Lecture 5. Detectors for Ionizing Particles: Gas Detectors Principles and Detector Concepts

Transcription

1 Lecture 5 Detectors for Ionizing Particles: Gas Detectors Principles and Detector Concepts

2 Dates Vorlesung 1 T.Stockmanns Vorlesung 2 J.Ritman Vorlesung 3 J.Ritman Vorlesung 4 J.Ritman Vorlesung 5 J.Ritman Vorlesung 6 J. Ritman (Akad. Feier?) Vorlesung 7 M.Mertens Vorlesung 8 T.Stockmanns Vorlesung 9 T.Stockmanns fällt aus Vorlesung 10 J.Ritman Vorlesung 11 T.Stockmanns Vorlesung 12 T.Stockmanns Vorlesung 13 J.Ritman Vorlesung 14 J.Ritman Besuch von COSY:

3 Introduction Sources of radiation Radioactive decay Cosmic Radiation Accelerators Content Interaction of Radiation with Matter General principles Charged particles heavy charged particles electrons Neutral particles Photons Neutrons Neutrinos Definitions Detectors for Ionizing Particles Principles of ionizing detectors Gas detectors Principles Detector concepts

4 Review of Lecture 4 Definitions Principles of Ionizing Detectors

5 Review Parameters of Particle Detectors? Sensitivity, Energy Resolution, Dead Time, Efficiency What is the Fano Factor? Detector Response?

6 Review Energy Loss Mechanisms? Excitation, Ionization Transport Mechanisms? Diffusion, Drift Difference between Electrons/Ions?

7 Review Avalanche Multiplication? Quenching? Choice of Gas?

8 Start of Lecture 5

9 Signal Generation Q(t), i(t) Parameters: creation x 0 = d/2 v + = ¼ v - When do you see a signal at the preamplifier? When the electrons reach the contact? When the ions reach the backside? Immediately when the ion/electron pair is created?

10 Signal Generation Q(t), i(t) Parameters: creation x 0 = d/2 v + = ¼ v -

11 Signal Generation Q(t), i(t) Parameters: creation x 0 = d/2 v + = ¼ v -

12 Signal Generation Q(t), i(t) Parameters: creation x 0 = d/2 v + = 1/3 v -

13 Signal Generation Q(t), i(t) Parameters: creation x 0 = ¾ d v + = 1/3 v -

14 Signal Generation To calculate the induced signal one has to solve the Poisson equation at each step on the drift of the electron-ion pair very complicated Solution (by Ramo and Shockley): i ( t ) q E with q: charge; E v w : weighting field; How to get the weighting field? Calculate the electrostatic field for each electrode by: removing the signal charge t w Q( t) i( ) d 0 v: velocity d x setting the electrode to U = 1V and all others to 0 V q x2 x1 E w

15 Weighting field Simple example: Only for setups with two electrodes the weighting field and the true field look alike. In general they are different! The actual electric field determines the path and the drift velocity of the carrier. The weighting field depends only on geometry and determines how the motion of the carrier couples to the electrode. weighting potential contour lines weighting field streamlines Segmented electrode

16 Induced signal in parallel plate detector

17 What happens in a tube? gas cathode b a anode E Which component creates the E threshold most charge in the preamplifier? Electrons or Ions? a 1/r Which creates the greatest current? r

18 Induced signal in a tube gas cathode b a anode E 1 V E E w r ln( b / a) r ln( b / a) E threshold 1/r with E: radial electric field, r: radial distance from axis, b: inside radius of cylinder, a: radius of wire Induced charge i Nqv( r) E a Induced current V Nqµ ( E) (ln( b / a)) w Nq µ ( E) E E r w µ ( E) 2 r electron current orders of magnitude greater than ion current because of the greater mobility and smaller radii but very fast (down to less than 1 ns) r Q Q el ions Q Q Q( t) i( ) d q 1 Nq ln( b / a) 1 Nq ln( b / a) el ions t 0 ln( x / a) ln( b / x) x a b x x2 x1 E 1 dr r 1 dr r w d x ln( x / a) Nq ln( b / a) ln( b / x) Nq ln( b / a) ln(2) ln(1000) 0.1

19 Time evolution of signal Ions have to drift back to cathode, i.e. dr is big. Signal duration limited by total ion drift time! Need electronic signal differentiation to limit dead time.

