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Gaseous Particle Detectors By Archana SHARMA CERN Geneva Switzerland UNAL BOGOTA COLUMBIA October 2012 1 Archana Sharma CERN, Geneva, Switzerland • HIGH • ENERGY • PARTICLES • ACCELERATORS • INTERACTIONS • DETECTORS What is a TeV ? Archana Sharma CERN, Geneva, Switzerland 7 TeV + 7 TeV 1 TeV = 1 Tera electron volt = 1012 electron volt Rate 40 MHz The LHC will determine the Future course of High Energy Physics 3 Introduction HEP experiments study the interactions of particles by observing collisions of particles Result: change in direction / energy / momentum of original particles And production of new particles Archana Sharma CERN, Geneva, Switzerland See http://cmsinfo.cern.ch/Welcome.html/ TOTAL Gaseous Detectors In CMS ~ 10,000 m2 Just in case you wonder why? Known particles Highly Expected Particles Methodology that disappeared after the Big Bang E=mc2 Hypothetical Or totally unsuspected ? 6 SUSY A Higgs Event in CMS Methodology 2 muons 2 electrons 7 Archana Sharma CERN, Geneva, Switzerland The ideal detector With an “ideal” detector, we can reconstruct the interaction, i.e. obtain all possible information on it. This is then compared to theoretical predictions and ultimately leads to a better understanding of the interaction and properties of particles For all particles produced, the “ideal detector” measures energy, momentum, type by : mass, charge, life time, spin, decays Archana Sharma CERN, Geneva, Switzerland Measure and derive The mass, velocity, energy and charge (sign) – from ‘tracking’ curvature in a magnetic field Negative charge Magnetic field, pointing out of the plane Positive charge The lifetime t from flight path before decay t Archana Sharma CERN, Geneva, Switzerland Different type of particles to be detected Charged particles – e-, e+, p (protons), p, K (mesons), m (muons) Neutral particles g (photons), n (neutrons), K0 (mesons), n (neutrinos, very difficult) Different particle types interact differently with matter (detector) (for example, photons do not interact with a magnetic field) Need different types of detectors to measure different types of particles Archana Sharma CERN, Geneva, Switzerland Principles of detection Interaction of a particle with detector Sensitive Material Measureable Signal Ionization Excitation e- p g e- p p p Particle trajectory is changed due to Bending in a magnetic field, energy loss Scattering, change of direction, absorption Archana Sharma CERN, Geneva, Switzerland Ionization signals by using Gaseous detectors: MWPC and its derivatives (Multi-Wire Proportional Chambers) Drift Chambers (DCs) TPC (Time Projection Chamber) Archana Sharma CERN, Geneva, Switzerland Charged Particle Radiation Fast Electrons b-particles emitted in nuclear decay energetic electrons produced by any process Heavy Charged Particles Energetic ions: alpha particles, protons Fission products Products of nuclear reactions 14 Neutral Particles Electromagnetic Radiation X-rays emitted by re-arrangement of electron shells in atoms Gamma rays from transitions in the nucleus Neutrons Generated in various nuclear processes Often subdivided into slow and fast neutron sources Electromagnetic Interaction of Particles with Matter Interaction with atomic electrons. Particle loses energy; atoms are excited or ionized. Interaction with atomic nucleus. Particle undergoes multiple scattering. Could emit a bremsstrahlung photon. If particle’s velocity is greater than the speed of light in the medium -> Cherenkov Radiation. When crossing the boundary between media, ~1% probability of producing a Transition Radiation X-ray. Stopping Power Linear stopping power (S) is the differential energy loss of the particle in the material divided by the differential path length. Also called the specific energy loss. Energy loss through ionization and atomic excitation Stopping Power of muons in Copper Particle Data Group Bethe-Bloch Formula m – electronic mass v – velocity of the particle (v/c = b) N – number density of atoms I – ‘Effective’ atomic excitation energy – average value found empirically Gas is represented as a dielectric medium through which the particle propagates And probability of energy transfer is calculated at different energies – Allison Cobb Particle Data Group Bethe-Bloch Formula Energy Loss Function Rel To mips 1.6 1.5 Fermi Plateau 1.4 Relativistic Rise 1.3 