1. Tracking detectors/1
F.Riggi

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 ( m)
large sensitive volume
4 geometrical acceptance
tedious photograph measurements
sensitive time  1 ms,
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 ( kV/cm), usually created around a thin anode wire ( 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 = mm
wire diameter  m
Spatial resolution d/12 =  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
Uppsala

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

51. Time-Projection-Chambers
- Electrons are used to detect the particle’s track
- They drift towards the readout chamber
- Drift path of electrons can be distorted by field inhomogeneities
- The electron clouds are detected by cathode pad chamber read out with high frequency ~ 10MHz
- The pad position gives two transverse coordinates
- The time of the electron cloud arrival is proportional to the longitudinal coordinate which is then determined from the time channel

52. Time-Projection-Chambers
-TPC is a 3D detector, like historical (bubble, spark, streamer) chambers but
the readout is fully electronic
has no pulsed very high-voltage
is faster, its speed is determined by maximum drift time which for large chambers is ~ 100  s (but still not fast enough)
spatial resolution depends on many parameters
drift length and diffusion constant
track angle with respect readout plane and pad row
primary ionization (electron statistics)
a typical value is ~ 500  m

53. Time-Projection-Chambers
NA49 TPC

54. Time-Projection-Chambers

55. Conclusions Historical devices for tracking
Development of detectors with electronic readout
Structure of large scale experiments with
large volume tracking devices (usually TPC) +
vertex detectors (usually silicon)