Particle Detectors. Summer Student Lectures 2007 Werner Riegler, CERN, History of Instrumentation History of Particle Physics

Transcription

1 Particle Detectors Summer Student Lectures 2007 Werner Riegler, CERN, History of Instrumentation History of Particle Physics The Real World of Particles Interaction of Particles with Matter, Tracking detectors Photon Detection, Calorimeters, Particle Identification Detector Systems 1

2 Detectors based on Ionization Gas Detectors: Transport of Electrons and Ions in Gases Wire Chambers Drift Chambers Time Projection Chambers Solid State Detectors Transport of Electrons and Holes in Solids Si- Detectors Diamond Detectors Gas Detectors 2

3 Gas Detectors with internal Electron Multiplication Principle: At sufficiently high electric fields (100kV/cm) the electrons gain energy in excess of the ionization energy secondary ionzation etc. etc. Elektron Multiplication: dn = N α dx α first Townsend Coefficient N(x) = N 0 exp (αx) α= α(e), N/ N 0 = A (Amplification, Gas Gain) N(x)=N 0 exp ( (E)dE ) In addition the gas atoms are excited emmission of UV photons can ionize themselves photoelectrons NAγ photoeletrons NA 2 γ electrons NA 2 γ 2 photoelectrons NA 3 γ 2 electrons For finite gas gain: γ < A -1, γ second Townsend coefficient Gas Detectors 3

4 Wire Chamber: Electron Avalanche Wire with radius (10-25 m) in a tube of radius b (1-3cm): Electric field close to a thin wire ( kV/cm). E.g. V 0 =1000V, a=10 m, b=10mm, E(a)=150kV/cm Electric field is sufficient to accelerate electrons to energies which are sufficient to produce secondary ionization electron avalanche signal. a b b Wire Gas Detectors 4

5 Gas Detectors with internal Electron Multiplication From L. Ropelewski 5

6 Wire Chamber: Electron Avalanches on the Wire Proportional region: A Semi proportional region: A (space charge effect) Saturation region: A >10 6 Independent from the number of primary electrons. Streamer region: A >10 7 Avalanche along the particle track. Limited Geiger region: Avalanche propagated by UV photons. Geiger region: A 10 9 Avalanche along the entire wire. Gas Detectors 6

7 Wire Chamber: Signals from Electron Avalanches The electron avalanche happens very close to the wire. First multiplication only around R =2x wire radius. Electrons are moving to the wire surface very quickly (<<1ns). Ions are difting towards the tube wall (typically 100 s. ) The signal is characterized by a very fast spike from the electrons and a long Ion tail. The total charge induced by the electrons, i.e. the charge of the current spike due to the short electron movement amounts to 1-2% of the total induced charge. Gas Detectors 7

8 Detectors with Electron Multiplication Rossi 1930: Coincidence circuit for n tubes Cosmic ray telescope 1934 Geiger Mode Position resolution is determined by the size of the tubes. Signal was directly fed into an electronic tube. Gas Detectors 8

9 Charpak et. al. 1968, Multi Wire Proportional Chamber Classic geometry (Crossection) : One plane of thin sense wires is placed between two parallel plates. Typical dimensions: Wire distance 2-5mm, distance between cathode planes ~10mm. Electrons (v 5cm/ s) are being collectes within in 100ns. The ion tail can be eliminated by electroniscs filters pulses 100ns typically can be reached. For 10% occupancy every s one pulse 1MHz/wire rate capabiliy! Gas Detectors 9

10 Charpak et. al. 1968, Multi Wire Proportional Chamber In order to eliminate the left/right ambiguities: Shift two wire chambers by half the wire pitch. For second coordinate: Another Chamber at 90 0 relative rotation Signal propagation to the two ends of the tube. Pulse height measurement on both ends of the wire. Because of resisitvity of the wire, both ends see different charge. Segmenting of the cathode into strips or pads: The movement of the charges induces a signal on the wire AND the cathode. By segmengting and charge interpolation resolutions of 50 m can be achieved. Gas Detectors 10

