Progress on a Gaseous Xe detector for Double Beta Decay (EXO) David Sinclair Xenon Detector Workshop Berkeley, 2009

EXO Gas participants The Full EXO collaboration is participating in both the liquid detector (EXO200) and the Gas phase detector. Main gas phase activity focused at Alabama, Bern, Carleton, Laurentian, Moscow, Stanford Objective – Based on the results of EXO200, and the demonstrated performance of liquid and gas detectors, decide on the optimal configuration for a detector at the ton (1-10) scale for neutrino-lsss double beta decay

Incentive for the gas phase approach Track information – Factor of 25 reduction in background at Gottard experiment from the identification of 2 Bragg peaks Excellent multisite identification Possibly improved energy resolution Different possible techniques for barium tagging

Fundamental Questions What are the performance characteristics of an optimized detector  Energy resolution  Tracking  Backgrounds  Ba tagging

Original Concepts Look at incremental improvements to the Gottard detector Gas TPC with micromegas gain stage Identify Ba with laser tag in high pressure gas as suggested in Danilov et al.

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

Problems with original Concept Ba is produced as Ba++ Ba++ is (probably) stable in pure Xe (demonstrated in Ar) Additives that would convert Ba++ to Ba+ will probably capture Ba+ Any quench gas is likely to destroy Ba ion Quench gas kills the scintillation light The laser scheme does not work in high pressure (but can be probably be modified)

New Concept Use a gas of pure Xe (or possibly a Xe-Ne mix) Use electroluminescence for gain  Nygren has pointed out the advantages for energy resolution  Only scheme that works in pure gas Drift Ba++ ion to a nozzle where it is extracted into vacuum and identified

Possible Concept for an electroluminescence readout with moderate tracking Design copied from Fermilab RICH counter CH4 Xe

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 We are constructing a detector to establish performance of EL for this application

August 31, 2009 Matt Bowcock 11 Present chamber design

Chamber Design Features Operation from vacuum to 10 bar Contain 1 MeV electrons above 1 bar Light readouts at both ends Anode gives tracking information Cathode end gives energy signal Probably use a teflon cylinder to improve light collection and give electrical insulation for field cage Trigger on scintillation to give full 3d images and location

Chamber status Chamber is in final design phase Fabrication start in new year Vacuum systems out for tender Process systems in design Aim for completion next summer

Barium Tagging – a new concept Try to extract the Ba++ ion from the high pressure gas Based on techniques used by radioactive beam facilities Inspiration came from work of a student M. Facina

Leuven Radioactive Beam Source Extraction Concept – A working Example

Leuven Experiment – making 71 Ni Beams Produce spallation of uranium target with protons Stop fragments in Ar gas at 0.5 b Flow Ar out orifice Ionize Ni using lasers at the orifice Accelerate ions through 40 kV and mass analyze selecting M=71 Measure gammas from accepted ions

Marius Facina PhD Thesis

Conclusions from Facina’s Data Ba++ is formed in the spallation/stopping process Ba++ ions are stable in Ar (~second) Ba++ ions can be trapped using the SPIG and released with ‘high’ efficiency

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 …. Can also change charge state and look for laser fluorescence

The Xe ions will be left behind Xe+ Ba++ Ba++ mobilities calculated by Larry Viehland

Can we use this? RIB facilities use He or Ar an ~0.5 b We want to raise the pressure to ~10 b We need to use Xe (or possibly Xe-Ne) However, Ba++ ions are preformed so we can use electric fields to guide them to the nozzle

New nozzle concept Most RIB facilities are using conducting nozzles Thus field terminates on the nozzle Development on insulated, multi-hole nozzles (Ross Willoughby, ChemSpace) Allows the velocity to reach sonic prior to fields reaching conductors Higher efficiencies claimed Small holes lead to smaller gas flows

Expansion of Gas through multi-hole nozzle

Detailed image. Holes are 50  m diameter and about 1 mm long Electric field is maintained within the channel Green => v ~ 0.8 sonic

Program for Ba Tagging in Gas Facility under design at Stanford to test the concepts. Similar to the extraction systems at RIB facilities except we are exploring the use of cryopumping to protect Xe Workshop being arranged March at Stanford

Progress on EL detection Progress has been made in 3 areas:  Demonstration of resolution of EL for alphas  Tests of CsI cathodes  Engineering work on the large detector

CsI Photocathode Tests Can we produce CsI cathodes Can we make stable cathodes What are the constraints (eg exposure to air) that we will have to work with

Schematic of the CsI test chamber CH 4 (~20torr) CsI coated pad Grid/mesh Quartz window Xe gas (760torr) Am source First: - Only look at the scintillation light in Xenon Upgrade: - Add a high field region on the Xenon side to create electroluminescence EXO Week, 08/31/09C. Hägemann Want to convince ourselves that the CsI concept will work in the large gaseous protoype

Data acquisition and analysis Xe signal to trigger EXO Week, 08/31/09C. Hägemann Xe grid signal CsI signal Histogram peak pulse height of the CsI signal Non-gaussian shape of the distribution due to distributions of photons on the readout pad with respect to the track angle (can’t cut on track angle currently) Record the mean and sigma of the distribution

Data acquisition and analysis Xe signal to trigger EXO Week, 08/31/09C. Hägemann Xe grid signal CsI signal Histogram peak pulse height of the CsI signal Non-gaussian shape of the distribution due to distributions of photons on the readout pad with respect to the track angle (can’t cut on track angle currently) Record the mean and sigma of the distribution Using T quartz =90%

1. Reproducibility of CsI Coating Compare runs with different CsI coatings – differ in exposure time to air Longest exposure shows large decrease in pulse height Second and third coatings very very similar in their response Need to minimize exposure to air!!! <30 minutes is currently not possible EXO Week, 08/31/09C. Hägemann V CsI = 700V, V Xe = 350V P CH4 = 30.1torr 2h exposure 30min

3. CsI Stability over time EXO Week, 08/31/09C. Hägemann Event # Signal constant over days

Summary/Conclusions Confident that we can reproduce CsI coating Heating of the readout pad needed to improve QE after exposure to air (either need to heat the pad or minimize exposure) Seem to be able to achieve ~20% QE, but need to verify with EL signals Response is stable over time  no flow seems to be needed  can live with other materials than SS, macor, peek Upgrade to be installed in the next 2 weeks (if mesh design works)  Larger Signals  Determine and cut on track direction  Test new grid holder design EXO Week, 08/31/09C. Hägemann