The PSI has one of the best neutron sources in the world, which is currently being upgraded to the latest standards. Christian Rüegg, Head of Neutronsand Myons Research, explains on site what is so special about the neutron process.

Neutron lenses – neutron guides that transport the particles from the source to the instruments – and several instruments are currently being upgraded at SINQ. (Photos: Basil Stücheli/ETH Board)

We all know about X-rays or photon beams. Re-searchers at the PSI also experiment with X-rays. However, Christian Rüegg, Head of the Neutrons and Myons Research Department, is responsible for another fluoroscopy method that works with neutrons. SINQ – the Swiss Spallation Neutron Source – which turned 20 in 2017, delivers these neutrons for this. Typical neutron sources are research reactors because free neutrons are also produced during nuclear fission. Obtaining neutrons with the help of a particle accelerator, such as at the PSI, is more complex. SINQ was the first facility to use spallation to produce a continuous neutron flow and is still the most powerful of its kind.

The investigation of samples with neutrons is not in itself different to using X-rays: you can also “illuminate” objects with a neutron source in order to look inside. Or you can measure how the neutrons change direction as they penetrate a sample. This, in turn, allows conclusions to be drawn about the finest regular structures down to individual atom level. “The art to this is in producing the neutrons in a very controlled way, decelerating them to the required energy and focusing them,” explains Rüegg. This is because neutron technology has to manage on fewer particles than X-ray technology. “We shouldn’t be too choosy when it comes to selecting them; otherwise, we will end up with too few particles and will not have the necessary intensity. The information gained is therefore unique and particularly valuable”. A state-of-the-art lens and sensitive and efficient detectors are therefore needed to capture as many generated and scattered particles as possible.

"Once everything has been refurbished, we'll have the source with the best lens in the world."

Neutron lenses – neutron guides that transport the particles from the source to the instruments – and several instruments are currently being upgraded at SINQ. A new instrument, the CAMEA (Continuous Angle Multiple Energy Analysis) neutron spectrometer,which the PSI built together with EPFL, is already finished and designed for optimum yield. The detection is described as “super efficient”. Rüegg is enthusiastic about it. “Once everything has been modified, we’ll have the source with the best lens, and our instruments such as the CAMEA will be among the most innovative in the world.” With this in mind, use is being made of the expertise of the PSI spin-off SwissNeutronics, which is making neutron optics future-safe with special focusing units and mirrors optimised for neutron beams. Some of these mirrors have up to 10,000 metal coatings. Innovative and much more accurate measurements will be possible from 2020. Rüegg says that not only the scientific expertise but more especially the technical know-how available at the PSI is crucial for this. The research institute is considered a powerhouse in this respect, with the best scientists and outstanding technicians. Many systems are built in-house in order to guarantee that the high technical requirements are met, and that the most innovative ideas for new instruments are implemented.

Researchers from the ETH Domain, Swiss universities and industry worldwide have unique opportunities to conduct experiments at PSI’s large-scale research facilities. Neutron scattering is often used to complement X-ray diffraction, for example to determine the position of hydrogen or lithium atoms, which are difficult to see in X-rays. This creates opportunities for a whole range of specific analytical applications, from basic research in physics and chemistry to applications in material and energy research or in medicine. The technology is highly efficient, for example, in battery research and in the in-situ observation of chemical processes. What makes neutrons even more interesting is their magnetic moment. Neutronscattering is, therefore, an important method for the investigation of magnetic structures and phenomena. It is possible to visualise layer structures in electronic modules and virtually to watch them during switching.

Christian Rüegg with his team Christian Kägi, Mechanics, Roman Bürge, Electronics, and Dieter Graf, Design, left to right

The rather weak interaction between neutrons and the sample being analysed is both a blessing and a curse. It makes it impossible to build detectors with an even finer resolution at present. But little interaction also means a great depth of penetration and non-destructive analysis, and they are invaluable advantages, for example, when examining large industrial metal parts or pieces of art.