Figure references are to the alternate second edition of Modern Physics by Weidner and Sells

(Allyn and Bacon, 1973)

The ideas and theories of nuclear and particle physics are linked to a large data base of experimental measurements. In particle physics, the fundamental experiment consists of producing collisions (interaction) between particles and carefully observing the results. New particles can be created, their rest mass (and rest energy) being derived from the kinetic energy of an accelerated particle (the situation is similar to pair production, except that the rest energy is supplied by a material particle rather than by a photon).

The required instrumentation can be broadly divided into accelerators which produce beams of high-energy particles and detectors which measure the properties of the particles produced. An accelerator normally uses stable particles which are electrically charged, allowing them to be accelerated to high energies by applying electrostatic fields. The particle beam has to travel in ultrahigh vacuum so that the particles are not scattered by air molecules on their way to the target. Electric and magnetic fields are also used to focus and steer the particle beam to its destination.

In a linear accelerator, the accelerated particles travel in a straight line to the target. The simplest form consists of an electron or ion source, and electrodes connected to a high potential difference. Voltages up to 1MV can be conveniently produced using a step-up transformer and a rectification (diode-capacitor) circuit. Voltages up to 30 MV are generated by a Van de Graaf generator, which uses a motor-driven belt made out of insulating material, onto which electrostatic charge is "sprayed" at the low-voltage end. In a tandem machine, the accelerating effect of the voltage is doubled; negative ions of a gas (e.g. hydrogen) are accelerated towards a thin carbon foil connected to a high positive voltage and while passing through the film the ions lose their electrons, becoming positive ions which are then repelled from the foil towards a target at ground potential; see Fig.8-19.

Because voltages are limited due to the problems of providing insulation, less direct schemes have to be used to accelerate particles to higher energies. One is the drift-tube accelerator in which the beam passes through a series of concentric hollow tubes connected to opposite poles of an alternating-voltage source. The charged particles are repeatedly accelerated by the electric field between the cylinders, provided the drift time in each tube is equal to half the period of the ac voltage; see Fig.8-20. Protons can be accelerated to 100MeV energy in such a machine.

In a waveguide accelerator, microwaves are guided down the tube containing the particle beam and their moving electric field accelerates the particles (which "ride" on the wave). At Stanford University, a 2-mile long waveguide accelerator accelerates electrons to 30 GeV kinetic energy (with a beam current of the order of 30 mA). Such electrons would lose energy rapidly (producing bresmstrahlung radiation) if they were accelerated in a curved path.

The cyclotron was invented by E.O Lawrence and M.S. Livingston in 1932. Heavy particles, such as protons or alpha-particles, are injected close to the centre C of two flat D-shaped conductors (dees) connected to an ac generator; see Fig.8-22. A perpendicular magnetic field causes the particles to move in a circle; they are accelerated twice per revolution by the electric field between the dees, so the radius of their orbit increases and they eventually emerge (with high energy) at the perimeter of the dees and are deflected out by an ejector plate E towards a target T (Fig.8-22). The angular frequency of rotation of particles of relativistic mass m and charge q in a magnetic field B is qB/m, independent of the radius R, so the ac fields continue to accelerate the particles provided their relativistic mass does not appreciably exceed their rest mass (equivalent to saying that the kinetic energy remains small compared to the rest energy). The latter condition limits the energy of protons (rest energy 1000 MeV) to about 25 MeV.

The synchrotron (Fig. 8-20) avoids the above limitation by varying the frequency of the accelerating field as the particles speed up. Particles are injected (in bunches) from an injection accelerator and are steered by bending magnets around a course of fixed radius R . To keep them on-course, the field B of the magnets must be increased as the particles accelerate, since the particle momentum must satisfy the condition mv = qBR. The momentum (and kinetic energy) of the particles is therefore limited by the magnet strength and the radius of the synchrotron. The Tevatron synchrotron at Fermilab (Illinois) accelerates protons to 800GeV energy with R = 0.62 miles.

Electrons undergoing circular motion in a synchrotron lose energy by the bremsstrahlung process, producing synchrotron radiation which has a wide range of wavelengths from the infrared to the x-ray region. Synchrotron x-rays are used for diffraction experiments and for x-ray lithography (to produce sub-micrometer integrated circuits, for example).

Early accelerators directed particles towards a fixed (stationary) target, such as liquid hydrogen or a solid metal. A disadvantage of this scheme is that, as predicted from momentum conservation, new particles created by collision must carry considerable momentum in the incident-beam direction. Therefore a large part of the kinetic energy of the incident particles goes into providing kinetic energy of the new particles and relatively little contributes to the creation of rest mass. For example, 1000GeV protons colliding with a fixed target would provide only 42 GeV of collisional energy to create new particles.

Since the 1960's, the trend has been to build collider machines in which two kinds of particles, travelling at comparable speeds in opposite directions, are allowed to collide by overlapping the beams. Two 1000GeV protons travelling with equal speeds in opposite directions could create a stationary particle with a rest mass of 2000 GeV in a head-on collision. Electron-positron colliders have been built in at Stanford (California), CERN (Geneva) and in China and Japan. A proton-antiproton collider operates at Fermilab (Illinois) and an electron-proton collider at HERA in Germany.

