1. Tracking detectors/1

2. Historical detectors for tracking • In the past, several techniques were used to track (and visualize) particles: • nuclear emulsions • cloud chambers • bubble chambers • spark chambers • streamer chambers

3. Nuclear emulsions Nuclear emulsions are among the oldest techniques used to track particles The passage of charged particles are recorded as a track of developed Ag-halide grains Single layers (about 600 m thick) or stacks with several layers

4. Nuclear emulsions MIP particles produce approximately 270 grains per mm of track length Measurement of grain density may give dE/dx For stopping particles, range may give the total energy Spatial precision: about 1 m No time information No fast analysis of tracks (visual observation)

5. Nuclear emulsions 32S at 6.4 TeV

6. Nuclear emulsions

7. Nuclear emulsions Range-energy relation in nuclear emulsions

8. Cloud chambers Cloud chambers are detectors filled with a gas and vapor mixture. A sudden expansion results in supersaturation of the vapor. After the passage of charged particles, droplets are formed and tracks can be photographed by suitable trigger systems. Large area detectors Track analysis tedious

9. Cloud chambers A Wilson chamber for cosmic rays, 1955

10. Cloud chambers Anderson and his cloud chamber

11. Cloud chambers Discovery of the positron in a cloud chamber by Anderson (August 2, 1932) while observing cosmic ray tracks

12. Bubble chambers In a bubble chamber a liquid is heated above its boiling point. A sudden expansion produces bubbles along the track of the particle. (Glaser, 1952) Need a trigger The track is photographed

13. Bubble chambers Advantages and disadvantages good spatial precision (10 - 150 m) large sensitive volume 4 geometrical acceptance tedious photograph measurements sensitive time  1ms, complicated operations, cryogenics, safety hardly to use at colliders

14. Bubble chambers Reconstruction of a  decay in a bubble chamber, CERN 1973

15. Bubble chambers 30 cm hydrogen bubble chamber (CERN), 1970

16. Bubble chambers Gargamelle bubble chamber, CERN, 1970

17. Bubble chambers An event in the Gargamelle bubble chamber

18. Bubble chambers BEBC bubble chamber, CERN 1977

19. Bubble chambers A reconstructed event in the BEBC hydrogen bubble chamber

20. Bubble chambers Measuring track angles by use of a protractor, CERN 1958

21. Bubble chambers Track analysis, CERN 1961

22. Bubble chambers Track analysis by computer CDC3100, CERN 1967

23. Bubble chambers Film analysis with Mirabelle chamber, CERN 1971

24. Bubble chambers ERASME measuring system for film analysis, CERN 1974

25. Spark chambers Spark chambers are made by a set of metallic plates inserted in a volume filled with a noble gas mixture External triggers are used to provide a high voltage pulse An avalanche discharge is produced forming a spark Track of sparks is photographed or recorded electronically

26. Spark chambers Advantages and disadvantages Spark chamber can be triggered Sensitive time ~ 1s Rather high intensity (~ 106 particles/s) Can be used without photographing on film Limited spatial resolution  300 m Relatively long dead time ~ 100 ms Pulsed high-voltage difficult to manage

27. Spark chambers Optical spark chamber, used at the PS11 experiment, CERN 1969

28. Spark chambers Arrangement for the use of a spark chamber

29. Spark chambers Cosmic trigger to a spark chamber

30. Spark chambers Cosmic trigger to a spark chamber

31. Spark chambers An educational way to visualize cosmic ray tracks, CERN Microcosm exhibition

32. Streamer chambers - In a streamer chamber (large gap spark chamber) a high-voltage system provides a 20 kV/cm field for a very short time ( 15 ns) - During such time sparks develop only close to the initial ions - Tracks of streamers are photographed on film - Streamer density can be used for particle identification below 1 GeV/c

33. Streamer chambers Advantages and disadvantages Streamer chamber can be triggered Sensitive time ~ 1 s Rather high intensity (~ 106 particles/s) Tedious film measurement Limited statistics Limited spatial resolution  300 m Relatively long dead time ~ 300 ms Pulsed very high-voltage difficult to manage

34. Streamer chambers Streamer chamber at the ISR intersection, CERN 1974

35. Streamer chambers ++e+ decay in streamer chamber

36. Streamer chambers 6.4 TeV S+Au event NA35 Experiment, CERN 1991

37. From old to new tracking detectors • Almost all tracking detectors discussed so far have been abandoned, due to: • - sensitive time and dead time, which limits the beam intensity and do not allow for high statistics • - limited resolution • - difficulties to handle and run these detectors • Modern tracking detectors are based on • - gas detectors with different technologies • - solid state detectors (silicon)

38. Gas detectors Principle of proportional counters: - electrons produced in ionization are directed in electrostatic field to the region of very high field (10-100 kV/cm), usually created around a thin anode wire (20 - 100 m) - between subsequent collisions they can gain enough energy to ionize further atoms - a chain of such reactions leads to formation of an avalanche of electrons and ions - charge liberated in avalanche to charge created in primary ionization is an amplification factor

39. Gas detectors • - in some region of electric field and gas pressure the amplification factor is a constant, i.e. does not depend on primary ionization • - therefore the measured pulse is proportional to the primary ionization (proportional region) • - the amplification factor reaches 104 – 106 • - charge carriers in avalanche produce by capacitive coupling a signal on anode wire • - main contribution to the signal comes from ions which moves slowly, not from electrons

40. Gas detectors • Most of gas detectors are based on the principle of proportional detector: • Multi-Wire Proportional chamber (MWPC) • Drift chambers • Straw tubes • Cathode strip or pad chambers • Time Projection Chambers (TPC) • Micro-Strip Gas Chambers (MSGC)

41. Multi-wire proportional chambers • Many proportional counters in one gas volume • The anode wires act as independent detectors • Typical dimensions • cathode - anode ~ 1 cm • wire pitch d = 1 - 2 mm • wire diameter 20 - 50  m • Spatial resolution d/12 = 300 - 600  m

42. Multi-wire proportional chambers Electric field calculations may be used to design the detector and to calibrate it by means of special programs (GARFIELD,…)

43. Multi-wire proportional chambers A MWPC used in CERN experiment PS17, 1970

44. Drift chambers • Drift chambers are proportional chambers with a large anode wire pitch (few cm) • electrons drift with a velocity up to ~ 5 cm/  s • the drift time to each wire allows position evaluation • time resolution of 1ns gives spatial precision of 50  m • Different configurations of cathode electrodes in order to achieve a constant field towards anode • Various geometries used: • planar, cylindrical, jet chamber • Worse timing and load characteristics compared to MWPC’s, left-right ambiguity

45. Drift velocity

46. Drift chambers Left-right ambiguity Solution: Use two stations with a proper shift

47. Drift chambers The focal plane detector used in the CLAMSUD magnetic spectrometer, including two drift-chambers Moscow 1992-1995 Uppsala 1995-2000

48. Wire ageing Some effect due to ageing of the wires must be cured for long term use of such detectors

49. Time-Projection-Chambers

50. Time-Projection-Chambers • - Time-Projection-Chambers (TPC) are large 3-D detectors made by a vessel of a gas with homogeneous electrostatic field (drift field) • - At the end of drift volume (i.e. one wall of the vessel) is a readout detector, usually cathode pad chamber • - When charged particles pass through the gas in the vessel electron - ion pairs are create • -Because of electrostatic field they do not recombine but start to move apart along the field lines • -Electrostatic field is chosen in a way that no multiplication occur (typically some 100’s V/cm) • -Electrons move much faster than ions • electron mobility ~ 1cm2V-1s-1 , for ions ~ 10-4cm2V-1s-1