Searching for Double Beta Decay with

the

Enriched Xenon Observatory

Lisa KaufmanUniversity of Maryland

For the EXO Collaboration

TAUP 2009

Overview

• Overview of EXO purpose

and design

• TPC Construction

• EXO-200 at WIPP

• Ba+ Tagging in LXe

• Future Plans

“EXO is a program aimed at building an enriched xenon double beta decay experiment with a one or more ton 136Xe source, with the particular ability to detect the two electrons emitted in the decay in coincidence with the positive identification of the 136Ba daughter via optical spectroscopy”

The EXO phased approach:

The EXO-200 Detector

Ba Ion Identification

Sensitivity

is efficiency

a is isotopic abundance

A is atomic mass

M is source mass

T is time

B is background

is resolution

To maximize sensitivity:

•Large mass

•Low background

•High detection efficiency

•Good energy resolution

In addition, identification of the daughter isotope would reject most sources of

background and confirm double beta decay.

S1/ 20

a

A

MT

B

1/ 2

Choice of Isotope

Natural isotopic abundance of 8.9 %

-Inexpensive enrichment

Q-value of 2479 keV

-favorable phase space

-above most naturally occurring -rays

EXO will search for the decay:136Xe 136Ba + 2e-

T1/20 > 1.2 x 1024 y (90% C.L.)

R.Bernabei et. al., Phys.Lett. 546B, 23 (2002)

F.Simkovic et al., Phys.Rev.C77:045503,2008

Why Xenon?

Xenon isotopic enrichment is easier: Xe is a gas and 136Xe is the heaviest isotope.

Xenon is “reusable”: can be re-purified and 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.

Energy resolution in LXe can be improved: scintillation light + ionizationanti-correlation.

… admits a novel coincidence technique: background reduction by Badaughter tagging.

6

EXO-200

Goals:- Look for 0νββ decay of

136Xe with competitive sensitivity

(T0ν1/2 > 6 x 1025 y, current limit: T0ν

1/2 > 1.2 x 1024 y)

- Measure the standard 2νββ decay of 136

Xe (Q = 2457.8 0.4 keV) andmeasure its lifetime (best upper limit to date: T2ν

1/2 > 1 x 1022 y

- Test backgrounds of large LXe detector at ~2000 m.w.e. depth- Test LXe technology and enrichment on a large scale- Test TPC components, light readout (518 LAAPDs), and radioactivityof materials, xenon handling and purification, energy resolution

[R. Bernabei et al., Phys. Lett. B 546 (2002) 23]

EXO-200 is a large LXe TPC with scintillation light readout. It uses a source of 200 kg of enriched xenon (80%

136Xe).

→ EXO-200 has no 136Ba++ identification ←

LXe Data Show Anticorrelation between

Scintillation and Ionization

Ionization alone:

σ(E)/E = 3.8% @ 570 keV or 1.8% @ Q(

Ionization & Scintillation:

σ(E)/E = 3.0% @ 570 keV or 1.4% @ Q(

E.Conti et al., Phys. Rev. B: 68 054201 (2003)

~570 keV

Bi-207 source1 kV/cm

•Timing of the event:

– Scintillation light gives

– t = 0 for drift time (z)•Position of the event:

Crossed wires at the

anode (x-y) collect

charge at t=z

•Event energy:

Ionization + scintillation

light

EXO-200

~1.5m

~1.5m

HFE7000

cooling/shielding fluid

Copper cryostat

Copper liquid

xenon vessel

Surrounded by 25 cm Pb shield

Copper Xe Vessel

• Very light (wall thickness 1.5

mm, total weight 15 kg), to

minimize material.

• All parts machined under 7 ft of

concrete shielding to reduce

activation by cosmic rays.

• Different parts are e-beam

welded together at Applied

Fusion. Construction of the

vessel with 55 welds has been

completed.

• End caps are TIG welded.

teflon light reflectors

flex cables on back

of APD plane (copper

on kapton, no glue)

field shaping rings

(copper)

acrylic supports

LAAPD plane (copper) and x-y wires

(photo-etched phosphor bronze)

Central HV plane

(photo-etched

phosphor bronze)

x-y crossed

wires, 60o

TPC Design

EXO-200 TPC Construction

EXO-200 TPC Construction

Flex cable etched, cut, and potted!

EXO-200 detector – done!

Massive effort on materials

qualification – database of

~330 entries

D.S. Leonard et

al.,arXiv:0709.4524 (NIMA)

The Crown Jewels of EXO-200

200 kg of xenon enriched to 80% = 160 kg of 136Xe:The most isotope in possession by any ββ0ν collaboration

Aerial View of the WIPP site

EXO

2150 ft (655 m)

Underground Facility

Waste Isolation Pilot Plant (WIPP)

Carlsbad, NM

muon flux at WIPP

(~ 1600 m.w.e.):

4.77×10-3 m-2 s-1

(3.10×10-3 m-2 s-1sr-1, ~15 m-2 h-1)

[Esch et al., NIM A 538 (2005) 516]

Underground Summer 2007

Clean room modules installed on jacks because the salt creeps over time

Cryogenics Systems

Cryogenics systems commissioning underway at WIPP

System during cryo-commissioning in Spring 2007 at Stanford

TPC Electronics

Physics resolution of 1.5 to 2% or 1500 to

2000 electrons

Goal: electronic noise < 800 e’s (adds in

quadrature)

Electronics chassis

Front end cards

•Each Front End card has up to 16

channels

• 3 FE Cards each for APD (37 channels),

HV (38 channels) and VG (38 channels)

• One chassis for each half of the chamber

Currently testing electronics with real EXO-200 chamber

20

TPC Shipment to WIPP

Aug 2009

concrete-shielded container

clean room

EXO-200 Sensitivity

•EXO-200 will also make the first observation of ββ2ν in Xe-136.

