Gaseous Micropattern Detectors

1. Gaseous Micropattern Detectors

2. Limitations of MWPCs Rate Limitations • Wire spacing limits position accuracy and two track resolution to ~1mm • Electrostatic instability limits the stable wire lengths • Widths of induced charges define the pad response function • Accumulation of positive ions restrict the rate capabilities

3. Multi-Step Chamber High Gain of Multi-Step Chamber • Divides the gain of the MWPC into two parts • First allow electrons produced by ionizing particles to ‘pre-amplify’ • Then proceed to the anode for further amplification. Chamber operation is more stable and provides higher gain.

4. Micro Strip Generation Micro-Strip Gas Chamber (MSGC) invented by Oed in 1988. A pattern of thin anodes and cathode strips on a insulating substrate with a pitch of a few hundred μm. Field Configuration in a MSGC Electric field from a drift electrode above and appropriate potentials applied.

5. Micro Strip Generation Removes positive ions from the vicinity of avalanches • ~30μm position resolution • Double track resolution of 400 μm • Good energy resolution • Applications in X-ray spectrometry and digital radiography High rate capabiity two orders of magnitude higher than MPWCs (106/mm2s)

6. Damage in MSGCs Damage in a MSGC Difficulties began when exposed to highly ionizing particles (charge 3x mip) Streamer to gliding discharge transition damaged strips Investigation showed that the streamer mode is stable in a MWPC because the electric field in the propagation direction is weak Field Along the Surface of MSGC Small anode-cathode distance in MSGC. High electric field at stream tip and along the surface. Streamer is followed by a voltage and ionization dependent discharge. Culprits are charging of surface defects, long-lived excited states and overlapping avalanches.

7. New Micropattern Era Microneedle Concept (1976) Microdot Chamber Schematic Ultimate gaseous pixel device with anode dotes surrounded by cathod rings. Very high gains (~106). Does not discharge up to very high gains. No observable gas gain due to fine needles (<<1μm) and small amplification region

8. Charpak and Giomataris Micro-Megas Very asymmetric parallel plate chamber. Uses the semi-saturation of the Townsend coefficient at high fields (100kV/cm) in several gas mixtures, to ensure stability in operation with mips. Electrons drifting from the sensitive volume into the amplication volume with an avalanche in the thin multiplying gap. Excellent energy resolution

9. Micro-Megas Energy Resolution of a Micro-Megas Detector Large area (40 x 40 cm2) Micro-Megas detector installed in the COMPASS experiment at CERN.

10. Compteur a Trous (CAT) Lemonnier et al. A narrow hole micro-machined in an insulator metallized on the surface as the cathode. Anode is the metal at the bottom of the hole. Electric Field Energy Resolution

11. Compteur a Trous (CAT) VIP μCAT Removing the insulator leaves the cathode as a micro-mesh placed with a thin gap above the readout electrode (μCAT). Gains of several 104. An ingenious scheme of readout from virtual pixels made by current sharing (20 times finer resolution compared to the reasout cell) giving 400 times more virtual pixels.

12. Gas Electron Multiplier (GEM) Sauli Chemical Etching Process Manufactured using standard printed circuit wet etching techniques. Comprise a thin (~50μm) Kapton foil, double-sided clad with copper and holes are perforated through. GEM Foil Two surfaces are maintained at a potential gradient; providing field for electron amplication and an avalanches of electrons.

13. Gas Electron Multiplier (GEM) When coupled with a drift electrode above and a readout electrode below, it acts as a micropattern detector. Electric Field Amplification and detection are decoupled  readout is at zero potential. This permits transfer to a second amplification device and can be coupled to another GEM. Avalanche across a GEM

14. Other Micropattern Detectors • Many other detectors following the GEM concept • Micro-Wire (μDOT in 3D) • Micro-Pin Array (MIPA) • Micro-Tube • Micro Well • Micro Trench • Micro Groove Gain with a Micro-Wire MIPA Array

15. MicroTube Detector Microtube • Combination of laser micro-maching and nickel electroplating • ~150μm diameter cathode • Anode tube machined through the well and plated alongside. • Electric field that increases rapidly at the anode, but no insulating material between cathode and anode. • Allow for higher gas gains, better stability (fewer discharges) and a reduction of charging effects. • Similar performance to μDOT and μCAT Field across a Microtube

