Glossary

Aare river

The river that runs between the two parts of the PSI campus, with entrances at PSI West and PSI East; SwissFEL stands on the Würenlingen bank. Near Brugg it is joined by the Reuss and the Limmat, a confluence known as the “water castle of Switzerland”. Read more →

Aargau canton

The canton in northern Switzerland in which Villigen and PSI lie. It had 743,643 inhabitants at the end of 2025, in 196 municipalities organised in eleven districts. Read more →

Beamline

The channel, with its own optics, that carries light or particles from a source to an experimental station, where researchers place their samples. A single ring such as the SLS feeds many beamlines at once, each set to the wavelength its users need. Read more →

Bragg peak

The sharp rise in the energy a proton deposits just before it comes to a stop in matter. Because the depth of the peak depends on the proton’s energy, the beam can be made to act at a chosen depth, which is the physical basis of proton therapy. Read more →

Center for Proton Therapy CPT

The part of PSI where tumours are irradiated with protons: eye tumours since 1984 and deep-seated tumours since 1996. Its beam comes from its own superconducting cyclotron, COMET, and patients are treated at Gantry 2, Gantry 3 and the eye station OPTIS2. This site describes the facility only and gives no medical advice. Read more →

Cyclotron

A circular particle accelerator in which magnets hold the particles on a spiral path while accelerating cavities give them a push on every turn. At PSI the large ring cyclotron of HIPA accelerates protons for research, and the compact superconducting cyclotron COMET supplies the beam for proton therapy. Read more →

Electronvolt eV

A unit of energy used alongside the SI, about 1.602 × 10⁻¹⁹ joule: the energy an electron gains when it is accelerated through a voltage of one volt. Accelerator energies are given in multiples such as MeV (a million) and GeV (a billion electronvolts); HIPA brings protons to 590 MeV. Read more →

Emittance

A measure of how spread out the particles of a beam are, in both position and direction; the smaller it is, the narrower the beam and the more concentrated the light it produces. The SLS 2.0 upgrade reduced the emittance of the electron beam by a factor of up to about 40, which is why PSI calls the new ring diffraction-limited. Read more →

ETH Domain

The group of Swiss federal institutions to which PSI belongs: the two Federal Institutes of Technology, ETH Zurich and EPFL, four research institutes (PSI, WSL, Empa and Eawag) and the ETH Board, which sets its strategy. Read more →

Femtosecond fs

One millionth of a billionth of a second, 10⁻¹⁵ s; the SI prefix femto was adopted in 1964. The X-ray flashes of SwissFEL last between 1 and 60 femtoseconds. Read more →

Gantry

A large rotating structure, at PSI weighing more than 200 tonnes, that carries the end of the proton beam line around the patient so that a tumour can be irradiated from any direction. PSI treats deep-seated tumours at Gantry 2 and Gantry 3. Read more →

HIPA High Intensity Proton Accelerator

PSI’s proton accelerator facility, running since 1974. Its large ring cyclotron brings protons to 590 MeV, about 80 per cent of the speed of light, and with a beam power of 1.4 megawatts it holds the world record for a cyclotron-based facility. The beam drives the neutron and muon sources. Read more →

Hotlab Hot Laboratory

PSI’s laboratory for examining highly radioactive materials, such as samples from nuclear reactors, in hot cells behind 1.5-metre concrete walls, handled with remote-controlled grippers. Inaugurated in 1963, it is Switzerland’s longest-running nuclear facility and the only one of its kind in the country. Read more →

Muon

An unstable elementary particle, a heavier relative of the electron, about 200 times as heavy, which decays within about two millionths of a second. At PSI muons are made from the decay of pions produced when the proton beam hits carbon targets. Read more →

Protein crystallography

A method of working out the three-dimensional structure of a protein from the way X-rays are scattered by a crystal of it. It is one of the best-known uses of the SLS, where more than 10,000 protein structures were solved during the lifetime of the first ring. Read more →

