Enriched Xenon Observatory (EXO) - SLAC...Phil Barbeau for the EXO collaboration Stanford University...

Phil Barbeau for the EXO collaborationStanford UniversityTAUP 2011, Munich

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

9/7/11 1

136Xe 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.

2Phil Barbeau, TAUP 20119/7/11

3

clean roomXe Vessel

Vacuum insulation

HFE (Heat transfer fluid)

Copper cryostat

25cm enclosure of low-activity lead

The EXO-200 Detector

46 cm

TPC

cryostat

Phil Barbeau, TAUP 20119/7/11

4

EXO-200 Time Projection Chamber (TPC) Basics

• 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.

Simulation of Charge Drift

U and V grids

v-wires (shielding grid)

u-wires (energy grid)

TPC Schematics


V-wires (shielding grid)

ionization drift trajectories

U-wires (energy grid)

5

EXO-200 TPCAPDs Cathode

Signal Cables

Field shaping rings

charge detection wires


crane rails

EXO-200 Installation Site: WIPP

6

• 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

★

EXO-200


EXO-200 was filled with Enriched Xe; data taking began in spring 2011

The data collected were used for immediate measurement of the 2νββ T1/2

of 136Xe and to begin energy resolution studies

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EXO-200 Enriched Low-Background Run in 2011

- Rn enclosure not yet in operation

- No Rn trap in Xe system

- Still missing part of front lead enclosure


214Bi – 214Po correlations in the EXO-200 detector

β-decay

α-decay

Rn Content in Xenon

α: strong light signal, weak charge signalβ: weak light signal, strong charge signal

8

Using the Bi-Po (Rn daughter) coincidence technique, we can estimate the Rncontent in our detector. The 214Bi decay rate is consistent with measurements from alpha-spectroscopy and the expectation before the Rn trap is commissioned.

Scintillation

Ionization


214Bi – 214Po correlations in the EXO-200 detector

β

Rn Content in Xenon

α: strong light signal, weak charge signalβ: weak light signal, strong charge signal

9

4.5 μBq kg-1

~1 per hour

T1/2 = 3.8 d

Using the Bi-Po (Rn daughter) coincidence technique, we can estimate the Rncontent in our detector. The 214Bi decay rate is consistent with measurements from alpha-spectroscopy and the expectation before the Rn trap is commissioned.


x-y distribution of events clearly shows excess near the source location

Sources:

137Cs, 60Co, 228Th

yz

x

Various calibration sourcescan be brought to severalpositions just outsidethe detector

10

Calibration Source Run


• Use sources to measure purity of LXe in TPC• Rapid achievement of ms lifetimes results is a clear benefit of recirculation.

Xenon Purity

maximum drift time ~ 100 μs


228Th Calibration

• 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%)

single - cluster multiple - cluster

γ γ

granularity from9 mm wire spacing

720720


• 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.

Energy Calibrations


Low-Background Spectra

• 31 live-days of data• 63 kg active mass • Signal / Background ratio 10:1

-as good as 40:1 for some extreme fiducial volume cuts

zoomed in

single - cluster multiple - cluster

T1/2 = 2.11·1021 yr (± 0.04 stat) yr (± 0.21 sys) [arXiv:1108.4193]

720720

720


2νββ

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.

T1/2 = 2.11·1021 yr (± 0.04 stat) yr (± 0.21 sys) [arXiv:1108.4193]15Phil Barbeau, TAUP 20119/7/11

Significantly shorter than previous limits reported: T1/2 > 1.0·1022 yr (90% C.L.) (R. Bernabei et al. Phys. Lett. B 546 (2002) 23)

and T1/2 > 8.5·1021 yr (90% C.L.) (Yu. M. Gavriljuk et al., Phys. Atom. Nucl. 69 (2006) 2129)

This Work


Summary

17

• All important subsystems of EXO-200 are working

• Low background physics data taking with enriched xenon has begun and already producing results!

• T1/2 = 2.11·1021 yr (± 0.04 stat) yr (± 0.21 sys) [arXiv:1108.4193]

• Backgrounds already very low (4·10-3 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


The EXO CollaborationD. Auty, M. Hughes, R. MacLellan, A. Piepke, K. Pushkin, M. Volk

University of Alabama, Tuscaloosa ALP. Vogel

California Institute of Technology, Pasadena CAA. 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, CanadaL.Kaufman

Indiana UniversityM. Moe

University of California, Irvine, Irvine CAC. Benitez-Medina, S. Cook, W. Fairbank, Jr., K. Hall, N. Kaufold,B. Mong, T. Walton

Colorado State University, Fort Collins COD. Akimov, I. Alexandrov, V. Belov, A. Burenkov, M. Danilov, A. Dolgolenko, A. Karelin, A. Kovalenko, A. Kuchenkov, V. Stekhanov, O. Zeldovich

ITEP Moscow, RussiaB. 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 MDJ. Cook, T. Daniels, K. Kumar, P. Morgan, A. Pocar, B. Schmoll, C. Sterpka, D. Wright

University of Massachusetts Amherst, Amherst MAD.Leonard

University of Seoul, Republic of KoreaM. Auger, D. Franco, G. Giroux, R. Gornea, M. Weber, J-L. Vuilleumier

Laboratory for High Energy Physics, Bern, SwitzerlandW. 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 CAP. 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


Backup Slides


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Assumptions:

Majorana neutrinos

EXO Sensitivity


Fiducial Volume Uncertainty

Red: True position in sim

Black: Data

Calibration Source at +X

Half spread: 9.3%

Calibration Source at +Y

Half spread: 7.1%

Active Detector

Uncertainty determined from the fidelity with which calibration events are reconstructed within a chosen volume as compared to simulation


Constraints from alpha spectroscopy

• Investigate alpha spectrum for scintillation signals from 238U• Calibrate spectrum with alphas in Rn chain• Can constrain contamination of 238U 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 234mPa which β decays with a Q-value of 2195 keV, which cannot then explain our LXe bulk signal


3D reconstruction threshold

23

• Events > 100 keV well above charge trigger and reconstruction thresholds

• 3D reconstruction still requires determination of t0 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


Energy Calibration: pair production event

24

• Identifying 3-site events from pair-production and annihilation provides 2 extra charge calibration peaks

-511 keV gammas are our lowest energy calibration sources -1592 keV pair production very similar topology to ββ decays


500

events

Pinpoint Source Location using a Compton telescope technique

• 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


Muon track in EXO-200

Cat

ho

de

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.


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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: 3.8% @ 570 keV or 1.8 %

@ Q(

Ionization & Scintillation: 3.0% @ 570 keV

or 1.4 % @ Q(E.Conti et al., Phys. Rev. B 68 054201 (2003)


Enriched Xenon Observatory (EXO) - SLAC...Phil Barbeau for the EXO collaboration Stanford University...

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