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 neutrons that are then channeled into beams and directed toward instruments used to carry out experiments. A higher power beam leads to a greater flux of neutrons, enabling greater spatial and temporal resolution and the study of small samples, such as proteins.
The ESS was proposed back in the late 1980s as a way to maintain Europe’s lead in this field, given planned 1=MW-caliber facilities in the United States and Japan. The imminent start of the ESS should also help counter growing worries about a “neutron drought” in Europe, as older neutron sources close. Building work on the ESS eventually began in 2014, after the project’s 15 partner countries agreed to foot the €1.84 billion construction bill. Plans at that point called for first neutrons in 2019, full 5-MW beam power for the first user experiments in 2023, with 16 of the instruments then available 2 years after that and the full complement of 22 instruments “a few years later,” according to ESS Director General John Womersley.
Those deadlines are now slipping owing to the project’s “initial operations phase,” which runs from 2019 to 2025, costing at least €150 million more than a €850 million forecast in 2014. At a meeting in June, the ESS council began evaluating scenarios to bring these costs down. Cost-cutting options include postponing the purchase of equipment needed to boost proton power to 5 MW and slowing the speed at which instruments reach full specification.
All of the scenarios envision the ESS operating with 2 MW of power in 2023 in order to guarantee what Womersley describes as “world leading performance” when experiments start up. But whereas one scenario delivers 5 MW by 2025, another foresees no rise in power by then. Similarly, although all the plans require 15 instruments to be installed by 2025, there are differences over how many of the additional seven will be constructed by that date. “We are retaining the project’s ultimate goals but changing the speed at which we achieve them,” he says.
Michael Preuss, a materials scientist at the University of Manchester in the United Kingdom and chair of the ESS science advisory committee, describes the delay in full power as “a very sensible thing to do.” He would prefer to expand the number of instruments rather than boosting power early on. In any case, he maintains, improvements to the design of the machine’s moderators—devices needed to slow neutrons down to the speeds that make them useful for research—will yield a neutron flux that is “almost as high” at 2 MW as it would have been at 5 MW.
Carlile says the project is going as he “would expect” for a large scientific facility built mainly with in-kind contributions. But he doesn’t think that progress with the moderators will compensate for the lower initial power, and he is worried that the cutbacks in power and instruments will be costly for European neutron science.
The ESS council plans to decide on a preferred scenario before the end of this year so that member nations can then agree to their shares of that budget in 2018.