Z.Djurcic, D.Leonard, A.Piepke Physics Dept, University of Alabama, Tuscaloosa AL

P.Vogel Physics Dept Caltech, Pasadena CA

A. Bellerive, M. Dixit, C. Hargrove, D. Sinclair Carleton University, Ottawa, CanadaW.Fairbank Jr., S.Jeng, K.Hall

Colorado State University, Fort Collins COM.Moe

Physics Dept UC Irvine, Irvine CAD.Akimov, A.Burenkov, M.Danilov, A.Dolgolenko, A.Kovalenko, D.Kovalenko, G.Smirnov, V.Stekhanov

ITEP Moscow, RussiaJ. Farine, D. Hallman, C. Virtue

Laurentian University, CanadaM.Hauger, F.Juget, L.Ounalli, D.Schenker, J-L.Vuilleumier, J-M.Vuilleumier, P.Weber

Physics Dept University of Neuchatel, Neuchatel SwitzerlandM.Breidenbach, R.Conley, C.Hall, A.Odian, C.Prescott, P.Rowson, J.Sevilla, K.Skarpaas, K.Wamba,

SLAC, Menlo Park CAE.Conti, R.DeVoe, G.Gratta, M.Green, T.Koffas, R.Leon, F.LePort, R.Neilson, S.Waldman, J.Wodin

Physics Dept Stanford University, Stanford CA

EEnriched nriched XXenon enon OObservatorybservatoryfor double beta decayfor double beta decay

Last decade: the age of ν physics

Discovery of ν flavor change:

• Solar neutrinos (MSW effect)• Reactor neutrinos (vacuum oscillation)• Atmospheric neutrinos (vacuum oscillation)• Loose ends: LSND results

So assuming that MiniBoone sees no oscillations,we know that:

•ν masses are non-zero,•There are 2.981±0.008 v (Z lineshape),•3 ν flavors were active in Big bang Nucleosynthesis

Drastically different mass scenarios are still allowed by the data

~2.8 eV From

tritium

en

dpoin

t(M

ain

tz an

d T

roitsk

)

~0.3 eVFrom

0νβ

β if ν is M

ajo

rana

~1 eVFro

m W

MA

P

Tim

e o

f flig

ht fro

m S

N198

7A

(PD

G 2

002)

23 eV

No real understanding why M so smallDirac vs Majorana neutrinos?Need a lepton number violating process…

decay can occur in two modes

a)Via the emission of 2’s:

A typical 2nd order nuclear process

b) A neutrinoless mode:Requires bothM0 and

decay a standard but small2nd order correction to regular decay

BUT in some cases is the onlyenergetically allowed

2 has beenobserved

experimentally

0 has neverbeen

observedexperimentall

y

0 sensitive to allneutrino masses

For 0 decay due to light Majorana ν masses:

where,

the effective Majorana neutrino mass,

FM and GTM nuclear matrix elements that canbe calculated,

0G a known phase-space factor,

02/1T the half-life time to be measured

12

0

2

0002/1

2,

FV

AGTee M

g

gMZEGTm

3

1

2

,i

iiieee mUm

2 spectrum(normalized to 1)

0 peak (5% FWHM)(normalized to 10-2)

0 peak (5% FWHM)(normalized to 10-6)

Summed electron energy in units of the kinematic endpoint (Q)

Detection of 0νββ DecayThe two e- energy sum is the primary tool

from S.R. Elliott and P. Vogel, Ann.Rev.Nucl.Part.Sci. 52 (2002) 115.

For further substantial progress we need tons of anappropriate isotope exposed for a long time

BUT there are problems

•In a bkgnd free environment mass sensitivity scales as

NtTm /1/1 02/1

•If bkgnd scales like Nt mass sensitivity scales as

4/102/1 /1/1 NtTm

Qualitatively new means are needed to suppressbkgnds and fully utilize the large fiducial mass

Xe is ideal for such a measurement• It is one of the easiest isotopes to enrich;

• Like argon, it represents a good ionization detecting medium;

• It exhibits substantial scintillation that can be used to complement the ionization detection;

• Can be re-purified during the experiment;

• No long lived Xe isotopes to activate;

• Its decay results in 136Ba that can be identified in its atomic form via techniques of high resolution optical spectroscopy.

