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A Gas Option for EXO

A Gas Option for EXO. David Sinclair EXO Week January 2006. Note – Most of the ideas here come from a discussion with Bill. Why Investigate a Gas Option. Gas counter would allow tracking of electrons Better rejection of gamma backgrounds Single site criteria Two Bragg peak criteria

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A Gas Option for EXO

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  1. A Gas Option for EXO David Sinclair EXO Week January 2006 Note – Most of the ideas here come from a discussion with Bill

  2. Why Investigate a Gas Option • Gas counter would allow tracking of electrons • Better rejection of gamma backgrounds • Single site criteria • Two Bragg peak criteria • Possibly better energy resolution? • Better prospects for in situ Ba tagging

  3. Gas Option Con’s • Much larger volume • Shielding more difficult • high pressure containment

  4. Objectives • To demonstrate a feasible concept for a gas phase Xe detector for bb decay • If successful, to build a 200 kg prototype • If really successful, expand to tonne scale

  5. Possible concept for a gas double beta counter Anode Pads Micro-megas WLS Bar Electrode Blue/red laser ->->->->->->->->->->->->->->->->->->->-> Wire cathode WLS Bars For 200 kg, 10 bar, box is 1.5 m on a side

  6. Challenges for a Gas EXO • Demonstrate good energy resolution in a gas • Decide on gas mixture – ‘to quench or not to quench’ • If quench, how to measure Z • Demonstrate gain and tracking

  7. Ionization test chamber Under construction at Carleton Should be completed and Taking date ~ 2 months Work much further Advanced at Neuchatel On micromegas

  8. Issues – Ba tagging • Need to demonstrate a robust Ba tagging scheme • Demonstrate Ba+ livetime in Xe gas • Demonstrate Ba++ -> Ba+ conversion • Measure Ba tagging efficiency • Demonstrate low false tagging probability

  9. Xe offers a qualitatively new tool against background: 136Xe 136Ba++ 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 107/s 2P1/2 650nm 493nm 4D3/2 • Important additional • constraint • Huge background • reduction metastable 80s 2S1/2

  10. Problems with in-situ tagging • Pressure broadening implies lower signal/laser power ratio (?x1000) • Lack of exact location means broad laser • Lack of exact location precludes focused detection • Time available for measurement is small • All lead to much greater background/signal compared with trapped source

  11. Background Rejection Options • 1) Pulse Blue and Red lasers out of phase, look for blue light only when blue laser is off (few microsecond periods) • AOM can give factor ~1000 contrast, using 2 should give 105 Can also consider Pockels cells. • Look in plane of polarization at 90 degrees only or use polaroid • Use narrow band filter to limit angular acceptance • Use atomic filtering • Take great care to minimize scattering – Use AR coated laser windows and lots of baffling

  12. Ba tagging Programme • Find effective source for Ba ions in Xe • Find fluorescence signal from Ba ions • Measure pressure shift/broadening for Ba ions in Xe • Measure Ba ion lifetime in Xe • Study background rejection techniques • Push towards single atom levels

  13. Ba ion production in Xenon • Evaporate Ba onto thin (125 ug/cm^2) Pt (on Mylar) • Focus pulsed laser onto back of Pt (405 nm, 50 mW, ~3 uS, few um) to heat to ~300C • Expect several hundred Ba atoms to be released per pulse – ions? • Exponential dependence on pulse width

  14. 405 nm laser 2 uS Laser window Lens Ba on Pt on Mylar Laser Beam Grid View Port PMT

  15. Lasers for gas phase • Need highest available power in tuned lasers • Wavelengths are most inconvenient • For blue use Krypton pump (Coherent Sabre 3W) and ring dye (899) with Coumarin 102 dye • For red use Verdi (532 nm 8W) and ring dye (DCM special)

  16. Ba tagging test chamber Ba is evaporated in the Green cross then moved To the lower cube 405 nm light enters back Of cube to creat Ba ions 493/650 laser light enters Windows in extended arms PMT faces view port Grid in front of viewport Drifts Ba ions into the beam

  17. Status of Ba studies • All optics and lasers on order or delivered • Main lasers due mid March • Design essentially complete on chamber • Almost entirely off the shelf components • Should be assembled and looking for Ba by summer

  18. Long Term Programme • Complete initial studies of both resolution and tagging ~ 1 year • Design/construct mini gas EXO • ~15 cm diameter • Include tracking • Include steerable laser • Use 134,137 Cs sources • Demonstrate full functionality of a double beta detector

  19. Longer Term plan • If everything works and if Gas option looks interesting • Build 200 kg gas prototype

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