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)