Big science facilities take a long time to come to fruition. The European Spallation Source (ESS) is no exception. Its origins go back to at least 1993, when the European Neutron Scattering Association, representing national neutron scattering societies and committees, began to campaign for a new, ambitious neutron spallation source to be built in Europe (https://europeanspallationsource.se/ess-mandate/ess-story). In 2003, a technical design concept was approved, which was followed by a lengthy host nation and site selection process. In May 2009, Lund, Sweden, was announced as the facility’s home, and construction broke ground in September 2014.

From a scientific point of view, the ESS is a no-brainer. Neutrons are excellent probes for studying condensed matter. As beams with the appropriate wavelengths, they provide access to structural and dynamical information. In addition, having a magnetic moment makes them a prime tool for investigations of magnetic materials.

In view of the recent closures of two long-serving European neutron sources, at the Laboratoire Léon Brillouin in France and the Helmholtz-Zentrum Berlin in Germany, a new neutron scattering facility in Europe is definitely no luxury. But the ESS will do much more than merely satisfy the demand for beamtime. It will advance neutron scattering on many levels.

The principle of nuclear spallation is to produce neutrons by shooting a beam of protons, accelerated to nearly the speed of light, at a heavy-metal target. The target’s atoms are excited and then de-excite by releasing a few tens of neutrons each. Spallation is much more involved and expensive than nuclear fission, the more traditional approach for generating neutrons, but not requiring a reactor — which would imply much more stringent safety and security concerns — is one obvious advantage. More fundamentally, spallation sources generate neutron pulses with a high brightness that are created only while the proton pulse hits the target, unlike the continuous operating mode of reactor sources, thereby creating a very high signal-to-noise ratio.

The ESS will operate with a rotating target: a stainless-steel disk with a diameter of 2.6 m, containing bricks of tungsten, weighing almost five tonnes in total, and spinning around at 23.3 rotations per minute. This unprecedented type of target is expected to provide a brilliance up to 20 times higher than what can be delivered by current spallation sources, and hence will make the ESS the brightest neutron source on the planet. Currently, target cooling tests are being carried out in the run-up to the ‘first beam on target’ milestone expected in 2025.

What is interesting, and perhaps a little surprising at first, is that when the ESS was formally given the green light in 2009, the suite of analysing instruments was left unspecified. Instead, a call for instruments was launched — a call that sought to solicit input from specialists from the neutron community1. Over the course of three proposal rounds in 2013, 2014 and 2015, a total of 39 proposals were peer-reviewed and ranked, prioritizing instruments that ensure early high-impact science, take full advantage of the strengths of the ESS’s neutron source, and cater for communities with currently limited neutron usage but large potential. This process led to the sign-off of a 15-instrument suite. It includes traditional workhorses such as small-angle neutron scattering and powder diffraction units, as well as dedicated imaging and engineering diffraction instruments enabling in situ and operando studies. An expansion to 22 instruments is foreseen and is to be concluded in a second phase of beamline construction.

The ESS project acknowledged early on the importance of data curation and analysis. In addition to the spallation site in Lund, a dedicated Data Management and Software Centre was established (https://europeanspallationsource.se/data-management-software-centre), headquartered at the Danish Technical University campus in Lyngby, Denmark. (Both Sweden and Denmark are host nations, carrying the bulk of the ESS’s cost.) The data centre is providing computing services during the ESS’s construction period and will manage the large amounts of data produced by ESS experiments, fostering the principles of open data, as well as developing and maintaining the necessary software. It also offers training on all things neutron data — a summer school will take place in September for the second year in a row, for example.

The ESS takes sustainability and environmental issues very seriously2. The building site in Lund boasts zero waste to landfill, biofuel-powered construction machinery and electrical power from renewable sources. Waste heat (mostly coming from cooling the target) will be used to heat offices and also channelled into the heating system of the local district. Radiation and other safety issues, as well as the disposal of radioactive waste, will be supervised by the relevant regulatory authorities. The main office building qualifies as outstanding according to a leading international environmental assessment method (https://europeanspallationsource.se/article/2021/07/08/ess-rated-outstanding-high-sustainability-standards), and the site’s landscape concept changed the former farmland into terrain with enhanced ecological value. A science and innovation campus located between the ESS and the nearby MAX IV synchrotron is in the works, and the adjacent urban area is rapidly being developed into a mixed-use neighbourhood with around 3,000 domiciles, and businesses estimated to provide employment for up to 20,000 to 25,000 people in the long term.

Planning big science infrastructure takes a long time indeed. But when done well, new standards are set, and the wait is worth it. Here’s looking forward to the first ESS scientific experiments in 2026.