11 Multi Wire Proportional Chamber Cathode strip: Width (1 ) of the charge distribution DIstance Center of gravity defines the particle trajectory. Avalanche (a) (b) Anode wire 1.07 mm Cathode s trips 0.25 mm C 1 C 1 C 1 C 1 C 1 C mm C 2 C 2 C 2 C 2 Gas Detectors 11

12 Drift Chambers 1970: Amplifier: t=t E Scintillator: t=0 In an alternating sequence of wires with different potentials one finds an electric field between the sense wires and field wires. The electrons are moving to the sense wires and produce an avalanche which induces a signal that is read out by electronics. The time between the passage of the particle and the arrival of the electrons at the wire is measured. The drift time T is a measure of the position of the particle! By measuring the drift time, the wire distance can be reduced (compared to the Multi Wire Proportional Chamber) save electronics channels! Gas Detectors 12

13 Drift Chambers, typical Geometries Electric Field 1kV/cm W. Klempt, Detection of Particles with Wire Chambers, Bari 04 Gas Detectors 13

14 The Geiger counter reloaded: Drift Tube ATLAS MDT R(tube) =15mm Calibrated Radius-Time correlation Primary electrons are drifting to the wire. Electron avalanche at the wire. The measured drift time is converted to a radius by a (calibrated) radius-time correlation. Many of these circles define the particle track. ATLAS Muon Chambers ATLAS MDTs, 80 m per tube Gas Detectors 14

15 The Geiger counter reloaded: Drift Tube Atlas Muon Spectrometer, 44m long, from r=5 to11m Chambers 6 layers of 3cm tubes per chamber. Length of the chambers 1-6m! Position resolution: 80 m/tube, <50 m/chamber (3 bar) Maximum drift time 700ns Gas Ar/CO 2 93/7 Gas Detectors 15

16 ATLAS Muon Chamber Front-End Electronics 3.18 x 3.72 mm Single Channel Block Diagram Harvard University, Boston University 0.5 m CMOS technology 8 channel ASD + Wilkinson ADC fully differential 15ns peaking time 32mW/channel JATAG programmable Designed around in 1997, produced in 2000, today 0.17um process rapidly changing technologies. Gas Detectors 16

17 Large Drift Chambers: Central Tracking Chamber CDF Experiment 660 drift cells tilted 45 0 with respect to the particle track. Drift cell Gas Detectors 17

18 Time Projection Chamber (TPC): Gas volume with parallel E and B Field. B for momentum measurement. Positive effect: Diffusion is strongly reduced by E//B (up to a factor 5). Drift Fields V/cm. Drift times s. Distance up to 2.5m! gas volume B drift y x E z charged track wire chamber to detect projected tracks Gas Detectors 18

19 ALICE TPC: Detector Parameters Gas Ne/ CO 2 90/10% Field 400V/cm Gas gain >10 4 Position resolution = 0.2mm Diffusion: t = 250 m cm Pads inside: 4x7.5mm Pads outside: 6x15mm B-field: 0.5T Gas Detectors 19

20 ALICE TPC: Konstruktionsparameter Largest TPC: Length 5m diameter 5m Volume 88m 3 Detector area 32m 2 Channels ~ High Voltage: Cathode -100kV Material X 0 Cylinder from composit materias from airplane industry (X 0 = ~3%) Gas Detectors 20

21 ALICE TPC: Pictures of the construction Precision in z: 250 m End plates 250 m Gas Detectors Wire chamber: 40 m 21

22 ALICE : Simulation of Particle Tracks Simulation of particle tracks for a Pb Pb collision (dn/dy ~8000) Angle: Q=60 to 62º If all tracks would be shown the picture would be entirely yellow! TPC is currently under Commissioning! Gas Detectors 22

23 ALICE TPC My personal contribution: A visit inside the TPC. Gas Detectors 23

24 Detectors based on Ionization Gas detectors: Transport of Electrons and Ions in Gases Wire Chambers Drift Chambers Time Projection Chambers Solid State Detectors Transport of Electrons and Holes in Solids Si- Detectors Diamond Detectors Solid State Detectors 24

