Cloud chamber

A cloud chamber is a track detector invented in 1912. It consists of a container filled with supersaturated vapor that produces condensation droplets due to adiabatic expansion, triggered externally with a piston, due to the sudden lowering of the temperature. This creates a cloud of vapor that is readily ionized when a charged particle passes through, leaving a short-lived trail of condensation where the particle passed. The chamber is photographed immediately after the passing to measure the track.

It has a typical resolutions from $\sim 0.5\text{ mm}$ to $\sim 0.3\text{ mm}$ in minimum understandable track thickness. Much information can be reconstructed from the track:

• the thickness is representative of the particle's kinetic energy and therefore speed, as lower energies will induce more ionization and therefore a thicker track (see Stopping power). This puts a sort of hard limit on the speeds a cloud chamber can visualize,
• the track length gives the particle's mass via the range,
• if a uniform magnetic field is applied, a Lorentz force makes the trajectory curve. The curvature radius depends on the particle's momentum $p$ and thus mass $m$.

A diffusion cloud chamber is a later evolution of the cloud chamber that features no pressure variation. Nowadays neither are used much, being superseded by bubble chambers and electronic tools

Discoveries#

The positron was discovered using a cloud chamber. A track passing through a slab of lead was found to be produced by a positively charged particle, possibly a proton, but measurements of both curvature and track length were consistent only with a particle with mass similar to that of an electron. See Electron > The positron.

The lambda particle was also discovered using a cloud chamber with cosmic rays. During an experiment, an anomalous bifurcation in tracks was observed. The bifurcation indicates a decay and thus a particle with a short mean lifetime. The absence of a track before the bifurcation indicates an electrically neutral particle. It was originally called $V_{0}$ because it produced a V-shaped bifurcation and was neutral ($Q=0$). The frequency of observation was consistent with a typical cross section for the strong interaction, but the mean lifetime was too long for a truly strong process; it had to be a weak decay. This strange property was described by a new quantum number called strangeness.