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 Particle Detectors Detector-Physics: Precise knowledge of the processes leading to signals in particle detectors is necessary. The detectors are nowadays working close to the limits of theoretically achievable measurement accuracy even in large systems. Due to available computing power, detectors can be simulated to within 5-10% of reality, based on the fundamental microphysics processes (atomic and nuclear crossections). 2

3 Electric Fields in a Micromega Detector Particle Detector Simulation Very accurate simulations of particle detectors are possible due to availability of Finite Element simulation programs and computing power. Follow every single electron by applying first principle laws of physics. Electric Fields in a Micromega Detector Electrons avalanche multiplication 3

4 Particle Detector Simulation I) C. Moore s Law: Computing power doubles 18 months. II) W. Riegler s Law: The use of brain for solving a problem is inversely proportional to the available computing power. I) + II) =... Knowing the basics of particle detectors is essential 4

5 Interaction of Particles with Matter Any device that is to detect a particle must interact with it in some way almost In many experiments neutrinos are measured by missing transverse momentum. E.g. e + e - collider. P tot =0, If the Σ p i of all collision products is 0 neutrino escaped. Claus Grupen, Particle Detectors, Cambridge University Press, Cambridge 1996 (455 pp. ISBN ) 5

6 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. 6

7 Classical Scattering on free Electrons Classical Scattering on Free Electrons: A charged particle with mass M, charge Z 1 e and velocity v passes an electron at distance b. Energy Loss by Excitation and Ionization 7

8 Classical Scattering on Free Electrons Targetmaterial: Atomic Mass A, Periodic Number Z, Density [g/cm 3 ] A gramm N A Atoms: Number of electrons/cm 3 n e =N A Z/A [1/cm 3 ] dx With E(b) db/b = -1/2 de/e E max = E(b min ) E min = E(b max ) = const. * 1/v 2 * ln v E max 2m e v 2 (M>>m e ), E min I (Ionization Energy) Energy Loss by Excitation and Ionization 8

9 Classical Scattering on Free Electrons 1/ The relativistic form of the electromagnetic field doesn t change the energy transfer, is just changes E max. This formula is up to a factor 2 and the density effect identical to the precise QM derivation: Bethe Bloch Formula: Density effect. Medium is polarized Which reduces the log. rise. Energy Loss by Excitation and Ionization 9

10 Small energy loss Fast Particle Cosmis rays: de/dx α Z 2 Small energy loss Fast particle Pion Large energy loss Slow particle Discovery of muon and pion Pion Pion Kaon 10

11 Bethe Bloch Formula 1/ Für Z>1, I 16Z 0.9 ev For Large the medium is being polarized by the strong transverse fields, which reduces the rise of the energy loss density effect At large Energy Transfers (delta electrons) the liberated electrons can leave the material. In reality, E max must be replaced by E cut and the energy loss reaches a plateau (Fermi plateau). Characteristics of the energy loss as a function of the particle velocity ( ) The specific Energy Loss 1/ρ de/dx firts decreaes as 1/ 2 increases with ln for =1 is independent of M (M>>m e ) is proportional to Z 1 2 of the incoming particle. is independent of the material (Z/A const) shows a pleateau at large (>>100) Energy Loss by Excitation and Ionization 11

12 Bethe Bloch Formula 1/ Bethe Bloch Formula, a few Numbers: For Z 0.5 A 1/ de/dx 1.4 MeV cm 2 /g for ßγ 3 Example 1: Scintillator: Thickness = 2 cm; ρ = 1.05 g/cm 3 Particle with βγ = 3 and Z=1 1/ρ de / dx 1.4 MeV de 1.4 * 2 * 1.05 = 2.94 MeV Example 2: Iron: Thickness = 100 cm; ρ = 7.87 g/cm 3 de 1.4 * 100* 7.87 = 1102 MeV Example 3: Energy Loss of a Carbon Ion with Z=6 and Momentum of 330 MeV/c/Nukleon in Water, i.e. βγ = p/m = 330/ β.33 de/dx 1.4 Z 2 /β MeV/cm Cancer Therapy! This number must be multiplied with ρ [g/cm 3 ] of the Material de/dx [MeV/cm] Energy Loss by Excitation and Ionization 12

