Detectors for High-Energy (Heavy-Ion) Experiments

Transcription

1 Lecture Week: Detectors for High-Energy (Heavy-Ion) Experiments 2.-5.April 2007 Hans Rudolf Schmidt, GSI Darmstadt 1

2 Gaseous Detectors (Basics) Ionization Mechanisms Transport Mechanisms Diffusion Drift Amplification Mechanisms 2

3 Power of Modern Gas Detectors Simultaneous detection of > 500 particles tracks in a Time Projection Chamber (TPC) Detection of 9 kev X-rays (image size 60 x 30 mm) employing Gas Electron Multipliers (GEM) 3

4 Gaseous Detectors in LHC Experiments ALICE: TPC (tracker), TRD (transition rad.), TOF (MRPC), HMPID (RICH-pad chamber), Muon tracking (pad chamber), Muon trigger (RPC) ATLAS: TRD (straw tubes), MDT (muon drift tubes), Muon trigger (RPC, thin gap chambers) CMS: Muon detector (drift tubes, CSC), RPC (muon trigger) LHCb: Tracker (straw tubes), Muon detector (MWPC, GEM) TOTEM: Tracker & trigger (CSC, GEM) 4

5 Ionizations Mechanisms (I) Primary and und secondary ionization: A charged particle ionizes or excites atoms primary, the energy distribution of the primary electrons is 1/E 2, electrons with energies above 100 ev ionized themselves secondary. on has: E ntotal = nprimär + nsekundär = = W n total n primär i de x dx W i W i = average energy loss per produced ion pair (W i > I 0 ) 30 ev E = total energy loss 5

6 Ionization - Detection Example: gas counter of 50 mm, filled with Argon; n prim 18 /(cm atm), n total cm ~ 360 e-ion pairs 360 electron-ion pairs are not easily detectable! amplifier noise is typical 1000 e - (ENC=equivalent noise charge)! increase number of electrons w/o additional noise production! gas amplification 6

7 Distribution of E Owing to their finite granularity (thickness) real detector do not measure de/dx, but the energy loss E in a layer of thickness x. How is the energy loss distributed E? In thin layer (or gases) one has some ionizations processes plus collisions with high energy transfer ( knock-on of δ-electrons). δ-elektron The distribution of the energy loss has fluctuation to high energy losses yielding a Landau distribution. 7

8 Landau Fluctuations The energy loss de/dx in thin layers (small surface density, i.e., gases) is not gaussian distributed! E W The energy loss de/dx is described by a Landau-distribution: < E> E ~ de dx D L(λ) = 1 2π e 1/ 2(λ+e λ ) most probable energy loss distribution of fractional deviations from most probable energy loss λ ( E E ) E optimization of the resolution (particle identification) via the truncated mean method later 8

9 Diffusion E=0 thermal Diffusion E>0 charge transport and diffusion 9

10 Diffusion The magnitude of the diffusion is a gas property and a very important criterion for the selection of gases for detectors. The (right) choice of the diffusion is optimizes the two-track and position resolution! pick-up pads 10

11 Diffusion w/o E-Field I After ionization, the electrons thermalize by collisions with the molecules of the gas. The average thermal energy at room temperature is: 3 3kT ε = kt = 40 mev v = 2 m ve = 11.5 cm µ s vi = cm µ s (A = 40) the energies are Boltzmann-distributed. F( ε) ~ ε e ε kt ε [ev] remark: the escape velocity is 1.1 cm/µs, therefore basically no light gases ( He) in the atmosphere. 11

12 Diffusion w/o E-Field II Diffusion in a field-free region is isotropic and the density is gaussian distributed: dn dx = N 1 2 4πDt e x 4Dt dn Ν The (1-dim) equation expresses which fraction of the charge (dn/n) is found after time t in dx. D is the diffusion constant. For linear and volume diffusion on has σ x = 2Dt vol x σ = 3σ = 6Dt σ x remark 1: the diffusion coefficient is defined 2D µ m σ = 2Dt= L=Dc L, Dc by: v drift cm t remark 2: the diffusion constant is defined by the continuity equation: N + Γ= 0 t Γ = N v D N 12

13 Diffusion (quantitative) Dependence of diffusion on the gas type: Diffusion in noble gases is large because there is little excitation (few degrees of freedom). In molecular gases with many excitation level large reduction of diffusion (e.g. in CO 2 - cold gas ) 13

