Neutron era

‘A new era will be opened’, recently appointed professor, Katia Pappas, proclaimed about the latest developments in her field. She made these comments last week during her inaugural address as professor at TU Delft. Pappas and her colleagues at the Delft Reactor Institute (RID) are preparing for the arrival of an ultra powerful neutron source in Sweden.

Neutrons are commonly used to peer inside materials. Because these particles are uncharged, they can penetrate deep into materials, and because they involve relatively little energy, a neutron will not destroy biological systems. The beams that institutes like RID generate, and with which they must work, are fairly weak, however. That will soon change.
Material science is expected to receive a big boost in about ten years’ time, when the world’s most powerful neutron source, the European Spallation Source (ESS), becomes operational in Lund, Sweden.

This source will provide beams that are fifty to hundred times more powerful than the beams currently produced by research reactors. The method used to produce neutrons differs fundamentally from the ‘traditional’ nuclear fission technique. In a spallation neutron source, neutrons are released when heavy metals, such as tungsten and mercury, are struck by protons.
RID hopes that the Netherlands will eventually be in line for beam time in Sweden, in exchange for its work on equipment that can be used with such particle sources. Sesans (Spin Echo Small Angle Neutron Scattering), an example of such equipment, uses an elegant quantum mechanical trick to create unprecedentedly accurate images of structures measuring between twenty nanometres and twenty micrometres.

Just like other neutron instruments, Sesans fires neutrons at a piece of material. Usually a location-sensitive detector detects how widely the neutrons are dispersed by the sample, which helps to visualise the material. In order to see in greater detail, however, a higher quality input beam is required. What makes Sesans special is that it uses spin echo, which involves taking each individual neutron and dividing it into two eigenstates. The quantum mechanic sum of these eigenstates gives the neutron its magnetic moment. The two separated eigenstates then probe the material.

“It’s rather difficult to visualise these quantum mechanical states in any way”, says Pappa’s colleague, dr. Jeroen Plomp. “Perhaps the best way would be to look at the divided neutrons as ghost neutrons, or to see the neutrons as some kind of cartoon characters that can split themselves in two.
“We use a magnetic field — like a prism, but magnetic — to split each neutron into a pair of ‘cartoon characters’ with opposite spin. These two characters then follow two different but parallel paths. The distance between the paths can be adjusted by changing the magnetic field that acts as a prism. This enables us to focus at different lengths.”
Building on their success, the Delft researchers constructed a second spin echo machine, called Offspec (off-specular reflectometer). Offspec’s special feature is that it does not fire the separated eigenstates straight through the sample, but instead compels them to bounce off the sample’s surface, so that they are reflected and fan out in a horizontal plane. This makes Offspec very good at scanning surface textures.

Pappas is now leading an even more ambitious project, called ‘Larmor’, which must ultimately result in the creation of a multifunctional spin echo machine that is capable of using eigenstate division to visualise atomic planes at high resolutions. These could be atoms situated in a metal lattice, for example. Pappas accepted her appointment at TU Delft with the proviso that she would also be given a neutron diffractometer, which will enable her to zoom in on materials down to a scale of one-tenth of a nanometre.

ESS’s high-powered beams offer unprecedented possibilities, according to Pappas: “You could compare ESS to a camera flash. Thanks to the powerful neutron beam, we can capture large numbers of images in quick succession, which enables us to see the movement inside materials. It’ll allow us to look at the way proteins work inside cells, for instance, or at the electron spins in new superconducting materials. When using the neutron beams from our research reactors, it’s like looking at a sample using a normal light bulb, as they need much longer imaging times.” 

 Stakelbeek en Heesen reden in mei over de E75 van Noorwegen, via onder andere Finland, Polen, Slowakijë, Hongarije en Macedonië, naar Kreta. “We wilden weten wat er te zien is van de Europese eenheid”, vertelt Stakelbeek.

De twee zagen de weg onderweg veel veranderen. “In het noorden is het relatief rustig, een soort dorpsweg. In het midden, bijvoorbeeld Polen, is het echt een transportweg met veel vrachtverkeer”, zegt Stakelbeek. “En in het zuiden maken ze van de pittoreske weg die E75 was, een rechte streep met tunnels.”

Aan het einde van de dag worden de foto’s geveild, dus de tentoonstelling duurt slechts één dag.