•If Heidelberg observation claim is correct, EXO-200 will see between 46 and 170 events, on a background of 40 events (5.0 σ to 11.7 σ effect).

1) Rodin, et. al., Nucl. Phys. A 793 (2007) 213-215

2) Caurier, et. al., arXiv:0709.2137v1

CaseMass

(ton)

Eff.

(%)

Run Time

(yr)

σE/E @ 2.5MeV

(%)

Radioactive

Background

(events)

T1/20ν

(yr, 90%CL)

Majorana mass

(eV)QRPA NSM

EXO-200 0.2 70 2 1.6 40 6.4 x 1025 0.1331 0.1862

Improves on previous 136Xe experiments by one order of magnitude and competitive

with the best ββ0ν experiments in the world.

Single Ba+ ion detection

Daughter identified by optical spectroscopy of Ba+, well studied in ion traps for more than 25 years [Neuhauser, Hohenstatt, Toshek, Dehmelt,

Phys. Rev. A 22 (1980) 1137]

- very specific signature (“Λ” shelving)

- cycling 493/650 nm transitions gives a fluorescence rate of ~108

Hz (in vacuum)

plenty of light!

493nm 650nm

2S1/2

2D3/2: metastable

~83s

2P1/2: ~7.9ns Ba+

CCD

e-

e-

e-

e- e-

e-

Quadrupole linear ion trap

Ba+ grabber

Ba+ Tagging Schematic for

Liquid Phase EXO

APDs Grid plane

Trapping

Vcos(Ωt) + U

U

Ba oven

DC

po

ten

tia

l [V

]

0 Volts

-5 Volts

BaBuffer gas

CCD

e- gun

Spectroscopy

lasersScope

Loading region in

the vacuum tank

e-gun

Ba oven

Tip loading accessMain turbo

port

Differentially

pumped aperture

M.Green et al., Phys Rev A 76 (2007) 023404

B.Flatt et al., NIM A 578 (2007) 409

~9σ discrimination in 25s integration

Trapping

0 ions

1 ion2 ions

3 ions

Liquid Phase Extraction

sensor

Cu cold finger

Au-coated leads

Vespel sleeves

W heater wire

2mmRead-out cables

Resonant Ion Spectroscopy

Cryogenic Probe

• Goal: 137Cs could be used as a tagged source of barium in LXe

for a complete test of the barium tagging method.

• 137Cs→137Ba+ + e- with T1/2 = 30.07 years

– 137Cs can be detected by the corresponding 662 keV gamma with a NaI

detector

• 137Cs comes in the form of CsCl salt, need to chemically isolate the Cs

byCsCl + Na → Cs + NaCl at 200°C

• Now: vacuum test

• Next: in 1 atm of Xe

• Then: in LXe

Cs

Source

goes

here

Na wire burning

CsCl

source

Ta foil for reaction

Current Cs source

test setup

Cesium Source Development at UMD

Xe system designed with

space above the LXe vessel

Cesium Source Development

EXO Sensitivities

The EXO CollaborationK. Barry, E. Niner, A. Piepke

University of Alabama, Tuscaloosa ALP. Vogel

California Institute of Technology, Pasadena CAM. Dixit, K. Graham, C. Green, C. Hagemann, C. Hargrove, E. Rollin, D. Sinclair, V. Strickland

Carleton University, Ottawa ON, CanadaM. Moe

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

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

ITEP Moscow, RussiaB. Aharmim, K. Donato, J. Farine, D. Hallman, U. Wichoski

Laurentian University, Sudbury ON, Canada

H. Breuer, C. Hall, L.J. Kaufman, D. Leonard, S. Slutsky,Y-R. Yen

University of Maryland, College Park MDK. Kumar, A. Pocar

University of Massachusetts Amherst, Amherst MAM. Auger, G. Giroux, R. Gornea, F. Juget, G. Lutter, J-L. Vuilleumier, J-M. Vuilleumier

Laboratory for High Energy Physics, Bern, SwitzerlandN. Ackerman, M. Breidenbach, R. Conley, W. Craddock, J. Hodgson, D. Mackay, A. Odian, C. Prescott, P. Rowson, K. Skarpaas, J. Wodin, L. Yang, S. Zalog

Stanford Linear Accelerator Center (SLAC), Menlo Park CAL. Bartoszek, R. Cooper, R. DeVoe, M. Dolinski, B. Flatt, G. Gratta, M. Green, F. LePort, M. Montero-Diez, R. Neilson,A. Reimer-Muller, A. Rivas, K. O’Sullivan, K. Twelker

Stanford University, Stanford CA

Schedule and Future Plans

All EXO-200 infrastructure is underground undergoing final testing

EXO-200 detector assembly complete, installation at WIPP in

2009

Will start running with natural Xe and switch to enriched Xe when

appropriate

Continuing development of Ba+ tagging technologies in LXe:1. Working on different ion grabbing techniques in parallel:

-Thin ice tip

- RIS tip

2. Developing a single Ba+ source in LXe for testing and calibration