Particle Detectors Summer Student Lectures 2007 Werner Riegler, CERN, werner.riegler@cern.ch History of Instrumentation ↔ History of Particle Physics � The ‘Real’ World of Particles � Interaction of Particles with Matter, Tracking detectors � Resistive Plate Chambers, Calorimeters, Particle Identification � Detector Systems � W. Riegler/CERN 1
Gas Detectors with internal Electron Multiplication Resistive Plate Chamber Parallel Plate Avalanche Chamber (PPAC) W. Riegler/CERN 2
Resistive Plate Chambers (RPCs) Keuffel ‘Spark’ Counter: High voltage between two metal plates. Charged particle leaves a trail of electrons and ions in the gap and causes a discharge (Spark). � Excellent Time Resolution(<100ps). Discharged electrodes must be recharged � Dead time of several ms. Parallel Plate Avalanche Chambers (PPAC): At more moderate electric fields the primary charges produce avalanches without forming a conducting channel between the electrodes. No Spark � induced signal on the electrodes. Higher rate capability. However, the smalles imperfections on the metal surface cause sparcs and breakdown. � Very small (few cm 2 ) and unstable devices. In a wire chamber, the high electric field (100- 300kV/cm) that produces the avalanche exists only close to the wire. The fields on the cathode planes area rather small 1-5kV/cm. W. Riegler/CERN 3
Resistive Plate Chambers (RPCs) � Place resistive plates in front of the metal electrodes. No spark can develop because the resistivity together with the capacitance (tau ~ e* ρ ) will only allow a very localized ‘discharge’. The rest of the entire surface stays completely unaffected. � Large area detectors are possible ! Resistive plates from Bakelite ( ρ = 10 10 -10 12 Ω cm) or window glass ( ρ = 10 12 -10 13 Ω cm). Gas gap: 0.25-2mm. Elektric Fields 50-100kV/cm. Time resolutions: 50ps (100kV/cm), 1ns(50kV/cm) Application: Trigger Detectors, Time of Flight (TOF) Resistivity limits the rate capability: Time to remove avalanche charge from the surface of the resistive plate is (tau ~ e* ρ ) = ms to s. Rate limit of kHz/cm 2 for 10 10 Ω cm. W. Riegler/CERN 4
ALICE TOF RPCs 130 mm active area 70 mm Several gaps to increase efficiency. Stack of glass plates. Small gap for good time resolution: 0.25mm. Fishing lines as high precision honeycomb panel spacers ! Flat cable connector (10 mm thick) Differential signal sent from PCB with cathode strip to interface card Large TOF systems with 50ps time pickup pads resolution made from window glass external glass plates and fishing lines ! 0.55 mm thick internal glass plates Before RPCs � Scintillators with (0.4 mm thick) very special photomultipliers – very PCB with expensive. Very large systems are anode pickup pads unafordable. Mylar film (250 micron thick) 5 gas gaps of 250 micron PCB with cathode M5 nylon screw to hold pickup pads fishing-line spacer Honeycomb panel (10 mm thick) connection to bring cathode signal Silicon sealing compound to central read-out PCB W. Riegler/CERN 5
Elektro-Magnetic Interaction of Charged Particles with Matter Classical QM 1) Energy Loss by Excitation and Ionization 2) Energy Loss by Bremsstrahlung 3) Cherekov Radiation and 4) Transition Radiation are only minor contributions to the energy loss, they are however important effects for particle identification. W. Riegler/CERN 6
Bremsstrahlung, semi-classical: A charged particle of mass M and charge q=Z 1 e is deflected by a nucleus of Charge Ze. Because of the acceleration the particle radiated EM waves � energy loss. Coulomb-Scattering (Rutherford Scattering) describes the deflection of the particle. Maxwell’s Equations describe the radiated energy for a given momentum transfer. � dE/dx W. Riegler/CERN 7
Proportional to Z 2 /A of the Material. 4 of the incoming Proportional to Z 1 particle. Proportional zu ρ of the particle. Proportional 1/M 2 of the incoming particle. Proportional to the Energy of the Incoming particle � E(x)=Exp(-x/X 0 ) – ‘Radiation Length’ 4 Z 2 ) X 0 ∝ M 2 A/ ( ρ Z 1 X 0 : Distance where the Energy E 0 of the incoming particle decreases E 0 Exp(-1)=0.37E 0 . W. Riegler/CERN 8
Critical Energy For the muon, the second lightest particle after the electron, the critical energy is at 400GeV. The EM Bremsstrahlung is therefore only relevant for electrons at energies of past and present detectors. Elektron Momentum 5 50 500 MeV/c Critical Energy: If dE/dx (Ionization) = dE/dx (Bremsstrahlung) Myon in Copper: p ≈ 400GeV Electron in Copper: p ≈ 20MeV W. Riegler/CERN 9
