EXO-GAS Detector

Status report for the SNOLAB EAC August 2007

EXO Canada Team

• Laurentian– J. Farine, D. Hallman, C. Virtue, U. Wichoski– Adam Blais (Summer Student)

• Carleton– M. Dixit, K. Graham, C. Hargrove, D. Sinclair– C. Green, E. Rollin (Grad. Students)– K. McFarlane (Engineer) L. Anselmo

(Chemist)

Heidelberg-Moscow Results for Ge double beta decay

57 kg years of 76Ge data Apply single site criterion

Normal and Inverted Mass Hierarchies

We need to develop new strategies to eliminate backgrounds to probe the allowed space

Barium tagging may offer a way forward

Inverted

Normal

EXO – Enriched Xenon Observatory

• Look for neutrino-less double beta decay in Xe• 136Xe --- 136Ba + e- + e-

• Attempt to detect ionization and the Ba daughter• Ba is produced as ++ ion• Ba+ has 1 electron outside Xe closed shell so

has simple ‘hydrogenic’ states• Ba++ can (?) be converted to Ba+ with suitable

additive

Advantages of Xe

• Like most noble gases/liquids it can be made extremely pure

• No long lived radioactive isotopes

• High Q value gives favourable rates

• Readily made into a detector

• Possible barium tagging to eliminate backgrounds

Liquid or Gas

Liquid

Compact detectorNo pressure vesselSmall shield -> lower purity reqd.

Gas

Energy resolutionTracking & multi-site rejectionIn-situ Ba tagging

Large CryostatPoorer energy, tracking resolutionEx-situ Ba tagging

Large detector

Needs very large shield

Pressure vessel is massive

Liquid Detector EXO 200

• Objectives– Prove the liquid detection concept– Measure decay rate for Xe– Test the HM claim for observation of

• Under construction at Stanford for deployment at WIPP

• Major engineering support from Vance Strickland

Status of 2Status of 2νν mode in mode in 136136XeXe

22νββνββ decay has never been observed in decay has never been observed in 136136Xe. Xe. Some of the lower limits on its half life are close to (and inSome of the lower limits on its half life are close to (and in

one case below) the theoretical expectation.one case below) the theoretical expectation.

T1/2 (yr)

evts/year in the 200kg prototype

(no efficiency applied)

Experimental limit

Leuscher et al >3.6·1020 <1.3 M

Gavriljuk et al >8.1·1020 <0.6 M

Bernabei et al >1.0·1022 <48 k

Theoretical prediction

QRPA (Staudt et al) [T1/2

max] =2.1·1022 =23 k

QRPA (Vogel et al) =8.4·1020 =0.58 M

NSM (Caurier et al)(=2.1·1021

)(=0.23 M)The 200kg EXO prototypeThe 200kg EXO prototype

should definitely resolve this issueshould definitely resolve this issue

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)

Xe offers a qualitatively new tool against background:Xe offers a qualitatively new tool against background:136136Xe Xe 136136BaBa++++ e e-- e e- - final state can be identified final state can be identified using optical spectroscopy using optical spectroscopy (M.Moe PRC44 (1991) 931)(M.Moe PRC44 (1991) 931)

BaBa++ system best studied system best studied(Neuhauser, Hohenstatt,(Neuhauser, Hohenstatt,Toshek, Dehmelt 1980)Toshek, Dehmelt 1980)Very specific signatureVery specific signature

““shelving”shelving”Single ions can be detectedSingle ions can be detectedfrom a photon rate of 10from a photon rate of 1077/s/s

•Important additionalImportant additional constraintconstraint•Huge backgroundHuge background reductionreduction

22PP1/21/2

44DD3/23/2

22SS1/21/2

493nm493nm

650nm650nm

metastable 80smetastable 80s

Anode PadsMicro-megas

WLS BarElectrode

For 200 kg, 10 bar, box is 1.5 m on a side

Possible concept for a gas double beta counter

Xe Gas

. . . . . . . .

. . . . . . . .PMT

Lasers

Grids

Anode PadsMicro-megas

WLS BarElectrode

For 200 kg, 10 bar, box is 1.5 m on a side

Possible concept for a gas double beta counter

Xe GasIsobutaneTEA

. . . . . . . .

