Particle detection and the decay law

Nuclear particles are invisible, so detectors register their effect on matter: ionisation, flashes of light, blackening of photographic emulsion. In a Geiger–Müller counter a high voltage is applied to a tube of low-pressure gas; if a particle ionises the gas, a brief discharge and a current pulse result, each pulse corresponding to one particle. In a scintillation counter a particle produces a flash of light in a crystal, which a photomultiplier turns into an electrical pulse. In a Wilson chamber tiny droplets form on the ions in supersaturated vapour, leaving a visible track; in a bubble chamber superheated liquid boils along the particle's path, giving a line of bubbles, and because the liquid is dense it also stops fast, high-energy particles. In photographic emulsion the path shows up as a row of dark grains after development. If the chamber is placed in a magnetic field the track bends: r = mυ/(|q|B); the radius gives the momentum and the direction of bending the sign of the charge. An alpha particle ionises gas strongly and leaves a thick, short track, a beta particle a thin, longer one; a gamma quantum leaves no track itself and is detected through secondary electrons.