13 Energy Loss as a Function of the Momentum Energy loss depends on the particle velocity and is independent of the particle s mass M. The energy loss as a function of particle Momentum P= Mcβγ IS however depending on the particle s mass By measuring the particle momentum (deflection in the magnetic field) and measurement of the energy loss on can measure the particle mass Particle Identification! Energy Loss by Excitation and Ionization 13

14 Energy Loss as a Function of the Momentum Measure momentum by curvature of the particle track. Find de/dx by measuring the deposited charge along the track. Particle ID 14

15 Range of Particles in Matter Particle of mass M and kinetic Energy E 0 enters matter and looses energy until it comes to rest at distance R. Independent of the material Bragg Peak: For >3 the energy loss is constant (Fermi Plateau) If the energy of the particle falls below =3 the energy loss rises as 1/ 2 Towards the end of the track the energy loss is largest Cancer Therapy. Energy Loss by Excitation and Ionization 15

16 Range of Particles in Matter Average Range: Example Kaon, p=700 MeV/c in Water: =1 [g/cm 3 ], Mc 2 =494 MeV =1.42 /Mc 2 R = 396g/cm 2 GeV R=195cm In Pb: =11.35 R=17cm Energy Loss by Excitation and Ionization 16

17 Relative Dose (%) Range of Particles in Matter Average Range: Towards the end of the track the energy loss is largest Bragg Peak Cancer Therapy Photons 25MeV Carbon Ions 330MeV Depth of Water (cm) Energy Loss by Excitation and Ionization 17

18 Fluctuations of the Energy Loss Fluctuation of the Energy Loss: Up to now we have calculated the average energy loss. The energy loss is however a statistical process and will fluctuate. Intermezzo: Crossections Crossection : Material with Atomic Mass A and density contains n = N A /A [Atoms/cm 3 ], where N A is the Avogadro Number. Considering the Atom to be a shere with Radius R: Atomic Crossection = R 2 A Volume with surface A 1 and thickness A 1 dx contains N=nA 1 dx Atoms. The total area of these Atoms is A 2 =N. Ratio of the areas p = A 2 /A 1 = N A /A dx = Probability that an incoming particle hits an atom in dx. The probability that a particle hits an atom between distances m*dx and m*dx+dx is: A 2 A 1 dx p(1-p) m p exp(-mp) = N A /A dx exp(- N A /A mdx) With x=m dx and = A/ N A we have the probability P(x)dx that a particle hits an atom between x and x+dx P(x)dx = 1/ exp(-x/ ) dx Mean Free path = x 1/ exp(-x/ ) dx = = A/ N A Average number of interactions per unit of length = 1/ = N A /A Energy Loss by Excitation and Ionization 18

19 Fluctuations of the Energy Loss Intermezzo,:Differential Crossection: d (E,E )/de Crossection that an incoming particle of Energy E loses the energy E in the interaction. Total Crossection: (E)= d (E,E )/de de Probability that in an interaction an amount of energy between E and E +de is lost: d (E,E )/de / (E) Average number of collisions per unit of length in which the particle loses and energy between E und E +de : N A /A d (E,E )/de Average Energy loss per unit of length: de/dx = N A /A E d (E,E )/de de Energy Loss by Excitation and Ionization 19

20 Fluctuations of the Energy Loss Fluctuation of the Energy Loss : x=0 x=d E X X X X X E - XX X X X X XXX X X XX X XXX X What is the probability that a particle of energy E looses the energy in a material of thickness D? Probability for an interaction between x und x+dx: P(x)dx = 1/ exp(-x/ ) dx with = A/ N A Probability to have n interactions in distance D is Poisson Distribution Energy Loss by Excitation and Ionization 20

21 d (E,E )/de / (E) Fluctuations of the Energy Loss Probability for loosing energy between E and E +de is given by the differential crossection d (E,E )/de / (E) Excitation and ionization Scattering on free electrons From P(n) and d (E,E )/de we can calculated P(E, ), the probability that a particle of energy E loses the energy when traversing a material of thickness D. P(E, ) cannot be expressed on closed form, but just as an integral: P(E, ) Landau Distribution Energy Loss by Excitation and Ionization 21