14 Diffusion: Magnetic Anisotropy B fields reduced the diffusion coefficient in direction of the magnet field: B = (0,0,B z ) Dz 1 = D Dx = Dy = 2 2 (1 + ωτ) E B 14

15 Transverse Diffusion in Electrical and Magnetic Fields drift length 15 cm σ T ;nob Feld σ ;B E T 15

16 Drift of Electrons in Electric Fields I Electrons (ions) drift with a constant equilibrium velocity along an external field. mechanism: Because of its small mass the electrons scatters isotropically from the (heavy) gas molecules, i.e., has lost after the scattering the direction of acceleration in the E- field. The velocity v drift from the E-field is given by (v drift <<v thermal ): v Drift = ee m τ τ = average time since last scattering The electron loses its gained energy in the next scattering, an equilibrium is established between energy gain from the field and energy loss from scattering; the electron drift with the constant macroscopic velocity v Drift. 16

17 Drift of Electrons in Electric Fields II The drift velocity saturates above a certain value or the drift field! The values of the E-field depends on the gas type and/or mixture. The value of the drift velocity depends also strongly on the gas type. The optimal working region for a gas detector is in the saturation region. 17

18 Drift of Electrons in Electric Fields III the average time between collision is given by 1 λ τ = = Nσv v ε = mv =ε thermal +ε E = kt +εe e - Ε εthermal εe vthermal vdrift εthermal εe vthermal vdrift v Drift ee ee 1 = τ= m m Nσ v thermal v x Drift 1 mv 2 2 = ε E Λε E = eex τ equilibriums condition v Drift E e 1 = N σε ( ) 3kTm v 2 Drift E e Λ( ε) = N m σε ( ) 2 Λ = average (%) energy loss per scattering 18

19 Drift of Electrons in Electric Fields IV σ = σ(ε)! σ~1/λ λ = λ(ε)! Ramsauer minimum, caused by interference effects, when wave length of the electrons (λ=h/p) is in the molecular range Excitation of internal degrees of freedom in CH 4 and Argon 19

20 Drift of Electrons in Electric Fields V v Drift qualitative complex gases 2 E e Λ( ε) v Drift(E/N, σε ( ), Λε ( )) = N m σε ( ) 2 E e v Drift(E/N, σε ( ), ε ) = N 2m ε σ ( ε ) noble gases E/N remarks: E/N for smaller drift field (E/N) 1/2 for larger drift field σ(ε) usually a complex function (Ramsauer effect) Λ(ε) usually a complex function because of molecular excitations for elastic scattering Λ(ε)=2 m e/ M 10-4 Λ(ε) 0 v drift 0 ε thermal = 40 mev ε E = 2.2 mev (E=400 V/cm ALICE TPC, v Drift =2.8 cm/µs) 20

21 Mobility of Electrons and Ions The definition of mobility ensures the it does not depend on trivial factors as the electric field or the gas density: µ v Drift V Drift E ee = τ m e µ = τ m typical values for the mobility of charge carriers are: cm µ e =10000 Vs 2 cm 2 µ ion =1 Vs 21

22 Drift in Electric and Magnetic Fields I forces on a charged particle: Lorentz force : qv B Coulomb force: qe stochastic force : ma s (t) particle moves on orbit with cyclotron frequency qb ω e ω= ; = = 17.6MHz/G m B m equation of motion is Langevin equation: mv = q(e + v B) + ma (t) s a(t) s = v τ Drift drift velocity ist const, i.e. no net acceleration 22

23 Drift in Electric and Magnetic Fields (E x B Effect) Solution: v µ E B (E B)B 2 2 D ( E i = + ωτ+ ω τ ) e µ= τ m e ω= B m 1+ω τ B B 4 10 ions ωτ= Bµ 1for electrons the drift velocity has three components: E, B, ExB, the direction of the drift velocity depends onωτ: ωτ <<1: drift velocity follows E-field ωτ >>1: drift velocity follows B-field special cases: E li B: E B: v v =µ E Drift = µ E 1+ω τ 2 2 tan α =ωτ (Lorentz angle) E 23

24 Electron Capture During its drift to the anode the electrons can disappear by recombination or formation of negative ions + e + I I (recombination) e + M M (electron capture) t a = 1 (p n ) a s Contaminations with electro-negative gases (O 2, halogens) are not acceptable for detectors with large drift times! ALICE TPC (250 cm drift, t=88 µs): O 2 < 5 ppm requiered 24

