Discovering Particles

Detection techniques

Physicists investigate particle interactions by placing devices known as particle detectors in a region where collisions are expected. Particles that emerge from the collision leave evidence of their passage through the detectors. Physicists analyse this evidence to reconstruct what happened in the collision, and in any subsequent particle decays. The approach is similar to that of a detective who, to reconstruct a sequence of events, analyses evidence left at a crime scene.

When a high-energy charged particle crosses a material, it may transfer energy to electrons in the material’s atoms. This results in ionisation if an electron gains enough energy to escape from its orbit, leaving behind a positively charge ion, or otherwise is referred to as excitation.

Effects relating to ionisation and excitation are exploited in devices known as tracking detectors, which are used to trace out particle trajectories. Ionisation may be seen, for example, from the blackening of photographic plates (nuclear emulsions); from the formation of liquid droplets in a vapour on the point of condensing (cloud chamber); from the formation of gas bubbles in a liquid close to boiling point (bubble chamber); from electric discharge (spark chamber); from electron-ion recombination to produce light (streamer chamber); from the accumulation of electric charge on sensor wires in a gas (multiwire proportional chamber, drift chamber); or from charge movement in a semiconductor (silicon- microstrip detector, charge-coupled device). In materials classed as scintillators, energy that an electron gains through excitation may be dissipated as light (scintillation counters, scintillating-fibre detectors).

Tracking detectors are often placed in a magnetic field, so that charged particles follow hellical paths, and the amount of curvature gives a measurement of particle momentum (product of mass and velocity). Signals recorded by early tracking detectors were recorded as photographic images, and were examined individually by teams of scanners. Signals from more-modern detectors are stored digitally, and are analysed using computers.

Tracking detectors contain only small amounts of material, so that the particles they measure pass through essentially undisturbed. A different approach is used in devices known as calorimeters. These are formed of dense material, designed to absorb all of the energy of an incident particle. The particle’s original energy is measured from the total amount of ionisation or excitation recorded. Thinner calorimeters are used to measure the energies of photons and electrons. Thicker calorimeters are used to measure the energies of hadrons.

Credits and licensing information