20 Detector Operation Modes

21 Operation modes Ionization chamber Geiger-Müller counter Proportional counter

22 Wire chamber operation modes Ionization Chamber operation point: voltage high enough to prevent recombination very simple and robust detector: no amplification signal proportional to the number of primary ion pairs but very small a single ionization cannot be processed by electronics suitable for the measurement of high rates / doses (nuclear industry, medical applications)

23 Ionization chamber An ionization chamber collects the generated charge (from ionization) without amplification I ~ radiation dose in the Ionization volume per time Impossible to measure a single ionization: electron charge (= - ion charge) = Coulomb (for 1 na 1000 ions / s) electronics needs at least Coulomb (fc) If we want to see a single ionization we need an amplification of at least 10,000 times without amplification of the noise

24 Wire chamber operation modes Proportional Chamber operation point: voltage high but still below saturation gain ~ 10 5 (typical) signal proportional to the number of primary ion pairs de/dx measurement possible in this mode signal mostly from ions drifting away avalanche clusters space point of primary ionisation is preserved space resolution use for particle detection in multi wire proportional chambers

25 Proportional Counter Gas amplification via secondary ionization: Amplification from 10 3 up to Small wire diameter necessary

26 Signal clusters in proportional chambers signal pulse recorded with high time resolution showing clusters of individual avalanches space correlation is maintained time scale ~10ns

27 Wire chamber operation modes Limited Proportionality operation point: gain > 10 6 strong secondary ionisation space charge effects destroy shape near wire 1 E r saturation of signal sets in this is sometimes wanted, when the number of particles is to be determined by the total signal height (e.g. slow protons give the same signal height as m.i.p.s - de/dx~1/ 2!)

28 Geiger-Müller counter Wire chamber operation modes operation point: G > 10 7 local avalanches don t exist any longer chain reaction atoms become excited and emit photons photons undergo photo effect secondary electrons secondary electrons are accelerated in the very high field regions produce avalanches everywhere no local space information obtained quenching gas (e.g. alcohols) needed to stop discharge continuous gas discharge signal independent of primary energy deposit

29 Avalanche creation in a Geiger counter photon induced discharge in a Geiger counter

30 Properties of Geiger-Müller Counter advantages: can detect radiation (if window thin!) large signals: V = Q / C 10 4 fc/ 10 pf = 1 V! det. eff. 100% operate in center of plateau! disadvantages: no space resolution large dead time low rate capability

31 proportional limited propotional Geiger proportional prop. signal start of Geiger pulses limited prop. Geiger pulses prop. signal Geiger WS 2005/06

32 proportional limited propotional Geiger

33 Streamer Mode Thick anode wires 100mm High voltage (5 kv) High quenching gas content > 50% Gas amplification typ Avalanche stays at the transmission point Long dead time Discontinous transition between proportiona and streamer mode

34 Comparison Guess the operation mode! Gas discharge particle (wire is vertical, particle trajectory is horizontal)

35 Comparison Solution: Gas discharge particle proportional Geiger-Müller Streamer (wire is vertical, particle trajectory is horizontal)

36 Detector Concepts

37 GEM GEM MWPC MSGC On this picture, we can see behind a lens, Georges Charpak and a particle detector Jet Chamber

38 Space point measurement with MWPC Multi-Wire-Proportional-Chamber (MWPC): Plane layer of proportional chambers without intermediate walls Anode wire, typ. gold plated tungsten (10-30 mm) Gas amplification up to 10 5 Typ. distance d = 2 mm Space resolution RMS = d / 12 ~ 600 mm Signal amplitude at the cathode depends from distance to avalanche

39 Space point measurement with MWPC Multi-Wire-Proportional-Chamber (MWPC): Plane layer of proportional chambers without intermediate walls Anode wire, typ. gold plated tungsten (10-30 mm) Gas amplification up to 10 5 Typ. distance d = 2 mm Space resolution RMS = d / 12 ~ 600 mm Signal amplitude at the cathode depends from distance to avalanche Typical parameters: L=5mm, d=1mm, a wire =20mm.

40 Second Coordinate Crossed wire planes: Ghost hits, restricted to low multiplicities, different stereo angles Charge division: Resistive wires (Carbon y 2k / m) track Q A Q B y L Q QB Q A B y L up to 0.4% L Time difference: y L track ( T ) 100 ps ( y) 4cm (OPAL) CFD T CFD One wire plane + 2 cathode planes:

41 ghost hits Hit ambiguities

42 Multi-Wire-Proportional-Chamber Field lines of a MWPC Scetch of the cathode readout of a MWPC: Pros and Cons: Simple and robust Wire instability via electro static repulsion and pass through of particles distortion of field If cathode readout is used: higher mechanical work load and more complicated signal processing

43 End of Lecture 5

44 Drift chambers Space point = drift velocity x drift time Resolution: approx µm

45 Drift chamber Comparable to MWPC, but much less wires Prinziple: propagation time of avalanche x = v t typ. resolution 100 mm More about drift chambers in a few minutes

