Aging Effect in Wire Chambers The degradation of operating conditions of wire chambers under sustained irradiation are the main limitation to the use of gas detector in high-energy physics. ‘Classical aging effects’ are deposits formed on electrode surfaces by chemical reactions in avalanche plasma near the anode. During gas avalanches many molecules break up and form free radicals (unionized atomic or molecular particles with one or more unsatisfied valence bonds). Free-radical polymerization is regarded at the dominating mechanism of wire chamber aging
Aging Effects (cont) Free radicals either recombine to form the original molecules or cross-linked molecular structures of increasing weight Leads to the formation of deposits (conducting or insulating) on electrode surfaces. • Decrease of the gas gain (due to modification of electric field) • Excessive currents • Sparking and self-sustained discharges • Radiation-induced degradation depends on • the nature and purity of the gas mixture • different additives and trace contaminants • materials in contact with the gas • materials of the electrodes • electric field configuration
Premature aging in Ar/CH4 Free radicals are hydrogen deficient and are therefore able to make bonds with hydrocarbon molecules. Therefore CH4 polymerizes in the avalanche plasma, which causes premature aging. Aging rate of Ar/hydrocarbon gases can be reduced by adding oxygen-containing molecules, which allows large systems to operate at low intensity with only a small performance loss. Not trustworthy for long-term, high-rate experiments.
Silicon Deposits Si-deposits on anode wires Silicon has been detected in the analysis of many wire deposits, although the source has not been clearly identified in all cases. Si-compounds can be found in many components including lubricants, adhesives, rubber, silicon-based grease, oils, O-rings, fine dust, gas impurities, diffusion pumps, molecular sieves, and many more. Most dangerous are Si-lubricant traces used for the production of gas system components. Cleaned by flushing the system with DME.
The Malter Effect Microscopic insulating layer deposited on a cathode from quencher dissociation products and/or pollutant molecules. Some metal oxide coatings, absorbed layers or even the cathode material itself may not be initially conducting enough inhibit neutralization of positive ions from the avalanche. These ions generate a strong electric field across the dielectric film and cause electron field-emission at the cathode. Positive feedback between electron emission at the cathode and anode amplification leads to the appearance of dark current, increased rate of noise pulses and finally exponential current growth (classical Malter breakdown) Adding water prevents Malter discharges, because water increases the conductivity of partially damaged electrodes
Malter Effect Malter Effect first imaged in the CRID RICH detector. Wire deposits in CH4+TMAE after a charge dose of 6x10-3 C/cm
Ionization Density Detector lifetime depends on ionization density and in turn on the irradiation rate, particle type and energy. Space-charge effects (at large current densities) reduce the electron energy in the avalanche and thuse the ion and radical density in the avalanche plasma. This can be related to the charge density and total energy dissipated in the detector from incoming particles. • Counting rate capabilities are limited by space-charge effects in the avalanches • gain reduction • formation of self-quenching streamers
Ionization Density Transition between proportional and streamer mode depends on both the HV but also the primary ionization density. α-particles may reach the streamer mode for moderate multiplication factors. Streamers produces a densely ionized and low resistivity plasma between the anode and cathode which may lead to spark events. Sparks may ‘prime’ the electrode surface and begin the build-up of deposits. Tips left from sparks enhance the electric field locally. The Malter current and electron jets from these may be as significant as heavily ionizing particles.
Aging from α-particles Energy loss per α track can be 103 times larger than primary ionization with X-rays or MIPS. This ages shows up as hair-like deposits within the irradiated area. Aging rate in Ar/CF4/CH4 ~100x higher in a 100 MeVα-beam than with a Fe55 sources with similar current densities.
High Voltage Dependence of aging properties on HV may relate to the electric field strength on the anode surface. This enhances inelastic processes and determines the electron temperature in avalanches. Detector lifetime decreases nearly exponentially with increase of anode voltage. Aging rate can also be accelerated by avalanche formation at the cathode (if field there > 10-20 kV/cm) Lifetime of ATLAS muon drift tubes as a function of HV for different particle fluxes
High Voltage Triple GEM detectors are more reliable and radiation hard than double GEMs, with the same total gain. This gain is due to the reduction in the electric strength across a single GEM. Triple GEM Detector of the COMPASS experiment GEMs age less than other wire-detectors because multiplication and readout are separate (gas amplification only within GEM holes) and the rate of impurity polymerization caused by the lower electric field.
Size of irradiated Area Detector lifetime depends on the size of the irradiated area. Aging rate increases in the direction of serial gas flow. Progressive deterioration of MIP efficiency in the direction of serial gas flow Some aggressive free radicals may diffuse within the irradiated area and react with other avalanche generated dissociative products, construction and electrode materials and enhancing aging effects with increasing gas usage.
