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 76 Ge dataApply 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 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

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 2ν mode in 136 Xe 2νββ decay has never been observed in 136 Xe. Some of the lower limits on its half life are close to (and in one case below) the theoretical expectation. T 1/2 (yr) evts/year in the 200kg prototype (no efficiency applied) Experimental limit Leuscher et al>3.6·10 20 <1.3 M Gavriljuk et al>8.1·10 20 <0.6 M Bernabei et al>1.0·10 22 <48 k Theoretical prediction QRPA (Staudt et al) [T 1/2 max ]=2.1·10 22 =23 k QRPA (Vogel et al)=8.4·10 20 =0.58 M NSM (Caurier et al)(=2.1·10 21 )(=0.23 M) The 200kg EXO prototype should definitely resolve this issue

Assumptions: 1)80% enrichment in 136 2)Intrinsic low background + Ba tagging eliminate all radioactive background 3)Energy res only used to separate the 0ν from 2ν modes: Select 0ν events in a ±2σ interval centered around the 2.481MeV endpoint Select 0ν events in a ±2σ interval centered around the 2.481MeV endpoint 4)Use for 2νββ T 1/2 >1·10 22 yr (Bernabei et al. measurement) *  (E)/E = 1.6% obtained in EXO R&D, Conti et al Phys Rev B 68 (2003) †  (E)/E = 1.0% considered as an aggressive but realistic guess with large light collection area collection area ‡ QRPA: 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 EXO neutrino effective mass sensitivity CaseMass (ton) Eff. (%) Run Time (yr) σ E 2.5MeV (%) 2νββ Background (events) T 1/2 0ν (yr, 90%CL) Majorana mass (meV) QRPA ‡ (NSM) # Conserva tive * 0.5 (use 1)2* (95) Aggressi ve †1† 0.7 (use 1)4.1* (21)

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

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

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%

Energy Spectrum for Xe + CH 4 (5%)

Xe + 5% CH 4

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 + Ba Grid 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)

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 detector 228 Th 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

Xe 200 kg at 18.2 psia Vacuum Around acrylic blocks ? H2O (3.3 psi psi) ~ 21.4 psia H2O (7.7 psi ) ~ 25.9 psia Acrylic Blocks 9 tonnes (Fills 25% of space) Crinkled Cubic Copper Liner 3,000 lb (if 0.1 inch thick) 10.2 feet each side Acrylic Cylindrical Shell 14.9 feet diameter, 12.2 feet high Water Tank 28 diameter for 2 meters H2O EXO GAS DETECTOR CONCEPT 200 Kg Elevation Plan View Note: Decreasing the Xe pressure to 1 bar requires increasing the copper tank to 11 foot sides. Water Shield 490 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 136 Xe Cu walls 0  decay, Q = 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 both L+G Options

Material screening - radon emanation tests Continued program at SNOLAB Sensitivity Rn/day, Rn/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 gases N 2 = Ar; Xe 23% lower

Radon Trap Development 1) ESC on EXO-200 Augmented with: –CO 2 trap –Rn source –Water vapour trap –Radon trap Mark I (LN 2 ) –Heat exchanger –Recirculation pump Study Rn removal efficiency: –In misc. gases Air/N 2 /Ar > Xe –Rn trap Mark I

Radon trap tests at ES-III (Stanford)  Mark I trap: 2” of SS wool at LN 2, multiple passes  efficiency too low (60% in 160 mbar N 2 ) - sets scope

Radon Trap Development 2) At SNOLAB 222 Rn and 210 Rn 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 220/222 Rn in N 2 /Ar/Xe Theory - KTG in binary, dilute mixture, calculate D 12 1D diffusion model with decay Experimental check  Diffusion length for 222 Rn at 1 atm: d = 2m in Ar; 1.2m in Xe L Gas at p,T