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Gaseous Detectors Introduction • Gaseous detectors are mainly used to measure the trajectory of a charged particle • When a magnetic field is applied the momentum can be determined by measuring the curvature of a particle track • If the momentum is determined than the energy loss of the particle can be related to its mass particle identification EXAMPLE: ZEUS Drift Chamber 50 MeV Inner radius 16 cm Outer radius 85 cm B-field = 1.5 Tesla 100 MeV radius = ρ= 1 ρ 0.3 B p ⋅ sinθ 500 MeV Typical example of modern gas-filled track detector. In the gas-filled volume, thin wires are fixed, and put at specific potential. A fast charged particle creates electron-ion pairs, and the electrons drift towards anode wire, causing a detectable (charge) signal. From the timing of these signals, accurate data on the track position can be obtained. EXAMPLE: ATLAS Drift tubes Inner radius 500 GeV 5m Middle radius 10 m Outer radius Sagitta = 5 mm Sagitta 15 m B-field = 0.6 Tesla radius = R Sagitta = L2/8R Another example of high-precision tracking: the position of a track is measured by groups of ‘drift tubes’. The track curvature, and therefore the momentum of the particle, follows from the sagitta. First Gaseous Detectors • Ionisation chamber Gas filled space with anode and cathode. A current flows after the passage or the absorption of a (possibly charged) particle. V t Discovery of cosmic rays V The first gas-filled detectors were ionisation chambers. Here, a static potential between two electrodes (for instance two plates, or a tube and a central wire) creates a static electric field. The gas can be ionised by passing charged particles, absorption of X-rays, or neutrons. In this ionisation process, electrons are separated from their atom or molecule, resulting in electronion pairs. Electrons drift towards the anode, and ions towards the cathode. This is an electric current. In 1912 it was noticed that if a tube + wire is charged and the battery is taken away, the potential still dropped, even when external radiation sources were taken away. Victor Hess concluded that the tube is still hit by ionisation radiation: cosmic rays. Ionisation chambers are still used today as smoke detector. The air-filled gap between two plates is crossed by alfa particles from a (low intensity!) source. These cause a measurable current. Smoke particles absorb some alfa’s, and, in addition, reduce the current due to recombination. As a result, the current decreases. First Gaseous Detectors • Geiger Counter: Hans Geiger, 1908, Manchester [100 years ago!!] wire • Gas ionisation • electron drift • electron avalanche • discharge propagation • (self) quenching Sequence of processes: - Ionisation of gas (for instance: dE/dX by passing charged particle) Separation of electrons and ions Drift of electrons along local electric field line towards anode (wire) Electron avalanche onto the wire Ions, created in avalanche, move backwards towards cathode tube [Discharge quenching, propagation: not relevant for proportional tubes, but important process in detectors in Geiger mode] The layout of a spark chamber Early Gaseous Detectors • Spark chamber Parallel conducting plates connected alternating to highvoltage supply (pulsed after external trigger) and ground. Filled with noble gas. High voltage causes avalanche formation leading to production of UV light seen as sparks • Streamer chamber Large gap spark chamber, with high voltage (typically about 500 kV) applied during short time after external trigger. Avalanches develop only in the vicinity of the initial ionization, resulting in short segments, "streamers",producing UV light. Energy loss • The energy loss depends on the particle and the medium it traverses • Fluctuations in the energy loss follow a Landau distribution • The average energy spent in the creation of an ion-electron pair, W, is roughly constant for a given gas dE W ⋅ N ionisation s = dx pathlength Energy loss • Primary energy loss by ionisation of atoms leads to creation of ion-electron pairs a gas volume. • Two step process: freed electrons can cause secondary ionisation (cluster, delta rays, number of electrons per cluster) Essential: - number of clusters per mm tracklength - number of electrons per cluster specific for gas (and density ρ, thus T, P!) The creation of electron-ion pairs is a two-step process. Per unit length, there is a specific probability that an electron is emitted from an atom or molecule. This electron may carry sufficient energy in order to release a 2nd or even 3rd electron. The energy may be so large that the electron creates a track next to the track of the original passing charged particle. Such an event is called a δ–ray. We now call a primary interaction a cluster: it contains at least one electron. A δ–ray is a cluster with many electrons along a deviating track. Z A δ E,W atomic number atomic mass specific density ionisation potentials dE/dX energy loss np number of clusters/cm nT total number of electrons/cm Parameters of gases which are often applied in gaseous detectors. Primary Ionization: HEED simulation 10. 