Tracking Detectors PDF

Tracking Detectors Historical tracking detectors Gas Detectors o Multiwire proportional chambers o Planar drift chambers o Cylindrical wire chambers Scintillating fiber trackers Semiconductor track detectors o Strip detectors o Hybrid pixel detectors...

Tracking Detectors Historical tracking detectors Gas Detectors o Multiwire proportional chambers o Planar drift chambers o Cylindrical wire chambers Scintillating fiber trackers Semiconductor track detectors o Strip detectors o Hybrid pixel detectors cfr. C. Grupen and B. Shwartz, Chapters 6 & 7; C. Grupen and I. Buvat, Chapter 12 Common modern particle detectors Fixed target geometry Collider geometry Look at collision products in a Particles to be detected over small open angle along beam full solid angle (4π) axis Detectors arranged around Planar detectors behind beam axis and particle particle interaction point, interaction point in an “onion” perpendicular to beam axis, i.e. structure “forward” detector configuration 2/74  LHCb forward CMS collider detector @ CERN detector @ LHC CERN LHC 3/74 Particle “jets” Tracks Energy Interaction Point Quark Jet Jet ≡ a collimated spray of high energy hadrons Quarks fragment into many particles forming a jet, depositing energy in both types of calorimeters Jet shapes narrower at high ET 4/74 Tracking in Particle Physics Tracking detectors are devices to measure and reconstruct trajectories of charged particles, which is important to understand physics processes in nuclear/particle/astro-particle physics experiments Measurement of particle trajectories provides information about: - interaction point, i.e. primary vertex - decay length and path of unstable particles - angular distributions - particle momenta (when used in combination with magnetic field) - particle types Most important tracker technologies include gas detectors, semiconductor detectors and scintillating detectors In modern experiments several tracking devices/technologies are usually combined together in very complex setups; actual trajectory reconstruction makes use of all available information and involves pattern recognition, track and vertex fitting 5/74 Tracking particles without disturbing them (too much …), and determining position of primary particle interaction vertex and secondary decays of particles, requires: Superb position resolution Highly segmented detectors to obtain high spatial resolution Large signal Small amount of energy to create signal quanta Thin detectors Close to interaction point Low mass Minimize multiple scattering (in detector, readout, cooling/support structures …)  Measure space points 6/74  From reconstructed particle tracks, deduce o vertex location(s), i.e. primary & secondary vertices o decay lengths o impact parameters Example of b-quark tagging: 7/74 b-quark tagging & lifetime measurement Discovery of top quark via secondary vertex finding e.g. → To measure lifetimes in picosecond regime requires spatial resolution of order 5-30µm 8/74 Historical track detectors Historical track detectors Following are a few examples of historical devices used in early cosmic ray and nuclear/particle physics experiments Mainly “optical devices”, in contrast to more modern “electronic” devices; sometimes quite labor intensive to take measurements, low count rates … Most of these devices are now part of physics demos or exhibitions … … although a few of these historical devices are still being used, e.g. nuclear emulsions and bubble chambers (for educational purposes) Big European Bubble Chamber @ CERN 10/74 Cloud chambers Wilson’s original cloud chamber @ Cavendish Laboratory museum (Cambridge) 11/74  One of the oldest tracking detectors … Operating principle: - Container filled with a gas-vapour mixture at vapour saturation pressure (e.g. air-water …) - Charged particles produce ionisation trail - After passage of particle an external trigger signal initiates fast expansion of the chamber, causing supersaturated vapour through adiabatic expansion - Trail of positive ions acts as seeds for condensation droplets - Trajectory is illuminated and photographed - Chamber needs to be recycled by recompression Cycle times from 1-10min → usage limited to low counting rate, rare event experiments 12/74  Variations of the expansion cloud chamber: - Multiplate cloud chamber, i.e. a regular cloud chamber with lead plates, used in air shower experiment - Diffusion cloud chamber, i.e. a temperature gradient provides a zone with permanently supersaturated vapour; charged particles produce a trail without need of external trigger; ions are removed using a clearing field 13/74 Bubble chambers Bubble chamber allow studying high complexity events with high spatial resolution, but still tedious analysis of pictures is needed … Operating principle: - Liquid (H2, D2, Ne …) inside pressure container close to boiling point - Before expected event, chamber volume is expanded by a