Transcript Progress on a Gaseous Xe detector for Double Beta Decay (EXO) David Sinclair
Progress on a Gaseous Xe detector for Double Beta Decay (EXO)
David Sinclair Report to the SNOLAB EAC August 2009
EXO Gas participants
Carleton – Construction of test detector 4 faculty (3 FTE) 1 (+1) RA 2 Students Laurentian – MC development 2 faculty (1 FTE) 1 RA Stanford, Alabama, Bern, Moscow
Objectives of the Gas Xe Program
Demonstrate a feasible concept for a gas phase Xe detector at the Tonne scale Energy Resolution < 1.5% to eliminate 2 n Tracking signal Identify 2 Bragg peaks Reject multisite events Locate event vertex Barium tag High efficiency Low false +ve
Fundamental Questions
1) What is the optimum pressure for such a detector High pressure => smaller detector Shorter tracks ? Ba identification Pressure Vessel Low pressure +> big detector Shielding issues Track containment
Fundamental Questions
What are the performance characteristics of an optimized detector Energy resolution Tracking Backgrounds Ba tagging
Original Concepts
Gas TPC with micromegas gain stage Identify Ba with laser tag in high pressure gas
Possible concept for a gas double beta counter
Micro-megas Electrode Xe Gas Isobutane TEA Anode Pads WLS Bar Grids
. . . . . . . .
. . . . . . . .
Lasers PMT For 200 kg, 10 bar, box is 1.5 m on a side
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
New Concept
Use a gas of pure Xe Use electroluminescence for gain Drift Ba++ ion to a nozzle where it is extracted into vacuum and identified
Concept for an electroluminescence readout
CH4 Xe Design copied from Fermilab RICH counter
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 No-one has demonstrated energy resolution in MeV range We propose to construct a detector to establish performance of EL for this application
We plan a 20 x 20 array of 2 cm pads on each end
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 ….
The Xe ions will be left behind
Ba++ and Xe+ Mobilities in Xe
1 0.8
0.6
0.4
0.2
0 0 50 100 150 200
E/N
Ba++ mobilities calculated by Larry Viehland Ba++ Xe+ 250
Extraction Concept – A working Example Leuven Radioactive Beam Source
Leuven Experiment
Produce spallation of uranium target with protons Stop fragments in Ar gas at 0.5 b Flow Ar out orifice Ionize Ni using laser 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 (Cont)
At end of pipe have an orifice leading to evacuated region Trap ions as they leave the gas using a two step process involving first an RF carpet and then a Sextupole Ion Trap (SPIG) Once the ion is in vacuum, use conventional techniques to identify it (Eg accelerate and measure energy, rigidity, velocity etc).
Can we use this?
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
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
Want to convince ourselves that the CsI concept will work in the large gaseous protoype Am source Xe gas (760torr) Quartz window First:
- Only look at the
scintillation light in Xenon Upgrade:
- Add a high field region on
the Xenon side to create electroluminescence CH 4 (~20torr) Grid/mesh CsI coated pad
EXO Week, 08/31/09 C. Hägemann
CsI Chamber Design
Mesh
EXO Week, 08/31/09
Grid
•
Collect Ionization for Trigger
•
Record signal from readout pad
•
Able to heat the readout pad to “purify” the CsI after air exposure Heater
C. Hägemann
CsI Chamber Design
Mesh
EXO Week, 08/31/09
Grid
•
Collect Ionization for Trigger
•
Record signal from readout pad
•
Able to heat the readout pad to “purify” the CsI after air exposure Heater
C. Hägemann
CsI Chamber Design
Mesh
EXO Week, 08/31/09
Grid
•
Collect Ionization for Trigger
•
Record signal from readout pad
•
Able to heat the readout pad to “purify” the CsI after air exposure Heater
C. Hägemann
CsI Evaporation & Assembly • Evaporation is done with the entire flange assembled (minus the grid) • Thickness of the coating is ~ few hundred nm • CsI coated pad is exposed to air during assembly of the chamber ~30 minutes • Once the chamber is assembled, heat the pad for at least 12h (at ~60 o C) while pumping vacuum on the chamber • Readout pad is cooled before filling with CH 4
EXO Week, 08/31/09 C. Hägemann
Questions to answer with the CsI Prototype
• Can we reproduce Quantum Efficiency of CsI between coatings • How does heating improve the response, is it necessary?
• Can we easily get QE ~ 15%?
• How does the signal behave over time, does the CsI or the CH4 contaminate?
• Do we need to flow the CH4 for purity?
• What energy resolution can we achieve when combined with EL?