22 Landau Distribution Landau Distribution P( ): Probability for energy loss in matter of thickness D. Landau distribution is very asymmetric. Average and most probable energy loss must be distinguished! Measured Energy Loss is usually smaller that the real energy loss: 3 GeV Pion: E max = 450MeV A 450 MeV Electron usually leaves the detector. Energy Loss by Excitation and Ionization 22

23 Landau Distribution LANDAU DISTRIBUTION OF ENERGY LOSS: PARTICLE IDENTIFICATION Requires statistical analysis of hundreds of samples Counts 4 cm Ar-CH4 (95-5) bars N = 460 i.p. Counts 6000 protons electrons 15 GeV/c 4000 FWHM~250 i.p For a Gaussian distribution: N (i.p.) N ~ 21 i.p. FWHM ~ 50 i.p N (i.p) I. Lehraus et al, Phys. Scripta 23(1981)727 Energy Loss by Excitation and Ionization 23

24 Particle Identification Measured energy loss average energy loss In certain momentum ranges, particles can be identified by measuring the energy loss. Energy Loss by Excitation and Ionization 24

25 Up to this Point we have learned the following: A charged particle leaves a trail of electrons and excited atoms along it s path. We have also quantified the amount of energy, which is given by the Bethe Bloch formula. This deposited energy is (to first order) only depending on the particles Z 12 and it s velocity v. It is (almost) independent of the particle s mass. 1/ de/dx is independent of the material and has a minimum at 1.4MeV cm 2 /g at 3. We can identify three characteristic regions as a function of the particle s velocity (or better ) 1) For non-relativistic velocities (v<<c) the deposited Energy goes as 1/v 2 2) After this minimum For v c the energy loss rises according to ln (relativistic rise) 3) For very large of the particle the energy loss shows a plateau due to polarization of the medium. The energy loss is a statistical process Landau Distribution. The energy loss is strongly asymmetric due to rare large energy transfers (delta electrons). Energy Loss by Excitation and Ionization 25

26 Detectors based on registration of excited Atoms Scintillators Scintillator Detectors 26

27 Detectors based on Registration of excited Atoms Scintillators: Charged particles leave a trail of excited Atoms (and ions) along their path. Emission of photons of by excited Atoms, typically UV to visible light. a) Observed in Noble Gases (even liquid!) b) Inorganic Crystals. c) Polyzyclic Hydrocarbons (Naphtalen, Anthrazen, organic Scintillators) ad b) Substances with largest light yield. Used for precision measurement of energetic Photons. Used in Nuclear Medicine. ad c) Most important cathegory. Large scale industrial production, mechanically and chemically quite robust. Characteristic are one or two decay times of the light emmission. Scintillator Detectors 27

28 Light yield is typically proportional to the energy loss. Saturation effects described by Birks law. k B = Typical light yield of scintillators: Energy (visible photons) few of the total energy Loss. z.b. 1cm plastikscintillator, 1, de/dx=1.5 MeV, ~15 kev in photons; i.e. ~ photons produced. Scintillator Detectors 28

29 Organic ( Plastic ) Scintillators Inorganic (Crystal) Scintillators Low Light Yield Fast: 1-3ns Large Light Yield Slow: few 100ns LHC bunchcrossing 25ns LEP bunchcrossing 25 s Scintillator Detectors 29

30 Photons are being reflected towards the ends of the scintillator. A light guide brings the photons to the Photomultipliers where the photons are converted to an electrical signal. By segmentation one can arrive at spatial resolution. Because of the excellent timing properties (<1ns) the arrival time, or time of flight, can be measured very accurately Trigger, Time of Flight. Scintillator Detectors 30

31 Typical Geometries: UV light enters the WLS material Light is transformed into longer wavelength Total internal reflection inside the WLS material transport of the light to the photo detector Scintillator Detectors From C. Joram 31

