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Detectors 1. Accelerators 2. Particle detectors overview 3. Tracking detectors Why do we accelerate particles ? (1) To take existing objects apart 1803 J. Dalton’s indivisible atom atoms of one element can combine with atoms of other element to make compounds, e.g. water is made of oxygen and hydrogen (OH) 1896 M. & P. Curie find atoms decay 1897 J. J. Thomson discovers electron 1906 E. Rutherford: gold foil experiment Physicists break particles by shooting other particles on them Why do we accelerate particles ? (2) To create new particles 1905 A. Einstein: energy is matter E=mc2 1930 P. Dirac: math problem predicts antimatter 1930 C. Anderson: discovers positron 1935 H. Yukawa: nuclear forces (forces between protons and neutrons in nuclei) require pion 1936 C. Anderson: discovers pion muon First experiments used cosmic rays that are accelerated for us by the Universe are still of interest as a source of extremely energetic particles not available in laboratories Generating particles Before accelerating particles, one has to create them electrons: cathode ray tube (think your TV) protons: cathode ray tube filled with hydrogen It’s more complicated for other particles (e.g. antiprotons), but the main principle remains the same Basic accelerator physics Lorentz Force: F = qE + q(vB) magnetic force: perpendicular to velocity, no acceleration (changes direction) electric force: acceleration Accelerators: Cockroft-Walton A (series of) voltage gap(s) Maximum energy of a single gap is 200 kV, limited by discharge CW accelerator at Fermilab: 750 kV Accelerators: Van de Graaf Van de Graaf generator: an electrostatic machine which uses a moving belt to accumulate very high voltages on a hollow metal globe 1: metallic sphere 2: electrode connected to 1 3: upper roller 4: belt (positive side) 5: belt (negative side) 6: lower roller 7: lower electrode (ground) 8: spherical device, used to discharge the main sphere 9: spark Surfing the electromagnetic wave Charged particles ride the EM wave create standing wave use a radio frequency cavity make particles arrive on time Self-regulating: slow particle larger push fast particle small push Surfing the electromagnetic wave How to create a standing wave ? Klystron (S. & R. Varian) electrons flow into cavity, excite eigen modes creates standing electromagnetic waves A similar device (magnetron) found in your microwave oven 325 MHz Klystron for Proton Driver Linac (Fermilab) Cyclotron 1929 E.O. Lawrence The physics: centripetal force mv2/r = Bqv Particles follow a spiral in a constant magnetic field A high frequency alternating voltage applied between Delectrodes causes acceleration as particles cross the gap Advantages: compact design (compared to linear accelerators), continuous stream of particles Limitations: synchronization lost as particle velocity approaches the speed of light the world largest cyclotron at TRIUMF (520 MeV protons) Synchrotron The idea: both magnetic field strength and electric field frequency are synchronized with the traveling particle beam particle trajectories confined to a thin vacuum beamline no large magnets, expandable synchrotron radiation limits its use for electrons Currently, accelerators of this type provide highest particle energies in the world Summary on accelerator types Electrostatic accelerators acceleration tube: breakdown at 200 keV Cockroft-Walton: improves to 800 keV AC driven accelerators linear: cavity design and length critical circular accelerators: cyclotron: big magnet, non-relativistic synchrotron: vacuum beamline, expandable, small magnets and cavities synchrotron radiation large for light particles Hadron vs electron colliders electron proton Point-like particle yes no Uses full beam energy yes no Transverse energy sum zero zero Longitudinal energy sum zero non-zero Synchrotron radiation large small Large Electron-Positron collider Location: CERN (Geneva, Switzerland) accelerated particles: electrons and positrons beam energy: 45104 GeV, beam current: 8 mA the ring radius: 4.5 km years of operation: 19892000 Tevatron Location: Fermilab (Batavia, IL) accelerated particles: protons and anti-protons beam energy: 1 TeV, beam current: 1 mA the ring radius: 1 km in operation since 1983 Large Hadron Collider Location: CERN (Geneva, Switzerland) accelerated particles: protons beam energy: 7 TeV, beam current: 0.5 A the ring radius: 4.5 km scheduled start: 2007 Future of accelerators International Linear Collider: 0.53 TeV awaiting directions from LHC findings political decision of location Very Large Hadron Collider (magnet development ?): 40200 TeV Muon Collider (source ?) 