The writer is a scientific commentator
For more than a decade, an extraordinary facility has been in the making in southern Sweden. The European Spallation Source, nearing completion in Lund 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 very important for science: Neutrons, the electrically neutral particles that sit alongside the positively charged protons in the nucleus of an atom, can be used to probe the nature and structure of materials, just as X-rays once revealed the structure of the DNA double helix. There are several neutron facilities in the world, including the USA, the UK and Japan.
But the superlative power of the ESS, which will undergo initial testing this year in preparation for experiments starting in 2026, could also provide a glimpse of something special: a neutron transforming into its antimatter equivalent, the anti-neutron. Seeing 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 senior scientist at the ESS, told me. The big bang, she explains, should have produced equal amounts of matter and antimatter, which later canceled each other out. “So maybe after the Big Bang, most of the universe was destroyed with only a little matter left.”
The challenge lies in explaining the residuals. One possibility is that matter can “oscillate” into antimatter and vice versa, and that this process somehow led to the excess we see today. Even the solitary observation of such a neutron conversion would be Nobel Prize-winning territory.
Neutrons, which are scattered from nuclei like balls spinning around a pool table, have long been used to peer into the heart of matter and materials. Scientists can infer the shapes and sizes of molecules and crystals by shining neutrons at them and measuring how the particles change energy, speed and direction after they meet. The more intense the neutron beam, the more detailed the structural information. Construction of a data and software management center in neighboring Denmark is under preparation; two Nordic countries contribute the most to the cost of 3.5 billion euros.
Neutrons offer advantages over X-rays and electrons, such as non-destructiveness. This makes them a valuable tool for examining fragile artifacts. In 1991, researchers at Oak Ridge National Laboratory in Tennessee used neutrons to study hair samples from Zachary Taylor, the 12th US president, to disprove theories that he was killed by arsenic poisoning.
Neutrons can also “see” small atoms such as hydrogen, making them useful for studying such samples as fuel cells. Their magnetic spin can be used to test magnetic materials. One of the planned applications, for example, is the development of more sensitive magnetic resonance imaging (MRI) scanners, which are used in cancer detection.
However, directing neutrons is no easy task; requires the splitting of atomic nuclei. This fracturing can be done using nuclear reactors or, as in the case of the ESS, a process known as nuclear spallation. The latter involves accelerating protons to nearly the speed of light and then slamming them into a heavy metal target (the ESS target is a rotating disk containing three tons of tungsten). This collision causes neutrons to “fall off” or be ejected. The released neutrons are then slowed down, cooled and directed for scientific use. Because of reactor limitations, spallation is considered the future of neutron science.
The ESS plant will initially operate at 2 MW, says Santoro, doubling the power of existing sources; then it will increase to 5 MW, producing 10 billion trillion neutrons per year. A more intense neutron beam should offer higher-resolution results and speed up experiments; The facility is expected to accelerate the advancement of more efficient batteries and more environmentally friendly plastics. Particle physicists around the world are also including the ESS in future research plans, with the “big science” facility seen as complementary to CERN in Geneva.
Santoro and colleagues need just one of those countless neutrons to change shape into an anti-neutron, creating a distinctive high-energy signature. “It’s like flipping a crazy number of coins, but we only need one signal,” she says, hoping that three or four years of work could yield the jackpot, as well as separate insights into phenomena like dark matter.
Amid the coming feast of neutron science—in biology, chemistry, materials science, drug development, archeology—we may one day learn how our universe began as cosmic debris.