1.2 1.1 1 10 100 Minimum ionizing particles (mips) 19 Archana Sharma CERN, Geneva, Switzerland Different Materials 20 Archana Sharma CERN, Geneva, Switzerland Average Ionisation Energy Few eV to few tens of eV 21 Energy-loss in Tracking Chambers The Bethe Bloch Formula tool for Particle Identification 22 Archana Sharma CERN, Geneva, Switzerland Straggling Mean energy loss Actual energy loss will scatter around the mean value Difficult to calculate – parameterization exist in GEANT and some standalone software libraries – Form of distribution is important as energy loss distribution is often used for calibrating the detector 23 Archana Sharma CERN, Geneva, Switzerland Straggling Dec 2008 24 Alfons Weber Archana Sharma CERN, Geneva, Switzerland Electrons Electrons are different light – Bremsstrahlung – Pair production 25 Archana Sharma CERN, Geneva, Switzerland Multiple Scattering Particles not only lose energy … but also they also change direction 26 Archana Sharma CERN, Geneva, Switzerland Range Integrate the Bethe-Bloch formula to obtain the range Useful for low energy hadrons and muons with momenta below a few hundred GeV Radiative Effects important at higher momenta. Additional effects at lower momenta. Radiation Length Mean distance over which an electron loses all but 1/e of its energy through bremsstralung also 7/9 of the mean free path for electronpositron pair production by a high energy photon Energy Loss in Lead Electron Critical Energy Energy loss through bremsstrahlung is proportional to the electron energy Ionization loss is proportional to the logarithm of the electron energy Critical energy (Ec) is the energy at which the two loss rates are equal Electron in Copper: Ec = 20 MeV Muon in Copper: Ec = 400 GeV! Photon Pair Production Differential Crosssection Total Cross-section What is the minimum energy for pair production? Probability that a photon interaction will result in a pair production Electromagnetic cascades Visualization of cascades developing in the CMS electromagnetic and hadronic calorimeters Muon Energy Loss For muons the critical energy (above which radiative processes are more important than ionization) is at several hundred GeV. Ionization energy loss Mean range Pair production, bremsstrahlung and photonuclear Muon Tomography Luis Alvarez used the attenuation of muons to look for chambers in the Second Giza Pyramid X-Ray Radiography for airport security Signals from Particles in a Gas Detector Signals in particle detectors are mainly due to ionisation And excitation in a sensitive medium – gas Also: Direct light emission by particles travelling faster than the speed of light in a medium – Cherenkov radiation Similar, but not identical – Transition radiation 35 Archana Sharma CERN, Geneva, Switzerland Cerenkov Radiation Moving charge in dielectric medium Wave front comes out at certain angle slow fast 1 cos c bn 36 Archana Sharma CERN, Geneva, Switzerland Transition Radiation Transition radiation is produced, when a relativistic particle traverses an inhomogeneous medium – Boundary between different materials with different diffractive index n. Strange effect – What is generating the radiation? – Accelerated charges Archana Sharma CERN, Geneva, Switzerland Transition Radiation (2) q1 medium v3 Before the charge crosses the surface, apparent charge q1 with apparent transverse vel v1 v1 vacuum q3 q2 v2 After the charge crosses the surface, apparent charges q2 and q3 with apparent transverse vel v2 and v3 38 From Interactions to Detectors Archana Sharma CERN, Geneva, Switzerland Archana Sharma CERN, Geneva, Switzerland Multiwire Proportional Chamber Archana Sharma CERN, Geneva, Switzerland Multiwire Proportional Chamber and derivatives Archana Sharma CERN, Geneva, Switzerland Signal Creation Charged particles traversing matter leaving excited atoms, electron or holes and ions behind. These can be detected using either excitation or ionization. Excitation Photons emitted by excited atoms can be detected by photomultipliers or semiconductor photon detectors Ionization If an electric field is applied in the detector volume, the movement of the electrons and ions induces a signal on metal electrodes. Signals are read out using appropriate readout electronics Signal Induction A point charge above a grounded metal plate induces a surface charge. q Total induced charge –q. -q Different charge positions results