Presentation is loading. Please wait.

Presentation is loading. Please wait.

Gaseous Tracking Detectors TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAAAAAAAA.

Similar presentations


Presentation on theme: "Gaseous Tracking Detectors TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAAAAAAAA."— Presentation transcript:

1 Gaseous Tracking Detectors TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAAAAAAAA

2 Peter Hansen, Lecture on tracking detectors P 2 Overview Straw trackers MWPCs TPCs Micropattern gas detectors Momentum measurement Thanks to Christian Joram and others from whom I borrowed slides

3 Peter Hansen, Lecture on tracking detectors P 3 Gaseous tracking detectors Provides economic tracking over large areas Measures primary ionization of charged tracks in the gas Works by having avalances of secondary ionization initiated when the primary ionization hits a small-area anode. This provides built-in amplification.

4 Peter Hansen, Lecture on tracking detectors P 4 Signal formation

5 P 5 The ATLASTransition Radiation Tracker Installation Design details Simulation and performance Calibration and Alignment Particle ID An example gaseous tracker

6 Peter Hansen, Lecture on gaseous tracking detectors P 6 Transition Radiation Tracker Length:Total6802 cm Barrel 148 cm End-cap257 cm Outer diameter206 cm Inner diameter cm # straws: Total Barrel End-cap # electronic channels Weight~1500 kg Barrel End-caps  Gas 70%Xe+27%CO 2 +3%O 2 Xe for good TR absorption CO 2 > 6% for maximum operation stability  Gas gain 2.5  10 4  Straw diameter - 4 mm  Wire diameter - 30 μm  Polypropylene foil/fibre radiators

7 Peter Hansen, Rome seminar, 04-May-07 P 7 protons Atlas CMS ATLAS

8 Peter Hansen, Lecture on geseous tracking detectors P 8 TRT performance B d 0  J/ψ K s 0 6 keV 0.3 keV MIP threshold – precise tracking/drift time determination TR threshold – electron/pion separation 90% electron efficiency pion rejection ~1 TR hit ~7 TR hits High-γ charged particles (e.g. electrons) emit transition radiation (X-rays) when they traverse the radiators, detected in the straw tubes as larger energy deposition (8-10 KeV) 20 GeV beam

9 Peter Hansen, Lecture on gaseous tracking detectors P 9 The TRT Straw

10 P 10 TRT commisioning The TRT barrel and end-caps were installed in their final position inside the cryostat in 2008 with all services. One “splash-event” in 2008 was very useful for timing all the channels relative to each other. Commissioning in 2009 with cosmic rays. The tracking resolution was found to be 160 microns. The High Threshold probability was found as expected.

11 The first collisions Dec2009 Peter Hansen, Lecture on gaseous tracking detectors

12 Peter Hansen, Lecture on tracking detectors P 12 The TRT gas 70% Xe (high amplification A=25000, absorbs X-rays) 27% CO 2 (quenches ultraviolet, does not polymerize) 3% O 2 (intercepts unwanted electrons) GasZA  E min WiWi dE/dxNpNp Xe g/cm eV22 eV1.23 MeV/(g /cm 2 ) 44 ion/cm Co

13 Peter Hansen, Rome seminar, 04-May-07 P 13 Drift of electrons in a B field New gas stabilizes drift velocity in B field. -and it does not eat glass

14 Peter Hansen, Rome seminar, 04-May-07 P 14 GEANT4 Simulation Primary clusters formed according to PAI model Custom TR physics process Fiber radiator Photon x-sect

15 Peter Hansen, Rome seminar, 04-May-07 P 15 Digitization simulation Digitization includes Diffusion and capture Avalance formation Electronics shaping Noise Reflections from ends Propagation along wire TOF and T0 fluctuation Threshold fluctuations

16 Peter Hansen, Rome seminar, 04-May-07 P 16 Custom ASIC readout

17 Peter Hansen, Lecture on tracking detectors P 17 The electric field According to Gauss, the capacity per unit length, C, and the anode voltage, V 0, determines the electric field: Integrating from the straw wall to the wire radius and equating The result to V 0, gives

18 Peter Hansen, Lecture on tracking detectors P 18 Avalance development The movement of a charge, Q, in a system with capacitance per unit length, C, by a distance dr gives a voltage signal v Almost all avalance electrons are created in the last mean free path

19 Peter Hansen, Lecture on tracking detectors P 19 Shaping The positive ions drift to the cathode gives rise to: This contribution is about 50 times larger than that of the electrons. But it is a slow signal. By terminating the wire in a resistance, the signal is differentiated with a time constant, RC. For the TRT, the rise-time is 8ns and the duration only 20ns.