5 clusters/mm at 7GeV λ 95µ µm nav 2.4 electrons/cluster long tail ! Values of dE/dX (left) expressed in clusters/mm, and the distribution of the number of single electrons within a cluster (right). Drift of charge • The electron mobility is much larger than the ion mobility, for example for pure Argon at 1 kV/cm it is – Electrons: r v drift = µE Electrons 0.4 10-3 cm2V-1m-1s-1 = 400 cm2V-1s-1 for electrons – Argons ions in Argon gas 1.5 cm2V-1m-1s-1 for Ar ions • The mobility depends on the field and (gas) pressure Fernow, page 212-213 The drift speed of electrons and ions follows the mobility relation vdrift = µE, where µ is a constant depending on the gas (mixture) and pressure (note that the mobility constants for electrons and ions are very different constants). For ions, this relation holds up to high values of E, but not for electrons. The collisions between electrons and atoms or molecules become inelastic, reducing the average speed of the electrons. The electron drift speed as a function of the electric field for a given gas mix can be obtained from simulations or from measurements done with calibration chambers. The drift velocity is an average of the erratic velocity v of the electrons or ions. If a magnetic field is applied, a Lorentz force is introduced in the form of the ‘cyclotron frequency’ ω: Drift time • General motion of charge in an EM-field r r r r dv = eE + ev × B m dt • But there is friction due to the gas r r r r r dv = eE + ev × B + Q( t ) m dt r • Solve for stationary operation or dv = 0 dt r v r eB and Q(t) = mA(t) • Define µ = r ω = m E The average of mA(t) is obtained by considering that this force is equal to <dp/dt>, which can be written as: Σ pi / tN = N mvD / (N <∆t>) = mvD / τ. The average force is balancing the force due to electric and magnetic fields, so mA(t) has to be replaced by -mvD / τ in the time averaged equation: eE + ev D × B − mv D / τ = 0 This can be written as: eτ v D = (E + v D × B) m The quantity eτ/m is equal to the mobility for P=P0 for the case that B = 0, as in that case: µ = vD/E =eEτ/(Em)=eτ/m, with τ the τ for the situation with drift field. From the equation it is seen that the definition of mobility can be generalized to make the mobility the proportionality constant between the drift velocity and the sizes of the electric and magnetic forces divided the electron charge. The equation is a vector equation in vD with as solution: vD = µ [E + µE × B +µ 2 (E • B)B] 1+ µ 2B2 For B perpendicular to E we have: µE µ 2EB v D,// = v = D, ⊥ 1+ µ 2B2 1+ µ 2B2 Therefore: v D, ⊥ / v D, / / = µB , i.e. drifting is still in a straight line, but under an angle of arctan (µB) with the electric field direction in the plane perpendicular to B. The angle is called the Lorentz angle and is independent of the electric field strength. With ω = eB/m (cyclotron frequency) is µΒ equal to ωτ. The expressions for B perpendicular to E then become: 1 1 -> the drift velocity in the field v D,// = µE = v D,//,no B 2 2 1+ ω τ 1+ ω 2 τ 2 direction decreases for increasing B ωτ ωτ v D, ⊥ = µE = v D,//,no B v D, ⊥ / v D, / / = ωτ 1+ ω 2 τ 2 1+ ω 2 τ 2 v µE v D = v 2D, ⊥ + v 2D, / / = = D,//,no2B 2 -> the total drift velocity decreases 2 2 for increasing B 1+ ω τ 1+ ω τ Operation Mode • Ionisation chambers are used for dosimetry: only the integrated signal over time is relevant (charge collection similar to solid state detectors, see next week) • Most particle detectors are in the proportional mode where the total number of secondary electrons is proportional to the number of primary electrons • Geiger counter discharges at every single ‘event’ Fernow, page 206 The development of avalanches With a wire potential below a certain value, no avalanche will occur, and all primary charge is simply collected onto the wire (gas gain = 1). With increased (positive) wire potential, avalanches develop and a larger charge signal is collected on the wire. Up to gas gains of 100 k, the charge signal is proportional to the primary ionisation. The detectors is in the proportional mode. Avalanche close to wire F.Sauli, CERN-77-09 Elementary gaseous detecor: The Proportional tube The charge signal from the wire is proportional to the primary ionistation charge Time development The charge signal development, as it appears on the wire of a proportional counter, is mainly determined by the displacement of the ions, created in the avalanche, moving from the wire surface towards the cathode tube. This causes the long ‘ion tail’ in the signal, which can be overcome by means of a differentiating filter. Multi Wire Proportional Chamber • • • • The signal on the wire provides the coordinate in 1-dimension The resolution is determined by the wire pitch P. No drift time: σ = P /√12 Typical dimension – Thickness 10 mm – Pitch 2 mm – Surface 1000 x 1000 mm Mind the electrostatic force between the