piston, reducing the pressure, causing superheated liquid - Charged particles cause bubble formation along track - Bubbles are illuminated and photographed 14/74  Pressure inside bubble chamber before expansion is several atmospheres Operation can be hazardous, using cooled hydrogen gas (explosion) or heated organic liquids (flammability) Bubble chamber are often operated inside magnetic fields to measure particle momenta; high spatial resolution of several micrometer Some drawbacks: - no possibility to trigger the chamber - difficult to produce them in a 4π geometry for storage-ring experiments - not massive enough to stop produced particles - muon/pion identification is difficult - magnetic field usually not suitable for high momenta - very time-consuming analysis of photographs Bubble chambers played big role in neutrino physics programs, e.g. Gargamelle @ CERN (first experimental observation of weak neutral currents, i.e. existence of Z0 boson, in 1973) 15/74 Discovery of the Ω- in a bubble chamber at Brookhaven National Laboratory (1964) 16/74 Streamer chambers Not be confused with streamer tubes (special mode of operation for special cylindrical counters)! Operating principle: - Two planar electrodes enclose a volume filled with counting gas - After passage of charged particle, a HV-pulse of high amplitude (~500kV), with short rise and decay time (~ns) and limited time duration (several ns) is applied - Ionization electrons start avalanches towards anode; avalanche formation gets interrupted after decay time of HV-pulse - Large amplitude HV-pulse gives very high amplification (~108) and yields streamers limited in space - Luminous streamers are photographed through one transparent electrode 17/74 Neon-flash-tube chambers Operating principle: - Neon- or neon/helium-filled tubes/spheres (glass or polypropylene-extruded plastic) placed between two metal electrodes - After passage of charged particle, a HV-pulse is applied, initiating a gas discharge in the tubes the particle went through, leading to glow discharge - Tubes are photographed or readout using pickup electrodes Typical tubes are ~2m long with diameter 5-10mm Spatial resolutions of several mm; long dead times of 30-1000ms Cosmic ray muons 18/74 Spark chambers Most commonly used track detector that could be triggered, before multiwire proportional and drift chambers were invented Operating principle: - Gas (typ. He-Ne mixture) filled volume with parallel plates, alternatingly grounded or connected to HV - External scintillators above and below chamber trigger HV-pulse - Gas amplification (~108-9) is chosen to cause spark discharges after a charged particle passed through - A clearing field removes ions in between two discharges 19/74  Efficiency close to 100% with Cosmic-ray muon appropriate time delay between particle passage and HV-pulse Events are recorded photographically, but electronic readout also possible Electrodes can be made of layers of wires to obtain track coordinates from discharged wires The clearing field needed to remove positive ions causes dead time of several milliseconds Demo spark chamber @ Cambridge University 20/74 Nuclear emulsions Nuclear emulsions consist of fine-grained silver-halide crystals (AgBr and AgCl) embedded in a gelatine substrate (similar to photographic paper) Operating principle: - Charged particle produces latent charge in the emulsion - Some halide molecules are chemically reduced to metallic silver due to the free charge carriers from the ionization process - A subsequent development process chemically reduces the silver-halide crystals; preferentially microcrystals which are already partly reduced are affected and transformed into elemental silver - A fixation process dissolves the remaining silver halide; the charge image, which is transformed into elemental silver particles remains - Evaluation of emulsion usually done with microscope, but automated readout is also possible Typical thickness of emulsions is 20-1000 microns, with sizes up 50x50cm2 Efficiencies is close to 100%; emulsions are permanently sensitive but cannot be triggered; spatial resolutions of ~µm are possible 21/74  Ionization profile of iron nucleus and a heavy nucleus (Z≈90) in a nuclear emulsion Interaction of a uranium nucleus in nuclear emulsion 22/74 Present tracking detectors Real CMS@LHC p-p collision event; high performance tracking is a non-trivial task … Tracking detectors Tracking at fixed target experiments: Tracking at collider experiments: Multi-layer MWPC or drift chamber cylindrical drift chamber 25/74 Single wire proportional counter 26/74 Multiwire proportional chamber Charpak’s MWPC @ National Museum of American History 27/74 28/74 Gain (G) variation with wire diameter (a) and gap length (L) 29/74  Avalanche formation in a multiwire proportional chamber is the same as in proportional counters Counting gases identical to the ones used for proportional counters, i.e. noble