Particle Physics: A Very Short Introduction by Frank Close

Author:Frank Close
Language: eng
Format: epub, pdf
ISBN: 9780191577901
Publisher: OUP Oxford
Published: 2016-10-28T00:00:00+00:00

From bubble chamber to spark chamber

A bubble chamber can provide a complete picture of an interaction, but it has some limitations. It is sensitive only when its contents are in the superheated state, after the rapid expansion. Particles must enter the chamber in this crucial period of a few milliseconds, before the pressure is reapplied to ‘freeze’ the bubble growth.

To study large numbers of rare interactions requires a more selective technique. In the 1960s, the spark chamber proved the ideal compromise.

The basic spark chamber consists of parallel sheets of metal separated by a few millimetres and immersed in an inert (less reactive) gas such as neon. When a charged particle passes through the chamber it leaves an ionized trail in the gas, just as in a cloud chamber. Once the particle has passed through, you apply a high voltage to alternate plates in the spark chamber. Under the stress of the electric field, sparks form along the ionized trails. The process is like lightning in an electric storm. The trails of sparks can be photographed, or their positions can even be recorded by timing the arrival of the accompanying crackles at electronic microphones. Either way, a picture of particle tracks can be built up for subsequent computer analysis.

The beauty of the spark chamber is that it has a ‘memory’ and can be triggered. Scintillation counters outside the chamber, which respond quickly, can be used to pinpoint charged particles passing through the chamber. Provided all this happens within a tenth of a microsecond, the ions in the spark chamber’s gaps will still be there, and the high-voltage pulse will reveal the tracks.

17. An image of one of the first observations of the W particle – the charged carrier of the weak force – captured in the UA1 detector at CERN in 1982. UA1 detected the head-on collisions of protons and antiprotons, which in this view came from the left and right to collide at the centre of the detector. The computer display shows the central part of the apparatus, which revealed the tracks of charged particles throughout the ionization picked up by thousands of wires. Each dot in the image corresponds to a wire that registered a pulse of ionization. As many as 65 tracks have been produced, only one of which reveals the decay of a W particle created fleetingly in the proton-antiproton collision. The track is due to a high-energy electron. Adding together the energies of all the other particles which reveals that a relatively large amount of energy had disappeared in the direction opposite to the electron, presumably spirited away by an invisible neutrino. Together, the neutrino and electron carry energy equivalent to the mass of the short-lived W particle.

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