Tracking Detectors at HEP

Transcription

1 Experimental Nuclear and Particle Physics Seminar Tracking Detectors at HEP László Oláh 5th December 2011

2 OUTLINE I. Introduction II. Gaesous Detectors III. Semiconductor Detectors IV. Applications

3 I. Introduction Nowday's prime motivations at HEP: Experimental study of the Higgs Mechanism consistency of Standard model above 1 TeV) (mathematical Study the QCD matter under extreme conditions of temperature and density and parton momentum fraction Explore the new energie domain (E= 7 TeV, L= 1027 cm-2s-1) Huge detector systems: Tracking and PID (measure the velocity and impulse) Meauring the impulse: p = Bqr Good spatial resolution close to the interaction point 3

4 HEP Detectors 4

5 Particle's in the Detector Muon = Tom Hanks, the long-range runner 5

6 Passage of Particles Through Matter Ionization: Bethe-Bloch formula describes the energie loss of charged particles: Interaction depends only on me electron mass 1/β: velocity dependence of interaction slower particle can interact more) (the β/sqrt(1-β) = 4 Mininimum Ionizing Particle (MIP) The β2: Relativistic Rise above the energie minimum The z2: charge basement particle selection Density Effect: at higher energies the electric field extends, so the distant-collison contribution increases as ln(βγ) However the medium becomes polarized and limit the field extension 6

7 Passage of Particles Through Matter Multiple Coulomb Scattering: A charged particle traversing a medium is deflected by many small-angles catters Most of this deflection is due to Coulomb scattering from nuclei: θ0 = 13.6 MeV/(βpγ)*Z*sqrt(x/X0)*[ ln(x/x0)] Cherenkov Radiation: Angle θc of Cherenkov radiation, relative to the particle s direction, for a particle with velocity βc in a medium with index of refraction n is cos(θc) = (1/nβ) Transition Radiation: Charged particle radiates if it's phase velocity is grater than the local phase velocityof light Charged particle radiates if it crosses suddenly from one medium to another with different optical properties particle with charge Ze crosses the boundary between vacuum and a medium with plasma frequency ωp is I = αz2γωp /3, where hωp/2π 20 ev Bremsstrahlung and pair production 7

8 Tracking Detectors Gaesous Detectors: Charge particles create electron-ion pairs along their path Number of electron-ion pairs: /cm (1 ac/cm) Charges are internally multiplied in order to provide a measerable signal Solid State Detectors: Charge particles create electron-hole pairs along their path Number of electron-hole pairs in Si: 8 x /cm (8 fc/cm) Charge is created by incoming particle is sufficient The induced signals are readout by dedicated readout electronics. The noise is caracterized by Equivalent Noisy Charge (ENC): charge signal at the input that produced an output signal equal to the noise. In Order to register a signal, the registered charge (~10 3 x q) must be >> ENC 8

9 II. Gaesous Detectors Why gas??? In electric field we can collect the electrons from large volume How can we collect the electrons? Geiger-Müller Tube in 1928 Ionizing Avalanche ( ) Electrons collected by anode wire 9

10 MWPC and Drift Chamber MWPC (Charpak, 1968): One plane of thin wires is placed between 2 paralel plates Wire distance 2-5 mm and distance between cathode planes 10 mm Ionization, avalanche and electrons are collected by the anode wires Bad mechanical tolerance! Drift Chamber (1970): Electrons are moving into the Sense Wires and produce an avalanche which induces a signal that is readout by electronics. The time between the passage of particle and arrival of the electrons at the wires is measured Drift Time is measure of the position of the particle Wire distance is reduced save electronic cahnnels 10

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12 Close Cathode Chamber Assymetric MWPC: Distance of lower cathode & wires: 1.5 mm 2 dimesional location: Lower cathode is segmented into 4 mm wide pads Field Wire wire distance 4 mm Sense wires (20-25 micron) & Field Wires ( micron) Typical applied voltages: ~ 1 kv at SW -500 V at Fw and Cathode Mechanical tolerance Low material budget: g Low coast 12

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14 Time Projection Chamber TPC = MWPC + Drift Chamber 3 dimensional track reconstruction Can measure the particle's track PID with measure the energie loss Homogen electric field by Field Cage B E : Can measure the impulse: p = Bqr Can decreasing the electron's drift 14

15 Wire Sructure of the TPC To much electrons are collected can deform the homogen electric field Shileding grid (0 potencial) and a gating grid above the wire plate We can close the gate with Alternate Voltage (left figure): Drifting electrons are collected by the gating grid Shielding grid close the avalanching region The incoming particle give the trigger signal and can open the gate (right figure) Electrons can reach the anode wires Slow ions can not go back to the drift volume if the gete is closed again 15

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17 Gas Electron Multiplier (GEM) 50 microns thick, Cu covered capton fólia with holes in every 70 microns Enough electric field in the holes to create avalanche with 300 V applied voltage Gain: 102 at least 2 or 3 layers GEM Excellent spatial resolution Flexible GEM with cilinder shape Other constructions: TGEM, Micromegas 17

18 Applied Gaesous Noble gas (Ar, Ne, etc.) No chemical activity Quenching gas (CO2, CH-s) 1 atomic gas: no rotation and no vibration modus elastic collisons and little energie loss 11.6 ev UV photons emitted by the Ar atoms UV photons collide to the cathode (Cu: 7.7 ev ''limit'' energie) and knock out electrons The ionisation becomes self-supporting O2 diffusion into the chamber (100 ppm) bind the electrons decreasing efficiency 18

19 II. Solid State Detector Solid state ionization chamber Ionization energie is proportional to the bandgap (1.12 ev in Si) 4.4 ev required energie to produce electron-hole pairs MIP forms 80 pairs per micron the charge deposition in 300 micron thick detector is about 4 fc well measurable quantity 19

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21 P-n junction Consists of 2 semiconductor regions of opposite doping type If external potencial applied with positive polarity onthe n side (and with negative polarity on the p side) the charges move to the opposite polarity electrodes Depleted zone near the junction Thermal equlibrium: Ndxn = Naxp Poisson equation: 21

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23 C-V Measurements If we increase the depletion voltage, the thickness of the depletion zone will increase Reaching the depletion thickness the capacitance will saturate. From CV measurements, the Effective doping and corresponding thickness have been calculated 23

24 Effective Doping Calculations 25

25 I-V Measurements Increasing the tickness of the depletion zone will increase the number of charge carriers and therefore the measured current will increase (I~d and d ~ V1/2 I ~ V1/2) Avalanche breakdown process: for high voltages the the accelareted charge carriers can crash with the silicon atoms and pump up the electrons from the valance bands, leading to an electon shower which increase the measured current 24

26 Thank You! :)