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Detectors

Explore the fascinating world of particle accelerators and their role in breaking apart and creating particles. Learn about various accelerator types, from electrostatic accelerators to synchrotrons, and discover the future possibilities of accelerator technology.

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Detectors

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  1. Detectors 1. Accelerators 2. Particle detectors overview 3. Tracking detectors

  2. 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

  3. 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

  4. 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

  5. Basic accelerator physics • Lorentz Force: F = qE + q(vB) • magnetic force: perpendicular to velocity, no acceleration (changes direction) • electric force: acceleration

  6. 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

  7. 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

  8. 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

  9. Surfing the electromagnetic wave

  10. 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)

  11. 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 D-electrodes 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)

  12. 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

  13. 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

  14. Hadron vs electron colliders

  15. Large Electron-Positron collider • Location: CERN (Geneva, Switzerland) • accelerated particles: electrons and positrons • beam energy: 45104 GeV, beam current: 8 mA • the ring radius: 4.5 km • years of operation: 19892000

  16. 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

  17. 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

  18. Future of accelerators • International Linear Collider: 0.53 TeV • awaiting directions from LHC findings • political decision of location • Very Large Hadron Collider (magnet development ?): 40200 TeV • Muon Collider (source ?) 0.54 TeV • lepton collider without synchrotron radiation • capable of producing many more Higgs particles compared to an e+e collider

  19. 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…

  20. Detectors 1. Accelerators 2. Particle detectors overview 3. Tracking detectors

  21. 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

  22. 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

  23. Natural particle detectors • A very common particle detector: the eye • detected particles: photons • sensitivity: high (single photons) • spatial resolution: decent • dynamic range: excellent (11014) • energy range: limited (visible light) • energy discrimination: good • speed: modest (~10 Hz, including processing)

  24. 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

  25. 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)

  26. 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

  27. 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 super-saturated state)

  28. 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)

  29. 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

  30. 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

  31. 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

  32. 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

  33. 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

  34. 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

  35. 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

  36. 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”)

  37. Generic HEP detector

  38. 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

  39. D: fairly typical HEP detector

  40. D: tracking system (1) • Vertex detector: Silicon Microstrip Tracker • four layers of silicon detectors intercepted with twelve disks + (recent addition) Layer 0

  41. D: tracking system (2) • Outer tracking detector: Central Fiber Tracker • sixteen double layers of scintillating fibers

  42. D: calorimeter • Liquid argon / uranium calorimeter, consisting of central and two end calorimeters

  43. D: outer muon system • The outermost part of the detector, surrounds the whole thing • Proportional Drift Tubes, Mini Drift Tubes • Central (Forward) muon SCintillators

  44. 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, …

  45. 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

  46. Detectors 1. Accelerators 2. Particle detectors overview 3. Tracking detectors

  47. 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)

  48. 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”)

  49. Ionization as a function of energy • Ionization probability gas dependant • General features: • threshold (~20 eV) • fast turn on • maximum (~100 eV) • soft decline eV

  50. 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 2.71019 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

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