20 Peter Hansen, Rome seminar, 04-May-07 P 20 Pulse shaping Problem: Long tail (much longer than the 25ns bunch spacing) from the positive ions moving outwards Solution: The ASDBLR front-end chip restores the baseline within ~20ns

21 Peter Hansen, Rome seminar, 04-May-07 P 21 A simple calculation of A Balancing concerns, the optimum gas amplification is In any cascade process, we have Leading to a total amplification of At ionization energy, W=22eV, Xe presents a cross-section of cm 2 and the electron has a mean free path of

22 Peter Hansen, Rome seminar, 04-May-07 P 22 A simple calculation of A The distance from the wire, where the avalance starts, is given by: The Townsend coefficient is assumed to be proportional to the kinetic energy of the electrons Assuming  =log 2 / at threshold, we have

23 Peter Hansen, Rome seminar, 04-May-07 P 23 A simple calculation of A Finally we get This leads for the TRT to the target amplification of at a voltage of 1513 Volts, probably by luck this is close to the true value 1530 Volts.

24 Peter Hansen, Lecture on tracking detectors P 24 Drift Chambers

25 Peter Hansen, Lecture on tracking detectors P 25 The driftvelocity - naively Assuming the electron is brought to halt at each collision and that the mean free path is independent of energy, we have at 1mm from a TRT wire: The correct answer is

26 Peter Hansen, Lecture on tracking detectors P 26 Complications The difference between prediction and fact is due to: Dependence on electron energy of cross-section (mainly the Ramsauer minimum around 1eV) The quencher gas. The magnetic field bending the drift-trajectories up/down Diffusion

27 Peter Hansen, Lecture on tracking detectors P 27 modifications from quencher gas

28 Peter Hansen, Lecture on tracking detectors P 28 Diffusion (no field case) The mean velocity of a particle in an ideal gas is given by Maxwell: According to kinetic theory, a collection of particles localized at x=0 at t=0, will later have a distribution: Where D is the diffusion coefficient

29 Peter Hansen, Lecture on gaseous tracking detectors P 29 The diffusion coefficient According to statistical mechanics: where the mean free path for an ideal gas is: By substitition:

30 Peter Hansen, Lecture on gaseous tracking detectors P 30 Diffusion in an electric field A classical argument by Einstein gives for an ideal gas in thermal equilibrium with the drifting ions: In practice, we parametrize the spread of the coordinate in the drift direction as: Where the characteristic energy can be calculated for known cross-sections and energy-losses of the electron-gas collisions.

31 Peter Hansen, Lecture on gaseous tracking detectors P 31 The TRT resolution The gasses in the TRT have a characteristic energy of about 2 eV. Thus we have for the coordinate perpendicular to the wires a spread of 0.114mm: For an average of 10 primary ion pairs, the distance of the closest electron to the wire has a spread of about 0.012mm The drift-time binning in 3.125ns contributes 0.043mm Noise and gain variations gives 0.035mm Uncertainties in wire position and time=0 gives 0.036mm All together this gives a coordinate resolution of about 0.132mm, in excellent agreement with detailled calculations – and with data.

32 Peter Hansen lecture on gaseous tracking detectors P 32 Drift time simulation The leading edge of signal gives the drift time of the ionization electrons and hereby the distance from the charged particle to the wire The simulation includes diffusion, Lorentz-forces, signal propagation and shaping, channel-to-channel fluctuations in threshold and noise amplitudes (deduced from the observed noise levels), the time structure of noise – and more

33 Peter Hansen lecture on gaseous trackling detectors P 33 CTB data and simulation Perfect agreement! But only if using an average threshold of 161eV - where previous it was 300eV. The explanation is probably new noise and threshold fluctuations in MC – but there is no profound understanding. Residuals fromThomas Kittelmann thesis Sigma=0.132mm 100 GeV pions

34 Peter Hansen, lecture on gaseous tracking detectors P 34 CTB data and simulation Also the Time Over Threshold is reasonably well simulated And the hit efficiency is predicted to 95% in agreement with data

35 Peter Hansen, Rome seminar, 04-May-07 P 35 Tracking performance in ATLAS

36 Peter Hansen, Rome seminar, 04-May-07 P 36 Calibration Calibration is concerned with T0, the R(t-T0) relation, the high threshold probability and noise removal. The ”V-plot” of time versus track impact position is used

37 Peter Hansen, Rome seminar, 04-May-07 P 37 Calibration The tip of the V yields T0 (and, if a single wire is plotted, also the wire position). The peak position in each 3ns bin of t-T0 yields R(t-T0) (Note that the average position is not good because of tails at long arrival times for tracks passing close to the wire)

38 Peter Hansen, Rome seminar, 04-May-07 P 38 Electron Identification in test beam Performace of combined pion rejection at 90% electron efficiency Universality of the HT probability

39 Peter Hansen, Lecture on tracking detectors P 39 Multiwire proportional counters In 1968 it was shown by Charpak that an array of many closely spaced anode-wires in the same chamber could each act as an independent proportional counter. This provided an affordable way of measuring particles over large areas, and the technique was quickly adopted in high energy physics. Later it has found applications in all kinds of imaging of X- rays or particles from radioactive decay.