In 1983, a proposal was put to the US Department of Energy to fund a proton-proton collider named the Superconducting Supercollider (SSC) with a collisional energy of 40 TeV (40 x 10^12 eV). It was to have a circumference of 83 km, using advanced superconducting magnets, and would generate a high collision rate (50 million collisions per second in up to six colliding regions). The cost was estimated at $8 billion, to be spread over 8 years of construction. The US President gave his approval in 1987 and construction began in 1990 at a site in Texas. By October 1994, when the project was 20% complete, it was cancelled by the US Congress because of concerns about cost and the US budget deficit.

Future high-energy research (e.g. on the Higgs-field problem) will therefore be based at CERN, where the 27km tunnel housing the positron-electron collider (LEP) has been refitted with 9 Tesla superconducting magnets to provide a proton collisional energy of 14 TeV.

Elementary particles cannot be seen directly but their presence can be detected through the ionizing effect of their electrical charge or (in the case of neutral particles) from the charged particles created by scattering. The simplest requirement is to count particles or to measure the particle flux (number of particles per unit area per second).

Gas-filled detectors (Fig.8-1) contain a wire electrode surrounded by a gas such as argon. Particles enter through a thin window and ionise gas molecules by collision. The ions move under the action of the electric field between a central electrode and the outer casing, generating a transient current which is measured in terms of the voltage generated across a resistor R . In the ionization chamber version, ions generated by each incoming particle are collected before they have time to recombine but the voltage pulse across R is too small to allow particle counting; instead, the flux of particles is measured in terms of the average current generated in R , provided the apparatus has been calibrated for that type of particle.

If the voltage to the central electrode is increased, the electric field inside the chamber accelerates the ions so much that they produce additional ions (as many as 10^6 per primary particle) by collision with gas molecules and a measurable pulse occurs across R for each particle which entered. This proportional counter allows particles to be counted directly, by counting voltage pulses. Moreover, the size of the pulse is proportional to the kinetic energy of the original particle, so the device can also be used to measure particle energies or to distinguish between different types of particles.

If the electrode voltage is further increased and a small percentage of halogen (e.g. Br) added to the argon in the chamber, any incoming particle (e.g. alpha, beta or gamma) will generate ions but the discharge is quenched (within 1 microsecond) by the halogen gas so a distinct pulse is produced across R. This Geiger counter can be used to count particles, irrespective of their type.

Scintillation detectors utilise the fact that a particle passing though a solid phosphor or liquid scintillating material produces visible-light photons when atoms of the scintillator (ionized by the particle) return to their ground state.The photons are detected by a photomultiplier tube (PMT), in which electrons released at a photocathode are amplified due to generation of secondary electrons at a series of dynode electrodes at increasing positive potential; see Fig. 8-3.

A Cerenkov detector uses the fact that a particle which is travelling faster than the speed of light (c/n) in a material of refractive index n generates Cerenkov radiation within a cone of directions around the particle path. The effect is similar to that of the shock wave produced by a supersonic aircraft. The radiation (usually visible light) is measured using a PMT. The cone angle q of the radiation is given by cos(q ) = (c/n)/Vp, allowing the possibility of measuring the particle speed Vp .

Nuclear emulsions are thick layers of photographic emulsion which darken (along the track of a particle) when chemically developed, due to silver deposited through the ionization of silver halide molecules. The length of the particle track can act as a measure of the particle energy. Nuclear emulsions can be attached to high-altitude balloons to detect the effect of cosmic rays arriving in the upper atmosphere.

The cloud chamber was invented by C.R.Wilson in 1907. Sudden expansion of water or alcohol vapour (withdrawal of a piston, triggered by a particle detector; see Fig.8-7) generates a supersaturated vapour in which small droplets of liquid can condense along the track of a particle, allowing the track to be photographed from scattered light. By applying a magnetic field B parallel to the direction of viewing, the track of a charged particle (charge q) is made to curve and the particle momentum p can be obtained by measuring the radius of curvature R and applying the formula p = mv = qBR.

The bubble chamber operates on similar principles but employs a container of superheated liquid (usually hydrogen), produced by a sudden reduction in pressure. Bubbles of vapour form along the track of a particle and are photographed, with a magnetic field applied. The bubble chamber has several advantages: the higher density in a liquid ensures a greater probability of collisions of the incoming particles, and the device can be retriggered at frequent intervals whereas the ions generated in a cloud chamber may take as long as a minute to completely clear from the chamber.

The spark chamber records the path of a charged particle as a series of small sparks in an inert gas, generated between electrodes by applying a potential difference just after the particle has passed through. The process can be repeated after only 10 microseconds. A wire spark chamber replaces the plate electrodes with two perpendicular sets of parallel wires and can record the path of a particle in three dimensions, making use of time delays in the current pulses produced by the discharge between each pair of wires.

Useful additional references:

Elementary Modern Physics, by R.T.Weidner and R.L. Sells (Allyn and Bacon, Alternate Second Edition, 1973)

From Quarks to the Cosmos, by L.M. Lederman and D.M. Schramm (Scientific American Library, 1995)