Proton charge radius

A single number describing the size of the cloud of electric charge of a proton, about 0.84 femtometres (10⁻¹⁵ m). In 2010 a team at PSI measured it with muonic hydrogen, an atom in which a muon replaces the electron, and found it about 4 per cent smaller than the value accepted until then, which started the “proton radius puzzle”. Read more →

PSI Paul Scherrer Institute

Switzerland’s largest research institute for natural and engineering sciences, in Villigen and Würenlingen. It was formed on 1 January 1988 by merging two federal institutes, EIR and SIN, and its particular task is to build and operate large research facilities open to outside researchers. At the end of 2025 it had about 2,300 employees. Read more →

SINQ Swiss Spallation Neutron Source

PSI’s neutron source, inaugurated in 1996, in which the proton beam from HIPA knocks neutrons out of a lead target by spallation. PSI describes it as the only continuous spallation neutron source in the world. Read more →

SLS Swiss Light Source

PSI’s synchrotron light source, in operation since 2001: electrons circle a storage ring with a circumference of 288 metres and give off light from infrared to X-rays for many experiments at once. The ring was rebuilt as SLS 2.0, inaugurated on 21 August 2025. Read more →

Spallation

A nuclear reaction in which a fast particle, such as a proton from an accelerator, strikes a heavy nucleus and makes it eject a number of neutrons; at SINQ each hit on a lead nucleus releases 10 to 20 of them. Unlike fission in a reactor, there is no chain reaction: without the proton beam, no neutrons are produced. Read more →

Spot scanning

A technique in which a narrow proton beam is moved point by point through a tumour, building up the required dose from many superimposed spots instead of shaping one broad beam. It was developed at PSI and first used on a patient there on 25 November 1996. Read more →

SwissFEL

PSI’s X-ray free-electron laser, a straight facility about 740 metres long in the forest on the Würenlingen bank of the Aare, inaugurated in 2016. It has two beamlines: Aramis for hard and Athos for soft X-rays. Read more →

Synchrotron

A ring-shaped accelerator in which magnets keep fast particles on a closed track. In a synchrotron light source such as the SLS, electrons travelling at almost the speed of light give off intense light, called synchrotron radiation, wherever their path is bent. Read more →

SμS Swiss Muon Source

PSI’s muon source, where the proton beam strikes two carbon targets; the pions produced there decay into muons, which are guided to instruments for materials research and particle physics. PSI describes it as the world’s most powerful continuous-beam facility for muon spin spectroscopy. Read more →

Ultracold neutrons UCN

Neutrons so slow, with kinetic energies below about 300 nanoelectronvolts, that they are reflected from the walls of suitable materials at any angle and can therefore be stored in a vessel for minutes. PSI’s UCN source, in operation since 2011, serves fundamental physics such as the search for an electric dipole moment of the neutron. Read more →

Undulator

A long row of magnets with alternating poles that forces passing electrons into a slalom, so that the light they emit is concentrated into a narrow, intense beam at chosen wavelengths. Undulators are used both in synchrotron rings and in X-ray free-electron lasers. Read more →

Villigen

A municipality in the canton of Aargau, between the Aare and the Geissberg hill, first recorded in 1247 and a wine-growing village since at least the 13th century. It had 2,323 inhabitants on 1 October 2026 and gave its name to PSI’s postal address, 5232 Villigen PSI. Read more →

Würenlingen

The municipality on the opposite bank of the Aare from Villigen. Reaktor AG, the forerunner of PSI’s reactor research institute EIR, was founded there in 1955, and SwissFEL stands in the forest on its side of the river. Read more →

XFEL X-ray free-electron laser

A straight accelerator in which electrons sent through a long chain of undulators emit X-rays that amplify themselves into flashes with the properties of laser light. The flashes last only femtoseconds, short enough to follow atoms and molecules in motion. Only a handful of such facilities exist; SwissFEL is one of them. Read more →

μSR muon spin rotation, relaxation or resonance

A family of techniques that use muons as tiny local magnetic probes: muons whose spins all point the same way are stopped inside a sample, and the way their spins turn reveals the magnetic fields around them. It is the main use of the SμS muon source. Read more →

Sources checked on: 3 November 2026