40 Peter Hansen, Lecture on tracking detectors P 40 Multiwire proportional counters

41 Peter Hansen, Lecture on tracking detectors P 41 Second coordinate –some ideas

42 Peter Hansen, Lecture on tracking detectors P 42 The TPC

43 Peter Hansen, Lecture on tracking detectors P 43 The TPC end-plate

44 Peter Hansen, Lecture on tracking detectors P 44 The TPC field cage Experience shows that the greatest challenge in a TPC is to maintain a constant axial electric field. This field is made by electrodes on the inner and outer cylinders of the Field Cage, connected to a resistor chain. Some useful elements are: A Gating Grid to avoid space-charges. Tight mechanical tolerances wrt the ideal cylindrical shape (while keeping the material budget low). Severe cleaniness, (the tiniest piece of fiber in the cage may short-cut two electrodes and distort the field.) As little as possible of insulator exposed to the drift-volume to avoid build-up of charge on the insulator. Perfect matching of equipotential surfaces at the end plane is needed to avoid transverse field components.

45 Peter Hansen, Lecture on tracking detectors P 45 Electron drift in E and B fields The TPC is immersed in a magnetic field parallel to the electric field. this Langevin equation becomes in the stationary state Introducing the mobility and the cyclotron frequency we get

46 Peter Hansen, Lecture on tracking detectors P 46 Electron drift in E and B fields Solving for the drift velocity: Since the coordinate resolution is high in the azimuthal direction, in order to measure the momentum well, a component of the velocity due to field distortions is very dangerous. But high  helps! At high , v D  is suppressed by powers of  except for the effect of a B  component. However, B  is zero on the average according to Ampere.

47 Peter Hansen, Lecture on tracking detectors P 47 Electron diffusion in E and B fields In the transverse projection an electron follows the arc of a circle with radius  = v T / , where the mean squared velocity projected onto the transverse plane is: After a time, t, the electron has reached a transverse distance of so the spread after one collision is:

48 Peter Hansen, Lecture on tracking detectors P 48 Electron diffusion in E and B fields After a longer time, the transverse spread is Thus in large magnetic fields the transverse diffusion is reduced by a factor 1+  2  2 e.g. for Ar/Ethane and B=1.5Tesla the reduction is a factor 50. This is what makes a TPC possible! Thus you can get a precision of about for about 30 points on each track over a 1m-2m radial distance from the collision point. Note that this is without any significant multiple scattering and with a good resolution also in the longitudinal direction. It is also relatively CHEAP, since it is mainly gas.

49 Peter Hansen, Lecture on tracking detectors P 49 The ALEPH TPC

50 Peter Hansen, Lecture on tracking detectors P 50 TPC calibration

51 Peter Hansen, Lecture on tracking detectors P 51 Why use silicon instead of gas

52 Peter Hansen, Lecture on tracking detectors P 52 Micropattern gaseous detectors Silicon detectors are still very expensive over large areas, and they suffer from radiation damage. Although they give more charges, they have no inbuilt amplification and are slow to read out. So if gaseous counters could be made small, fast and precise, they would be quite attractive. The way to go is to employ the same precision methods for fabricating micro-structures in silicon on the gas detector readouts.

53 Peter Hansen, Lecture on tracking detectors P 53 Micro gaseous detectors

54 Peter Hansen, Lecture on tracking detectors P 54 Gas Electron Multipliers (no spark)

55 Peter Hansen, Lecture on tracking detectors P 55 Two GEM’s are better than one

56 Peter Hansen, Lecture on tracking detectors P 56 Thin gap chambers

57 Peter Hansen, Lecture on tracking detectors P 57 Resistive plate chambers

58 Peter Hansen, Lecture on tracking detectors P 58 From coordinates to momenta Given a number of ”track hit-coordinates” from the various detector elements, the parameters of the track is determined from a least squares fit. In a solenoidal field geometry, the parameters are those of a helix: The track position at closest approach (perigee) to the beamline (3 parameters) The angle of the track to the beamline (polar angle) The signed curvature +-1/R

59 Peter Hansen, Lecture on tracking detectors P 59 Momentum measurement

60 Peter Hansen, Lecture on tracking detectors P 60 Momentum accuracy

61 Peter Hansen, Lecture on tracking detectors P 61 Multiple scattering

62 Peter Hansen, Lecture on tracking detectors P 62 Total momentum error

63 Peter Hansen, Lecture on tracking detectors P 63 Technology choices Since the TPC is so superior why is it not used at the LHC? This is because it is too slow. It is in fact used in ALICE who do not depend on a very high intensity beam. Why then are the micropattern gas detectors not used? Well, they are for trigger chambers. But for the main trackers the technology was deemed too risky at the time of decision.


Download ppt "Gaseous Tracking Detectors TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAAAAAAAA."

Similar presentations


Ads by Google