in different charge distributions but the total charge stays –q. q -q Signal Induction for Moving Charges Segment the grounded metal plate into grounded individual strips. The surface charge density from the moving charge does not change with respect to the infinite metal plate. The charge on each strip depends on the charge position. If the charge is moving, current flows between the strips and ground. q q -q -q Charge Generation in a Gas Amount of ionization produced in a gas is not very great. A minimum ionizing particle (m.i.p.) typically produces 30 ion pairs per cm from primary ionization in commonly used gases (e.g. Argon) The total ionization is ~100 ion pairs per cm including the secondary ionization caused by faster primary electrons. Primary ionization Secondary ionization Charge Collection Cathode Charge is produced near the track. Electric Field Apply an electric field to move charge to electrodes. Charge is accelerated by the field, but loses energy through collisions with gas molecules. Overall, steady drift velocity of electrons towards anode and positive ions towards the cathode. Anode Ion Mobility Ions drift slowly because of their large mass and scattering cross-section. Similar spectrum to the Maxwell energy distribution of the gas molecules. Average drift velocity (W+) increases with the field strength (E) and decreases as the gas pressure, P, increases. A pressure increase leads to a shorter mean free path (distance during which an ion is accelerated before losing its energy in a collision). The ion mobility, μ+, defined as μ+=W+(P/E), is constant for a given ion type in a given gas. Electron Drift Velocity The dependence of the electron drift velocity on the electric field varies with the type of gas used. Electron Diffusion Electron longitudinal (dashed) and tranverse (solid) diffusion. Ionization Chamber Geometry Cathode Parallel Plate Ionization Chamber Anode Anode wire Circular Cathode Cylindrical Ionization Chamber Charge Multiplication At sufficiently high electric fields (50100kV/cm) electrons gain energy in excess of the ionization energy, which leads to secondary ionization, etc. Townsend Coefficient Townsend Coefficient Computed values of the Townsend coefficient as a function of the electric field for different gases. Avalanche Positive Ions Number of electrons and ions increases exponentially and quickly forms an avalanche. Electrons move more rapidly than ions and development a tight bunch at the head of the avalanche. Electrons: close to the wire Anode wire Ions move significantly more slowly and have typically not moved from their original position when the electrons reach the anode. Types of Avalanches Proportional region: A=103-104 LHC Semi-proportional region: A=104-106 Saturation region: A > 108 (independent of the number of primary electrons) 1970’s Streamer region: A > 107 Avalanche along the particle track Limited Geiger region: Avalanche propagated by UV photons Geiger region: A = 108 Avalanche along the entire wire 1950’s Proportional Counters Space charge effects arise when the electron and ion density is so large that they modify significantly the electric field locally and reduce the ionization probability. For low gains, this is unimportant and the size of the signal charge is proportional to the initial ionization. A detector operated in such a way is called a proportional counter. Time Development of a Signal Electron avalanche occurs very close to the wire, with first multiplication occuring ~2x the wire radius. Electron move to the wire surface very quickly (<<1ns), but the ions drift to the tube wall more slowly (~100 μs). Characterized by a fast electron spike and a long ion tail Total charge induced by the electrons amount to only ~1-2 % of the total charge. Properties of Gases Properties of commonly used gases Introduction Particle physics experiments rely on the detection of charged and neutral particles by gaseous electronics A suitable gas mixture within an electric field between electrodes detects charged particles Ionizing radiation passing through liberates free charge as electrons and ions moving due to the electric field to the electrodes. The study of the drift and amplification of electrons in a uniform (or non-uniform field) has been an intensive area of research over the past century. Requirements for Gas Mixtures Fast: an event must be unambiguously identified