Not just any old iron

The ring cyclotron of the PSI proton accelerator, seen from above. © Paul Scherrer Institute, Markus Fischer

Forty years ago, engineers and researchers at the Paul Scherrer Institute built a proton accelerator. Today, it still belongs among the best research facilities in the world of physics. By Simon Koechlin

​Nothing ventured, nothing gained. This saying is certainly true in modern physics. In the 1960s, Jean-Pierre Blaser, Hans Willax and their team at ETH Zurich wanted to build a new kind of accelerator for protons, the positively charged constituents of every atomic nucleus. Many simply shook their heads at the idea. “Well-known physicists said: ‘That can’t work’”, says Klaus Kirch, the Head of the Laboratory of Particle Physics at the Paul Scherrer Institute (PSI). But Blaser and Willax wouldn’t be put off. Their proton accelerator was built – with a loan of nearly 100 million francs – and began operating 40 years ago. Today, it’s still one of the most successful research facilities in the world.

This is partly thanks to the courage and ambitions of Blaser and Willax, but also thanks to the fact that PSI has been continually developing and improving the accelerator. This is discussed by Andreas Pritzker in his History of SIN, which tells the story of the development of the proton accelerator at PSI’s forerunner, the Swiss Institute for Nuclear Physics (SIN). In his view, it is successful because it can be used in many other research fields besides particle physics. The proton accelerator was built for ‘medium energy physics’. The aim was to create so-called pions. “These particles are important in holding together protons and neutrons in the atomic nucleus”, says Kirch. In order to create and investigate pions, you first take a hydrogen molecule and split it; this is how protons are formed. These are then sent through three accelerators, the last of which is the so-called ring cyclotron. There the particles fly round in a circle and are accelerated using eight magnets. This means of acceleration, which was revolutionary in Blaser’s and Willax’s day, needs considerably less energy than the acceleration of particles in a straight line, Kirch explains. “In terms of efficiency, the proton accelerator at PSI is probably the best in the world today”.

Near loss-free proton streams

In total, the protons travel some 180 times around the ring accelerator. Their speed
reaches roughly 80 percent of the speed of light, and then, as the researchers describe
it, they are ‘extracted’ from the accelerator. The protons are first pointed at ‘carbon
targets’. This collision creates pions and muons, the ‘heavy brothers’ of electrons.
Then they continue their path towards a lead target, knocking neutrons off it. If the
protons come into contact with another material, it becomes radioactive. It is a real
art to direct the proton stream out of the accelerator without any such losses, says
Kirch. Over the years, the physicists at PSI have greatly increased the efficiency of the equipment. Today, it runs so ‘cleanly’ that 99.99 percent of the accelerated protons are available for experiments.

Refining a standard model

With a diameter of some 15 metres, the ring accelerator is certainly impressive.
Compared with other large installations in the field of particle physics, however, it
is rather modest. But this proves that good research is still possible, even when far removed from the mighty dimensions of particle accelerators such as the LHC at CERN in Geneva. Yet only PSI’s particle accelerator creates the quantities of muons needed to deliver results in the search for specific, rare instances of decay in these particles. “These experiments have produced results that are really fundamental”, says Kirch. “They help us to test and refine the standard model of elementary particle physics”. And only at PSI do the muons travel slowly enough to be used to investigate thin layers of material. In order to study the magnetic properties of a material, for example, muons are brought to its surface. When they decay, the researchers can use the direction in which the decaying particles travel to make deductions about the magnetic fields of the material. “These experiments are not usually intended to discover any direct practical applications but to examine materials which might be suitable for use as superconductors or in new storage media and hard drives”, says Kirch. In total, more than 500 researchers use the proton accelerator every year to carry out experiments with muons.

There are just as many users for the Swiss Spallation Neutron Source SINQ, which is used to study neutrons knocked out of lead by protons. Because neutronshave to be slowed down for experiments, the equipment is situated in a tank of heavy water. Neutrons are a unique tool for investigating the magnetic structures of materials. It is also possible to use them to create internal images of archaeological artefacts without causing any damage. The accelerator even has ‘healing properties’. Exactly 30 years ago, PSI began to use proton beams to treat patients with certain types of cancer. “At the beginning, the protons for this proton therapy came directly from the accelerator”, says Kirch. Demand grew with the success of the therapy – the cure rate for eye tumours is roughly 98 percent – and so today there is a dedicated accelerator.

More than ever before, the proton accelerator is proving its worth as a high-quality
research facility – and Kirch says that’s what really impresses him. “How is it that
a piece of equipment can last for 40 years without landing on the scrap heap?” This
is in part thanks to the research environment in Switzerland, which allows for greater continuity. It has meant that the ever-motivated scientists at PSI have been able to improve and expand the proton accelerator over the years. The latest example
dates back to 2011, when it started to produce ultracold neutrons. There will be
further adjustments and improvements in coming years. But no revolutionary alterations are planned at present. It’s more a matter of making it more reliable, says
Kirch – so that the proton accelerator can remain indispensable for yet another generation of researchers.

Simon Koechlin is the editor-in-chief of the magazine Die Tierwelt and a science journalist.

(From "Horizons" No .101, June 2014)