AD – The Antiproton Decelerator

Manufacturing antimatter

The Antiproton Decelerator is a unique machine providing low-energy antiprotons for studies of antimatter, in particular for creating anti-atoms. Previously, ‘antiparticle factories’ at CERN or elsewhere consisted of a chain of accelerators, each performing one of the steps needed to provide antiparticles for experiments. Now the AD performs all the tasks alone, from producing the antiprotons to delivering them to the experiments.

The starting point, surprisingly, is a beam of protons from the Proton Synchrotron (PS), which is fired into a block of metal. The energy of the collisions is enough to create a new proton-antiproton pair about once in every million collisions. The antiprotons are produced travelling at almost the speed of light and have too much energy to be useful for making anti-atoms. 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 focussing magnets keeps the antiprotons on the same track, while strong electric fields decelerate them. 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 anti-atoms for the first time. Currently it serves three experiments that are studying antimatter: ALPHA, ASACUSA and ATRAP. The ACE experiment also uses antiprotons, in this case to assess their suitability for cancer therapy.