25 Gas Amplification if the drift velocity of the electrons is large enough for secondary ionization, an avalanche develops. The condition is: λ ee > I 0 I0 = ev, λ = 1 2µ m E I 0 > = λ 5 10 V/cm In chambers with this wires (10 µm) this condition is reached close to the surface of the wire. 25

26 Gas Amplification The number of produced ions/unit path length is called 1. Townsend coefficient (α) α= 1 λ = Nσ ionisation dn(x) = n(x) α dxź n(x) = n 0 e αx the amplification is A = n(x) n 0 = e αx The signal is proportional to the primary ionization. The amplification in the proportional region is typically 10 3 to 10 6, i.e., one has 13 to 20 generations (duplication in each collision assumed). The charge is therefore essentially produced in the last few λ (mean free path ~ µm), i.e., within as few ns. This is important for the signal development at the anode. The proportional region ends at αx 20 n A 10 prim 8 (Raether Limit) 26

27 Gas Amplification - 2 nd Townsend Coefficient The Raether-Limit is reached relatively fast by the production of photons in the UV-range. The photons are produced by de-excitation of atoms or molecules, which were excited in collisions. n0 = primary electron γ=probability, that1 e is produced per photon n0a = elektron multiplication in collisions n Aγ= photo electrons 0 n 0 A λ = n 0 A (Aγ) n = n 0 1 Aγ n 0 γ is called the second Townsend coefficient For Aγ 1 the expression diverges, the signal becomes independent from the strength of the primary ionization (Geiger-Müller region) 27

28 Gas Choice The atoms of the operating gas (e.g. Ar) in proportional chambers can be excited in the avalanche: * e+ Ar Ar Ar+ hν De-excitation of noble gases only via emission of UV γ s of high energy, e.g., 11.6 ev in argon. This is larger the the work function of electrons in metal (copper: 7.7 ev). The photo electron initiates a new avalanche: stationary discharge Cure: admixtures of so-called quencher (molecules, e.g., CH 4 ), which absorb the photons before they can reach the cathode. or: transfer of the excitation energy in collisions Ar + + CH 4 Ar + CH 4 + broad absorption (many rotational- and vibrational degrees of freedom). De-excitation via dissociation!! (-> Ageing) 28

29 Geiger-Müller Region At very high voltages UV photons initiate massive secondary avalanches (in the gas and the cathode surface). An ion tube develops around the anode wire, the amplification is >10 10, independent of the primary ionization trigger counter, efficiency 100% for single particles, but no energy information. Stopping of the discharge: high load resistor (charging of the capacitor by an external HV source >> ion drift time ~ ms) large dead time admixture of (> 10%) quencher, reduction of UV range to < 100 µm (self-extinguishing counter) reduction of the dead time to < 100 µs 29

30 Streamer Discharge I In the so-called limited proportional range and large fraction of quencher gases ( 50%) one has local avalanches Streamer (UV-photons are absorbed and contribute only locally to the avalanches). gas amplification > 10 8 large signal, several particle can be registered simultaneously but: no energy loss measurement, charge independent of primary ionization granular hit counters 30

31 Streamer Discharge II proportional geiger streamer wire wire wire discontinuous transition from proportional to streamer modus at 3.5 kv both discharge modi possible above 4 kv (dependant of the gas) broad plateau with ε=100% (efficiency) 31

32 Summary: SWPC Modi of Operation Ionizations modus collection of all charges not charge multiplication amplification ~ 1 measurement difficult Proportional modus multiplication of primary ionization detected signal if proportional to primary ionization energy loss (de/dx) measurement possible; secondary avalanches have to be suppressed (quenched) amplification ~ Limited proportional modus enhanced UV-photo emission, production of streamers, mixture of primary and secondary avalanches strong quencher necessary large pulses, measurement easy amplification ~ Geiger modus massive UV-photo emission propagation of the avalanche along the anode wire discharge stopped via load resistor (HV voltage drop) amplification >

33 Gaseous Detectors (Applications) Ionization Chamber Proportional Chamber Signal Generation Streamer Tubes Multi-wire Proportional Chambers Signal Generation Drift chambers TPC Miniaturization GEM, MGC, MSGC Resistive Plate Chambers Ageing 33

34 Ionizations Chamber Principal scheme of an ionization chamber: the most simple arrangement are two parallel plates (capacitor) in an gas volume. Field E=U 0 /d such, that the charges are transported without multiplication and recombination losses to the anode and cathode, respectively. 34

