The famous Gargamelle bubble chamber, featured in the May 1968 issue, operated from 1970 to 1976 with a muon-neutrino beam produced by the Proton Synchrotron, before moving to the Super Proton Synchrotron until 1979. In July 1973, the Gargamelle collaboration presented the first direct evidence of the weak neutral current, and the chamber body is now an exhibit in CERN’s Microcosm garden.
In the April issue of CERN COURIER we reproduced a photograph of the arrival of the first piece of the new heavy liquid bubble chamber ‘Gargamelle’. The 140 ton base-plate for the magnet was towed onto the site by two tractors in a 48-wheel convoy on 31 March. It seems an appropriate time to say something about this significant addition to CERN’s research equipment.
Gargamelle has been conceived principally as an instrument for research on neutrinos. The fascination of these elusive particles has been brought out in several previous articles in CERN COURIER (see particularly the article by C.A. Ramm, vol. 6, p. 211). They are the most abundant particles in the universe and their study will tell us much about the weak interaction, the only one in which they take part. Their interactions are so rare that ten years ago, our present ability to observe neutrinos was unimaginable. By 1963, it had become possible at high-energy accelerators, where large, refined detection equipment was already in use, to ‘see’ about one neutrino an hour from the millions that the accelerator produced. This will have increased by the early 1970s to something like 10 000 per day and the study of neutrinos will be on the same footing as that of most other particles. At CERN, Gargamelle will be one of the important contributors to this advance.
For neutrino experiments, a heavy liquid bubble chamber has two advantages over a hydrogen bubble chamber:
i) It presents a more dense target so that there are more particles with which the neutrino can interact;
ii) The distance a neutral particle travels in the liquid before producing charged particles (which leave tracks giving information about the parent neutral particle) is shorter. Many important neutrino interactions – such as the elastic scattering of an antineutrino and a proton producing a neutron – yield neutral particles, and the ability of the heavy liquid chamber to give information on them is therefore invaluable.
A heavy liquid chamber is less favourable than the hydrogen chamber in the complexity of the target it presents to the incoming beam and in the accuracy with which the particle tracks can be measured. Also, it is worth adding here that modified hydrogen chambers are now coming into vogue containing hydrogen/neon mixtures or a hydrogen target surrounded by a hydrogen/neon mixture, which compromise between the advantages and disadvantages of pure hydrogen and heavy liquid.
The main detector in the neutrino experiments previously carried out at CERN has been the CERN heavy liquid bubble chamber, which has a volume of 1180 litres. Gargamelle is much bigger with 10 000 litres of useful volume. In a uniform neutrino beam the event rate would be proportional to the volume for the same liquid. In fact, Gargamelle will contribute about a factor of seven to the rate at which neutrino interactions can be observed.
The new chamber is being designed and built at the Saclay Laboratory in France, with help from Ecole Polytechnique, Orsay and industry and is being given to CERN who are providing its buildings and supplies. As mentioned above, the first piece arrived recently and the other components will arrive during the course of the next year. The magnet is coming directly to be assembled at CERN. The other components will be first assembled and tested at Saclay. It is hoped to have the chamber in operation at the end of 1969.
Description of the chamber
The main features of the chamber are as follows: the body (which is almost ready for delivery) is a welded cylinder with dished ends, 1.85 m in diameter and 4.5 m long, with the axis of the cylinder in the direction of the beam. It is constructed of low carbon steel, 60 mm thick increasing to 150 mm in the region of the ports. Its total volume is 12 m3 of which 10 m3 is ‘useful volume’, i.e. can be seen by two cameras. Two diaphragms, made of polyurethane elastromer 4 m by 1 m, running in the direction of the axis on one side of the chamber are used to vary the pressure on the liquid. The liquid can be pure propane (when the chamber would contain 5 tons of liquid) to freon (15 tons) or any intermediate mixture. Four fish-eye lenses, with an angle of view of 110° are set in apertures in each diaphragm; each set of four have their images recorded on a single film. There are 21 xenon flash tubes distributed over the chamber behind diaphragms to give ‘dark field’ illumination (see CERN COURIER vol. 7, p. 144).
The chamber is surrounded by a magnet designed to produce a field of 19 kG. The magnet yoke, weighing 800 tons, serves as support for the chamber, the expansion system and the coils. The two sets of coils weigh 80 tons each and are mounted vertically; the field direction is horizontal.
The name Gargamelle is taken from the satirical novel ‘Gargantua’ by Rabelais (1534) in which Gargamelle was the mother of the giant Gargantua. She gave birth to Gargantua through her ear. The association of headaches with Gargamelle is appropriate even in modern times. The construction of the new chamber has created many problems for its makers. Bringing forth the data from Gargamelle will also cause some headaches. The direct interpretation of the events recorded on the two films will be much more complicated than with smaller bubble chambers. New scanning and measuring techniques will be essential and already, under the auspices of the Gargamelle Users’ Committee, much development is in progress.
Gargamelle, in combination with the increases in repetition rate and intensity per pulse of the proton synchrotron and the refinements incorporated in the new neutrino beam-line, should make the coming years of neutrino research at CERN very fruitful ones.
- This article was adapted from text in CERN Courier vol. 8, May 1968, pp95–96