For E γ >>m e c 2 =0.5MeV : λ = 9/7X 0 Average distance a high energy photon has to travel before it converts into an e + e - pair is equal to 9/7 of the distance that a high energy electron has to travel before reducing it’s energy from E 0 to E 0 *Exp(-1) by photon radiation. W. Riegler/CERN 10
Electro-Magnetic Shower of High Energy Electrons and Photons W. Riegler/CERN 11
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Electro-Magnetic Shower of High Energy Electrons and Photons W. Riegler/CERN 13
Calorimetry: Energy Measurement by total Absorption of Particles W. Riegler/CERN 14
Calorimetry: Energy Measurement by total Absorption of Particles Liquid Nobel Gases (Nobel Liquids) Scintillating Crystals, Plastic Scintillators (sampling) W. Riegler/CERN 15
EM Calorimetry Crystals Noble Liquids W. Riegler/CERN 16
EM Calorimetry Direct CP violation experiments NA48, KTeV � Excellent EM Calorimetry for π 0 measurement. X 0 = 4.7cm ρ = 2.41 g/cm 3 ρ M = 5.5cm X 0 = 1.85cm ρ = 4.51 g/cm 3 ρ M = 3.5cm W. Riegler/CERN 17
18 Hadronic Showers W. Riegler/CERN
19 Hadron Calorimeters W. Riegler/CERN
20 Sampling Calorimeters W. Riegler/CERN
21 W. Riegler/CERN
Particle Identification W. Riegler/CERN 22
dE/dx Measured energy loss ‘average’ energy loss In certain momentum ranges, particles can be identified by measuring the energy loss. W. Riegler/CERN 23
Time of Flight (TOF) NA49 combined particle ID: TOF + dE/dx (TPC) W. Riegler/CERN 24
Cherenkov Radiation W. Riegler/CERN 25
Ring Imaging Cherenkov Detector W. Riegler/CERN 26
27 LHCb RICH W. Riegler/CERN
Transition Radiation W. Riegler/CERN 28
Detector Systems, Selected Experiments ALICE: Heavy Ion Experiment at CERN Donut: Neutrino Experiment at Fermilab CNGS: Long Baseline Neutrino Experiment CERN/Gran Sasso Amanda: Neutrino Experiment at the Southpole AMS: Particle Physics Experiment in Space Thanks to Heinrich Schindler W. Riegler/CERN 29
ALICE A heavy Ion Experiment at the LHC. W. Riegler/CERN 30
31 ALICE W. Riegler/CERN
ALICE Alice uses ~ all known techniques! π /K TPC + ITS K/p (dE/dx) e / π π /K e / π TOF K/p π /K HMPID K/p (RICH) 0 1 2 3 4 5 p (GeV/c) π /K TPC (rel. rise) π /K/p K/p TRD e / π PHOS γ / π 0 1 10 100 p (GeV/c) W. Riegler/CERN 32
ALICE TPC LHC: dN ch /dy = 2000 - 4000 ALICE 'worst case' scenario: NA49 dN ch /dy = 8000 STAR STAR 33
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AMANDA A ntarctic M uon A nd N eutrino D etector A rray W. Riegler/CERN 35
AMANDA South Pole W. Riegler/CERN 36
AMANDA W. Riegler/CERN 37
AMANDA Look for upwards going Muons from Neutrino Interactions. Cherekov Light propagating through the ice. � Find neutrino point sources in the universe ! W. Riegler/CERN 38
AMANDA Event Display Up to now: No significant point sources but just neutrinos from cosmic ray interactions in the atmosphere were found . � Ice Cube for more statistics ! W. Riegler/CERN 39
DONUT Detector for Observation of Tau Neutrino. W. Riegler/CERN 40
41 DONUT W. Riegler/CERN
DONUT W. Riegler/CERN 42
DONUT Tau lepton has very short lifetime and is therefore identified by the characteristic ‘kink’ on the decay point. W. Riegler/CERN 43
DONUT One of the 4 tau candidates. Emulsion resolution 0.5um ! W. Riegler/CERN 44
CERN Neutrino Gran Sasso (CNGS) W. Riegler/CERN 45
CNGS If neutrinos have mass: ν e Muon neutrinos produced at CERN. See if tau neutrinos arrive in Italy. ν μ ν τ W. Riegler/CERN 46
CNGS Project CNGS (CERN Neutrino Gran Sasso) � A long base-line neutrino beam facility (732km) � send ν μ beam produced at CERN � detect ν τ appearance in OPERA experiment at Gran Sasso � direct proof of ν μ - ν τ oscillation (appearance experiment) W. Riegler/CERN 47
48 CNGS W. Riegler/CERN
Neutrinos at CNGS: Some Numbers For 1 day of CNGS operation, we expect: protons on target 2 x 10 17 pions / kaons at entrance to decay tunnel 3 x 10 17 ν μ in direction of Gran Sasso 10 17 ν μ in 100 m 2 at Gran Sasso 3 x 10 12 ν μ events per day in OPERA ≈ 25 per day ν τ events (from oscillation) ≈ 2 per year W. Riegler/CERN 49
CNGS Layout 800m 100m 1000m 26m 67m vacuum p + C → (interactions) → π + , K + → (decay in flight) → μ + + ν μ W. Riegler/CERN 50
51 CNGS W. Riegler/CERN
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