The author is a scientific commentator
For more than a decade, an extraordinary facility in southern Sweden has been taking shape. The European Spallation Source in Lund, which is nearing completion and funded by 13 European countries including the UK, will use the world’s most powerful linear proton accelerator to produce the world’s most powerful neutron source.
This is important for science: Neutrons, electrically neutral particles found alongside positively charged protons in atomic nuclei, can be used to probe the properties and structure of materials, just as X-rays once revealed the double helix structure of matter. . There are multiple neutron facilities around the world, including in the United States, United Kingdom, and Japan.
But the ESS’s extraordinary capabilities, which will undergo initial testing this year in preparation for experiments starting in 2026, may also allow us to see something special: neutrons turning into their antimatter equivalents, known as antineutrons. Discovering this could solve one of the biggest mysteries in fundamental physics: Why is there more matter than antimatter in the universe?
“We shouldn’t exist,” Valentina Santoro, a particle physicist and ESS senior scientist, told me. She explains that the Big Bang should have produced equal amounts of matter and antimatter, which then canceled each other out. “So, maybe after the Big Bang, most of the universe was annihilated, leaving just a little bit of matter.”
The challenge is to explain the rest. One possibility is that matter can “oscillate” into antimatter and vice versa, a process that is in part responsible for the surplus we see today. Even a single observation of this neutron conversion would be the domain of a Nobel Prize.
Neutrons scatter out of atomic nuclei like balls on a pool table and have long been used to peer into the core of matter and materials. Scientists can infer the shape and size of molecules and crystals by directing neutrons toward them and measuring how the particles change energy, speed and direction after encountering them. The stronger the neutron beam, the more detailed the structural information. In preparation, a data management and software center is being built in neighboring Denmark; the two Nordic countries are the main contributors to the €3.5 billion cost.
Compared with X-rays and electrons, neutrons have the advantage of being non-destructive. This makes them valuable tools for detecting fragile artifacts. In 1991, researchers at Oak Ridge National Laboratory in Tennessee used neutrons to study hair samples from Zachary Taylor, the 12th president of the United States, to disprove the theory that he was murdered by arsenic poisoning.
Neutrons can also “see” small atoms such as hydrogen, which makes them useful for studying samples such as fuel cells. Their magnetic spins can be used to detect magnetic materials. For example, one planned application is the development of more sensitive magnetic resonance imaging (MRI) scanners for cancer detection.
However, marshalling neutrons is not an easy task. It requires the splitting of atomic nuclei. This rupture can be accomplished through a nuclear reactor or, as in the case of the ESS, through a process called nuclear spallation. The latter involves accelerating protons to nearly the speed of light and then crashing them into a heavy metal target (the ESS target is a spinning disk containing three tons of tungsten). This collision causes the neutrons to “fall off” or be ejected. The released neutrons are then slowed down, cooled and used directly for scientific purposes. Due to reactor limitations, spallation is seen as the future of neutron science.
Santoro said the ESS facility would initially operate at 2MW, twice the power of existing power sources; it would then rise to 5MW, producing 10 billion trillion neutrons per year. Stronger neutron beams should provide higher-resolution results and speed up experiments; the facility is designed to accelerate progress in more efficient batteries and greener plastics. Particle physicists around the world are also including ESS in their future research plans, with the “big science” facility seen as a complement to CERN in Geneva.
Santoro and colleagues only needed one of countless neutrons to morph into an antineutron, creating a unique high-energy signature. “It’s like flipping a lot of coins, but we only need one signal,” she says, hoping that a three- or four-year run will lead to big prizes and individual insights into phenomena like dark matter.
In an upcoming feast of neutron science—in biology, chemistry, materials science, drug development, archaeology—we may one day understand how our universe began as cosmic remnants.