46 Micro-strip gas chambers Made possible by modern photolithography techniques cathode anode general layout

47 Micro-strip gas chambers Drift electrode higher rates possible Anode strip ~5 mm Glass support Cathode width: Anode width: Pitch: 90 mm 10 mm 200 mm Back plane Cathode strips

48 Micro-strip gas chambers + ions are sucked by cathode strips ~no space charge effects + resolution x ~ 30 µm (factor ~10-20 better than MWPC) - gas gain limited by e - emission from cathode to < 6x R/O needs VLSI electronics - problem: substrate charges up (glass is not a perfect insulator) & interactions with substrate sparks

49 MICROMEsh GASeous Detector Thin-gap parallel plate chamber

50 Gas Electron Multiplication - GEM Thin metal-coated polymer foil chemically pierced by a high density of holes. On application of a voltage gradient, electrons released on the top side drift into the hole, multiply in avalanche and transfer the other side. Proportional gains above 10 3 are obtained in most common gases. Thickness: V: Hole Diameter: Pitch: ~ 50 mm V ~ 70 mm ~140 mm

51 GEM Double GEM + readout pads Same gain at lower voltage Less discharges

52 2 mm Resistive Plate Chambers 10 kv spacer bakelite (melamine phenolic laminate) Double and multigap geometries improve timing and efficiency pickup strips Gas: C 2 F 4 H 2, (C 2 F 5 H) + few % isobutane (ATLAS, A. Di Ciaccio, NIM A 384 (1996) 222) Time dispersion 1..2 ns suited as trigger chamber Rate capability 1 khz / cm 2 15 kv Two operation modes: Streamer and avalanche Streamer mode is very fast and does not require amplification electronics but the rate capability is limited to O(100 Hz/cm 2 ) Avalanche mode has a 10 times lower signal ( electronic amplification needed) but is able to handle khz/cm 2

53 Drift Chambers scintillator DELAY Stop Start TDC Resolution determined by diffusion, path fluctuations, electronics primary ionization statistics drift anode (N. Filatova et al., NIM 143 (1977) 17) low field region drift high field region gas amplification Measure the arrival time on wire x v ( t t ) d - 0 To be solved: Left-Right ambiguity

54 Planar Drift Chambers Optimize geometry constant E-field Choose drift gases with little dependence v D (E) linear space - time relation r(t)

55 Cylindrical Drift Chambers

56 t (ns) t (ns) Drift Chambers MAGNETIC FIELD EFFECTS: DISTORSIONS IN DRIFT CHAMBERS

57 Jet Drift Chamber r-f point from drift time ( ~0.1mm) z-position from charge sharing ( ~1cm), runtime differences or stereo wires left-right ambiguity solved by displacement of anode wires Lorentz angle because of crossing E and B field Hier: K 0 S,K +,p +

58 Jet Drift Chamber

59 Straw-Tube Tracker Aluminium-Mylar-Film, 30 µm thick Gas: Ar/CO 2, p ~ 2 bar self supporting and stretching at 1 bar overpressure 1 m Cathode Anode wire (20µm, +2kV) p~1.2bar ~1kg 1 cm

60 isochrones Straw-Tube Tracker

61 Straw-Tube Tracker ~5000 straws in ~26 layers 15 kg total weight 1.5 m

62 Time Projection Chamber (TPC) Large volume active detector. full 3-D track reconstruction x-y from wires and segmented cathode of MWPC z from drift time and de/dx gas volume with E & B fields B y x E drift σ r 200 μ m σ z 1mm z (E~ V/cm/atm) Drift velocity ~ 5-7 cm/ms wire chamber to detect projected tracks

63 TPC Setup Electrons are collected at the gating grid until the gate is opened by a trigger. After the grid the e - are accelerated (avalanches). e - are collected at the wire and ions at the pads. Space resolution improved by chevron pads:

64 TPC Setup

65 TPC Question: Why is the TPC gated? Gate open Gate closed Gate wires Cathode wires V g = 150 V Anode wires Cathode pads

66 TPC Space charge problem from positive ions, drifting back to medial membrane gating Gate open Gate closed Gate wires Cathode wires V g = 150 V Anode wires Cathode pads

67 TPC Usually B E improvement of diffusion Drift length 1m Rather (very) stringent requirement on homogeneity of E and B field Space charge by ions Slow detector t D ~ ms The ALICE TPC 420 cm E B

68

69 Solenoidal Tracker At RHIC STAR Transp ort

70 STAR in CAVE