. . . . . . . .PMT

Lasers

Grids

Triggers

• Level 1 – Light => event in fiducial volume– Light => energy = Q +- 10%

• Level 2– Ionization => energy = Q +- 3%– 2 Bragg peaks– Single site event

• Determine Ba location• Start search for Ba

Gas Option for EXO

• Need to demonstrate good energy resolution (<1% to completely exclude ) tracking,

• Need to demonstrate Ba tagging– Deal with pressure broadening– Ba ion lifetime– Ba++ -> Ba+ conversion– Can we cope with background of scattered

light

Tasks to design gas EXO

• 1) Gas Choice– Measure Energy resolution for chosen gas– (Should be as good as Ge but this has never

been achieved)– Measure gain for chosen gas– Measure electron attachment for chosen gas– Understand optical properties– Measure Ba++ -> Ba+ conversion– Measure Ba+ lifetime

Tasks to design EXO Gas

• 2) TPC Design– What pressure to use– What anode geometry to use– What chamber geometry to use– What gain mechanism to use– Develop MC for the detector– Design electronics/DAQ

Tasks to design EXO Gas

• 3) Ba Tagging– Demonstrate single ion counting– Understand pressure broadening/shift– Understand backgrounds– Fix concept

Tasks to design EXO Gas

• 4) Overall Detector concept– Fix shielding requirements and concepts– Design pressure containment– Fix overall layout

Gas Properties

• Possible gas – Xe + iso-butane + TEA• Iso-butane to keep electrons cold, stabilize

micromegas/GEM• TEA

– Converts Ba++ -> Ba+• Q for TEA + Ba++->TEA+ + Ba+* ~ 0

– Converts 172 nm -> 280 nm?– ? Does it trap electrons?– ?Does it trap Ba+?

Measuring Gas properties

• Gridded ion chamber being used to measure resolution, drift of electrons using alpha source

AnodeGrid

Field Rings

Source

Movable source holderContacts rings with wiper

Gridded Ion Chamber

Progress on energy resolution – Pure Xe, 2 Bar

Xe Energy Spectrum 3cm 2b 5992

0

50

100

150

200

500 510 520 530 540 550 560 570 580 590 600

Energy (MeV)

Co

un

ts

Alpha spectrum at 2 b pressure.

= 0.6%

Energy Spectrum for Xe + CH4 (5%)

Corrected Energy Spectrum (10 cm)

0

50

100

150

200

450 460 470 480 490 500 510 520 530 540 550

Energy (arb. units)

Co

un

ts

Amplitude vs risetime

0

0.1

0.2

0.3

20 40 60 80 100 120 140

rise time (16 ns/bin)

Am

plit

ud

e (

Arb

. un

its

)

Amplitude and resolution vs source distance

0.40.450.5

0.550.6

0.650.7

0.750.8

0 5 10 15 20

Source distance (cm)

resolution

Peak Amplitude

Xe + 5% CH4

Xe + Isobutane Peak vs Drift Distance

547539

518

497490

500

510

520

530

540

550

0 5 10 15 20

Drift (cm)

Pea

k am

pli

tud

e

Note: (1) peak width was constant at ~0.6% over the range(2) Gas was not purified but was spec’d at 99.9%

Current status on energy resolution

• Ionization in gaseous Xe gives adequate energy resolution, even for alpha particles.

• We can now use this to explore gain options

Studying Ba ions in high pressure Xe gas

- - - - - - - - - - - - - - - - - -

- - - - - - - - - - - - - - - - - -

Thin (5 m) Pt wire + BaGrid 1

Grid 2

PMT

__ __ __ __ __ __ __ __ __ __ __ __ __ Laser Beams

Pulse red and blue lasers out of phase with each other

Filter

Ion production in test cell (detection using Channeltron)Ion detection from hot Pt wire

1

10

100

1000

10000

100000

1000000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Time since heating pulse (s)

Co

un

ts p

er

1 m

s b

in

Progress on Ba tagging

Problems with Proposed technique

• It appears that the D state de-excites through collisions on a timescale short compared to our laser pulsing