two wires The Multi Wire Proportional Chamber (Wire Chamber). A particle crossing the detector plane vertically will, in general, result in one wire to give a (negative) charge signal. Tilted tracks crossing more than one ‘cell’, associated with a wire, may ‘fire’ more (adjacent) wires. Drift Chamber • After the MWPC introduction of the drift time measurement • To improve the electric field configuration field wires are used The drift chamber. The moment of passage of a particle is known from another detector (i.e. a scintillator). The primary electrons drift towards the anode wire in a homogeneous electric field. Arrived at the anode wire an avalanche occurs and a charge signal appears. The time period between the scintillater signal and the signal from the wire and is called the drift time. This drift time can be converted to a drift distance if the drift velocity is (empirically) known. In order to provide a homogeneous drift field, more wires are applied, put a suitable potential, possibly by means of a resistor chain. Drift velocity as a function of the electric field for several gas mixing rations Ar/Ethane In the right-hand figure above, a multi-sampling drift chambers is shown. The position of a track, crossing a drift cell, can be accurately reconstructed. - Lorentz Angle + B is perp. to image Due to the Lorentz force, the direction of drifting electrons is affected by the presence of a magnetic field (this field is required for the measurement of the momentum of charged particles). In planar drift chambers (depicted above), the Lorentz force can be compensated by a tilt of the electric field. The potential of the field wires is carefully set. Cylindrical Wire chamber • In a Drift Chamber the drift is orthogonal to the wires and also to the magnetic field. Lorentz angle is important B E in plane electron With field wires individual driftcells can be created Cylindrical Drift Chamber C.Joram. CERN Academic Training 1997-1998 Drift chambers have been made for many applications, in a large variety of geometries. An important application has been the vertex detector (see Semiconductor Detectors). These detectors are placed as close as possible to the interaction point of collider experiments. These detectors must measure tracks with the best possible spatial resolution, and must be as light as possible in order not to disturb tracks (multiple scattering). Diffusion Diffusion. Primary electrons on a track left by a passing particle drift towards the anode wire. Allthough the primary electrons have a common drift velocity, they will spread due to the stochastic drift process. This diffusion is proportional to the square root of the drift time, and thus the drift distance. The diffusion has a Gaussian character and the spatial spread can be described by σdiff. Per electron the diffusion can be approached as σdiff = σd sqrt(D), where D equals the drift length (often in cm), and σd a constant (gas property) in µm/sqrt(cm). The values for longitudinal and transversal diffusion are different, in general. Gas Gain • The increase in the number of electron after travelling a distance y+dy dN = αN dy α = Townsend first multipication coefficient • The electron multiplication or gain is M(x) = e αx • The maximum gain is up to 107 Fernow, page 214 Performance (=precision) of Drift Chambers ‘Spatial resolution’ ‘ time resolution’ - timing measurement: slewing; avalanche fluctuations, noise - fluctuation in # primary electrons - electron diffusion during drift - variation in drift velocity - E-field variation - Temperature and pressure variations wire el. el. δt Track Sci el. Beam Cross t0 Time measurement: - time interval (tstop – tstart) All (cable) delays to be measured? The calibration of drift chambers. First the time measurements must be corrected for the delays due to signal propagation over wires and cables, and for the propagation of the particle itself. The calibration system is usually integrated in the design of the electronic readout system. Drift Time Spectrum Irradiate chamber homogeneously (with cosmics) time spectrum dift distance t0 tmax D-T relation Dmax R-T relation t0 tmax drift time The drift velocity is the main parameter to be known when operating drift chambers. In a planar drift chamber with a constant drift velocity, and thus a linear relation between drift time and drift distance (linear r-t relation), the drift time spectrum has a flat distribution if we take care for a homogeneous irradiation (by particle tracks, or by a radioactive source). In an experiment one can often use the data itself for obtaining these spectra. Round ATLAS drift tube dia. 30 mm Above the drift time spectrum of a drift tube is shown. Since the electric field is proportional with 1/R, the drift velocity is by no means constant within the tube. The empirical r-t relation can be obtained by integration of the drift time spectrum. Y :: 1/ Vdrift Vdrift = dR/dt D-t relation is integral of drift time spectrum The r-t relation of a drift