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

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

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

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

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

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

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

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

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

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) 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 10 5 Can also consider Pockels cells. AOM can give factor ~1000 contrast, using 2 should give 10 5 Can also consider Pockels cells. Look in plane of polarization at 90 degrees only or use polaroid Look in plane of polarization at 90 degrees only or use polaroid Use narrow band filter to limit angular acceptance Use narrow band filter to limit angular acceptance Use atomic filtering Use atomic filtering Take great care to minimize scattering – Use AR coated laser windows and lots of baffling Take great care to minimize scattering – Use AR coated laser windows and lots of baffling

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

Ba ion production in Xenon Evaporate Ba onto thin (125 ug/cm^2) Pt (on Mylar) 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 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? Expect several hundred Ba atoms to be released per pulse – ions? Exponential dependence on pulse width Exponential dependence on pulse width

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

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

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

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

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

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