gases (Ar, Xe) with admixtures of CO2, CH4, isobutane …; gas amplification of 105 and time resolution of 4ns achievable Usually, analogue information on the wires is not used, i.e. only signal thresholds are set; for e.g. d = 2mm, spatial resolution is d σ= ( x) = 577 µ m 12 Electrostatic repulsion is limiting wire spacing d; a minimum wire 2 tension T is required to stabilize wires:   for e.g. l = 1m, V = 5kV, requires V ⋅l  2  1  wire stretching with about 50g T ≥  4πε 0  π L 2π r   d    2 − ln i    d  d   Consider also sag due to gravity for horizontal wires 30/74  Spatial resolution of MWPC can be improved via cathode segmentation and measurement of analogue signals in cathode segments Resolutions up to ≈50µm are achievable For x-y configuration the problems of ghost hits appears 2nd cathode plane can fix this … 31/74 Planar Drift chambers In drift chambers one measures Δt, i.e. the time between the passage of particle through the chamber and the arrival at the anode wire of the charge cloud induced by the primary ionization electrons Field or potential wires are introduced between anode wires to produce suitable drift field; drift velocity of electrons needs to be known precisely along drift path or x= v − ⋅ ∆t ∫ x = v − (t )dt Compared to MWPC, drift time measurement allows to reduce number of anode wires, or to improve spatial resolution using small anode-wire spacings 32/74 The first drift chamber [A.H. Walenta, J. Heintze and B. Schürlein (1971)] 33/74 Field formation Remember the MWPC field lines: Regions of low E-field, which is a problem … Modified MWPC: introduce field wires to avoid low field regions, i.e. long drift-times - Field wires are at negative potential - Anode wires are at positive potential - Cathode planes are at zero potential 34/74 Principle of an adjustable field multi-wire drift chamber Introduction of voltage divider via cathode wire planes for larger chambers - very few (or only one) anode wires for large drift volumes - space point resolution limited by mechanical accuracy [for large chambers: σ ≈ 200 μm] 35/74 Drift-time-space relation in a large drift chamber (80x80cm2) with only one anode wire 36/74 Alternative: field formation by charging insulated chamber walls (insulating foil mounted on cathode facing drift space) with positive ions Electrodeless drift chamber [Allison et al., 1982] Chamber requires some charging time, i.e. sufficient positive ions need to accumulate on chamber walls Before charging up, field line end at cathode After charging time, no field lines end at cathode Finite resistance of insulator, i.e. some field lines end at cathode, to avoid overcharging 37/74 Spatial resolution Typical spatial resolutions σx=50-100 µm i.e. not limited to cell size - Primary ionization statistics: how many ion pairs, ionization fluctuations dominates close to the wire - Diffusion: diff. constant, drift length dominates for large drift length → Possible improvements: - Electronics: noise, shaping - Increase N by increasing pressure characteristics - Decrease D by increasing pressure constant contribution, independent of drift length 38/74  Statistical production of electron-ion pairs is important for particle trajectories perpendicular to the chamber i.e. electron-ion pair closest to the anode wire is not necessarily produced on the connecting line between anode and potential wire … … large drift-path differences for tracks close to anode wire, but minor effect for distant tracks 39/74 Left-right ambiguity Time measurement cannot distinguish whether particle has passed right or left from a wire, i.e. the "left-right ambiguity" → Solutions: separate wires, inclined wires, staggered wires, multiple (at least two) drift cell layers displaced relative to each other … 40/74 Example of staggering in a cylindrical drift chamber: 41/74 [Other drift-based chambers] Several types of chambers exist using the electron drift principle, e.g. the Multistep avalanche chamber, consisting of two MWPCs operating at low amplification and separated by a large drift region: - A particle will leave small signals in both MWPCs, but electrons from avalanche in MWPC 1 have a small probability ε to drift towards MWPC2 - Long drift path (several 100s of ns) leaves sufficient time to decide if MSAC for event is interesting and to open tracking in (pulse) the grid WA98 experiment at the CERN - 2nd amplification in MWPC2 gives SPS 106∙ε, i.e. large enough to trigger readout electronics of the chamber Such “gas delays” nowadays replaced by electronic delay circuits 42/74 Cylindrical wire chambers Collider experiments need maximum solid angle coverage, i.e. hermeticity → a cylindrical wire arrangement is needed First experiments used cylindrical multigap spark chambers and multiwire proportional chambers At present, drift chambers have been adapted for measurement of particle trajectories: - cylindrical drift chambers (wire layers form cylindrical