chamber with hexagonal cells, obtained from the drift time spectrum. Note the significant tail, associated with very long drift times. This is due to areas in the drift cell with a very low electric field. Time Projection Chamber (TPC): 2D/3D Drift Chamber The Ultimate Wire (drift) Chamber track of charged particle E-field (and B-field) Wire plane Wire Plane + Readout Pads Pad plane Time Projection Chamber • Large drift volume of several meters to measure accurate drift time. • Electron drifts parallel to B-field • Avalanche induces signal on sense wires & readout pads Electron drift Avalanche Readout pads Sense wires The principle of the Time Projection Chamber. If the magnetic field is parallel to the electric field, the diffusion is reduced, much to the benefit of the precision of track position measurements. With a magnetic field in the direction of the drift field the transverse diffusion will be reduced: ρ sin θ/2 under influence of the magnetic field the distance transverse to the direction of the fields between successive collisions φ is equal to 2ρ sin(θ/2), with θ=vT∆t/ρ. ρ Projected on one of the transverse directions the result is: 2ρ sin (vT∆t/(2ρ) ) cos φ. The average of the square of the displacement in one of the transverse directions becomes: s 2 ⊥,1D 1 direction = ∫ 2π 0 cos 2 φ dφ ∫ 2π 0 dφ ∞ vTt ∫ (2ρsin 2ρ ) 0 Averaging over all angles -> π/(2π) =1/2 Without magnetic field: D T.B = Dno B s 2 ⊥,1D = 2 θ/2 1 2τ 2 v 2T e− t / τ τ 2 v 2T dt = = 2 2 2 2 1+ τ v T /ρ 1+ ω 2 τ 2 τ ∫ 2π 0 cos 2 φ dφ ∫ 2π 0 dφ ∞ ∫v 0 2 2 T t e−t / τ dt = v 2T τ 2 τ 1+ ω 2 τ 2 Gas Mixtures: Ageing • At the electrodes (anode or cathode) a deposit can develop. This reduces the liftime of the detector. The deposit can be formed by polymers. Addition of gas with a low ionisation potential can be put to avoid polymerisation (like propylalcohol (C3H7OH) or methylacohol(OCH3CH2) • If instead an inorganic quenching gas is used polymerisation can be avoided altogether (like CO2 which is has the advantage of being un-inflammable but it is ‘toxic’) The negative effect of the deposit (in terms of the variation of the signal amplitude) is about proportional with the total collected charge per unit surface on the electrode (or, more practically, the total collected charge per unit wire length). Ageing is very important for chambers placed in a high-radiation environment. × 100% C.Joram. CERN Academic Training 1997-1998 Unsolved issue for gaseous detectors: ageing. × 100% C.Joram. CERN Academic Training 1997-1998 Micro Pattern Gas Chambers (MPGCs) Get rid of wire: granularity, 2D versus 1D - MSGC - Micromegas - GEM - GridPix chambers Micro Strip Gas Counter Wire chambers: MSGCs: granularity ~ 1 mm granularity 200 µm No more wires!! The Micro Strip Gas Counter. Here, the wires are replaced by strips printed on a glass substrate. These ‘wires’ can be placed with a pitch of ~0. 1 mm. Not often applied: …sparks……! The electric field of a MSGC. Note the local strong field at the glass’ surface near the anode strip. This causes instabilities which are hard to prevent. Micro Patterned Gaseous Detectors (MPGDs (MPGDs)) • High field created by Gas Gain Grids GEM • Most popular: GEM & Micromegas Micromegas improved granularity : wire chambers react on COG of many electron clouds/clusters Micro Pattern Gas Detectors. Here, the wires with their strong E-field close to their surface, ate eliminated and replaced by perforated foils. The perforated Micromegas foil is placed parallel to the readout anode, at a distance of 50 µm. By applying some 400 V between the foil and the anode, electron avalanches, and thus gas amplification occurs. A drift field above the foil takes care for the transport of primary electrons into the holes of the foil. Time Projection Chamber (TPC): 2D/3D Drift Chamber The Ultimate Wire (drift) Chamber track of charged particle E-field (and B-field) Wire plane Wire Plane + Readout Pads Pad plane The MPGD can be well applied as Time Projection Chamber. 55Fe Cathode (drift) plane Micromegas Drift space: 15 mm Baseplate MediPix2 pixel sensor Brass spacer block Printed circuit board Aluminum base plate Very strong E-field above (CMOS) MediPix! Layout of the GridPix detector, where the readout anode plane is replaced by a pixel chip. Each hole in the Micromegas foil is associated with its own pixel. He/Isobutane 80/20 Modified MediPix GridPix: the electronic bubble chamber 14 mm δ-ray! Efficiency for detecting single electrons: < 95 % A cosmic muon observed with the GridPix detector. Note the δ- ray. Resistive Plate Chambers cheap, fast signal, good timing (but noisy) + - 10 kV Principle of the Resistive Plate Chamber. It consists of two parallel conducting plates with high-resistive layers at their inside face. The high-potential difference appears at the surface of the high-resistive layers, and a strong electric avalanche field exists between the layers. The primary ionistation due to a passing charged particle causes a detectable avalanche.