NA48: Wiring up for Change

The future of the NA48 experiment is coming right down to the wire, that is, the wires which the Collaboration is installing in the clean room of Hall 887 on the Prévessin site. Six days a week, technicians are working in shifts to rebuild the experiment's drift chambers. The original chambers were damaged when a section of a vacuum tube imploded at the end of 1999.
A year ago, CERN gave the green light for this essential part of the spectrometer to be rebuilt, so the NA48 experiment, which studies CP violation (see box), still has a bright future ahead of it.

In January 2000, teams from NA48 began a race against the clock. Their objective: to be ready by July 2001, when the beam is due to be on line again. Easier said than done, since the teams at CEA/Saclay near Paris, who built the first chambers seven years ago, have since been disbanded. 'We have still got the drawings and production details', says Allain Gonidec, NA48's technical coordinator, 'but the experience of the technicians involved at that time is invaluable for understanding techniques and the pitfalls to be avoided'. Information from technicians still working at Saclay allowed a transfer of expertise to CERN. Fifteen people from the Dubna Institute in Russia, the Institutes of Ferrara, Florence, Pisa and Turin in Italy, and from CERN have been hard at work ever since. And Jean Heitzmann, an engineer from Saclay, has put his retirement on hold to lend the teams a hand.
 

With a total of 110 kilometres of wire to be soldered to a very high degree of precision, the rebuilding of the four chambers represents a mammoth task. Each chamber contains four groups of wires oriented in different directions, known as 'views'. But the complexity of the task doesn't end here, as each view itself contains six planes of 240 wires, each soldered onto an octagonal frame inside a circle measuring 2.7 metres in diameter. By the time they have finished, the technicians will have made over 48,000 solderings in total.
Judging by the fineness of the wires, this is not a task for the faint-hearted. The field wires, which set the electric field that causes the electrons to drift, measure 120 microns in diameter. Hardly enormous, but big in comparison to the tungsten sense wires, which are a mere 20 microns in diameter - about three times finer than a human hair - and which have to be positioned and soldered to within 20 microns. Given the limitations of human eyesight at such a scale, the technicians are obliged to use an optical bench fitted with a magnifying camera mounted on a rail. Produced with the help of CERN's Metrology Group, this bench stores the wires' theoretical positions in its memory. When positioning and soldering a wire, the technician can see its correct position magnified by a factor of about a thousand on a screen.

After a layer of wires has been soldered, their tension is checked before the whole assembly is cleaned using a special sable brush. The layer is then subjected to a high-voltage test, which burns off any roughness and residual dust. The final step is to insert a graphite-coated mylar foil between each view. The foils are made at CERN by a team of four technicians with the help of a professional painter.
Despite the very delicate nature of the work, rapid progress is being made. 'One chamber is already ready, another is undergoing cosmic rays testing, a third is in production and the fourth is being dismantled' says Allain Gonidec.
In parallel, a new vacuum tube designed by CERN's TA2 Group is being machined in the MF Group's workshops. Having learned its lesson from the 1999 implosion problem, the Collaboration abandoned carbon fibre in favour of aluminium. 'We were obliged to find a good compromise between mechanical strength and minimising the amount of material used in order to reduce the background' explains Allain Gonidec. The tube is therefore thin - 1.1 millimetres - but reinforced at intervals to increase its stress resistance.