32 The frequent use of Scintillators is due to: Well established and cheap techniques to register Photons Photomultipliers and the fast response time 1 to 100ns Semitransparent photocathode glass Schematic of a Photomultiplier: PC Typical Gains (as a function of the applied voltage): 10 8 to e - Typical efficiency for photon detection: < 20% For very good PMs: registration of single photons possible. Example: 10 primary Elektrons, Gain electrons in the end in T 10ns. I=Q/T = 10 8 *1.603*10-19 /10*10-9 = 1.6mA. Across a 50 Resistor U=R*I= 80mV. Scintillator Detectors 32

33 Fiber Tracking Planar geometries (end cap) Light transport by total internal reflection n 1 typ. 25 m core polystyrene n=1.59 cladding (PMMA) n=1.49 n 2 typically <1 mm Circular geometries (barrel) High geometrical flexibility Fine granularity Low mass Fast response (ns) (R.C. Ruchti, Annu. Rev. Nucl. Sci. 1996, 46,281) Scintillator Detectors From C. Joram 33

34 Fiber Tracking Readout of photons in a cost effective way is rather challenging. Scintillator Detectors From C. Joram 34

35 Magnetic Spectrometers for Momentum Measurement of Charged Particles A charged particle describes a circle in a magnetic field: Limit Multiple Scattering Magnetic Spectrometers 35

36 Magnetic Spectrometers for Momentum Measurement of Charged Particles ATLAS: N=3, sig=50um, P=1TeV, L=5m, B=0.4T p/p ~ 8% for the most energetic muons at LHC Magnetic Spectrometers 36

37 Magnetic Spectrometers for Momentum Measurement of Charged Particles: Multiple Scattering. Statistical (quite complex) analysis of multiple collisions gives: Probability that a particle is defected by an angle after travelling a distance x in the material is given by a Gaussian distribution with sigma of: 0 = /( cp[gev/c])*z 1 * x/x 0 X 0... Radiation length of the materials Z 1... Charge of the particle p... Momentum of the particle Magnetic Spectrometers 37

38 VERTEX Detectors By direct measurement of the decay-position of the paticle ona can measure the lifetime and therefore identify the particle. Mainly for Charm, Beauty Hadron and Tau-Lepton Physics. Typical Lifetimes: bis s Typical decay distances: μm Typical required Resoution: ~10 μm ` Silicon Strip or Pixel Detectors. Vertex Detectors 38

39 Detectors based on registration of Ionization: Tracking in Gas and Solid State Detectors Charged particles leave a trail of ions (and excited atoms) along their path: Electron-Ion pairs in gases and liquids, electron hole pairs in solids. The produced charges can be registered Position measurement Tracking Detectors. Cloud Chamber: Charges create drops photography. Bubble Chamber: Charges create bubbles photography. Emulsion: Charges blacked the film. Gas and Solid State Detectors: Moving Charges (electric fields) induce electronic signals on metallic electrons that can be read by dedicated electronics. In solid state detectors the charge created by the incoming particle is sufficient. In gas detectors (e.g. wire chamber) the charges are internally multiplied in order to provide a measurable signal. Tracking Detectors 39

40 The induced signals are readout out by dedicated electronics. The noise of an amplifier determines whether the signal can be registered. Signal/Noise >>1 The noise is characterized by the Equivalent Noise Charge (ENC) = Charge signal at the input that produced an output signal equal to the noise. ENC of very good amplifiers can be as low as 50e-, typical numbers are ~ 1000e-. In order to register a signal the registered charge must be q >> ENC i.e. typically q>>1000e-. Gas Detector: q=80e- too small. Solid stated detector: q= e- O.K. Gas detector needs internal amplification in order to be sensitive to single particle tracks. An amplifier measures the total charge in case the integration time is larger than the drift time of the charge. Without internal amplification it can only be used for a large number of particles that arrive at the same time (ionization chamber). Tracking Detectors 40

41 Principle of signal induction by moving charges: A point charge q at a distance z 0 Above a grounded metal plate induces a surface charge. The total induced charge on the surface is q. Different positions of the charge result in different charge distributions. The total induced charge stays q. q q -q The electric field of the charge must be calculated with the boundary condition that the potential φ=0 at z=0. For this specific geometry the method of images can be used. A point charge q at distance z 0 satisfies the boundary condition electric field. -q The resulting charge density is (x,y) = 0 E z (x,y) (x,y)dxdy = -q I=0 Signal induction by moving charges 41

