Bubble Chamber

  A particle detector of major importance during the initial years of high-energy physics. The bubble chamber has produced a wealth of physics from about 1955 well into the 1970s. It is based on the principle of bubble formation in a liquid heated above its boiling point, which is then suddenly expanded, starting boiling where passing charged particles have ionized the atoms of the liquid. The technique was perfected to work with high precision in large volumes of different liquids, embedded in a magnetic field. As liquids, they used many varieties, from the simplest nuclei, free of Fermi motion (H₂) to low-interaction length ``heavy liquids'' like propane (C₃H₈) or freons (Dupont's trade mark for fluor compounds, e.g. CF₂Cl₂ or CF₃Br). The liquid in a bubble chamber served simultaneously as target and as detector with a solid angle coverage; stereo cameras recorded data on film. The technique was honoured by the Nobel prize award to D. Glaser in 1960. For details, see [Shutt67].

Even today, bubble chamber photographs provide the aesthetically most appealing visualization of subnuclear collisions; the above figure (from [Harigel94]) shows a historical event: one of the eight beam particles (K^- at 4.2 GeV/c) which are seen entering the chamber, interacts with a proton, giving rise to the reaction

followed by the decays

and

Some chambers were built with an embedded track-sensitive target of a different (heavier) liquid; others were operated at a high repetition rate, and used in conjunction with a spectrometer of electronic detectors coupled to some trigger logic.

Bubble chamber film was scanned and measured by humans, later often assisted by computers. The projectors for scanning and measuring required substantial investment, and the teams operating them were impressive; the sharing of tasks with film as carrier of information allowed large international collaborations to emerge, with many vital tasks decentralized.

As large-volume high-precision detectors with electronic data recording became available, and physics required ever more complex triggers, and as colliders became the high-energy accelerators of choice, retirement time arrived for bubble chambers. A complete and comprehensive obituary exists in the form of conference proceedings [Harigel94]. The contribution of the technique to physics, and its role in setting up major international collaborations and in developing large-scale analysis programs is uncontested, and also this aspect was highlighted by a Nobel prize (Alvarez 1968).