In Sweden, big money and big science combine to produce tiny particles

Just 50,000 cubic metres of concrete, 6000 tonnes of rebar, 40 kilometres of pipe for air distribution and 2000 kilometres of cables: these are the raw numbers behind the construction of the European Spallation Source (ESS), currently being built in the outskirts of the historical university town of Lund in Sweden. 

When complete, the ESS will be the world’s most powerful facility for the generation of neutron beams for science. It is currently half-completed and when finished will cover an area equivalent to seven football fields.

The ESS project was kick-started by the European Commission in August, 2015. Nearly half the construction cost is being borne by Sweden and Denmark. Other European member and observer countries are providing the rest.

When complete, like the CERN in Geneva, the ESS’s “engine” will be a particle accelerator, but linear rather than circular and stretching for 600 metres. Unlike CERN, it will not be used to produce strange, exotic and possibly unknown particles, but simpler, more familiar, and well-known neutrons. These humble particles, in fact, are the probes of a powerful technique capable of extracting unique information. The neutrons will be used in 15 highly specialised instruments – most still being constructed – to illuminate a large range of scientific subjects, from medicine to archeology, and from life science to physics and engineering.

Neutrons are particles that together with protons and electrons make up an atom. The mean lifetime of free neutrons is less than 15 minutes – not very long compared to those of their charged companions, but much longer than other non-elementary particles.

Neutrons also have another special property: they have no charge, but they do have a “magnetic moment”, which means that they behave like tiny magnets. They are scattered by atomic nuclei, but their interaction with most materials is fairly weak, making them perfect to study magnetic structures and dynamics at the atomic scale.{%recommended 670%}

In other words, neutrons can be used to pinpoint the composition of matter without invasive chemical labelling, but at the same time they pass easily through most substances in a non-destructive way. This makes them useful probes for a very broad range of applications, from the industrial development of new materials to the study of complex and delicate biological structures.

Neutron scattering enables researchers to study the structure and dynamics of atoms and molecules over an enormous range of distances, from several micrometres to just one-hundred-thousandth of a single one. The same applies to time, with the process lasting from milliseconds to ten-million-millionths of a millisecond. This provides a rich combination of structural and dynamic information.

Realising these attractive perspectives, however, requires a huge number of the particles freed from their nuclear bonds. This is where “spallation” comes in. 

There are two methods to obtain “magic” neutron beams. The first, nuclear fission, requires a lot of energy and can be dangerous. The second is spallation, a process in which fragments of material are ejected as the result of an impact by an incoming force. The concept was first outlined in 1937 by Nobel laureate Glenn Seaborg in his doctoral thesis.

When energetic particles, for example protons, interact with atomic nuclei in the spallation process, lighter particles are ejected from the target as the result of nuclear reactions. Most of these are neutrons. 

The spallation method thus uses an accelerator to shoot protons at a target, beginning with charged molecular hydrogen ions produced by an ion source. The hydrogen gas is heated through rapidly varying electromagnetic fields so that the electrons evaporate from the hydrogen molecules, leaving the protons. 

These are then injected into a linear particle accelerator and released as a high-energy pulse that strikes the target, where spallation occurs. The energy of the resulting spalled neutrons, however, is too high, so they need to be slowed down and finally to be guided through beam lines to the instruments and target samples.

In the ESS 600-metre-long linear accelerator, particles will reach nearly the speed of light. At the end of the pipe, they will smack into a colossal target weighing 11 tonnes and incorporating a three-tonne rotating wheel of tungsten. This is a new technology for spallation sources, which includes a cryogenic system cooled by helium and a moderator-reflector system where the neutrons are slowed down to useable speed. After this point, the neutron beamlines direct the neutrons to the instruments.

A lot of heat will be generated at ESS, for example by the cryogenic systems used to cool the superconducting cavities in the accelerator. The tungsten reaches a temperature of several hundred degrees when in operation. In addition to free neutrons for research, the spallation process produces a lot of waste heat – which will be recycled and used to heat homes in Lund. 

The facility is set to welcome its first researchers in 2023. Once fully operational, up to 3000 scientists will use the ESS’s neutrons beams every year.