Useful Guidelines for Wire Detectors • Carefully choose construction materials that are radiation hard and have appropriate outgassing properties • Limited set of aging resistant gases can be used in high intensities experiments: CF4, C02,O2, H20. • Validate assembly procedures and ensure maximal cleanliness • Carefully control any anomalous activity in the detectors: dark currents, changes of anode current and remnant activity in the chamber when the beam goes away • If aging effects are observed add oxygen-based molecules; operations with CF4 decreases risk of Si polymerization • Surface conductivity of electrodes is important because it relates closely to operational capability at high ionization densities
RPCs and Aging Efficiency Drop with increasing current The two B-factories, Belle and BaBar use RPCs in streamer mode. High currents appeared in Belle’s RPCs almost immediately upon installation. This was due to the formation of hydrochloric acid formed due to the presence of high levels of water. Cured by replacing plastic rubes with copper cones
STM Imaging of Anodes Virgin Glass Surface “Good” anode “Bad” anode Surfaces etched by HF acid formed from water and Freon in the gas
Aging of BaBarRPCs The BaBarRPCs were made of Bakelite coated with Linseed oil. A permanent reduction in efficiency was caused by the lack of polymerization in the linseed oil and the formation of oil droplets under the high temperatures and currents.
Images of RPC Aging Damaged Cathode Raw Glass Damaged Anode AFM scans Raw Glass Damaged Electrode SEM scans
Aging in Gaseous PhotoDetectors Gaseous photodetectors need to ensure efficient conversion of UV photons into electrons and to detect single photoelectrons. Systematic aging studies of TMAE and TEA vapors. TMAE has best quantum efficiency, but high gas gain loss due to deposits on anode wires. TMAE has a larger aging rate because the molecule is more fragile. The aging rate in both is inversely proportional to the anode wire diameter.
Damage to MSGCs Discharges measured in the CMD MSGC prototypes Strip Damage due to discharges and sparks
Gaseous Particle Detectors: • By Archana SHARMA • CERN Geneva Switzerland • March 2009 • Troisieme Cycle • EPFL Lausanne, Switzerland
Chapter I • 1.1 Introduction • 1.2 Units and Definitions, Radiation Sources • 1.3 Interaction of radiation with Matter • Chapter II • 2.1 General Characteristics of gas detectors, Electronics for HEP detectors • 2.2: Transport Properties • 2.3: Wire-based Detectors
Chapter III 3.1 Resistive Plate Chambers for Tracking 3.2 Aging and Long Term Operation 3.3 Micropattern Detectors Chapter IV 4.1 Measurements of Energy, Momentum, Time of Flight 4.2 Designing a HEP Experiment 4.3 Applications Outside Particle Physics
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
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.
Micro Strip Generation Field Configuration in an MSGC 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. Electric field from a drift electrode above and appropriate potentials applied.
Micro Strip Generation Removes positive ions from the vicinity of avalanches High rate capabiity two orders of magnitude higher than MPWCs (106/mm2s) • ~30μm position resolution • Double track resolution of 400 μm • Good energy resolution • Applications in X-ray spectrometry and digital radiography
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.
New Micropattern Era Microneedle Concept (1976) Microdot Chamber Schematic Ultimate gaseous pixel device with anode dotes surrounded by cathode 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
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
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.
Lemonnier et al. Compteur a Trous (CAT) 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
Compteur a Trous (CAT) VIP μCAT 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. Removing the insulator leaves the cathode as a micro-mesh placed with a thin gap above the readout electrode (μCAT). Gains of several 104.
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. Two surfaces are maintained at a potential gradient; providing field for electron amplification and an avalanche of electrons.
Gas Electron Multiplier (GEM) Electric Field When coupled with a drift electrode above and a readout electrode below, it acts as a micropattern detector. Avalanche across a GEM 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.
Other Micropattern Detectors Gain with a Micro-Wire • Many other detectors following the GEM concept • Micro-Wire (μDOT in 3D) • Micro-Pin Array (MIPA) • Micro-Tube • Micro Well • Micro Trench • Micro Groove MIPA Array
MicroTube Detector Microtube • Combination of laser micro-machining 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
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 Vertex Reconstruction • Can obtain higher gains with poorly quenched gases (lower operating voltage and higher diffusion) • lowers charge density • Lowers photon feedback probability Safe operation of a combination of an MSGC and a GEM has been demonstrated up to gains of ~10000s
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
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.
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
MICROMEGAS Xrays MICROMEGAS detectors have been developed for X-ray imaging. Vertebra scanned with a MICROMEGAS Operate with pure Xenon at atmospheric pressure
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
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
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
Cherenkov Ring Imaging Very high gains observed with cascade of four GEMs and using pure ethane as the operating gas
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
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.