The world’s most powerful source of neutron beams will be less than half as powerful as planned when the facility begins scientific experiments in 2023. The European Spallation Source (ESS), under construction in Lund, Sweden, was designed to reach 5 megawatts (MW), but ballooning costs means that it will only achieve 2 MW in 6 years’ time—a reduced level that will likely limit the range of scientific studies it can carry out.
Although the ESS council, the project’s main decision-making body, is considering plans that would boost power to 5 MW by 2025, some scientists fear that the facility will remain stuck at 2 MW for good. “There are some people with persuasive voices who say you don’t need 5 MW,” says Colin Carlile, a physicist at Uppsala University in Sweden and former ESS director. “But theirs are siren voices. It would be tragic if that happens.”
Like x-rays, beams of neutrons are a way for scientists to explore the atomic structure of materials. But where x-rays scatter off the cloud of electrons surrounding an atom, neutrons scatter off atomic nuclei. That capability helps scientists, for example, to locate hydrogen, which, with only one electron, is a more elusive target for x-rays. Neutron beams can also differentiate between nuclei of different isotopes. And, because neutrons carry a spin, they can reveal the magnetic properties of the material in question.
Most neutron sources are nuclear reactors that generate neutrons through fission. But in recent years scientists have increasingly turned to spallation sources, in which an accelerated beam of protons breaks apart the nuclei of atoms in a solid target, stripping off…