with its bunch crossing Leads to compromise between high drift velocity and large primary ionization statistics Drift velocity saturated or have small variations with electric and magnetic fields Well quenched with no secondary effects like photon feedback and field emission: stable gain well separated form electronics noise Fast ion mobility to inhibit space charge effects Electron Transport Properties With no electric field, free electrons in a gas move randomly, colliding with gas molecules with a Maxwell energy distribution (average thermal energy 3/2 kT), with velocity v When an electric field is applied, they drift in the field direction with a mean velocity vd Energy distribution is Maxwellian with no field, but becomes complicated with an electric field vd Noble Gases Electrons moving in an electric field may still attain a steady distribution if the energy gain per mean free path << electron energy Momentum transfer per collision is not constant. Electrons near Ramsauer minimum have long mean free paths and therefore gain more energy before experiencing a collision. Drift velocity depends on pressure, temperature and the presence of pollutants (e.g. water or oxygen) Cross-section for electron collisions in Argon Poly-atomic gases Poly-atomic molecular and organic gases have other modes of dissipating energy: molecular vibrations and rotations Electron collision cross-sections for CO2 In CO2 vibrational collisions are produced at smaller energies (0.1 to 1 eV) than excitation or ionization Vibrational and rotational crosssections results in large mean fractional energy loss and low mean electron energy Mean or ‘characteristic electron energy’ represents the average ‘temperature’ of drifting electrons Electron Diffusion Electrons also disperse symmetrically while drifting in the electric field: volume diffusion transverse and longitudinally to the direction of motion In cold gases, e.g. CO2, diffusion is small and the drift velocity low and unsaturated: non-linear space-time relation Warm gases, e.g Ar, have higher diffusion. Mixing with polyatomic/organic gases with vibrational thresholds between 0.1 and 0.5 eV reduces diffusion vd Lorentz Angle B Due to the deflection effect due to a B field perpendicular to the E field, the electron moves in a helical trajectory with lowered drift velocity and transverse dispersion The Lorentz angle is the angle the drifting electrons make with the electric field Large at small electric field but smaller for large electric fields Linear with increasing magnetic field Gases with low electron energies have small Lorentz angle F Properties of Helium Diffusion Drift HeliumEthane Lorentz Angle for Helium-Isobutane Neon Longitudinal Diffusion Constant for NeCO2 mixtures Diffusion in Argon Transverse Diffusion in Ar-DME mixture No B field With B field Transverse Diffusion in Ar-CH4 Argon Drift Velocity for Pure Argon Possible gas for single photon detectors Lorentz Angle in Ar/CO2 Xenon In medical imaging, the gas choice is determined by spatial resolution: CO2 added to improve diffusion Pure Xenon XenonCO2 DME Transport Parameters for Pure DME Low diffusion characteristic s and small Lorentz angles used to obtain high accuracy Lorentz angle in DME-based mixtures Introduced as a better photon quencher than isobutane. Absoption edge of 195nm: stable operation with convenient gas multiplication factors High gains and rates without sparking. Townsend Coefficient Mean number of ionizing collisions per unit drift length Helium-Ethane DME/C O2 Ion Transport Properties vi Ion mobility E/p Electric field Ion drift velocity pressure Constant up to rather high fields Pollutants Pollutants modify the transport parameters and electron loss occurs (capture by electro-negative pollutants) The static electric dipole moment of water increases inelastic crosssection for low energy electrons thus dramatically reducing the drift velocity Electron capture phenomenon has a non negligible electron detachment probability Mean electron capture length Wire-based Detectors Geiger-Muller Counter Tube filled with a low pressure inert gas and an organic vapor or halogen and contains electrodes between which there is a voltage of several hundred volts but no current. Anode is a wire passing through it. Cathode is the walls. Avalanches in a Geiger Discharge Ionising radiation passing through the tube ionizes the gas. The