0.54 TeV lepton collider without synchrotron radiation capable of producing many more Higgs particles compared to an e+e collider Conclusions Motivation for particle acceleration understand matter around us create new particles Particle accelerator types electrostatic: limited energy AC driven: linear or circular Modern accelerators TeVatron, LHC accelerators to come: ILC, VLHC, muon collider… Detectors 1. Accelerators 2. Particle detectors overview 3. Tracking detectors Detectors and particle physics detectors allow one to detect particles experimentalists study their behavior new particles are found by direct observation or by analyzing their decay products theorists predict behavior of (new) particles experimentalists design the particle detectors Overview of particle detectors What do particle detectors measure ? spatial location trajectory in an EM field momentum distance between production and decay point lifetime energy momentum + energy mass flight times momentum/energy + flight time mass Natural particle detectors A very common particle detector: the eye detected particles: photons sensitivity: high (single photons) spatial resolution: decent dynamic range: excellent (11014) energy range: limited (visible light) energy discrimination: good speed: modest (~10 Hz, including processing) Photographic paper 1895 W. C. Röntgen: sensitivity to high energy photons (X-rays) invisible to the eye working medium: emulsion Properties: detected particles: photons sensitivity: good spatial resolution: very good dynamic range: good no online recording no speed resolution The Geiger counter 1908 H. Geiger passing charge particles ionize the gas ions (electrons) drift towards cathode (anode) cause an electric pulse, can be heard in a speaker Properties: detected particles: charged particles (electrons, ,…) sensitivity: single particles spatial resolution: none (detector size) – can be fixed dynamic range: none – can be fixed speed: high (determined by charge drift velocity) The cloud chamber 1911 C. T. R. Wilson (1927 Nobel Prize) the first tracking detector (tracking=many spatial measurements per particle) Principle of operation: an air volume is saturated with water vapor pressure lowered to generate super-saturated air charge particles cause saturation of vapor into small droplets can be observed as a “track” photographs allow longer inspection The cloud chamber Properties: detected particles: charged particles (electrons, ,…) sensitivity: single particles spatial resolution: excellent dynamic range: good as particle slows down, droplets occur closer to each other if placed inside a magnet, can observe curled trajectories speed: limited (need time to recover the supersaturated state) Photographic emulsions Rarely used in modern experiments due to principal restrictions: cannot be read out electronically used to need a lot of technicians looking at photographs by eye – inefficient, boring, and error prone today using pattern recognition software (think OCR) cannot be used online One advantage is excellent spatial resolution (<1 m) Were used in the -neutrino discovery (DONUT, 2000) Modern detector types Tracking detectors detect charged particles principle of operation: ionization two basic types: gas and solid Scintillators sensitive to single particles very fast, useful for online applications Calorimeters measure particle energy usually measure energy of a bunch of particles (“jet”) modest spatial resolution Particle identification systems recognize electrons, charged pions, charged kaons, protons Tracking detectors A charged track ionizes the gas 10—40 primary ion-electron paris multiplication 3—4 due to secondary ionization typical amplifier noise 1000 e— the initial signal is too weak to be effectively detected ! as electrons travel towards cathode, their velocity increases electrons cause an avalanche of ionization (exponential increase) The same principle (ionization + avalanche) works for solid state tracking detectors dense medium large ionization more compact put closer to the interaction point very good spatial resolution Calorimetry The idea: measure energy by total absorption also measure location the method is destructive: particle is stopped detector response proportional to particle energy As particles traverse material, they interact producing a bunch of secondary particles (“shower”) the shower particles undergo ionization (same principle as for tracking detectors) It works for all particles: charged and neutral Electromagnetic calorimeters Electromagnetic showers occur due to Bremsstrahlung: similar to synchrotron radiation, particles deflected by atomic EM fields pair production: in the presence of atomic field, a photon can produce an electron-positron pair excitation of electrons in atoms Typical materials for EM calorimeters: large charge atoms, organic materials important parameter: radiation length Hadronic calorimeters In addition to EM showers, hadrons (pions, protons, kaons) produce hadronic showers due to strong interaction with nuclei Typical materials: dense, large atomic weight (uranium, lead) important parameter: nuclear interaction length In hadron shower, also creating non detectable particles (neutrinos, soft photons) large fluctuation and limited energy resolution Muon detection Muons are charged particles, so using tracking detectors to detect them Calorimetry does not work – muons only leave small energy in the calorimeter (said to be “minimum ionization particles”) Muons are detected outside calorimeters and additional shielding, where all other particles (except neutrinos) have already been stopped As this is far away from the interaction point, use gas detectors Detection of neutrinos In dedicated