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 76 Ge dataApply 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 136 Xe ---  136 Ba + 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 detector No pressure vessel Small shield -> lower purity reqd. Gas Energy resolution Tracking & multi-site rejection In-situ Ba tagging Large Cryostat Poorer energy, tracking resolution Ex-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: 136 Xe 136 Ba ++ e - e - final state can be identified using optical spectroscopy (M.Moe PRC44 (1991) 931) Ba + system best studied (Neuhauser, Hohenstatt, Toshek, Dehmelt 1980) Very specific signature “shelving” Single ions can be detected from a photon rate of 10 7 /s Important additionalImportant additional constraint constraint Huge backgroundHuge background reduction reduction 2 P 1/2 4 D 3/2 2 S 1/2 493nm 650nm metastable 80s

Anode Pads Micro-megas WLS Bar Electrode 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 Pads Micro-megas WLS Bar Electrode For 200 kg, 10 bar, box is 1.5 m on a side Possible concept for a gas double beta counter Xe Gas Isobutane TEA.... 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

Anode Grid Field Rings Source Movable source holder Contacts rings with wiper Gridded Ion Chamber

Progress on energy resolution – Pure Xe, 2 Bar 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 109 Cd 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 2 P 1/2 4 D 3/2 2 S 1/2 493nm 650nm metastable 80s Original concept Pulse 493 nm laser to Excite D state Then pulse 650 nm Laser to un-shelf D state 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 10 5 ions/pulse using an old N 2 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  = E 2 D  / 2 Correction field Ec = E 2 D  / 2 [  o 3 D ] Ec = E 2  / 6  o

Selecting ions based on 

FAIMS in non uniform field For a non-uniform E field Say E = E o (1 +  y) Then there is a restoring field Ec = E o 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

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 MeV gamma background ( 136 Xe  Q value is at the Compton edge for 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