EXOGAS Detector Status report for the SNOLAB EAC
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. Mc. Farlane (Engineer) L. Anselmo (Chemist)
Heidelberg-Moscow Results for Ge double beta decay 57 kg years of 76 Ge data Apply single site criterion
Normal and Inverted Mass Hierarchies
Inverted Normal 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 Gas Compact detector No pressure vessel Small shield -> lower purity reqd. 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 2 decay rate for Xe – Test the HM claim for observation of 0 • 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) theoretical expectation. T 1/2 (yr) evts/year in the 200 kg prototype (no efficiency applied) 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 QRPA (Staudt et al) [T 1/2 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) Experimental limit Theoretical prediction The 200 kg EXO prototype should definitely resolve this issue
EXO neutrino effective mass sensitivity 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: 4) Select 0ν events in a ± 2σ interval centered around the 2. 481 Me. V endpoint 5) 4) Use for 2νββ T 1/2>1· 1022 yr (Bernabei et al. measurement) Case Mass (ton) Eff. (%) Run Time (yr) σE/E @ 2νββ 2. 5 Me. V Background (%) (events) Conserva tive 1 70 5 1. 6* Aggressi ve 10 70 10 1† 0. 5 (use 1) T 1/20ν (yr, 90%CL) 2*1027 0. 7 (use 1) 4. 1*1028 Majorana mass (me. V) QRPA‡ (NSM)# 33 (95) 7. 3 (21) s(E)/E = 1. 6% obtained in EXO R&D, Conti et al Phys Rev B 68 (2003) 054201 s(E)/E = 1. 0% considered as an aggressive but realistic guess with large light collection area ‡ QRPA: A. Staudt et al. Europhys. Lett. 13 (1990) 31; Phys. Lett. B 268 (1991) 312 # NSM: E. Caurier et al. Phys Rev Lett 77 (1996) 1954 * †
Xe offers a qualitatively new tool against background: 136 Xe 136 Ba++ e- e- final state can be identified using optical spectroscopy (M. Moe PRC 44 (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 107/s • Important additional constraint • Huge background reduction 2 P 1/2 650 nm 493 nm 4 D 3/2 metastable 80 s 2 S 1/2
Possible concept for a gas double beta counter Anode Pads Micro-megas WLS Bar Electrode Xe Gas Grids . . . . For 200 kg, 10 bar, box is 1. 5 m on a side Lasers PMT
Possible concept for a gas double beta counter Anode Pads Micro-megas Electrode Grids Xe Gas Isobutane TEA . . . . For 200 kg, 10 bar, box is 1. 5 m on a side WLS Bar Lasers PMT
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 2 ) 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
Movable source holder Contacts rings with wiper Field Rings Source Grid Anode Gridded Ion Chamber
Progress on energy resolution – Pure Xe, 2 Bar s = 0. 6% Alpha spectrum at 2 b pressure.
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 mm) Pt wire + Ba - - - - - Grid 1 Laser Beams _____________ - - - - - Grid 2 Filter PMT Pulse red and blue lasers out of phase with each other
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 Laser Beam Lens PMT 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
EXO GAS DETECTOR CONCEPT 200 Kg Crinkled Cubic Copper Liner 3, 000 lb (if 0. 1 inch thick) 10. 2 feet each side Plan View Acrylic Cylindrical Shell 14. 9 feet diameter, 12. 2 feet high Water Tank 28 diameter for 2 meters H 2 O Vacuum Around acrylic blocks ? Water Shield 490 tonnes water If filled without internals Acrylic Blocks 9 tonnes (Fills 25% of space) H 2 O (3. 3 psi + 18. 2 psi) ~ 21. 4 psia Xe 200 kg at 18. 2 psia H 2 O (7. 7 psi + 18. 2) ~ 25. 9 psia Note: Decreasing the Xe pressure to 1 bar requires increasing the copper tank to 11 foot sides. Elevation
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=2 R to minimize S/V • Filled with 136 Xe • Cu walls • 0 decay, Q = 2457. 8 ke. V • Differentiate e–/ /both crossing fid. vol.
Uncertainties obtained from 20 independent simulations. + Points include detailed low energy processes, scintillation and E=1 k. V/cm (. . 30 x CPU cycles).
2 / 0 differential c at edges • Simulations for 1 T 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 10 220 Rn/day, 20 222 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) Ø Mark I trap: 2” of SS wool at LN 2, multiple passes Ø 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 Gas at p, T L 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 • 1 D diffusion model with decay Experimental check Ø Diffusion length for 222 Rn at 1 atm: d = 2 m in Ar; 1. 2 m in Xe
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