EXO Gas

Progress and Plans

October, 2008

David Sinclair

EXO Collaboration

Canada Carleton, Laurentian

USA Alabama, Caltech, Colorado State, UC irvine, Maryland, Massachusetts, SLAC, Stanford

Switzerland Bern

Russia ITEP

EXO People

Canadian Team

Laurentian Jacques Farine, Doug Hallman, Ubi Wichoski

Carleton Madhu Dixit, Kevin Graham, Cliff Hargrove, David

Sinclair Christina Hagemann (RA Arrives 2 weeks) Etienne Rollin (PhD Student) Chad Greene, James Lacey (MSc students)

Currently 3 undergraduate thesis/project students

New effort for the gas phase

NSF grants to Stanford and Alabama for RA,s students to work on gas EXO

New EXO collaborators at ITEP who have just completed a Xe TPC project

Possible collaboration with Spanish group

Heidelberg-Moscow Results for Ge double beta decay

57 kg years of 76Ge data Apply single site criterion

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

Barium tagging may offer a way forward

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

EXO 200

A 200 kg liquid xenon detector is nearing completion at WIPP

We play a major role in this project and there is on-going activity at SNOLAB supporting this project

This talk will focus on the gas counter as this is a potential candidate for a SNOLAB project

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

Program as stated last year

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 almost 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+?

Progress This Year

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%

Program with Gridded Ion Chamber Response for many gas mixtures measured New data on drift velocities in Xe + Methane,

isobutane, TEA Some electron attachment measured but may

be due to gas impurities

First efforts with Micromegas Grid and anode of chamber replaced by

micromegas Collaboration with Saclay and CERN to

produce micromegas Using new ‘microbulk’ form of micromegas as

this is thought to offer best resolution Ion density with alphas too high for this

technology – resolution ~ 1.7% Switch to betas

Spectroscopy with micromegas

109Cd source

22 keV

Status of Micromegas

Energy resolution of 4% observed for 22 keV x-ray is promising (-> 0.4% at 2 MeV)

Microbulk technology is not sufficiently robust for this application

Xe requires high fields for gas gain and lifetime of the micromegas is hours for these fields

Will attempt again with the T2K style micromegas

Progress on Detector Simulations Double beta events being simulated in Xe

gas using GEANT and EGS Tracks are ugly!

Containment of tracks

Case for mixed gas

There is incentive from previous slides to investigate a mixed gas (Ne-Xe or He-Xe)

Tracks in the lighter are straighter Better containment for given amount of

(expensive) xenon

Ratio of projected track to the total track length

Measuring the scintillation light signal

Energy and position response for scintillation light

Light from gas mixtures

(this slide intentionally left blank)

Measuring scintillation light in Xe gas mixtures It appears that any quench gas in Xe kills the

scintillation light It appears that the mechanism is not

absorption of the photons but interaction between Xe dimers and the additives which de-excite the dimers.

Barium tagging

22PP1/21/2

44DD3/23/2

22SS1/21/2

493nm493nm

650nm650nm

metastable 80smetastable 80s

Original conceptPulse 493 nm laser toExcite D stateThen pulse 650 nm Laser to un-shelf Dstate

Does not work!

New Concept for Laser Tagging in High Pressure The D state is quenched by gas interactions

in ns So – use only blue laser, look for red light

Barium fluorescence Observed

Status of tagging

A number of linewidth measurements made with the arc source

Changing from an arc source to a laser ablation source

We have demonstrated production of about 105 ions/pulse using an old N2 laser

We are about to modify chamber to introduce this source

New Detector Concept

We have some as yet unresolved issues with the original concept

We do not get scintillation light with quenchers but we cannot have gas gain without

We are concerned that additives such as TEA will give us gas purification difficulties so how do we convert Ba++ to Ba+ and we do not know that TEA like additives will not form molecules or clusters with the Ba ions

New Concept

Identify the barium production by extracting the ion into vacuum and using conventional techniques to identify a mass 136, ++ ion.

Expect this to be unique to Ba Operate the detector in pure noble gas (Xe or

Xe+Ne) Use electroluminescence in place of gas

electron gain

Concept for an electroluminescence readout

Design copied from Fermilab RICH counter

Electroluminescence Demonstration EL is a well studied technique in noble gases and

mixed noble gases EL is preferred over electron proportional counters

for gamma ray detectors In Ne + Xe all of the light comes out at the Xe

scintillation wavelength (175 nm) for admixtures of >1% Xe

No-one has demonstrated energy resolution in MeV range

We propose to construct a detector to establish performance of EL for this application

We plan a 20 x 20 array of 2 cm pads on each end

Barium Identification

Because of the complexity of the electron tracks in Ba, it will be hard to determine exactly where the Ba is produced.

We have some volume within which it will be contained.

Transport that ‘volume’ to the edge of the detector

Stretch and squeeze it using field gradient into a long pipe

Barium Identification (Cont)

At end of pipe have an orifice leading to evacuated region

Trap ions as they leave the gas using a Sextupole Ion Trap (SPIG)

Once the ion is in vacuum, use conventional techniques to identify it (eg Wein filter + quadrupole MS or TOF + rigidity or ….

Critical Design Point

What is the efficiency for getting the ion out of the pipe and trapped by the spig?

We will start by simulations for the trap varying trap geometry, pressures, gas mix

Possibly do tests on existing traps Look at improving delivery of ions down pipe

using RF carpets or FAIMS

RF Carpets RF Funnels

Riken Ion Source

Gas cell length is 1 m

Gas is He at 100 torr

RF is 150 V at 10 MHz

RF Carpet operating at low pressure (10’s of mb)

MSU Source

Ion path near the orifice

Problems with RF carpets

These devices work best with low pressure, light gases

We need to work with at least a substantial fraction of Xe and we would like to work at or above atmospheric pressure

FAIMS for EXO

Field Asymmetric Ion Mass Spectrometer

Concept

FAIMS Operation

Deflection during 1 cycle = E D (hi - lo) Let (E) = 0 + E Then = E2 D / 2 Correction field Ec = E2 D / 2 [ o 3 D ] Ec = E2 / 6 o

Selecting ions based on

FAIMS in non uniform field

For a non-uniform E field Say E = Eo (1 + y) Then there is a restoring field Ec = Eo

2 () y

Coaxial cylinders ion selection

Mass Spec on Hydrolyzed Yeast

Is FAIMS useful for EXO

Would explore a different geometry with focusing to center of pipe

Need data on mobility of Ba++ in Xe, (Ne) The technique is used at atmospheric

pressure and tested to 2 bar Need to explore impact of longitudinal drift

field

Only data found to date in doubly charged ion mobility

Mobilities for Xe+, Xe++ in Ne

012345678

0 50 100 150

E/N

Mo

bili

tie

s

Xe+

Xe++

Where Might This Lead

We are aiming at a detector design at 200 kg scale

Would be world’s first ‘background free’ double beta decay experiment – competitive with the best in the world for sensitivity

Would be a test of concept for a ton scale detector

Requirements for the Detector Needs to be deep underground to avoid

cosmogenic production of radioactive Cs Needs to be well shielded to cut the 2.614

MeV gamma background (136Xe Q value is at the Compton edge for 2.614 MeV gammas) – Water shield

Size depends on the pressure and gas mix Would likely occupy much of Cryopit

What do we want from EAC

Overwhelming endorsement for the ongoing R&D program

Continued SNOLAB support Part time technician to operate and maintain the

lasers Engineering support

Note that a request for a large detector underground is likely next year – candidate for the Cryopit