free electrons are accelerated by the field. The avalanche begins as these in turn ionise more. Cathode Anode wire Cathode MWPC Grid of parallel thin anode wires between two cathode planes. Electrons drift to the anodes and are amplified in avalanche. Drift of ions produced in the avalanche induces a negative charge on the wire and positive charges on surrounding electrodes. Positive induced charge on adjacent wires overcomes the negative charge due to the large capacitance between the wires Two-Dimensional Readout An MWPC with the cathode strips perpendicular to the wires. Charge profile recorded on both anodes and cathodes. Centre of gravity provides X and Y projections: Xi;Yi: Strip coordinates Qi(X), Qi(Y): Charge on strips Q(X), Q(Y): Total Charge 2D readout essential for medical imaging applications. Drift Chambers D An alternating sequence of wires with different potentials, there is an electric field between the ‘sense’ and ‘field’ wires. The electrons move to the sense wires and produces an avalanche which induces a signal read out by the electronics. The time between the passage of the particle and the arrival of the electrons is measured measure of the particles position. Can increase the wire distance to save electronics channels. Straws If a single wire breaks in an MWPC the entire detectors is impacted. A solution is to replace the volume, with arrays of individual single-wire counters, known as straws. Typically a wire is strung between two supports within a thin straw (either metallic or with the internal surface coated with a metal) Portion of the ATLAS TRT End Cap MDTs The ATLAS barrel muon spectrometers uses Monitored Drift Tubes. These reconstruct tracks to 100 μm accuracy. ATLAS MDTs MDTs can also be used for making music! MDT pipe organ made by Henk Tieke from NIKHEF, Amsterdam. Time Projection Chamber (TPC) A TPC is a gas-filled cylindrical chambers (with parallel E and B field) with MWPCs as endplates. Drift fields of 100-400 V/cm Drift times 10 -100 μs Distance up to 2.5 m Gas volume B drift Event display of a Au-Au collision in STAR at RHIC. Typically ~200 tracks per events. E Wire chamber Modern TPCs STAR TPC ALICE TPC Gas for TPCs A common gas filling used is 90% Argon, 10% CH4, but this has saturated drift velocity at low fields and transverse diffusion is reduced with a magnetic field. Best choice is CF4 because it has low diffusion even without a magnetic field. Requires high drift fields. Computed with MAGBOLTZ S. Biagi, NIM A42(1999) 234 Cherenkov Radiation Photons are emitted by a charged particle moving faster than the speed of light in a medium at an angle which depends on the particle’s velocity: β=1/n cos(θ) θ These are reflected on a spherical mirror. The radius of the ring is R = rθ/2 Cerenkov Radiation in the core of a nuclear reactor RICH Detectors ALICE HMPID LHCb RICH Detector Can be used for particle identification together with tracking detectors COMPASS RICH Event Display Array of 8 MWPCs with CsI photocathodes Resistive Plate Chambers Place resistive plates (Bakelite or window glass) in front of the metal electrodes Sparks cannot develop because the resistivity and capacitance will allows only a very localized discharge. Large area detectors can be made Rate limit of kHz/cm2 CMS RPCs Limitations of MWPCs Rate Limitations 1.1 MWPC-Rate1 0120 0 1 Relative Gain 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 0.9 0.8 0.7 0.6 rate (Hz/mm 2 ) 0.5 10 3 10 4 10 5 10 6 10 92 7 Multi-Step Chamber Chamber operation is more stable and provides higher gain. High Gain of Multi-Step Chamber 10 7 MSC-Trend Nov26,2 000 10 6 Gain 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. V =3 kV 10 V = 2.5 kV 2 5 2 V = 2 kV 2 10 10 10 4 3 2 0 0.5 1 1.5 2 2.5 3 3.5 4 V (kV) 1 93 Micro Strip Generation Micro-Strip Gas Chamber (MSGC) invented by Oed in 1988. Field Configuration in an MSGC 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. 94 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 95 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 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. Field Along the Surface of MSGC 1.4 10 8 -1 ) Electric Field (V cm Investigation showed that the streamer mode is stable in a MWPC because the electric field in the propagation direction is weak 1.2 10 8 1 108 8 107 6 107 4 107 2 107 0 0 96 20 40 60 80 x (µm) 100 New Micropattern Era Microneedle Concept (1976) No observable gas gain due to fine needles (<<1μm) and small amplification region 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 97 gains. 