42 Principle of signal induction by moving charges: If we segment the grounded metal plate and if we ground the individual strips the charge density doesn t change. q V The charge induced on the individual strips is now depending on the position z 0 of the charge. If the charge is moving there are currents flowing between the strips and gorund. -q The movement of the charge induces a current. -q I 1 (t) I 2 (t) I 3 (t) I 4 (t) z 0 (t)=z 1 -v*t Signal induction by moving charges 42

43 In order to calculate the signals the Poisson equation must be calculated for all different positions of the charge q difficult task. Theorem (1) (Reciprocity theorem, Ramotheorem): The current induced on a grounded electrode (n) by the movement of a charge q along a trajectory x(t) can be calculated the following way: One removes the charge q and brings electrode (n) to potential V 0 while keeping all the other electrodes at ground potential. This defined an electric field E n (x) ( Weighting field of electrode n). The induced current is then Signal induction by moving charges 43

44 Example: Elektron-Ion pair in gas I 2 E 1 =V 0 /D E 2 =-V 0 /D z E -q, v e q,v I Z=D Z=z 0 I 1 = -(-q)/v 0 *(V 0 /D)*v e - q/v 0 (V 0 /D) (-v I ) = q/d*v e +q/d*v I I 2 =-I 1 Z=0 I 1 I 1 The induced current is proportional to the velocity of the charges. T e T I t Q tot 1= I 1 dt = q/d*v e T e + q/d*v I *T I = q/d*v e *(D-z 0 )/v e + q/d*v I *z 0 /v I = q(d-z 0 )/D + qz 0 /D = q e +q I =q The induced charge depends on the position from where the charge starts moving. The total induced charge on a specific electrode, once all the charges have arrived at the electrodes, is equal to the charge that has arrived at this specific electrode. Signal induction by moving charges 44

45 Theorem (2): In case the electrodes are not grounded but connectd by arbitrary active or passive elements one first calculates the currents induced on the grounded electrodes and places them as ideal current sources on the equivalent circuit of the electrodes. V 2 R Z=D R I 2 q, v 1 -q,v 2 Z=z0 C V 1 R Z=0 R I 1 Signal induction by moving charges 45

46 By using thin wires, the electric fields close to the wires are very strong (e.g kv/cm). In these large electric fields, the electrons gain enough energy to ionize the gas themselves Avalanche a primary electron produces electrons measurable signal. 46

47 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 47

48 Transport of Electrons in Gases: Drift-velocity Ramsauer Effect Electrons are completely randomized in each collision. The actual drift velocity along the electric field is quite different from the average velocity of the electrons i.e. about 100 times smaller. The driftvelocity v is determined by the atomic crossection ( ) and the fractional energy loss ( ) per collision (N is the gas density i.e. number of gas atoms/m 3, m is the electron mass.): Because ( )und ( ) show a strong dependence on the electron energy in the typical electric fields, the electron drift velocity v shows a strong and complex variation with the applied electric field. v is depending on E/N: doubling the electric field and doubling the gas pressure at the same time results in the same electric field. Transport of charges in gases 48

49 Transport of Electrons in Gases: Drift-velocity Typical Drift velocities are v=5-10cm/ s ( m/s). The microscopic velocity u is about ca. 100mal larger. Only gases with very small electro negativity are useful (electron attachment) Noble Gases (Ar/Ne) are most of the time the main component of the gas. Admixture of CO 2, CH 4, Isobutane etc. for quenching is necessary (avalanche multiplication see later). Transport of charges in gases 49

50 Transport of Electrons in Gases: Diffusion An initially point like cloud of electrons will diffuse because of multiple collisions and assume a Gaussian shape. The diffusion depends on the average energy of the electrons. The variance σ 2 of the distribution grows linearly with time. In case of an applied electric field it grows linearly with the distance. Electric Field x Solution of the diffusion equation (l=drift distance) Thermodynamic limit: Because = (E/P) 1 P F E P Transport of charges in gases 50