• This would allow a different approach

• Use cw blue laser and look for red fluorescence lines

• Red sensitive PMT on order

Si detector228Th

Lens

PMT

Laser Beam

Concept for single ion fluorescence of Ra

Plans (Dreams)

• We are working to address the technical issues associated with a large gas Xe double beta decay detector

• If all goes well we will seek funding to build a 200 kg gas detector with Ba tagging

Xe200 kg

at 18.2 psia

VacuumAround acrylic

blocks ?H2O

(3.3 psi + 18.2 psi)~ 21.4 psia

H2O(7.7 psi + 18.2)

~ 25.9 psia

Acrylic Blocks9 tonnes

(Fills 25% of space)

Crinkled Cubic Copper Liner3,000 lb (if 0.1 inch thick)

10.2 feet each side

Acrylic Cylindrical Shell14.9 feet diameter,

12.2 feet highWater Tank28 diameter

for 2 meters H2O

EXO GAS DETECTOR CONCEPT200 Kg

Elevation

Plan View

Note: Decreasing the Xe pressureto 1 bar requires increasing the coppertank to 11 foot sides.

Water Shield490 tonnes water

If filled without internals

Longer term plans

• If things go really well we can consider a ton scale detector.

• Could be either liquid or gas• If Ba tagging works very well then

incentive to use separated isotope Xe is weaker

• A detector of several tons could be accomodated in either the cube hall or the cryopit.

EXO Progress Update

Laurentian University

Jacques Farine

EXO Gas Option Simulation

First step: containment efficiencies

• Pressure and mass dependence

• Cylinder, take H=2R to minimize S/V

• Filled with 136Xe

• Cu walls

• 0 decay, Q = 2457.8 keV

• Differentiate e–//both crossing fid. vol.

Uncertainties obtained from 20 independent simulations. + Points include detailed low energy processes, scintillation and E=1kV/cm ( .. 30x CPU cycles).

2 / 0 differential c at edges

• Simulations for 1T at 5 atm, equator

• 10,000 evts ea.• Contam. of 2 in 0

increases towards the edge

• > Optimize fiduc. volume and/or vary fraction of contamination

Next steps

• Add chemical composition / drift / attenuation / absorption / attachment // light+charge readout

• Add backgrounds as source of singles• Write code to detect Bragg peaks • For single/double separation, determine:

– Contamination / sacrifice– Effect of Bremsstrahlung

• Light collection options > E resolution

Studies related to bothL+G Options

Material screening - radon emanation tests

• Continued program at SNOLAB• Sensitivity 10 220Rn/day, 20 222Rn/day• Measure EXO-200 plumbing• No substantial source• Clean !

Characterize counters for Ar/Xe

• Allow for:– Absolute emanation

measurements– Diffusion studies in

• Absolute cross-calibration between gasesN2 = Ar; Xe 23% lower

Radon Trap Development1) ESC on EXO-200• Augmented with:

– CO2 trap

– Rn source

– Water vapour trap

– Radon trap Mark I (LN2)

– Heat exchanger

– Recirculation pump

• Study Rn removal efficiency:– In misc. gases Air/N2/Ar > Xe

– Rn trap Mark I

Radon trap tests at ES-III (Stanford)

Mark I trap: 2” of SS wool at LN2, multiple passes efficiency too low (60% in 160 mbar N2) - sets scope

Radon Trap Development

2) At SNOLAB

• 222Rn and 210Rn sources development

• Radon extractor board as trap testbed

• Refrigerator purchased

• Cold head integration underway

• Xenon purchased

• Xe plumbing assembly initiated (w/ RCV vessels)

• ESC integration underway

Xenon heat exchanger

in construction

Diffusion of Rn in Xe

Reduction factor along dead legs• Known, irreducible source term• Want max. ingress rate at distance L• For 220/222Rn in N2/Ar/Xe

Theory - KTG in binary, dilute mixture, calculate D12

• 1D diffusion model with decay

Experimental check Diffusion length for 222Rn at 1 atm:

d = 2m in Ar; 1.2m in Xe

LGas at p,T