The Antiproton Decelerator

The Antiproton Decelerator (AD) provides low-energy antiprotons mainly for studies of antimatter. Previously, “antiparticle factories” at CERN and elsewhere consisted of chains of accelerators, each performing one of the steps needed to provide antiparticles for experiments. Now the AD performs all the tasks alone, from making antiprotons to delivering them to the experiments.

The starting point is a beam of protons from the Proton Synchrotron (PS), which is fired into a block of metal. The energy from the collisions is enough to create a new proton-antiproton pair about once in every million collisions. The antiprotons produced travel at almost the speed of light and have too much energy to be useful for making antiatoms. They also have a range of energies and move randomly in all directions. The job of the AD is to tame these unruly particles into a useful low-energy beam.

A ring of bending and focusing magnets keeps the antiprotons on the same track, while strong electric fields slow them down. Passing the antiprotons through clouds of electrons – a technique known as “cooling” – reduces the sideways motion and the spread in energies. Finally, when the antiprotons have slowed down to about 10% of the speed of light, they are ready to be ejected. One “deceleration cycle” is over: it has lasted about one minute.

In 2002 the AD made headlines around the world when the ATHENA and ATRAP experiments successfully made large numbers of antiatoms for the first time. Currently it serves four experiments that are studying antimatter: AEGIS, ALPHA, ASACUSA and ATRAP. The ACE experiment also uses antiprotons, to assess their suitability for cancer therapy.