decay rate is consistent with measurements from alpha-spectroscopy and the expectation before the Rn trap is commissioned. 9

10 Calibration Source Run Sources: 137 Cs, 60 Co, 228 Th z y x Various calibration sources can be brought to several positions just outside the detector x-y distribution of events clearly shows excess near the source location 10

11 Xenon Purity maximum drift time ~ 100 μs Use sources to measure purity of LXe in TPC Rapid achievement of ms lifetimes results is a clear benefit of recirculation. 11

12 single - cluster 228 Th Calibration multiple - cluster granularity from 9 mm wire spacing γ γ Calibration runs compared to simulation - GEANT4 based simulation - charge propagation - scintillation propagation - signal generation - energy resolution parameterization is added in after the fact There are no free parameters for these comparisons (worst agreement is +8%) 12

13 Energy Calibrations After purity correction, calibrated single and multiple cluster peaks across energy region of interest (511 to 2615 kev) -uncertainty bands are systematic Point-like depositions have large reconstructed energies due to induction effects - observed for pair-production site (similar to β and ββ decays ) - reproduced in simulation Peak widths also recorded and their dependence on energy is parameterized. 13

14 Low-Background Spectra single - cluster multiple - cluster zoomed in 2νββ live-days of data 63 kg active mass Signal / Background ratio 10:1 -as good as 40:1 for some extreme fiducial volume cuts T 1/2 = yr (± 0.04 stat) yr (± 0.21 sys) [arxiv: ] 14

15 Low-Background Spectra constant in time 2νββ signal is clearly in the LXe bulk, while other gamma background contributions decrease with increasing distance from the walls. T 1/2 = yr (± 0.04 stat) yr (± 0.21 sys) [arxiv: ] 15

16 This Work Significantly shorter than previous limits reported: T 1/2 > yr (90% C.L.) (R. Bernabei et al. Phys. Lett. B 546 (2002) 23) and T 1/2 > yr (90% C.L.) (Yu. M. Gavriljuk et al., Phys. Atom. Nucl. 69 (2006) 2129) 16

17 Summary All important subsystems of EXO-200 are working Low background physics data taking with enriched xenon has begun and already producing results! T 1/2 = yr (± 0.04 stat) yr (± 0.21 sys) [arxiv: ] Backgrounds already very low ( counts kg -1 kev -1 yr -1 in 0νββ region) without full lead wall, Rn exclusion tent, Rn trap or full 3D reconstruction Stay tuned: improved energy resolution using charge & scintillation signals; upgraded pattern recognition 17

18 The EXO Collaboration D. Auty, M. Hughes, R. MacLellan, A. Piepke, K. Pushkin, M. Volk University of Alabama, Tuscaloosa AL P. Vogel California Institute of Technology, Pasadena CA A. Coppens, M. Dunford, K. Graham, P. Gravelle, C. Hagemann, C. Hargrove, F. Leonard, K. McFarlane, C. Oullet, E. Rollin, D. Sinclair, V. Strickland Carleton University, Ottawa ON, Canada L.Kaufman Indiana University M. Moe University of California, Irvine, Irvine CA C. Benitez-Medina, S. Cook, W. Fairbank, Jr., K. Hall, N. Kaufold, B. Mong, T. Walton Colorado State University, Fort Collins CO D. Akimov, I. Alexandrov, V. Belov, A. Burenkov, M. Danilov, A. Dolgolenko, A. Karelin, A. Kovalenko, A. Kuchenkov, V. Stekhanov, O. Zeldovich ITEP Moscow, Russia B. Beauchamp, D. Chauhan, B. Cleveland J. Farine, J. Johnson, U. Wichoski, M. Wilson Laurentian University, Sudbury ON, Canada C. Davis, A. Dobi, C. Hall, S. Slutsky, Y-R. Yen University of Maryland, College Park MD J. Cook, T. Daniels, K. Kumar, P. Morgan, A. Pocar, B. Schmoll, C. Sterpka, D. Wright University of Massachusetts Amherst, Amherst MA D.Leonard University of Seoul, Republic of Korea M. Auger, D. Franco, G. Giroux, R. Gornea, M. Weber, J-L. Vuilleumier Laboratory for High Energy Physics, Bern, Switzerland W. Feldmeier, P.Fierlinger, M. Marino Technical University of Munich, Garching, Germany N. Ackerman, M. Breidenbach, R. Conley, W. Craddock, S. Herrin, J. Hodgson, D. Mackay, A. Odian, C. Prescott, P. Rowson, K. Skarpaas, M. Swift, J. Wodin, L. Yang, S. Zalog Stanford Linear Accelerator Center (SLAC), Menlo Park CA P. S. Barbeau, L. Bartoszek, J. Davis, R. DeVoe, M. J. Dolinski, G. Gratta, F. LePort, M. Montero-Diez, A.R. Muller, R. Neilson, K. O Sullivan, A. Rivas, A. Saburov, D. Tosi, K. Twelker Stanford University, Stanford CA 18

19 Backup Slides 19

20 EXO Sensitivity Assumptions: Majorana neutrinos 20

21 21 Fiducial Volume Uncertainty Uncertainty determined from the fidelity with which calibration events are reconstructed within a chosen volume as compared to simulation Active Detector Red: True position in sim Black: Data Calibration Source at +Y Half spread: 7.1% Calibration Source at +X Half spread: 9.3%

22 Constraints from alpha spectroscopy Investigate alpha spectrum for scintillation signals from 238 U Calibrate spectrum with alphas in Rn chain Can constrain contamination of 238 U in bulk LXe by searching for 4.5 MeV alphas < 0.3 counts per day in our fiducial volume -The same limit applies to its daughter 234m Pa which β decays with a Q-value of 2195 kev, which cannot then explain our LXe bulk signal 22

23 Events > 100 kev well above charge trigger and reconstruction thresholds 3D reconstruction still requires determination of t 0 from scintillation signal Compare ratio of fully reconstructed events to triggered events to determine reconstruction efficiency Early software threshold ~700 kev Recent dramatic decrease with change in APD bias voltages ~300 kev 3D reconstruction threshold 23

24 Energy Calibration: pair production event Identifying 3-site events from pairproduction and annihilation provides 2 extra charge calibration peaks -511 kev gammas are our lowest energy calibration sources kev pair production very similar topology to ββ decays 24

25 Pinpoint Source Location using a Compton telescope technique 500 events Detector measures E, x, y, z for each site Use scattering formula From each site a cone is drawn and adding up these cones produces the image to the right 25

26 Cathode Muon track in EXO-200 One of the two TPC modules U and V wires A track from a cosmic-ray muon in EXO-200. The horizontal axis represents time (uncalibrated for now) while the vertical is the wire position (see sketch). V wires see inductive signals while U wires collects the charge. The muon in the present event traverses the cathode grid, leaving a long track in one TPC module and a shorter one in the other. 26

27 Liquid Xenon Calorimetry When ionizing radiation enters liquid xenon, it creates many Xe + and e - pairs and Xe*, some of the Xe + and Xe* undergo recombination and give off 175nm VUV photons or heat. Ionization alone: 570 kev or 1.8 Q( Ionization & Scintillation: 570 kev or 1.4 Q( E.Conti et al., Phys. Rev. B (2003) 27