2 Signal CreationCharged particles traversing matter leaving excited atoms, electron or holes and ions behind. These can be detected using either excitation or ionization.ExcitationPhotons emitted by excited atoms can be detected by photomultipliers or semiconductor photon detectorsIonizationIf 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
3 Signal InductionA point charge above a grounded metal plate induces a surface charge.-qqTotal induced charge –q.Different charge positions results in different charge distributions but the total charge stays –q.
4 Signal Induction for Moving Charges Segment the grounded metal plate into grounded individual strips.-qqThe 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.
5 Signal TheoremsPlacing charges on a metal electrodes results in certain potentials. A different set of charges results in a different set of potentials.Reciprocity Theorem
6 Signal TheoremsCharge induced on an electrode is independent of the actual path.Once all charges have arrived at the electrodes, the total induced charge in the electrodes is equal to the charge that has arrived at this electrode.If one electrode encloses all the others, the sum of induced currents is always zero.
7 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 ionizationSecondary ionization
8 Charge Collection Cathode Electric Field Anode Charge is produced near the track.Electric FieldApply 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
9 Ion MobilityIons 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.gilmoreAnd table from pdgA 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.
10 Electron Drift Velocity The dependence of the electron drift velocity on the electric field varies with the type of gas used.pdg
11 Electron DiffusionElectron longitudinal (dashed) and tranverse (solid) diffusion.pdg
13 Charge Multiplication At sufficiently high electric fields (100kV/cm) electrons gain energy in excess of the ionization energy, which leads to secondary ionization, etc.Townsend CoefficientrieglerAmplification/Gas Gain
14 Townsend CoefficientComputed values of the Townsend coefficient as a function of the electric field for different gases.pdg
15 Avalanche Positive Ions Electrons: close to the wire Anode wire Number of electrons and ions increases exponentially and quickly forms an avalanche.Positive IonsElectrons move more rapidly than ions and development a tight bunch at the head of the avalanche.gilmoreElectrons: close to the wireIons move significantly more slowly and have typically not moved from their original position when the electrons reach the anode.Anode wire
16 Types of Avalanches LHC 1970’s 1950’s Proportional region: A=103-104 Semi-proportional region: A=Saturation region: A > 108(independent of the number of primary electrons)1970’sStreamer region: A > 107Avalanche along the particle trackrieglerLimited Geiger region: Avalanche propagated by UV photons1950’sGeiger region: A = 108Avalanche along the entire wire
17 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.gilmore
18 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).Gilmore/rieglerCharacterized by a fast electron spike and a long ion tailTotal charge induced by the electrons amount to only ~1-2 % of the total charge.
19 Properties of GasesProperties of commonly used gasespdg
20 2.2 Transport Parameters of Operational Gas Mixtures
21 IntroductionParticle physics experiments rely on the detection of charged and neutral particles by gaseous electronicsA suitable gas mixture within an electric field between electrodes detects charged particlesIonizing 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.
22 Requirements for Gas Mixtures Fast: an event must be unambiguously identified with its bunch crossingLeads to compromise between high drift velocity and large primary ionization statisticsDrift velocity saturated or have small variations with electric and magnetic fieldsWell quenched with no secondary effects like photon feedback and field emission: stable gain well separated form electronics noiseFast ion mobility to inhibit space charge effects
23 Electron-Ion Pair Production in a Gas An ionizing particles passing through a gas produces free electrons and ions in amounts that depend on the atomic number, density and ionization potential of the gas and energy and charge of the incident particleNp: number of primary electron pair per cm.Nt: total number of electron ion pairs (from further ionization)
24 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 vvdWhen an electric field is applied, they drift in the field direction with a mean velocity vdEnergy distribution is Maxwellian with no field, but becomes complicated with an electric field
25 Noble GasesElectrons moving in an electric field may still attain a steady distribution if the energy gain per mean free path << electron energyCross-section for electron collisions in ArgonMomentum 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)
26 Poly-atomic gases Electron collision cross-sections for CO2 Poly-atomic molecular and organic gases have other modes of dissipating energy: molecular vibrations and rotationsIn CO2 vibrational collisions are produced at smaller energies (0.1 to 1 eV) than excitation or ionizationVibrational and rotational cross-sections results in large mean fractional energy loss and low mean electron energyMean or ‘characteristic electron energy’ represents the average ‘temperature’ of drifting electrons
27 Electron DiffusionElectrons also disperse symmetrically while drifting in the electric field: volume diffusion transverse and longitudinally to the direction of motionIn cold gases, e.g. CO2, diffusion is small and the drift velocity low and unsaturated: non-linear space-time relationvdWarm gases, e.g Ar, have higher diffusion. Mixing with polyatomic/organic gases with vibrational thresholds between 0.1 and 0.5 eV reduces diffusion
28 Lorentz AngleBDue 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 dispersionFThe Lorentz angle is the angle the drifting electrons make with the electric fieldLarge at small electric field but smaller for large electric fieldsθLinear with increasing magnetic fieldGases with low electron energies have small Lorentz angle
29 Properties of Helium Helium-Ethane Lorentz Angle for Helium-Isobutane DriftDiffusion
30 NeonLongitudinal Diffusion Constant for Ne-CO2 mixtures
31 Diffusion in Argon Transverse Diffusion in Ar-DME mixture Transverse Diffusion in Ar-CH4No B fieldWith B field
32 Argon Lorentz Angle in Ar/CO2 Drift Velocity for Pure Argon Possible gas for single photon detectors
33 XenonXenon-CO2In medical imaging, the gas choice is determined by spatial resolution: CO2 added to improve diffusionPure Xenon
34 DME Transport Parameters for Pure DME Low diffusion characteristics and small Lorentz angles used to obtain high accuracy
35 Lorentz angle in DME-based mixtures Introduced as a better photon quencher than isobutane.Absoption edge of 195nm: stable operation with convenient gas multiplication factorsHigh gains and rates without sparking.
