1. Sheffield : R. Hollingworth, D. Tovey R.A.L. : R.Luscher Development of Micromegas charge readout for two phase Xenon based Dark Matter detectors Contents: TPC Workshop, Paris 2004 P.K.lightfoot, University of Sheffield Configuration of the test cell for single phase operation Objective of this research Configuration of the test cell for dual phase operation First results of two phase system

2. Objective of this research TPC Workshop, Paris 2004 P.K.lightfoot, University of Sheffield Current and future Xenon based UKDMC detectors feature a large number of PM tubes known to be the biggest source of background Zeplin II UCLA design

3. Objective of this research TPC Workshop, Paris 2004 P.K.lightfoot, University of Sheffield Current and future Xenon based UKDMC detectors feature a large number of PM tubes known to be the biggest source of background Neutron background in large scale xenon detectors for dark matter searches - M.Carson

4. Objective of this research TPC Workshop, Paris 2004 P.K.lightfoot, University of Sheffield A possible solution would be to use an electrical rather than an optical readout system such as a Micromegas or GEM Advantages: Low cost, high radiopurity, small size, robust, capability for high spatial resolution Disadvantages: Not as yet demonstrated to be a replacement for PMTs in two phase Xenon dark matter detectors 45mm active area 200 micron diameter 50 micron pillars with 1mm spacing 25 micron diameter holes at a 50 micron pitch

5. Objective of this research TPC Workshop, Paris 2004 P.K.lightfoot, University of Sheffield Scintillation photons are generated by both nuclear and electron recoils These generate electron via photoemission (QE=30%) from two CsI photocathodes deposited on the upper cathode surface and the lower surface of the GEM The ionisation track from the electron recoil is drifted onto the amplification stage allowing discrimination Nuclear recoil signal events contain no (for low drift field) primary ionization between these two pulses.

6. Configuration of the test cell for single phase operation TPC Workshop, Paris 2004 P.K.lightfoot, University of Sheffield Purification rig Helium based cooling system Cryostat

7. Configuration of the test cell for single phase operation TPC Workshop, Paris 2004 P.K.lightfoot, University of Sheffield Inside the cryostat: PTFE lined target 80mm high by 80mm internal diameter Fill tube Cartridge heaters Cooling coil

8. Configuration of the test cell for single phase operation TPC Workshop, Paris 2004 P.K.lightfoot, University of Sheffield Gain was found by comparing the signal voltage to charge generated by Fe-55 With the mesh grounded, the anode signal was decoupled through a 47pF capacitor, passed through an Amptek A250 charge sensitive amplifier and shaping amplifier and into a DAQ

9. Configuration of the test cell for single phase operation TPC Workshop, Paris 2004 P.K.lightfoot, University of Sheffield In a double phase system it is critical that a quencher is used to limit UV feedback in the gas region where the Micromegas operates Methane (b.p. -161°C) may be used but as can be seen, the proportion used is critical to the gain on the Micromegas Why are we interested in the single phase operation of Micromegas? It is therefore critical that the quencher used be a gas at liquid xenon temperatures (-110°C) and pressures (1.5 - 2 bar) However it is also critical that the quencher is not present in the liquid phase to avoid attenuating the scintillation photons

10. Configuration of the test cell for single phase operation TPC Workshop, Paris 2004 P.K.lightfoot, University of Sheffield Gain in single phase Xe/Me gas environment at a cathode of 2500V Dependency of the gain on the amplification field, gas pressure and composition for a fixed drift field (400V/cm)

11. Configuration of the test cell for single phase operation TPC Workshop, Paris 2004 P.K.lightfoot, University of Sheffield Dependency of the gain on the drift field (4cm gap), gas pressure and composition for a fixed amplification field (535V/50 microns)

12. Configuration of the test cell for single phase operation TPC Workshop, Paris 2004 P.K.lightfoot, University of Sheffield The greatest gains were observed at the lowest pressures (1 bar) The gain increased exponentially as the amplification field was increased The gain dropped exponentially with increasing pressure The addition of quenchers reduced the gain for a constant amplification field but increased the stability allowing higher voltages and greater gains to be reached Increasing the drift field increased the gain. However above 300V/cm the drift field had only a limited effect on the gain Conclusions:

13. Configuration of the test cell for dual phase operation TPC Workshop, Paris 2004 P.K.lightfoot, University of Sheffield It is crucial that the charge generated in the liquid reaches the anode. This is achieved by setting up a 400V/cm drift field between the cathode and the underside of the GEM The Xenon/ methane blend was purified through a SAES getter In a double phase Xenon detector the GEM is positioned just below the liquid surface. The charge is drawn from the liquid surface using an elevated electric field established by the pd between the upper GEM electrode and the mesh. Criteria for success The liquid level is defined by the capacitance between the GEM and the mesh Xenon is cooled using a compressed Helium supply and controlled using cartridge heaters

14. Configuration of the test cell for dual phase operation TPC Workshop, Paris 2004 P.K.lightfoot, University of Sheffield First tests have not as yet featured CsI photocathodes within the target GEM cathode PTFE cylinder Micromesh (both this and woven steel type tested) spacer anode

15. Configuration of the test cell for dual phase operation TPC Workshop, Paris 2004 P.K.lightfoot, University of Sheffield Drift region Extraction zone GEM Amplification zone

16. TPC Workshop, Paris 2004 P.K.lightfoot, University of Sheffield First results of two phase system Dependency of gain on the GEM and amplification field for 2 bar gas Xe/Me blend at -90 degC

17. TPC Workshop, Paris 2004 P.K.lightfoot, University of Sheffield First results of two phase system The pulses could be stopped either by removing the source or reducing the extraction field above the liquid surface At a vapour pressure of 1050 torr, a set point of -102°C and an anode to mesh pd of 650V pulses were seen from an external Co57 source positioned under the cathode Increasing the amplification field resulted in an increase in the pulse height Typical output from A250 charge amplifier Typical output from A250 and shaping amplifier mV gain is x 1323

18. TPC Workshop, Paris 2004 P.K.lightfoot, University of Sheffield First results of two phase system Problem is believed attributed either to:- Charging-up effects on the pillars Kapton resistance increases at low temperature. This may restrict charge leakage through the pillars and onto the mesh at liquid Xenon temperatures Condensation of Xenon in the avalanche region Condensing Xenon vapour (which naturally migrates to regions of large field non- uniformity due to the high polarisability of the Xe atom) may block the mesh holes After 20 minutes of operation in two phase mode, the pulses cease Remaining issues

19. TPC Workshop, Paris 2004 P.K.lightfoot, University of Sheffield Conclusion Research is on going We have characterized charge readout devices in Xenon / Methane blends for pressures up to 2.5 bar We have constructed and optimised charge readout devices specifically for operation at elevated pressures by carefully selecting hole size, pitch and separation We have demonstrated extraction and amplification of ionisation from liquid Xenon albeit for short periods The drift field can be optimised to ensure maximum gain Methane has been used as a quencher in two phase operation The percentage of Methane has been optimised for maximum gain and stability of the Micromegas at pressures between 1.4 to 2.0 bar

20. TPC Workshop, Paris 2004 P.K.lightfoot, University of Sheffield Next steps CsI will then be applied to the GEM and cathode and the detector operated in two phase xenon Following successful sustained operation of the detector the apparatus will be scaled up to accept a 250 x 250mm two dimensional strip readout Solve long term stability problem CsI coated GEM