16. Other Micropattern Detectors Assembled GEM+MSGC Studies have shown that discharges in the presence of highly ionizing particles appear in all micropattern detectors at gains of a few thousand • Can obtain higher gains with poorly quenched gases (lower operating voltage and higher diffusion) • lowers charge density • Lowers photon feedback probability Vertex Reconstruction Safe operation of a combination of an MSGC and a GEM has been demonstrated up to gains of ~10000s

17. Larger GEMs Discharge Probability for single, double and triple GEMs Triple GEMs operate even more stably in poor hadronic beam environments Larger GEMs are segmented to reduce capacity and limit the energy in the discharge

18. MSGCs for X-ray Imaging Images of a snail shell taken with an MSGC operating with Xe-Ch4 at 4 bar Conventional film radiography has excellent spatial resolution but limited dynamic range Conventially storage and display media are the same. The film image can saturate and the display contrast is fixed at the time of exposure. A digital system has infinite dynamic range and the display contrast can be varied at will.

19. TPC Readout Fractional ion feedback in the TPC drift volume For the TESLA experiment at the ILC, a double or triple GeM is under consideration. It boasts a fast electron signal, minimal magnetic distorting effects and suppression of ion feedback. Special hexagonal pads are being developed to provide 50 x 60 μm resolution

20. MICROMEGAS Xrays MICROMEGAS detectors have been developed for X-ray imaging. Vertebra scanned with a MICROMEGAS Operate with pure Xenon at atmospheric pressure

21. Protein Crystallography SAXS X-ray diffraction patterns of Cytochrome C with different levels of contamination Rapid analysis of single crystal structures with X-ray diffraction studies using MSGCs Crystal structures of organic molecules can be determined in minutes using position and time information Fast time resolved measurements off a time variation of the SAXS pattern of a protein sample in 10 ms. X-ray diffraction insensities

22. Protein Crystallography SAXS Diffraction pattern of a lipid membrane made with the VIP detector. Complex algorithms made for the cell border and superimposing several shots allow a high degree of detail to be obtained

23. Digital Mammography Benefits of the early detection of cancer are obvious. Small tumours usually detected in routine radiographic scanning of the body Current equipment limited by contrast difference between malignant and benign tissues Combination of an x-ray converter, a MSGC and visible photocathode shows great promise. Single photon detection with a CsI photocathode coupled to 3/4 GEMs in tandem and very large gains obtained in Ar

24. Cherenkov Ring Imaging Very high gains observed with cascade of four GEMs and using pure ethane as the operating gas

25. Scintillation Light Imaging A novel application was developed by integrating a MSGC in a gas proportional scintillation counter. Scintillation images of alpha tracks in Ar-CF4 A reflective CsI photocathode was deposited on the microstrip plate surface of the MSGC that serves as the VUV photosensor for the scintillation light from xenon GPSC

26. X-ray imaging: Radiology and Diagnostics 13 kV X-ray absorption radiography of a fish bone taken at 2 atm using a GEM + MSGC combination. Radiography of a small bat using GEM and 50µm x 50 µm 2d-readout 3 mm x 10 mm 50 kV x- ray image of a digit of a mouse.

27. Imaging of Polarized X-rays Measurements of X-ray polarization are used to investigate pulsars, synchrotron nebulae, etc Photoelectrons from GCP Polarimeter Emission direction of the primary electron depends on the incident X-ray polarization, it can be measured Some X-ray polarimters have been developed using GCPs and GEMs

28. GEM for Plasma Diagnostics Time resolved plasma diagnostics are made with a GEM and individual pixel readout. Imaging the dynamics of fusion plasmas has been attempted at the FrascatiTokamak Upgrade to exploit the sensitivity of the GEM to soft X-rays. Counts integrated in 50 μs for four adjacent pixels at the FrascatiTokamak Reconstruction of photoelectrons with a GEM+ micropixel readout

29. Conclusion and Outlook • Multiwire chambers have matured since their introduction over the last few decades, with several applications in particle physics and diagnostics of various kinds. • The last decade has seen several novel developments in Micropattern Gaseous Detectors. • Understanding of the discharge mechanisms in these devices has also improved allowing amelioration of their design. • Progress in manufacture of customized readout boards has evolved revolutionizing the potential applications of these detectors in radiology, diagnostics, astrophysics and other fields.