25 Solid State Detectors Originally: Solid state ionization chambers in Crystals (Diamond, Ge, CdTe ) Primary ionization from a charged particle traversing the detector moves in the applied electric field and induced a signal on the metal electrodes. Principle difficulty: Extremely good insulators are needed in order to suppress dark currents and the related fluctuations (noise) which are hiding the signal. Advantage to gas detectors: 1000x more charge/cm (density of solids 10 3 times density of gas) Ionization energy is only a few ev (up to times smaller than gas). Solid State Detectors 25

26 Typical thickness a few 100μm Diamond Detector Velocity: μ e =1800 cm 2 /Vs, μ h =1600 cm 2 /Vs, 13.1eV per e-h pair. Velocity = μe, 10kV/cm v=180 μm/ns Very fast signals of only a few ns length! Charges are trapped along their path. Charge collection efficiency approx 50%. Diamond is an extremely interesting material. The problem is that large size single crystals cannot be grown at present. The technique of chemical vapor deposition can be used to grow polycrystalline diamonds only. The boundaries between crystallites are probably responsible for incomplete charge collection in this material. Solid State Detectors 26

27 Silicon Detector Velocity: μ e =1450 cm 2 /Vs, μ h =505 cm 2 /Vs, 3.63eV per e-h pair. ~11000 e/h pairs in 100μm of silicon. However: Free charge carriers in Si: T=300 K: n = 1.45 x / cm 3 but only 33000e-/h in 300 m produced by a high energy particle. Why do we use Si as a solid state detector??? Solid State Detectors 27

28 Silicon Detector used as a Diode! p n doping n-type p-type Solid State Detectors 28

29 Si-Diode used as a Particle Detector! At the p-n junction the charges are depleted and a zone free of charge carriers is established. By applying a voltage, the depletion zone can be extended to the entire diode highly insulating layer. If an ionizing particle produced free charge carriers in the diode they drift in the electric field an produce an electric field. As silicon is the most commonly used material in the electronics industry, it has one big advantage with respect to other materials, namely highly developed technology. Solid State Detectors 29

30 Silicon Detector ca m Fully depleted zone readout capacitances SiO 2 passivation 300 m N (e-h) = /100μm Position Resolution down to ~ 5μm! Solid State Detectors 30

31 Silicon Detector Every electrode is connected to an amplifier Highly integrated readout electronics. Two dimensional readout is possible. Solid State Detectors 31

32 Picture of an CMS Si-Tracker Module Outer Barrel module Solid State Detectors 32

33 2,4 m CMS Tracker Layout Inner Barrel & Disks TIB & TID - Outer Barrel -- TOB- End Caps TEC 1&2- Total Area : 200m 2 Channels : Solid State Detectors 33

34 CMS Tracker 34

35 Silicon Drift Detector (like gas TPC!) bias HV divider Collection drift cathodes ionizing particle pull-up cathode Solid State Detectors 35

36 Resolution ( m) Silicon Drift Detector (like gas TPC!) Anode axis (Z) Drift time axis (R-F) Drift distance (mm) Solid State Detectors 36

37 Pixel-Detectors Problem: 2-dimensional readout of strip detectors results in Ghost Tracks at high particle multiplicities i.e. many particles at the same time. Solution: Si detectors with 2 dimensional chessboard readout. Typical size 50 x 200 μm. Problem: Coupling of readout electronics to the detector. Solution: Bump bonding. Solid State Detectors 37

38 Bump Bonding of each Pixel Sensor to the Readout Electronics ATLAS: 1.4x10 8 pixels Solid State Detectors 38

39 Pixel Detector Application: Hybrid Photon Detector Solid State Detectors 39

40 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. 40

41 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 Solid State Detectors 41

42 Proportional to Z 2 /A of the Material. Proportional to Z 14 of the incoming particle. Proportional zu 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 X 0 M 2 A/ ( Z 14 Z 2 ) X 0 : Distance where the Energy E 0 of the incoming particle decreases E 0 Exp(-1)=0.37E 0. 42

43 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 MeV/c Critical Energy: If de/dx (Ionization) = de/dx (Bremsstrahlung) Myon in Copper: Electron in Copper: p 400GeV p 20MeV 43

44 For E >>m e c 2 =0.5MeV : = 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. 44

45 Electro-Magnetic Shower of High Energy Electrons and Photons 45

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