A Subatomic Super-Cannon
The European Spallation Source (ESS) is a machine built on a colossal scale to study the universe at its smallest. At its core is a linear proton accelerator nearly 600 meters long. This accelerator fires protons, the positive particles in an atom's nucleus, down a tunnel until they reach approximately 96% of the speed of light. At the end of this path, the high-energy protons—traveling in pulses 2.86 milliseconds long at a rate of 14 times per second—smash into a target.
The target is not a simple block of material. It is a 4.9-tonne wheel, 2.5 meters in diameter, made of the dense metal tungsten. This wheel rotates to withstand the intense energy of the 5-megawatt proton beam. The impact of the protons on the tungsten nuclei is a violent process called spallation. The term comes from mining, meaning to chip away fragments from a larger rock. Here, the process chips neutrons away from the tungsten atoms. This method is safer than nuclear fission and produces a higher number of usable neutrons for the energy consumed. The result is a neutron beam up to 100 times brighter than any other facility on Earth.
An Atomic Microscope
Once freed, the high-energy neutrons are directed through moderators of water and cryogenic hydrogen, which slow them down to useful energy levels. From there, they are guided down beamlines to an initial suite of 15 specialized scientific instruments, with plans to expand to 22. When these neutrons scatter off a sample material placed in one of the instruments, detectors record their change in direction and speed. This data allows scientists to create a map of a material's atomic structure and motion.
This capability has applications across numerous scientific fields. In medicine, it can reveal the structure of viral proteins, helping develop new drugs. In energy research, it allows scientists to observe the movement of ions inside a battery, leading to more efficient designs. The instruments at ESS will enable investigations into engineering materials, magnetic phenomena, and even archaeological artifacts. Construction on the facility began in 2014, with the scientific user program scheduled to start in 2027. Once fully operational, ESS expects to host up to 3,000 visiting researchers each year.