Neutrons are building blocks of atomic nuclei, even our body owes 50% of its mass to neutrons. Astronomers find neutron stars in our universe, which consist of an extremely dense superfluid neutron condensate. Such stars are even denser than atomic nuclei, so that a teaspoon of their interior would have the mass of about 10 billion tons. Individual neutrons can be produced in the nuclear fission process inside nuclear reactors or as a result of spallation in heavy-metal targets bombarded by highly energetic protons. These same principles underlie the production of neutrons for research purposes.

Unlike neutrons inside atomic nuclei, free neutrons are unstable with a mean lifetime of just under 15 min. However, their thermal velocities at room temperature are of the order of 2 km/s, which means that a typical thermal neutron would have enough time to cover the distance from Dresden to Madrid before it decays. Trapping neutrons in a container for long enough times to study their decay process therefore represents a technical challenge and requires much lower temperatures of the order of a few mK. These so-called ultracold neutrons move with pedestrian velocities and can be contained in traps and manipulated using Earth's gravity.

A collaboration of at least 17 European countries is currently working on the construction of the world's next-generation neutron source in Lund, Sweden. The 1.843 billion EUR European Spallation Source project (ESS), due to be completed by 2025, will become the world’s most powerful neutron source. It will provide up to 30 times higher neutron flux than any currently available neutron facility, offering the best conditions for the world-class multi-disciplinary research and new groundbreaking experiments.