51 Transport of Electrons in Gases: Diffusion The electron diffusion depends on E/P and scales in addition with 1/ P. At 1kV/cm and 1 Atm Pressure the thermodynamic limit is =70 m für 1cm Drift. Cold gases are close to the thermodynamic limit i.e. gases where the average microscopic energy =1/2mu 2 is close to the thermal energy 3/2kT. CH 4 has very large fractional energy loss low low diffusion. Argon has small fractional energy loss/collision large large diffusion. Transport of charges in gases 51

52 Drift von Ions in Gases Because of the larger mass of the Ions compared to electrons they are not randomized in each collision. The crossections are constant in the energy range of interest. Below the thermal energy the velocity is proportional to the electric field v = μe (typical). Ion mobility μ 1-10 cm 2 /Vs. Above the thermal energy the velocity increases with E. V= E, (Ar)=1.5cm 2 /Vs 1000V/cm v=1500cm/s=15m/s times slower than electrons! Transport of charges in gases 52

53 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 53

54 Detectors with 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(x) = N(x) α dx α first Townsend Koeffizient N(x) = N 0 exp (αx) α= α(e), N/ N 0 = A (Amplification, Ga) 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 54

55 Cylinder Geometry for Wire Chambers 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. Gas Detectors 55

56 Detectors with Electron Multiplication 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 Avalache along the particle track. Limited Geiger Region: Avanache propagated by UV photons. Geiger Region: A 10 9 Avalanche along the entire wire. Gas Detectors 56

57 Detectors with Electron Multiplication Proportional Semi proportional Saturation Streamer Limited Geiger region Geiger region Gas Detectors 57

58 Detectors with Electron Multiplication 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 drifting 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 58

59 Detectors with Electron Multiplication Rossi 1930: Coincidence circuit for n tubes Cosmic ray telescope 1934 Position resolution is determined by the size of the tubes. Signal was directly fed into an electronic tube. Gas Detectors 59

60 Charpak et. al. 1968, Multi Wire Proportional Chamber Classic geometry (Crossection) : One plane of thin sense wires is placed between two parallel plates. Tyopica dimesions: 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 60

61 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 61

62 Cathode Strip: Width (1 ) of the charge distribution DIstance Center of Gravity defines the particle trajectory. Ladungslawine (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 62

63 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 63

64 Drift chambers: typical geometries Electric Field 1kV/cm W. Klempt, Detection of Particles with Wire Chambers, Bari 04 Gas Detectors 64

65 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 65

66 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 66

67 ATLAS MDT 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 Gas Detectors 67

68 Diffusion Charge Fluctuation Simulation of ATLAS muon tubes: Switch off individual components. Gas Gain Gas Pressure Position resolution depends on: Diffusion (Pressure) Gas Gain Fluctuation of Primary Charge. Electronics Gas Detectors 68

69 Large Drift Chambers: Central Tracking Chamber CDF Experiment 660 drift cells tilted 45 0 with respect to the particle track. drift cell Gas Detectors 69

70 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 with E & B fields B drift y x E z charged track wire chamber to detect projected tracks Gas Detectors 70

71 Important Parameters for a TPC Transverse diffusion ( m) for a drift of 15 cm in different Ar/CH 4 mixtures Position resolution momentum resolution. Number of pad rows and pad size. Homogeneous, parralel E and B field. X 0 Choice of material to limit multiple scattering. Energy resolution particle Identification. Gas Detectors 71

72 ALICE TPC: Detektorspezifikationen Gas Ne/ CO 2 90/10% Field 400V/cm Gas Gain >10 4 Position resolition = 0.2mm Diffusion: t = 250 m Pads inside: 4x7.5mm Pads outside: 6x15mm B-field: 0.5T cm Gas Detectors 72

73 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 airplace inductry (X 0 = ~3%) Gas Detectors 73

74 ALICE TPC: Pictures of the construction Precision in z: 250 m End plates 250 m Wire Chamber: 40 m Gas Detectors 74

75 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! Gas Detectors 75

76 ALICE TPC My personal contribution: A visit inside the TPC. Gas Detectors 76

77 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 77