35 Proportional Counter (SWPC) Field strength in proportional counters ~ V/cm In the proportional region the primary signal is amplified (~ ). In planar proportional counters the amplitude depends on the location of the primary ionization, i.e., no relation between the signal and the energy loss of the particle. Solution: cylindrical proportional counter, amplification only in the immediate vicinity of the anode wire, i.e., the signal become independent of the location of the primary ionization and depends only in the energy loss. 35

36 Cylindrical Counter: Signal Generation I W = q ϕ(r) d ϕ(r) dw = q dr dr 1 2 W = lcv0 dw = lcv0dv 2 d ϕ(r) q lcv0dv = q dr dv = dr lcv 0 d ϕ(r) dr dr potential energy of a charge in the potential ϕ change of the pot. energy at relocation dr energy of a cyl. capacitor induced voltage change In cylindrical (proportional)-counters field an potential are related as: CV0 1 E(r) = 2 πε r CV0 r ϕ (r) = ln 2πε a 2πε C = ln b a ( ) E = ϕ capacity/unit length - r a b 36

37 Cylindrical Counter: Signal Generation II Relation of induced voltage from electron and ions: a q dϕ q a+ rm = = V dr ln lcv dr 2πεl a 0 a+ r m b + q dϕ q b V = dr ln lcv = 0 dr 2πε l a + r a+ r m m + q V = V + V = lc V V + = a+ r ln a b ln a + r m m typical values for a cylindrical proportional counter: a=10 µm (diameter for the wire) b=10 mm r m =1µm - r a V - /V + <1% b The signal in cylindrical proportional counters comes mainly from the movement of the ions! 37

38 Cylindrical Counter: Signal Generation III Calculation of the voltage for the ion part: r(t) dv q r(t) V(t) = dr = ln dr 2πεl a r(0) v drift dr µ ioncv0 1 = =µ ione(r) = dt 2πε r rdr = µ CV 2πε ion 0 dt r(t) r(t) t µ ioncv0 rdr rdr = dt 2πε r(0) a 0 2 µ ioncv0 r(t) = a + t πε 12 q µ CV q t V(t) = ln 1+ t = ln 1+ 4πεl πεa 4πεl t ion

39 Cylindrical Counter rise time of individual pulses: ~<1 ns decay time (given by RC-constant):~20 ns PASA (Preamplifier-Shaper): 200 ns 39

40 Streamer Tubes Application of streamer discharges in (Iarocci) streamer tubes as large multiplicity counters or in calorimeters. Typical gas mixture in streamer tubes is argon-isobutene (50%, 50%), therefore no Geiger mode, but local discharge (spark) at the anode wire. Relatively simple construction and production: extruded, rectangular plastic housing thick anode wire readout from the wire or inductive coupling to a cathode pad (2-dim position information) 40

41 Multi-Wire Proportional Chamber (MWPC) cathode Equipotential lines are only different from rotational geometry at large distances from wire multiplication of SWPC in a single gas volume cathode parallel wires (typical d=2 mm wire spacing) gives in addition position resolution d σ= ~0.5mm 12 G. Charpak, Nobelpreis

42 Signal Generation at Cathode Strips/Pads w avalanche at the anode produces mirror charge at the cathode strips/pad calculation of the lateral extension of the mirror charge (surface charge density σ(x)) on the cathode yields: q 1 σ (x) = 2D cosh( πx / D) x+ w/2 P(x) = σ(x ')dx ' = pad response function x w/2 sampling of the induced charges (PRF) improves the position resolution significantly 42

43 CSC Cathode Strip Chamber precise measurement of the x- coordinate via center-of-gravity method determination of PRF (e.g. gauss fit) 2 layers of chambers allows 2-dimensional position determination 43

44 Drift Chambers Spatial information on the track location of a particle by measuring the drift time of the electrons to the anode relative to a time t 0. t 0 given by e.g. trigger scintillator or bunch crossing time (LHC) The resolution is essentially determined by diffusion, fluctuation of the primary ionization and readout electronics. The drift velocity is constant in planar geometry the (E=constant). Additional cathode (field) wires between the anode wire homogenizes the field up to the amplification layer. The position is given by: d= d0 + v d (t1 t 0) 44

45 Radial Drift Chambers At collider radial drift chambers are the choice adapted to the collider geometry cylindrical drift chambers jet chambers time projection chambers cylindrical drift chambers: anode- und cathode wires in axial direction (along beam or/and magnetic field): wire geometry: field lines around anode wire 45

