Particle Detectors

A particle detector is a device used to detect, track and identify ionising particles, such as those produced by nuclear decay. These ionising particles must be charged initially, and are then ionised when the charged particle passes through a substance. A particle detector can only detect charged particles. There are two types of particle detectors cloud and bubble chambers.

Cloud Chamber

Cloud chambers work using a super-cooled vapour (something that is still a gas below its condensation temperature. The ions left condense and form vapour trails, deep yet short tracks mean large amounts of ionisation (these could be from alpha particles) and faint, long tracks are from particles with less ionisation (e.g. beta particles).

Bubble Chamber

Similar to cloud chambers, bubble chambers have liquid hydrogen, kept this way by putting it under high pressure. If the pressure is suddenly reduced, bubbles of gas will start to form where there is a trail of ions. Bubbles grow in size as the chamber expands, until they are large enough to be seen or photographed. Several cameras are mounted around it, allowing a three-dimensional image of an event to be captured.

Magnetic Fields

A charged particle moving in a field will experience a force, making it follow a curved track. With positive and negative particles curving in opposite directions. These show up in spiral shapes like this. The equation for a charged particle in a magnetic field is p=mv/Bq, where p is the momentum, m is the mass, v the velocity, B the magnetic field strength and q the charge.

Chamber Photographs

Whilst only charged particles show up, when a neutral particle decays into two charged particles, these will show up on chamber photographs in a V shape. We can determine the charge of these particles using the magnetic field. Bubble chamber diagrams can contain a lot information.

Drawbacks
 * The need for a photographic readout rather than three-dimensional electronic data makes it less convenient, especially in experiments which must be reset, repeated and analyzed many times.
 * The superheated phase must be ready at the precise moment of collision, which complicates the detection of short-lived particles.
 * Bubble chambers are neither large nor massive enough to analyze high-energy collisions, where all products should be contained inside the detector.