Tracking detectors/1
Dec 17, 2015
Tracking detectors/1
Historical detectors for tracking
In the past, several techniques were used to track (and visualize) particles:
nuclear emulsionscloud chambersbubble chambersspark chambersstreamer chambers
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
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 informationNo fast analysis of tracks (visual observation)
Nuclear emulsions
32S at 6.4 TeV
Nuclear emulsions
Nuclear emulsions
Range-energy relation in nuclear emulsions
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
Cloud chambers
A Wilson chamber for cosmic rays, 1955
Cloud chambers
Anderson and his cloud chamber
Cloud chambers
Discovery of the positron in a cloud chamber by Anderson (August 2, 1932) while observing cosmic ray tracks
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
Bubble chambers
Advantages and disadvantages
good spatial precision (10 - 150 m) large sensitive volume 4 geometrical acceptance tedious photograph measurements sensitive time 1 ms, complicated operations, cryogenics, safety hardly to use at colliders
Bubble chambers
Reconstruction of a decay in a bubble chamber,
CERN 1973
Bubble chambers
30 cm hydrogen bubble chamber (CERN), 1970
Bubble chambers
Gargamelle bubble chamber, CERN, 1970
Bubble chambers
An event in the Gargamelle bubble chamber
Bubble chambers
BEBC bubble chamber, CERN 1977
Bubble chambers
A reconstructed event in the BEBC hydrogen bubble chamber
Bubble chambers
Measuring track angles by use of a protractor,
CERN 1958
Bubble chambers
Track analysis, CERN 1961
Bubble chambers
Track analysis by computer CDC3100,
CERN 1967
Bubble chambers
Film analysis with Mirabelle chamber,
CERN 1971
Bubble chambers
ERASME measuring system for film analysis,
CERN 1974
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 sparkTrack of sparks is photographed or recorded electronically
Spark chambers
Advantages and disadvantages
Spark chamber can be triggered Sensitive time ~ 1s 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
Spark chambers
Optical spark chamber, used at the PS11 experiment,
CERN 1969
Spark chambers
Arrangement for the use of a spark chamber
Spark chambers
Cosmic trigger to a spark chamber
Spark chambers
Cosmic trigger to a spark chamber
Spark chambers
An educational way to visualize cosmic ray tracks,
CERN Microcosm exhibition
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
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
Streamer chambers
Streamer chamber at the ISR intersection,
CERN 1974
Streamer chambers
++e+ decay in streamer chamber
Streamer chambers
6.4 TeV S+Au event
NA35 Experiment,
CERN 1991
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)
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
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
Gas detectors
Most of gas detectors are based on the principle of proportional detector:
Multi-Wire Proportional chamber (MWPC)Drift chambersStraw tubesCathode strip or pad chambersTime Projection Chambers (TPC)Micro-Strip Gas Chambers (MSGC)
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
Multi-wire proportional chambers
Electric field calculations may be used to design the detector and to calibrate it by means of special programs (GARFIELD,…)
Multi-wire proportional chambers
A MWPC used in CERN experiment PS17, 1970
Drift chambers
Drift chambers are proportional chambers with a large anode wire pitch (few cm)
electrons drift with a velocity up to ~ 5 cm/ sthe drift time to each wire allows position evaluationtime 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
Drift velocity
Drift chambers
Left-right ambiguity
Solution: Use two stations with a proper shift
Drift chambers
The focal plane detector used in the CLAMSUD magnetic spectrometer, including two drift-chambers Moscow 1992-1995Uppsala 1995-2000
Wire ageing
Some effect due to ageing of the wires must be cured for long term use of such detectors
Time-Projection-Chambers
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
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
Time-Projection-Chambers
-TPC is a 3D detector, like historical (bubble, spark, streamer) chambers but
the readout is fully electronichas no pulsed very high-voltageis 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 parametersdrift length and diffusion constanttrack angle with respect readout plane and pad rowprimary ionization (electron statistics)a typical value is ~ 500 m
Time-Projection-Chambers
NA49 TPC
Time-Projection-Chambers
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)