EXO Week, 08/31/09 C. Hägemann
Data acquisition and analysis
• Xe signal to trigger
Histogram peak pulse height of the CsI signal Xe grid signal 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
EXO Week, 08/31/09 C. Hägemann
Data acquisition and analysis
• Xe signal to trigger
Histogram peak pulse height of the CsI signal Xe grid signal 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
EXO Week, 08/31/09
Using T quartz =90%
C. Hägemann
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
2h exposure 30min 30min V CsI P CH4 = 700V, V Xe = 30.1torr
= 350V
EXO Week, 08/31/09 C. Hägemann
2. How does heating improve response • Heating the readout pad while pumping a vacuum for 12h right after assembly improves the signal significantly • Does continues heating improve the signal further?
no heating 12h heating
EXO Week, 08/31/09 C. Hägemann
3. CsI Stability over time
Run over several days, plot PH vs Event #: V
EXO Week, 08/31/09
Event #
C. Hägemann
Chamber upgrade • • • Add electroluminescence region on the Xe side New design for the mesh/grid assemblies Add field shaping rings on the Xe side • • • Create larger signals to accurately determine the QE of the CsI Better energy resolution with improved meshes/grids Allow us to cut tracks depending on their angle
EXO Week, 08/31/09 C. Hägemann
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, 08/31/09 C. Hägemann
Test Chamber Design
A chamber with EL readout and tracking large enough to contain 1 MeV electrons is being designed Nominal design has 400 @ 2 cm x 2 cm pads at each end Use end with EL region for tracking Use far end for energy
Chamber body
August 31, 2009 Matt Bowcock 43
Chamber Specifications
Field cage length: 780 mm Field cage diameter: 535 mm Operating pressure: Vacuum to 10 Bar.
Readout on both ends.
Gasses: Xenon and CH4 Requires TSSA approval
Readout Package Option 1
August 31, 2009 Matt Bowcock 45
Readout Package Option 1
Good Straight forward gas pressure control.
Shorter overall chamber length. Not so good Extremely heavy ~ 500 lbs.
Hard to coat readout pads.
Difficult to handle.
Package part of chamber boundary.
Deformation of readout due to pressure.
Readout Package Option 1
August 31, 2009 Matt Bowcock 47
Readout Package Option 1
August 31, 2009 Matt Bowcock 48
Readout Package Option 1
February, 2009 Vance Strickland, P. Eng.
49
Readout Package Option 2
Good Lighter: ~ 100 lbs.
Easier to coat readout pads.
Easier to handle.
Package separate from chamber boundary.
Almost no deformation from pressure.
Not so good Complicated pressure control scheme.
Much longer overall chamber length.
Readout Package Option 2
August 31, 2009 Matt Bowcock 51
Readout Package Option 2
August 31, 2009 Matt Bowcock 52
Readout Package Option 2
August 31, 2009 Matt Bowcock 53
Readout Package Option 2
February, 2009 Max deflection ~ 0.0002” Vance Strickland, P. Eng.
54
Outstanding Design Issues
Coating or depositing grids on quartz window.
High voltage connections to window.
Finalize process system.
Cleanroom chamber handling system.
?
Current Process System Diagram
August 31, 2009 Peter Liimatainen, P.Eng.
56
Cleanroom Rehabilitation
Barium Tagging
Work on characterization of Ba+ transitions in high pressure gas continues
Ba+ Spectroscopy
We want to measure how the transitions broaden and shift with increasing pressure and determine any change in branching ratio An arc source is not optimum for this because the arc changes the local environment Developing a laser ablation/ionization source This will come to SNOLAB shortly for final measurements
Image showing laser beam Laser is 349 nm, 5 ns, 100 m J (Some smoke and mirrors Were used!)
SIMION Simulation
Ba Extraction
Feasibility of Ba extraction into vacuum has been discussed further with experts.
Ba++ reaction cross sections with water vapour have been measured at York (Dietre Bohme) Paper in final draft Figures consistent with ~second lifetime Concept for extraction developed further
Ba extraction summary
Design of test facility developing Work will shift to Stanford where we will concentrate the effort on Ba tagging Will study Ba++ detection Look at techniques to convert Ba++ to Ba+ outside of the Xe so the existing laser tag can be used (long shot) look at laser tag of Ba++
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 m diameter and about 1 mm long Electric field is maintained within the channel Green => v ~ 0.8 sonic
Milestones & Organization
Milestones
Long Term Plans
By 2012 we plan to have a decision on the best way to go to the tonne scale for neutrino-less double beta decay We will probably need a water shielded detector The SNOLAB Cryopit would be an ideal location to site such a detector and could house either a gas or liquid detector
Requests to SNOLAB
1) Continued use of the EXO lab on surface 2) Possible use of space underground (?ladder space) for prototype in ~ 2 years 3) Note potential bid for Cryopit in ~3 years