neutrino experiments, rely on their interaction with material interaction probability extremely low need huge volumes of working medium In accelerator experiments, detecting neutrinos is impractical – rely on momentum conservation electron colliders: all three momentum components are conserved hadron colliders: the initial momentum component along the (anti)proton beam direction is unknown Multipurpose detectors Today people usually combine several types of various detectors in a single apparatus goal: provide measurement of a variety of particle characteristics (energy, momentum, flight time) for a variety of particle types (electrons, photons, pions, protons) in (almost) all possible directions also include “triggering system” (fast recognition of interesting events) and “data acquisition” (collection and recording of selected measurements) Confusingly enough, these setups are also called detectors (and groups of individual detecting elements of the same type are called “detector subsystems”) Generic HEP detector D detector at Fermilab D detector is one of two large multipurpose detectors at Fermilab (another one is CDF) name = one of six intersection points D: fairly typical HEP detector D: tracking system (1) Vertex detector: Silicon Microstrip Tracker four layers of silicon detectors intercepted with twelve disks + (recent addition) Layer 0 D: tracking system (2) Outer tracking detector: Central Fiber Tracker sixteen double layers of scintillating fibers D: calorimeter Liquid argon / uranium calorimeter, consisting of central and two end calorimeters D: outer muon system The outermost part of the detector, surrounds the whole thing Proportional Drift Tubes, Mini Drift Tubes Central (Forward) muon SCintillators D: other elements Magnet: a central solenoid magnet (2 T) and outer toroid magnet Luminosity scintillating counters Central and forward preshower Forward proton detector (Roman pots) Data acquisition, trigger system, … Conclusions Particle detectors follow simple principles detectors interact with particles most interactions are electromagnetic imperfect by definition but have gotten pretty good crucial to figure out which detector goes where Three main ideas track charged particles and then stop them stop neutral particles finally find the muons which are left Detectors 1. Accelerators 2. Particle detectors overview 3. Tracking detectors Gas detectors As a charged particle crosses a gas volume, it creates ionization electrons get kicked out of atoms the rest of atom becomes electrically charged (ion) In absence of external field, ions and electrons recombine back to neutral atoms electrons drift to anode ions drift to cathode E = V/r ln(b/a) Ionization Affected by many factors gas temperature gas pressure electric field gas composition Important parameters: ionization potential mean free path Some gases eat up electrons (“quenchers”) Ionization as a function of energy Ionization probability gas dependant General features: threshold (~20 eV) fast turn on maximum (~100 eV) soft decline eV Mean free path Average distance an electron travels before it hits an atom – determined by gas density At ambient pressure (1013 hPa), air density is 2.71019 molecules/ccm, and mean free path is 68 m At high vacuum (10—3…10—7 hPa), mean free path is 0.1…1000 m What happens after ionization ? After collision, ions (electrons) thermalize and travel until neutralized through electron (ion), wall, negative ion (other molecule) Mean free path for electrons ~4 times longer than for ions Ions diffuse slowly, electrons diffuse quickly Diffusion velocity depends on gas Avalanche Steps of an avalanche: a primary electron proceeds towards the anode, experiencing ionizing collisions due to the lateral diffusion, a drop-like avalanche, surrounding the wire, develops electrons are collected during ~1 ns a cloud of positive ions slowly migrates towards the cathode Ionization chamber Low voltage, no secondary ionization – just collect ions example: smoke detector radiation source (Am-241) emits -particles they pass through ionization chamber, creating current smoke absorbs -particles and interrupts current Proportional counter Higher voltage, tuned to provide proportional regime: each avalanche is created independently from others total amount of charge created remains proportional to the amount of charge liberated in the original event, which in turn is proportional to the particle’s kinetic energy Spark chamber Device similar to Geiger counter Ionizing particles produce sparks along its way that can be photographed and used later for reconstruction of tracks My diploma work was done on the ITEP’s 3m magnet spectrometer equipped with spark chambers Regimes in a tracking chamber Gas tracking detectors: summary detector voltage avalanches regime ionization chamber low no single ion collection proportional counter medium isolated proportional Geiger-Müller counter high maximal saturated Multi Wire Proportional Chamber 1968 G. Charpak (1992 Noble Prize) the idea: make a proportional counter with a lot of anodes placed between two cathode planes by looking at which wires were fired, can determine position of the particle if the proportional