22PP1/21/2

44DD3/23/2

22SS1/21/2

493nm493nm

650nm650nm

metastable 47smetastable 47s

Optical detection of a 136Ba+ atom (M. Moe PRC44(1991)931)

Resonant laser detection is:

•Highly sensitive, yielding >107

photons per atom;•Highly selective;•Extensively used in the atomic physics community.

Provides additionalconstraint

Huge bkgnd reductionpump probe

Detector R&D Program

• Single ion Ba+ tagging at different Xe pressures;

• LXe energy resolution;• LXe purification for long e- lifetime and

radioimpurities;

• Ba ion lifetime and grabbing from LXe;

• 136Xe Isotopic enrichment;

• Procurement and characterization of low radioactivity

materials;

• Construction/operation of a 200kg enriched 136Xe prototype detector;

EXO spectroscopy lab

Ion TrapIon Trap

493nm laser493nm laser650nm laser650nm laser

CCD Image of Ba+ ions in the trap

Trapedge

Zero ion backgroundZero ion background

All above in UHV;Perform the sameexperiment in noblegas atmosphere

Millikan type experiment with the ion trap

1Lt Test Chamber

PMT

Ionization ReadoutPreamplifier

Energy Resolution Measurement Setup

Reconstruct energy as linearcombination of ionization andscintillation signals

There are indications thatcorrelations between the twovariables help improve energyresolution; J.Seguinot et al. NIM A354 (1995) 280

Clear anti-correlation on event-to-event basis is seen…

A linear combination of ionization and scintillation WILL optimize resolution

First E

XO published result i

n Phys.Rev.B

Resolution is optimized by a ~100-150 ‘mixing angle’

Ionization onlyIonization only

Ionization combinedIonization combinedwith scintillationwith scintillation

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

3.3%@570keV3.3%@570keVor 1.6%@2.5MeVor 1.6%@2.5MeV

Xe purification studies-Continuous Xe Recirculation

This is already the This is already the largest non-fissile largest non-fissile

isotope enrichment isotope enrichment program ever program ever entertained!entertained!

First 200 kg pilot production started in the Summer of 2001 First 200 kg pilot production started in the Summer of 2001 and was successfully completed in May 2003 and was successfully completed in May 2003

Xe leak monitoringXe leak monitoringstationstation

200kg 136Xe Prototype is an important step

• Need to test the detector technology, particularly the LXe option;

• Essential to understand backgrounds from radioactivity;

• Necessary to measure the 2 “background” mode;

• Test the production logistics and quality of 136Xe;

• 2000kg of natural Xe are already available by our collaborators at ITEP;

• Already a respectable (20x) decay experiment (no Ba-ion tagging at this stage);

Detector

• ~60 liters enriched liquid 136Xe,

– In low background teflon vessel– Surrounded and shielded by ~50 cm radially low

background thermal transfer fluid– Contained in a low background Cu double walled

vacuum insulated cryostat– Shielded by ~ 5 cm very low background Pb– Further shielded by ~20 cm low background Pb– Located ~800 m below ground in NaCl deposit – WIPP in

Carlsbad, New Mexico.

• Detector is a liquid TPC with photo-detectors to provide start time and improve energy resolution of the β’s.

