A proton beam and what it knocks loose

The proton accelerator

The High Intensity Proton Accelerator facility, HIPA, accelerates protons in three stages. A Cockcroft-Walton pre-accelerator, which also contains the proton source, raises them from 60 to 870 kiloelectronvolts. A small ring cyclotron, Injector 2, brings them to 72 MeV. Finally the large ring cyclotron, about 15 metres across, with eight sector magnets, accelerates them over 186 revolutions to 590 MeV, or about 80 per cent of the speed of light.

With a beam power of 1.4 megawatts and a beam current of up to 2.4 milliamperes, HIPA holds the world record for a cyclotron-based accelerator facility. It accelerates about 10 trillion protons per second and supplies around 20 user facilities with secondary particles: pions, muons, neutrons and ultracold neutrons.

SINQ: neutrons from lead

In nature, neutrons are bound inside atomic nuclei, so they have to be knocked out first. At the Swiss Spallation Neutron Source SINQ, the proton beam strikes a block of lead. Each hit heats a lead nucleus, which then ejects 10 to 20 neutrons, a process called spallation. These neutrons are too fast for experiments, so the target sits in a tank of heavy water that slows them down; a second tank of heavy hydrogen at minus 250 °C produces even slower, cold neutrons for studying larger structures.

PSI describes SINQ as the only continuous spallation neutron source in the world. Construction began in 1989 and it was inaugurated in 1996.

Why neutrons

Neutrons have no electric charge, so they interact mostly with atomic nuclei and penetrate deep into materials. They carry a magnetic moment and act like tiny compass needles that sense how magnetic atoms are arranged. They also show hydrogen clearly, which X-rays barely see, and that matters for biological materials, water and organic substances. Their wavelength is similar to the size of the structures being studied, so scattering experiments can resolve details far too small for a microscope.

SμS: muons from carbon

For the Swiss Muon Source SμS, the proton beam strikes two carbon targets. The collisions produce pions, which decay after about 26 nanoseconds into a muon and a neutrino. Muons are themselves unstable and decay within a few millionths of a second, so they have to be produced continuously during an experiment.

Muons made this way all spin in the same direction and are slow enough to stop inside a sample, which makes them sensitive probes of magnetism in solids. Particle physicists use muons and pions to test predictions of the Standard Model. PSI describes SμS as the world’s most powerful continuous-beam facility for muon spin spectroscopy.