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. Excellent energy resolution Electrons drifting from the sensitive volume into the amplication volume with an avalanche in the thin multiplying gap. 98 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. 99 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 100 Compteur a Trous (CAT) 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. VIP An ingenious scheme of readout from virtual pixels made by current sharing (20 times finer resolution compared to the reasout cell) giving 400 times101 more virtual pixels. Gas Electron Multiplier (GEM) 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. 102 Gas Electron Multiplier (GEM) Electric Field When coupled with a drift electrode above and a readout electrode below, it acts as a micropattern detector. 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 103 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 104 MicroTube Detector Microtube Combination of laser micromachining 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 105 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 106 Larger GEMs Triple GEMs operate even more stably in poor hadronic beam environments Discharge Probability for single, double and triple GEMs Larger GEMs are segmented to reduce capacity and limit the energy in the discharge 107 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. 108 TPC Readout It boasts a fast electron signal, minimal magnetic distorting effects and suppression of ion feedback. Fractional ion feedback in the TPC drift volume 0.25 pos Ion DGem P 16.9.99 Fractional Ion Feedback For the TESLA experiment at the ILC, a double or triple GeM is under consideration. 0.2 0.15 0.1 Special hexagonal pads are being developed to provide 50 x 60 μm resolution 0.05 Measurement S. Bachmann NIM 99 simulation A. Sharma unpublished 99 0 Drift Field (V/cm) -0.05 0 500 1000 1500 2000 109 MICROMEGAS Xrays MICROMEGAS detectors have been developed for X-ray imaging. Vertebra scanned with a MICROMEGAS Operate with pure Xenon at atmospheric pressure 110 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 111 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 112 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 113 Cherenkov Ring Imaging Very high gains observed with cascade of four GEMs and using pure ethane as the operating gas 114 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 115 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. 3 mm x 10 mm 50 kV xray image of a digit of a mouse. Radiography of a small bat using GEM and 50µm x 50 µm 2dreadout 116 Imaging of Polarized X-rays Measurements of X-ray polarization are used to investigate pulsars, synchrotron nebulae, etc Some X-ray polarimters have been developed using GCPs and GEMs Photoelectrons from GCP Polarimeter Emission direction of the primary electron depends on the incident Xray polarization, it can be measured 117 GEM for Plasma Diagnostics Imaging the dynamics of fusion plasmas has been attempted at the Frascati Tokamak Upgrade to exploit the sensitivity of the GEM to soft X-rays. Time resolved plasma diagnostics are made with a GEM and individual pixel readout. Counts integrated in 50 μs for four adjacent pixels at the Frascati Tokamak Reconstruction of photoelectrons with a GEM+ micropixel readout 118 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 Aging Effects in Wire Chambers 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 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 MeV αbeam than with a Fe55 sources with similar current densities. 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. 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. Triple GEM Detector of the COMPASS experiment Damage to MSGCs Discharges measured in the CMD MSGC prototypes Strip Damage due to discharges and sparks Triple GEM Ageing test Gain of 2x104 Total integrated charge of 13 C/cm2 is expected in 10 years of operation in LHCb 50 MHz/cm2 X-rays, in 10 days a total charge of 20 C/cm2 was integrated Less than 5% change in the chamber behavior CMS high-η - maximum integrated charge LHCb 129 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. 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