46 Example: OPAL Jet Chamber 46

47 Straw Tubes A variant of cylindrical drift chambers are the so-called straw tubes: individual drift tubes of 5-10 mm diameter advantage minimizing the risk of broken wires high counting rates because of short ion collection times compact high position resolution, often used a vertex detector 47

48 Time Projection Chamber I E, B 3-dimensional reconstruction particle track (electronic bubble chamber) rϕ: MWPC with cathode pads z: drift time to readout plane determination of the drift time via sampling analog-to-digital converter (as, for example in mobile phones) therefore temporal resolution of many tracks induced signal on pad ~ de/dx momentum measurement (curvature in field) 48

49 Time Projection Chamber II HIGH MULTIPLICITY COSMIC RAY IN ALICE TPC signal seen in one pad charge time (10 MHz sampling 100 ns/bin) 49

50 Gating Grid Ion feed back from amplification region into drift volume leads to space charge effects: electric field is distorted by presence of many positive ions ( Track Distortions ) Solution: gating grid Effect: opening of the gate by trigger only for interesting events (reduces load of readout chamber) inhibits ion feed back 50

51 Miniaturization advantage of gas detectors: small radiation length good surface/price ratio flexible geometry good energy and position resolution problems: rate limitations by the time it needed to evacuate of positive ions = multi wire proportional chamber = micro strip gas chamber = mesh gas chamber solution: miniaturization, reduction of the detector cell while keeping the field geometry (reduction of the ion path to the cathode) wire based chambers cannot be reduced further for of electromechanical reasons but: with chemical etching small corresponding structures can be produced on a substrate 51

52 Gas Electron Multiplier GEM = Gas Electron Multiplier cathode-anode-structure with amplification zone realized with double-sided perforated metallized polyamide foil 70 µm 140 µm 52

53 GEM: Charge Transportation ELECTRONS: I in IONS + I drift DIFFUSION LOSSES I out I out = I in M T Ion Feedback = (gain x transparency) + I drift I out 53

54 GEM: Multi-Step amplification Employing multi-layer GEMs high amplification can be reached at moderate voltage/foil (reduction of the spark risk) DRIFT ED DRIFT GEM 1 GEM 2 GEM 2 READOUT ET1 TRANSFER 1 ET2 TRANSFER 2 EI INDUCTION 54

55 GEM: Readout principle of the readout readout structure and amplification structure are decoupled and can be optimized independently COMPASS TOTEM three-fold GEM-foils in the COMPASS und TOTEM experiments future: replaces MWPC in Linear Collider TPC seminar talk 55

56 Micro Strip Gas Chamber MSGC = Micro Strip Gas Chamber 200 m thin metallic anode and cathode strip on insulating material (glass, polyamide problems: relatively large probability for discharges (sparks) for heavily ionizing particles at regions of high field (edges) charging up of insulator, therefore field distortions and risk of spark solution: amplification in several steps slightly conducting substrate, z.b Ωcm) 56

57 Resistive Plate Chambers Parallel plate counter (ppc) operate in the Geiger-Modus... If a parallel plate chamber is operated at a field which is above the break-though limit (> 5 MV/m bar) spark are initiated (spontaneous or by ionizing particles) resistive anode + extremely fast pulse (rise time <10-9 s), well suited for TOF + extremely large pulse (>1 V), amplification not necessary - self destructing capacitor becomes conducting by plasma filament completely discharged; for large areas (=capacitor) the current through the filament is so large that crater a formed on the surface of the counter - long dead time E 0 E threshold solution: streamer/spark counter with localized discharge use of anode with high specific resistivity ( Ωcm); e.g., special glass ( Pestov -Glas) capacitor discharges and quenches only locally optimized photon absorption by a quencher mix dramatically enhanced global counting rate capability dead area 57

58 RPC Signal pickup (x) India Ink Glass plates gap ~ 2mm 8 kv Signal pickup (y) Spacer India Ink characteristic: robust counter with good time resolution (1-2 ns) large surface, cheap production 2-dim. position resolution (with appropriate readout) high (global) count rate application: trigger counter, tracking layer 58

59 Micromegas micro mesh above readout structure (typical anode strips) produced field as in parallel plate counter E a /E i ~50 ensures electron transparency and ion feed back suppression 100 m position resolution 59

60 Ageing of Gas Counters Free radicals from cracking of organic gas admixtures (quencher) near the anode, often catalytic effect of gas contaminants formation of polymers, which are deposited on the anode anode: deposits increase wire diameter decrease or changing electric field variable amplification and energy resolution cathode: dipole formation by insulating layer field emission (Malter effect) 60