The return of photographic emulsion

In the 1970s, nuclear emulsions were very much 'old' technology. There were much more efficient ways of detecting sub-nuclear particles. The reaction products from every pulse of an accelerator could be monitored by electronic counters; particle tracks were made visible as they passed through spark chambers; bubble chamber photographs provided beautiful pictures of interactions as they occurred.

However, nuclear emulsions had one very important advantage over all other techniques at that time — their large resolving power. A length of 1 mm looks 'like a mile' under a microscope, while it could barely be resolved in any other form of detector at that time.

Eric Burhop (1911-1980) of University College London realised that it might be possible to re-construct the creation and subsequent decay of a charmed particle by combining the advantages of different experimental techniques. In emulsion it is difficult to find the interesting events of special interest, unless one knows exactly where to look. The complete scanning under microscope of a 10-litre stack might well take one observer 100 years to complete. There might only be one or two charm-producing interactions in a whole stack — trying to find them would be worse than looking for a needle in a haystack!

The task of finding the events would be much easier, if it were somehow possible to pinpoint them to within a small volume of the order of 1 cm3. Burhop organised a collaboration of research groups from universities in Brussels, Dublin, London, Strasbourg and Rome in a combined effort to look for charm, by incorporating nuclear emulsions and spark chambers in a hybrid detector. Strasbourg had special expertise in 'wide-gap' spark chambers. These had much better directional resolution than conventional spark chambers, but were more difficult to construct and operate. They could be used to map the trajectories of product particles and project them back to their origin to within a volume of a few mm3. The Rome group had a long tradition in both emulsion and electronics. Electronic circuits would trigger the spark chambers at the instant of an interesting interaction. L'Université Libre de Bruxelles, University College London and University College Dublin had participated in joint emulsion experiments over the previous 20 years.

Interactions involving high-energy neutrinos were theoretically the most efficient way of creating charmed particles. Neutrinos of course are uncharged, and weakly interacting. Nearly all neutrinos reaching us from the sun go right through the earth and continue onwards across the universe! In fact only about one neutrino in 100,000 will interact on its way from one side of the earth to the other. On the rare occasions when a neutrino interacts with a nucleus, there are practically no restrictions on the kind of matter which will be created out of energy. (When hadrons interact two bundles of quarks crash together, and 'ordinary' quarks are more likely to turn their energy into other 'ordinary' quarks.)

In January 1976, the equipment was brought across the Atlantic to the Fermilab accelerator in Batavia, near Chicago, and exposed to a neutrino beam of high energy and intensity over a period of three months. During that time the counters clicked 250 times to register that debris from what was presumed to be a neutrino interaction had come from the emulsion

Neutrinos they are very small.

They have no charge and have no mass.

And do not interact at all.

The earth is just a silly ball?

To them, through which they simply pass.

John Updike

Figure 17.1 First picture of the track of a charmed particle. Courtesy of European Emulsion Collaboration.

stack. The emulsions were brought back, developed and shared out among the participating laboratories, and the search began in the areas reconstructed from tracks in the spark chambers. By November 1976, a total of 29 neutrino interactions had been located. One of these, found in Brussels, was of great interest and is reproduced in Figure 17.1.

The incoming neutrino interacts with the nucleus of an atom in nuclear emulsion (probably silver or bromine). Fragments of the nucleus scatter in all directions (short, dark tracks). Particles created out of energy shoot forwards towards the right at a speed close to the speed of light, leaving light tracks of minimum ionisation.

Nuclear emulsion is a three-dimensional medium, and in the photograph most of the tracks go out of focus almost immediately. Figure 17.1 is a mosaic of photographs in which the grains of the charmed particle track, and of its subsequent decay products, are kept in focus. Having travelled about 0.2 mm the charmed particle spontaneously breaks up into three charged decay products.

To estimate the lifetime of the charmed particle we assume that it was travelling at about 0.9 times the speed of light. This gives a value of 10-12 s, taking relativistic time dilation into account.

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