surfaces) - jet chambers (drift spaces segmented in azimuthal direction - time-projection chambers (trajectory information drifts to circular end- plate detectors) Cylindrical drift chambers are operated in magnetic fields to determine particle momenta from the bending radius of the track 43/74 Cylindrical drift chambers Characteristics: All wires stretched in axial (z-) direction, i.e. along the direction of magnetic field Potential wires between two anode wires and between adjacent readout layers Potential wire Ø≈100µm, while anode wire Ø≈30µm → Cylindrical symmetry → trapezoidal drift cell geometry Require: Simple space-time relation given by E, B field and drift cell geometry 44/74  Several thousand wires stretched between two end plates which take the whole wire tension that can amount of several tons for large chambers Cylindrical drift chamber for the central detector at Intersection 8 of the CERN Intersecting Storage Rings 45/74 Various cylindrical drift chamber geometries 46/74 Determining “z”-coordinates, i.e. measure coordinate along the signal wire Charge-division method, i.e. measuring either signal amplitude or their propagation time (with fast electronics) at both ends of the anode wire longer collection path = larger resistivity shorter collection path = smaller signal = smaller resistivity = larger signal → accuracies of order 1% of wire length 47/74  Spiral-wire delay lines of diameter smaller than 2mm, with tunable electrical characteristics (delay/m, attenuation …), stretched parallel to sense wire; may also help resolving left-right ambiguities if placed between closely spaced wires → accuracies of order 0.1% along the wires Stereo wires, i.e. anode wires stretched not exactly parallel to cylinder axis, but tilted by a small angle, e.g. resolutions of σz = 3mm can be obtained for stereo angle γ ≈ 4° Stereo wires appear to sag hyperbolically wrt. axial anode wires, “Hyperbolic chambers” 48/74 Effect of magnetic field on electron drift paths PLUTO@DORIS/DESY 3.5-5GeV e+e- storage ring; first superconducting solenoid in the world 49/74 Alternative drift chamber geometries Cylindrical configuration of thin-wall straw-tube or chambers multi-wire drift module with 70 drift cells in one carbon-fibre container of 30mm diameter advantage: one broken wire = one channel missing one broken wire   can kill large region of detector The straw tubes have Reconstruction of a particle diameters of 5-10mm passage through a multi-wire and are frequently operated at overpressure; drift module; the circles indicate the measured drift these detectors allow for times of the fired anode wires; spatial resolutions of the particle track is a tangent 30µm to all drift circles 50/74 ATLAS straw tubes (Transition Radiation Tracker) 300000 thin-walled proportional-mode drift tubes (wound Kapton + tungsten wire) Drift tubes Silicon semiconductors Single point resolution: 120-130 µm Points per track: ~30 over a long lever arm (extend silicon tracker) Spaces in between tubes filled with fibers or foils to create Transition Radiation, leading to additional energy deposit in gas, i.e. higher signals; effect strongest for electrons, i.e. possibility for particle identification 51/74 Momentum resolution: effect of TRT 52/74 Jet drift chamber Accurate measurement of energy loss by ionization using as large a number of anode wires as possible, i.e. large-volume drift chamber made of multiple independent cells, with a single wire plane in a moderate drift volume Example of JADE @ PETRA / DESY-Hamburg: energy loss of each particle measured by 48 wires, parallel to magnetic field detector made of 24 radial segments, each subdivided into smaller drift regions of 16 anode wires each field formation made by potential strips at sector boundaries electric field perpendicular to counting wire planes and to magnetic field → Lorentz angle α = 18.5° for 0.45 T magnetic field (≡ electron deflection angle due to B-field) chamber operated at 4 atm to increase sensitivity coordinate along wire determined using charge-division method staggered wires to resolve left-right ambiguity 53/74 1979: gluon discovery at PETRA (JADE, Mark-J, PLUTO) from 3-jets event 54/74 Example of OPAL @ CERN LEP e+e- collider (1989-200) Z-Chambers (drift chambers to measure track z-coordinates) Silicon Microvertex Detector Jet drift chamber inside pressure vessel (P = 4 bar) and solenoid (B = 0.435 T) Vertex Drift Chamber Jet-Chamber 55/74 OPAL Central Vertex Chamber precision along the drift direction is typically ~100µm, and can be better than 50µm with pressurized gas; precision by charge division along the wire is a few centimetres very good two-track resolution using multi-hit electronics, hence the name “jet chamber“ typical two-track resolution is 1-3 mm 56/74 OPAL Jet Drift Chamber 57/74 58/74 Time Projection Chamber colliding beam A Time Projection Chamber (TPC) particles is divided in two halves via a central