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The four accelerators at PSI
How acceleration works
- Electric fields give charged particles a push again and again – in a circular accelerator on every turn, in a linear accelerator in many sections one after another.
- Magnets steer the particles along their path or keep them on a circle.
- Every time a charged particle is deflected, it emits light: synchrotron light. The faster (more energetic) the particle and the stronger the deflection, the harder (shorter-wavelength) and brighter the light – up to X-rays.
- Protons radiate far less than electrons of the same energy, because they are about 1836 times heavier (the radiation falls with the fourth power of the mass). That is why electrons are used as light sources (SLS, SwissFEL) and protons to create other particles and for therapy (HIPA, PROSCAN).
HIPA – High Intensity Proton Accelerator
Three stages (Cockcroft-Walton, Injector 2 cyclotron, Ring cyclotron) bring protons to 79 % of the speed of light, with up to 2.4 mA (1.4 MW) – a world record for cyclotron facilities. The beam produces neutrons (SINQ), muons (SμS) and pions for materials research and particle physics.
Pions and muons: The beam first hits two rotating graphite wheels, Target M (5 mm) and Target E (40 mm). There pions are created, which decay into muons. The muon source SμS uses the muons, among other things for μSR measurements (muons as tiny magnetic-field probes in materials). In the 1980s the targets were made of beryllium; they could not withstand the beam and have been made of graphite since the 1990s. HIMB (High-Intensity Muon Beams) is planned, with about 100 times more muons.
Isotopes for medicine: HIPA protons are also used directly to produce radioactive isotopes; this already happens today at the IP2 irradiation station with part of the 72 MeV beam. For this purpose TATTOOS (“Targeted Alpha Tumour Therapy and Other Oncological Solutions”) is being built, a new facility that directs part of the 590 MeV beam (about 100 µA) onto tantalum targets. Construction and commissioning are planned for 2029–2032, together with the University and University Hospital of Zurich. The first products are to be the terbium isotopes Tb-149, Tb-152 and Tb-155.
- Tb-149: alpha therapy (destroys tumour cells over a short distance)
- Tb-152: PET imaging
- Tb-155: SPECT imaging
- Tb-161: beta therapy; already used in studies at University Hospital Basel, but produced with neutrons outside PSI, not with HIPA
The idea is called theranostics: the same element first to find the tumour, then to irradiate it precisely from inside.
Location: PSI West (exact coordinate not found in OSM, approximate point). Sources: psi.ch – HIPA, psi.ch – Targets, Meson targets (beryllium history), psi.ch – TATTOOS, psi.ch – TATTOOS (IMPACT), psi.ch – Radiopharmacy (timeline), Sci. Rep. 2024 (Tb isotopes), Kiselev (HIMB)
PROSCAN / COMET – proton therapy
COMET is a superconducting cyclotron, in operation since 2007, used only for medicine. A graphite degrader sets the energy between 70 and 250 MeV, depending on how deep a tumour lies. Treatment takes place at Gantry 2, Gantry 3 and the eye station OPTIS2.
Location: PSI West, Center for Proton Therapy (OSM “Proton Therapy (WPTA)”). Sources: Kiselev 2021 (PSI), Cyclotrons 2007
SLS – Swiss Light Source
Storage ring with a circumference of 288 m. Since the upgrade to SLS 2.0 (shutdown from October 2023, about 15 months) it runs at 2.7 GeV instead of the former 2.4 GeV, with light about 100 times brighter. The light is produced in bending magnets (broad spectrum, e.g. superbends) and in undulators: rows of magnets that guide the electrons in a slalom and thus produce very bright light at selected wavelengths.
Examples: protein crystallography at the beamlines PXI, PXII and PXIII (structure of proteins, e.g. for medicines). Tomography at S-TOMCAT and I-TOMCAT (3-D X-ray images down to below one micrometre).
Sources: psi.ch – Storage ring, Willmott 2024 – SLS 2.0, psi.ch – TOMCAT
SwissFEL – X-ray free-electron laser
A 740 m long linear accelerator. The electrons then pass through long undulators (Aramis: 12 of them over 60 m) and emit laser-like, coherent X-ray flashes lasting only 1 to 60 femtoseconds. Aramis (hard X-rays) runs at 2.2 to 6.2 GeV (design: max. 5.8 GeV), Athos (soft X-rays) at about 3 GeV.
Location: in the Würenlingen forest, adjoining the PSI site (east side of the Aare, municipality of Würenlingen). Sources: psi.ch – SwissFEL, Paraliev 2022
The sites are also in the GPS list on the PSI page.