Detector

2D Detector schematic

APD plane below crossed wire array

Cryostat Cross Section

Condenser

Xenon Heater

should be

on this area

Xenon

Chamber

Support

Outer Copper Vessel

Inner Copper Vessel

FC-87

FC-87

1” thick Thermal Insulation (MLI-vacuum), not shown to scale

Outer Door

Inner Door

Xenon

Chamber

Refrigerant feedthroughsHeat Transfer Fluid In/Out

Detector Full View

Simplified xenon handling system diagram

An experimental facility for EXO

WIPP : Waste Isolation Pilot PlantCarlsbad NM

•Enriched Xe in hand.

•Clean rooms in commercial production.

•WIPP agreement, including Environmental Impact, complete.

•Swiss collaborators building cryostat.

•Xe purification and refrigeration issues through R&D, purchasing of components.

•Detector vessel, readout, and electronics being engineered.

•EXO could be in WIPP by Summer 2005, if technically limited.

Status

Assumptions: Assumptions: 1)1) 200kg of Xe enriched to 80% in 136200kg of Xe enriched to 80% in 1362)2) σσ(E)/E = 1.6% obtained in EXO R&D, Conti et al Phys Rev B 68 (2003) 054201(E)/E = 1.6% obtained in EXO R&D, Conti et al Phys Rev B 68 (2003) 0542013)3) Low but finite radioactive background: Low but finite radioactive background: 20 events/year in the ±220 events/year in the ±2σσ interval centered around the 2.481MeV endpoint interval centered around the 2.481MeV endpoint4)4) Negligible background from 2Negligible background from 2νββνββ (T (T1/21/2>1·10>1·102222yr R.Bernabei et al. measurement)yr R.Bernabei et al. measurement)

EXO 200kg prototype mass sensitivityEXO 200kg prototype mass sensitivity

Case Mass(ton)

Eff.(%)

Run Time(yr)

σE/E @ 2.5MeV

(%)

Radioactive

Background

(events)

T1/20ν

(yr, 90%CL)

Majorana mass(eV)

QRPA (NSM)

Prototype 0.2 70 2 1.6* 40 6.4*1025 0.18 (0.53)

Assumptions: Assumptions: 1)1) 80% enrichment in 13680% enrichment in 1362)2) Intrinsic low background + Ba tagging eliminate all radioactive backgroundIntrinsic low background + Ba tagging eliminate all radioactive background3)3) Energy res only used to separate the 0Energy res only used to separate the 0νν from 2 from 2νν modes: modes: Select 0Select 0νν events in a ±2 events in a ±2σσ interval centered around the 2.481MeV endpoint interval centered around the 2.481MeV endpoint4)4) Use for 2Use for 2νββνββ T T1/21/2>1·10>1·102222yr (Bernabei et al. measurement)yr (Bernabei et al. measurement)

** (E)/E = 1.6% obtained in EXO R&D, Conti et al Phys Rev B (E)/E = 1.6% obtained in EXO R&D, Conti et al Phys Rev B 68 (2003) 05420168 (2003) 054201†† (E)/E = 1.0% considered as an aggressive but realistic guess with large light(E)/E = 1.0% considered as an aggressive but realistic guess with large light collection areacollection area‡‡ QRPA: A.Staudt et al. Europhys. Lett.13 (1990) 31; Phys. Lett. B268 (1991) 312QRPA: A.Staudt et al. Europhys. Lett.13 (1990) 31; Phys. Lett. B268 (1991) 312## NSM: E.Caurier et al. Phys Rev Lett 77 (1996) 1954 NSM: E.Caurier et al. Phys Rev Lett 77 (1996) 1954

EXO neutrino effective mass sensitivityEXO neutrino effective mass sensitivity

Case Mass(ton)

Eff.(%)

Run Time(yr)

σE/E @ 2.5MeV

(%)

2νββBackgroun

d(events)

T1/20ν

(yr, 90%CL)

Majorana mass(meV)

QRPA‡ (NSM)#

Conservative 1 70 5 1.6* 0.5 (use 1) 2*1027 33 (95)Aggres

sive 10 70 10 1† 0.7 (use 1) 4.1*1028 7.3 (21)