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Semiconductor Detectors for Particle Physics

Semiconductor Detectors for Particle Physics

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Semiconductor Detectors for Particle Physics

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  1. Semiconductor Detectorsfor Particle Physics T. Bowcock

  2. This week • HEP experiments • Photo Detectors • Strip Detectors • Pixel Detectors T. Bowcock

  3. HEP Experiments • Small introduction to HEP experiments • Accelerators • Detectors • Will look at specific experiments in last lecture T. Bowcock

  4. Accelerators • Primary tool T. Bowcock

  5. Linac Electron Gun T. Bowcock

  6. Linac • Buncher T. Bowcock

  7. Linac Cavities T. Bowcock

  8. Linac Continued T. Bowcock

  9. Cyclotron T. Bowcock

  10. Cyclotron T. Bowcock

  11. Synchro-Cyclotron(Synchrotron) T. Bowcock

  12. Focussing T. Bowcock

  13. Storage Rings T. Bowcock

  14. Storage Rings T. Bowcock

  15. Facilities • CESR (Cornell, USA) , e+e- at 10GeV (symmetric) • DESY ( Hamburg, Germany), ep asymmetric (27 and 920) GeV • PEP2 (SLAC, Stanford USA) – e+e- at 10GeV (asymmetric) • KEK e+e- at 10GeV asymmtric • Tevatron (FNAL, Chicago, Illinois) p-pbar at 2TeV • LHC (CERN, Geneva, Switzerland) pp at 7TeV/beam • Other facilities at Brookhaven, Beijing, …. T. Bowcock

  16. Large Hadron Collider T. Bowcock

  17. LHC T. Bowcock

  18. Experimental Output Simulated Higgs event - ATLAS W event – UA1 T. Bowcock

  19. Detectors T. Bowcock

  20. Detector Function • Measure the energy carried by electrons and photons in each direction from the collision. • Measure the energy carried by hadrons (protons, pions, neutrons, etc.) in each direction. • Identify which charged particles from the collision, if any, are electrons. • Identify which charged particles from the collision, if any, are muons. • Identify whether some of the charged particles originate at points a few millimetres from the collision point rather than at the collision point itself (signalling a particle's decay a few millimetres from the collision point). • Infer (through momentum conservation) the presence of undetectable neutral particles such as neutrinos. • Have the capability of processing the above information fast enough to permit triggering about 10-100 potentially interesting events per second out of the billion collisions per second that occur, and recording the measured information. • The detector must also be capable of long and reliable operation in a very hostile radiation environment. T. Bowcock

  21. Detector Structure T. Bowcock

  22. Detector Structure T. Bowcock

  23. Photodetectors • Photodetectors perform the function of converting optical photons into electrical signals. The process within the semiconductor during this conversion is essentially the carrier generation by incident light followed by the sensing of these carriers. T. Bowcock

  24. Wavelength-Energy T. Bowcock

  25. Routes to Absorption T. Bowcock

  26. Absorption versus Wavelength T. Bowcock

  27. Photoconductor T. Bowcock

  28. Photoconductor In thermal equilibrium number recombining equal number generated T. Bowcock

  29. Gain Photoconductor T. Bowcock

  30. Photoconductors • Photoconductors are limited by the dark current in the active region. • To get extended wavelength sensitivity impurities may be added so the detector is more extrinsic in its behaviour. • However these impurities add to the dark current in the sensor hence limiting the sensitivity of the sensor • Another limitation is the recombination in the sensitive region. • solution to this are Blocked Impurity Band detectors. T. Bowcock

  31. Photodiodes • High impedance through the depletion region • High impedance at elevated temperatures, compared with photoconductors • Little recombination noise • Great gain that allows photon counting. T. Bowcock

  32. P-i-n photodiode • One of the simplest kind of photodiodes is the p-i-n photodiode in which an intrinsic piece of semiconductor is sandwiched between two heavily (oppositely) doped regions. T. Bowcock

  33. P-i-n • The two charge sheets (on the n+ and p+) sides produce a field which, even without an external field supplied, will tend to separate charges produced in the depleted region. • The separated charges will be swept to either terminal and be detected as a current provided that they are not recombined. T. Bowcock

  34. Avalanche Photodiode • If a high enough reverse bias is applied to the photodiode avalanche multiplication may take place. • The multiplication will result in substantial gain in the diode. • A larger reverse-bias voltage results in a larger gain. • It should be noted that increasing the reverse-bias voltage also results in increased noise levels. • Stochastic process T. Bowcock

  35. Applications • Calorimeters (e.g. CMS experiment at CERN) • Crystals (CLEO at Cornell, Babar SLAC) • Radiation Sensors • Optical Coupling for decoupling electrical signals T. Bowcock

  36. Strip Detectors • Reverse biased diode T. Bowcock

  37. Diode Behaviour T. Bowcock

  38. Diodes with overvoltage T. Bowcock

  39. Charge Velocity T. Bowcock

  40. Charge velocity depends BC’s T. Bowcock

  41. Velocity/Current moving charge with velocity in the circuit I s given by Thus a simple expression can be formed that gives the induced current at any time i.e. T. Bowcock

  42. Currents in Diodes s Figure 6: Signal for separation of an electron-hole in a diode T. Bowcock

  43. Ramo’s Theorem • Ramo’s Theorem • Above we assumed a form for the induced current on one of the electrodes. In fact a general theorem exists (called Ramo’s or the Schokley-Ramo Theorem) that in its general form states that the current induced in electrodej due to a moving charge at point P is • The derivative is that of the electrostatic potential that occurs when we put a unit voltage on electrode j and keep all other electrodes at zero potential and is known as the weighting field. T. Bowcock

  44. Weighting Field Figure 7: Weighting filed for a strip detector. Note the field is high close to the strip coordinate (at 300 and thus the currents are induced close to the high field region i.e. strip. T. Bowcock

  45. MIPS • Ionizing Particles • The rate of ionization loss is given by the Bethe-Bloch Equation T. Bowcock

  46. Protons in Si T. Bowcock

  47. MIPS • For a Si the dE/dx (for protons) is minimum at about 0.4keV which would predict the creation of 110electron holes pairs/micron. • The actual figure is measured to be about 80 e-h pairs per micron. • The difference between the two being accounted for by energy lost by the incident particle to the lattice via phonon interactions. T. Bowcock

  48. Current in diode • For a 300 micron thick sensor we expect of order 24,000e/h pairs to be created on the passage of a minimum ionizing particle through the sensor. Given a collection time of approximately 2ns the peak current generated due the deposition of this charge is approximately given by T. Bowcock

  49. Landau Distribution Most probable charge ≈ 0.7 mean T. Bowcock

  50. -Vb>-Vdep -Vb<-Vdep p+ p+ n n n+ n+ Vb>Vdep Vb<Vdep p+ p p n+ Partially depleted Fully depleted Fully depleted Partially depletion Diode Bulk connection. T. Bowcock