mode is used, can determine particle’s energy + improve position resolution (by interpolation) drift chambers: measure time of arrival of the electron avalanche improve position resolution + provide a timing reference point MWPC electric field Homogeneous field away from anode wires Field near wires very sensitive to their position from G. Charpak’s Noble lecture MWPC design Constraints precise position measurements require precise and small wire spacing homogeneous fields require small wire spacing large fields require thin wires geometric tolerances cause gain variations Geometry and problems required precision: sub millimeter long chambers need strong wires (W/Au plated) and high tension to minimize sagging Choice of gas It’s a magic low working voltage high gain operation good proportionality high rate capability long lifetime fast recovery price … Operation conditions Pressure: slightly above atmospheric avoid incoming gas “pollution” a large tracker is not really air tight not too high (difficult to maintain) Temperature: slightly lower than room t. avoid large temperature gradients affected by environment (e.g. cooling of nearby systems) Limitations of chambers High occupancy: OK used in Alice (heavy ion collisions at LHC) Radiation hardness tough but manageable (need gas flow) Speed is a problem for LHC applications (25 ns bunch crossing) ion drift is limiting factor can be addressed with special technologies (GEM) Time Projection Chamber (RHIC) Brookhaven Nat’l Lab, Relativistic Heavy Ion Collider Shown: Gold-Gold collision Solid state detectors Basic operation principle same as gas detectors gas liquid solid Density low moderate high Atomic number low moderate moderate Ionization energy moderate moderate low Signal speed moderate moderate high Silicon detectors Solid state tracking detectors: semiconductor diodes with reverse bias normally there is no current (except very low “dark current”) a charged particle creates a track of carriers (electron-hole pairs) along its way charge pulse Why silicon ? Low band gap width: 1.12 eV (large number of charge carriers / unit energy loss) Energy to create an e/h pair: 3.6 eV (an order of magnitude smaller than ionization energy for gases) high carrier yield low Poisson noise no gain stage required better energy resolution and high signal Why silicon ? (cont’d) High density and atomic number reduced range of secondary particles can build thin detectors better spatial resolution High carrier mobility typical charge collection times <30 ns no slow component (ions) Excellent mechanical rigidity Industrial fabrication techniques Detector and electronics can be integrated Problems Cost proportional to area covered most of the cost is moving to read out channels Material budget for complex detectors can be as large as ~1—2 radiation lengths affects calorimeters behind the detector affects tracking accuracy (multiple scattering) Typically need cooling to reduce leakage current (thermal energy = 1/40 eV) Radiation hardness What is it ? particles damage silicon crystal structure band gap decreases leakage currents increase gain drops detector looses efficiency and precision What to do ? exchange detectors ATLAS: replace inner detector after 3 yrs of operation switch to radiation hard technology (e.g. diamonds) Diode strip detectors Idea (1980’s): divide the large-area diode into many small strip-like regions and read them out separately Typical strip pitch p = 20—few hundred m Position measurement precision: digital readout: = p/12 analog readout: = p/(S/N) (S = signal, N = noise) -function Let a particle pass the detector between two strips (i) and (i+1) at coordinate x = xi…xi+p If strip (i) collects charge qi, and strip (i+1) collects charge qi+1, (x) = qi/(qi+qi+1) ideally, (x) = 1, x<xi+p/2, and (x) = 0, x>xi+p/2 in practice, it’s not true: finite charge cloud size (~5 m) charge capacitance between strips non-uniform electric field Lorentz shift If a detector is placed in magnetic field (parallel to its strips), charge careers are deflected as they drift towards the strips introduces systematic shift of the measured position signal gets spread between several strips increases cluster sharing (bad) with analog readout, improves spatial resolution (good) Double sided readout detectors Idea: use both types of carriers to make two position measurements for the same amount of material n-side charge Usually cross strips 2-dim measurement From charge correlation can resolve ambiguities p-side charge Pixel detectors Provide 3-dim points with very high precision main issue is readout can read out individual pixels or entire rows/columns Electrodes are close ! low full bias low collection distance no charge spreading fast charge sweep out Pixel vs strip detector operation SiO2 +ve -ve +ve -ve -ve -ve p+ h+ h+ e- n E W3D pixel detector E e- n+ +ve strip detector W2D Pixel detector at ATLAS Conclusions Tracking detectors detect charged particles measure arrival time and charge deposition derive 3 dimensional location and energy Design inner detectors: silicon (strip/pixel), highest track density resolution (tens of m) outer detectors: gas detectors, lower resolution (hundreds of m)