The detectors used in modern particle physics experiments can be divided into two broad groups.Counters, which simply register the passage of a charged particle and track recording devices that provide images of the paths of particles.
The emulsions from which photographic film is made consist of chemical grains that react due to the ionisation of the atoms by the passage of charged particles. Development of the film causes the particle tracks to show up as dark lines. Photographic film provides a permanent record of charged particle events and has been extensively used in studying the charged particles produced by cosmic ray events. Particle tracks produced by nuclear emulsions are often very short due to their high photographic grain densities which quickly bring the particle to a halt. For this reason it is usually necessary to view them under a microscope.
|One of the first cloud chambers|
Charged particle tracks can be made visible by means of a cloud chamber. This device, invented by C.T.R. Wilson (1869-1959) in 1911, was an early invention for studying radioactivity enabling photographs to be taken of alpha beta and gamma rays. A chamber is filled with air containing saturated water or alcohol vapour. If the vapour is rapidly cooled by expanding the gas by means of a piston then, when a charged particle passes through the chamber, it will ionise the air molecules causing condensation, which show up as tracks.
|Bubble chamber image|
Liquids are better than gases for detecting particles because their much greater density means they contain many more nuclei with which particles can interact. The bubble chamber was invented in 1952 by an American physicist Donald Arthur Glaser (1926-) and consists of a tank filled with a liquid such as liquid hydrogen that is maintained at a temperature just below its boiling point. If the pressure of the liquid is suddenly lowered then the liquid begins to boil.
When liquid hydrogen changes state in this way it is said to be in a superheated state. It will start to boil but not immediately. During this initial superheated condition, if a high energy charged particle passes through the chamber then it will ionise atoms of the liquid along its path ejecting electrons in the process. The electrons deposit all their energy in the liquid and trigger the formation of bubbles. High speed cameras photograph the tracks and then within a few hundredths of a second, the chamber is then recompressed which raises the boiling point and quenches the bubbles. The cycle is repeated for the next passage of charged particles.
|A scintillation detector|
These are detectors in which a flash of light of scintillation is produced when charged particles or photons pass through certain materials. It was this kind of detector that Rutherford and his co-workers used in their alpha-particle scattering experiments. A typical scintillator material is a sodium iodide (NaI), although some plastics and even liquids can be used as scintillation materials.
The NaI crystal is attached to a device called a photomultilpier tube (PMT) which consists of a series of dynodes arranged in an evacuated glass tube. The flash of light falls on the first dynode and ejects electrons by the photoelectric effect. These are then accelerated through a potential difference to the next one where more electrons are released by a secondary emission. As a result, an electron amplification process occurs at each dynode stage, right through to the final electrode where a useful current is produced.
|A spark chamber|
This detector consists of parallel metal plates, separated by a few mm and immersed in an inert gas. When a charged particle passes through, it ionises the gas atoms leaving a trail of ions. If a high voltage is applied to every other plate immediately after the ion trail has been formed, then sparks like miniature lightning bolts form along the trail, revealing the path of the particle. Spark chambers can be used in conjunction with a scintillation counter placed outside the chamber which note the arrival of charged particles and trigger the electric discharges between the plates.
|"Multiwire Proportional Chamber|
Multiwire Proportional Chambers and Drift Chambers
The development of the spark chamber is the wire chamber. Here the metal plates are replaced by sheets of fine wires a few mm apart. When a charged particle passes through, an ion trail is formed. However, the pulse of current is sensed by the wires nearest to the spark and an electrical pulse is sent down the wire which is amplified. Here, photographing the particle track, as in a spark chamber is unnecessary, since the current pulses provide all the information about the particles' trajectory in a form that can be readily processed by a computer. Wire spark chambers can be operated 1000 times faster than bubble chambers, allowing fast decay events to be imaged.
The speed and precision of particle detectors was greatly improved by the multiwire proportional counter (MWPC) and the drift chamber. These were developed at CERN in the 1970s by a Frenchman, Geroges Charpak (1925-). The MWPC consists of a "sandwich" of fine parallel wires such that a middle layer is held at a positive potential of 3-5kV with respect to two outer layers. The sandwich is filled with an inert gas and then, when a charged particle passes through it, an avalanche of ionisation electrons is produced near the vicinity of the nearest wire in the central layer. The position of the wire that generates the current pulse locates a point along the particle's track and by placing several MWPC sandwiches together, the path of a particle can be reconstructed.
|A drift chamber|
A MWPC can easily record extremely fast particle events (as many as a million particles per second!) and are now a standard method of detection in particle physics experiments. Drift chambers are similar in construction to the MWPC but the wires in the central layer are further apart. These wires consist of alternated 'field' and 'sense' wires in which the voltages across them, produce an electric field in which the electrons 'drift' at a constant velocity towards the nearest sense where they initiate a current pulse. While not as fast as a MWPC, with drift chambers it has proved possible to measure the position of particle tracks to an accuracy of 50 microns. (1 micron=10-6m).