April 22nd, 2013

CERN Headquarters

Our group met outside the CERN headquarters building, waiting to board a CERN shuttle bus to see the first of the Large Hadron Collider (LHC) detector experiments we were to visit. “CERN” is an acronym of “Conseil Européen pour la Recherche Nucléaire (European Council for Nuclear Research), which organised the laboratory between 1952 and 1954. The official name of the organisation, as it appears on the building, is “Organisation européenne pour la recherche nucléaire (European Organization for Nuclear Research). The acronym “CERN” was retained, both for historical reasons and the fact that acronyms of the official name are awkward in both French and English.

Between two of the CERN headquarters buildings is a park in which some of the hardware from CERN's past is exhibited.

The Gargamelle bubble chamber was used in conjunction with the original CERN Proton Synchrotron to study neutrino interactions. Unlike most bubble chambers which use liquid hydrogen as the medium, Gargamelle was filled with liquid halon (bromotrifluoromethane) which, being denser, increased the probability of neutrinos interacting with it. It was Gargamelle which confirmed the existence of weak neutral currents in 1974.

The Big European Bubble Chamber (BEBC) was the largest bubble chamber in the world when it entered service in 1973, and incorporated what was then the largest superconducting magnet. The chamber contained 35 cubic metres of liquid medium (hydrogen, deuterium, or a neon-hydrogen mixture depending on the experiment) and recorded interactions from the beam of the Proton Synchrotron and later the Super Proton Synchrotron. When retired in 1984, it had produced 6.3 million photographs of particle interactions in the chamber.

Here we are looking up into the BEBC from below. The cylindrical portion at the bottom is where the piston operated. By retracting the piston, the pressure in the chamber was reduced to the point where the fluid was momentarily above its boiling point. Charged particles would serve as a nucleation source for bubbles, which would track the motion of the particles. The superconducting magnet caused particle tracks to curve, allowing the mass and charge of the particles to be deduced from photographs of these tracks.

This piston was activated for each photograph taken in the BEBC, a total of 13 million strokes over its 11 years in service. Each stroke exerted a force of 350 tonnes. When you are doing something that violent so many times, there's always the possibility of things ending badly, so the building which housed the BEBC was designed with a roof which would be blown off in case of a hydrogen explosion. Fortunately, that never happened.

The actual acceleration of electrons and positrons in the Large Electron-Positron Collider was performed in radio frequency cavities like these. A total of 128 such cavities were distributed around the accelerator ring. The copper sphere at the top stored the acceleration energy between particle bunches, saving energy and reducing heat dissipation in the cavity.

This Cockcroft-Walton voltage doubler, initially installed in 1964, provided the 600 kilovolt high voltage supply which provided the first acceleration of protons to the linear accelerator which fed the Proton Synchroton.

This wooden building, said to be the largest in the world, was originally built for Expo.02 in Neuchâtel, as the Palais de l'Equilibre, a celebration of anti-human “sustainable development”. After the end of the exposition, it was dismantled and re-assembled at CERN to be a visitor and outreach centre. We did not have an opportunity to visit it.

by John Walker May 12th, 2013

This document is in the public domain.