36 Townsend CoefficientMean number of ionizing collisions per unit drift lengthHelium-EthaneDME/CO2
37 Ion Transport Properties Ion drift velocityElectric fieldpressureConstant up to rather high fields
38 PollutantsPollutants modify the transport parameters and electron loss occurs (capture by electro-negative pollutants)The static electric dipole moment of water increases inelastic cross-section for low energy electrons thus dramatically reducing the drift velocityMean electron capture lengthElectron capture phenomenon has a non negligible electron detachment probability
40 Geiger-Muller Counter 1928Tube 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 DischargeIonising 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.From Knoll/wikipediaCathodeUV photonAnode wireCathode
41 Multiwire Proportional Chamber Invented by Georges Charpak in Nobel Prize in 1992.From Sauli
42 MWPC The particle ionizes the gas producing electrons and free ions. The liberated electrons move rapidly move towards the anode wire and the ions towards the cathode plansMore electrons are liberated which in turn ionise the gas. An avalanche of charges is produced giving rise to and electric pulse on the anode wire.From Charpak Nobel prize lecture
43 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
44 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:From SauliXi;Yi: Strip coordinatesQi(X), Qi(Y): Charge on stripsQ(X), Q(Y): Total Charge2D readout essential for medical imaging applications.
45 Applications of MWPCsLow dose X-ray digital radiography scanner based on the MWPCApplications include crystal diffraction, beta chromatography and dual energy angiographyFilm of congenital hip dislocation in a 7-year old boy.Satisfactory visualization of femoral architecture and bone structure
46 Applications in Medical Imaging Activity in a vasopressine-labelled rat’s brain (from CERN-Geneva hospital).E. Tribollet et al, Proc. Natl. Acad. Sci. USA 88(1991) 1466SauliRegional uptake of deoxyglucose in a dog’s heartM.G. Trivelli et al,Cardiovasclar Res. 26(1992) 330
47 Drift ChambersDAn 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.
48 Typical Geometries of Drift Chambers W. Klempt. Detection of Particles with Wire Chambers, Bari ‘04
49 StrawsIf 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)From Sauli and TRT picture from: upenn group webpagePortion of the ATLAS TRT End Cap
50 MDTsThe ATLAS barrel muon spectrometers uses Monitored Drift Tubes. These reconstruct tracks to 100 μm accuracy.Sauli + picture from ATLAS image collection
51 ATLAS MDTs MDTs can also be used for making music! MDT pipe organ made by Henk Tieke from NIKHEF, Amsterdam.
52 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 V/cmDrift times μsDistance up to 2.5 mGas volumedriftBEvent display of a Au-Au collision in STAR at RHIC. Typically ~200 tracks per events.EWire chamber
54 Gas for TPCsA 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 MAGBOLTZS. Biagi, NIM A42(1999) 234
55 Cherenkov RadiationPhotons 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)θInspired by Sauli but modifiedReactor is Idaho’s Advanced Test ReactorThese are reflected on a spherical mirror. The radius of the ring is R = rθ/2Cerenkov Radiation in the core of a nuclear reactor
56 RICH Detectors ALICE HMPID LHCb RICH Detector Alice picture from SabaLHCb RICH diagram from Oxford physics websiteCan be used for particle identification together with tracking detectors
57 COMPASS RICH Event Display Array of 8 MWPCs with CsI photocathodes From SauliArray of 8 MWPCs with CsI photocathodes
58 Time Resolution Time Resolution of Wire Chambers It takes the electrons some time to move from their creation point to the wire. Fluctuations in this primary deposit and diffusion times to ~5nsIf one uses a parallel plate geometry with a high field, the avalanche starts immediately so that time resolutions down to 50 ps can be achieved. These detectors can be used for triggering.
59 Resistive Plate Chambers Place resistive plates (Bakelite or window glass) in front of the metal electrodesSparks cannot develop because the resistivity and capacitance will allows only a very localized discharge.From CMS outreach site + rieglerCMS RPCsLarge area detectors can be madeRate limit of kHz/cm2
60 ALICE TOF DetectorLarge Time-Of-Flight (TOF) system with 50 ps time resolution made from window glass and fishing line (high precision spacers)rieglerBefore RPCs were available, very expensive photomultipliers were used with scintillators