µ- electrode; end plates usually consist of multiwire proportional chambers e- e+ or micro-pattern gaseous detectors; large electric field drifting (100-400 V/cm) between central electrons electrode and end plates; parallel µ+ magnetic field (~1.5 T) suppresses particlesµfrom + diffusion perpendicular to electric field collisions (Larmor radii < 1 µm) Position (→ r, φ coordinate) and time of arrival (→ z coordinate along cylinder axis) of primary electrons measured in end plates, allowing 3-dimensional track reconstruction Analogue anode wire signal provides information on specific energy loss, i.e. particle identification 59/74  No constructional elements other than counting gas, i.e. optimal for minimizing multiple scattering and photon conversions Typical counting gas Argon:methane (90:10) Use of additional gate to avoid effect of strong space charge from positive ions drifting back to central electrode: - gate normally closed, only opened for short time after external event trigger and then closed again - electrons cannot enter amplification region without trigger - after triggered events, positive ions cannot drift back into detector volume ALEPH TPC Gate principle 60/74  TPCs can be made very large, i.e. diameter ≥ 3m, length ≥ 5m; large number of analogue readout channels, i.e. #anode wires ≈ 5000 and #cathode pads ≈ 50000 → several hundred samples can be obtained per track and therefore excellent determination of curvature and energy loss, i.e. very good for particle identification Drawback of TPCs is the large electron drift time, up to 40µs (for 2m drift path), and the analogue readout requiring several microseconds Typical spatial resolution σz = 1 mm and σr,ϕ = 160 µm; z-coordinate determination requires accurate knowledge of drift velocity (calibration using UV- laser induced ionization tracks) TPCs can be operated also with liquid noble gases, e.g. liquid-argon TPCs: - acts like an electronic version of bubble chambers, with possibility of 3-dim. event reconstruction; resolution is ~100 µm - can serve as calorimeter at the same time - permanently sensitive - can intrinsically provide trigger signal from scintillation light produced in liquid noble gas - require ultrapure argon and high-performance low-noise preamplifiers (no gas amplification occurs in the counting medium) 61/74 ALEPH TPC @ LEP/CERN 62/74 Scintillating Fiber Tracker Stack of scintillating fibers, readout individually, used a tracking detector; can also be thin capillaries filled with liquid scintillator Spatial resolution (≤ 100 µm) can exceed that of drift chambers; with short scintillation flash decay time even possible alternative to gas-discharge detectors that are slow to due slow electron drift Fiber diameters from ~mm down to 60µm; requires photosensitive readout (multi-anode PMTs, CCD cameras, silicon APD) → experiments with high particle rates requiring high time and spatial resolution 63/74 Scintillating fiber tracker @ Japan Proton Accelerator Research Complex (J-PARC) for E10 & E22 experiments 64/74 Semiconductor Track Detectors width of an Gaussian distribution equivalent to a “ flat” probability distribution with 1mm pitch 65/74 Hybrid Pixel Detectors 66/74 CMS@LHC/CERN: fully silicon Tracker   θ  Pseudo-rapidity: η ≡ − ln  tan  2      67/74  Inner pixel detector (1 m2 active area) Outer strip detector (200 m2) Operated in a 4T magnetic field multiple scattering due to the large amount of material (0.5 X0 on average) Single hit resolution: 8 - 64µm Transverse momentum resolution: 0.7 - 5% Large number of hits per track (The error bars reflect the RMS of the distribution for many tracks with smeared primary vertices in the given η range) 68/74 ATLAS@LHC/CERN: Inner Detector Pixel Detectors Barrel and Forward Semiconductor Trackers (SCT): silicon microstrips Transition Radiation Tracker (TRT): straw tubes 69/74 ATLAS@LHC/CERN: Pixel Detectors 3 barrel layers + 3 disks pixels of 50µmx400µm 280 µm thick Total of 2.2M pixels, i.e. largest assembly so far space resolution: ~8-14 µm 70/74 Comparing ATLAS / CMS barrel tracking Charge sharing used to determine where charged particle passed through the detector; in this way one can reach spatial resolution much smaller than strip or pixel size 71/74 Muon momentum resolution (expected) ATLAS CMS Transverse momentum resolution degrades with particle momentum 72/74 ZEUS@HERA/DESY-Hamburg: Silicon Micro-vertex Detector (1992-2007) 3 layers (barrel) + 4 wheels (forward) silicon thickness: 330µm high resistivity n-type silicon with readout p+ strips, AC coupled to the readout electronics readout pitch: 120µm space resolution: expected = 120/√12 = 8.3µm measured = 7.5 µm → improvement obtained adding five intermediate p+ strips between two readout strips by capacitive charge division 73/74 BaBar@SLAC Stanford: Silicon Vertex Tracker (1999-2008) 5 layers two-sided silicon strip 300µm thick pitch: 50-200 µm space resolution: 10-30 µm 74/74