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The writer is a scientific commentator
For more than a decade, a remarkable installation has been taking shape 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 proton linear accelerator to produce the world’s most powerful neutron source.
This is very important for science: neutrons, the electrically neutral particles that sit alongside 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 double helix structure of DNA. There are several neutron facilities around the world, including the United States, the United Kingdom and Japan.
But the superlative power of the ESS, which will be the subject of the first tests this year with a view to experiments which will begin in 2026, could also offer a glimpse of something special: a neutron transforming into its antimatter equivalent, an anti- neutron. Spotting it could solve one of the biggest mysteries of 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 would have subsequently canceled each other out. “So maybe after the Big Bang, the majority of the universe was wiped out with just a little matter remaining.”
The challenge is explaining the remains. One possibility is that matter can “oscillate” into antimatter and vice versa, and that this process somehow leads to the surplus we observe today. Even a solitary observation of such a neutron conversion would be Nobel Prize territory.
Neutrons, which scatter nuclei like balls smacking on a billiard table, have long been used to peer into the heart 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 more intense 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 this cost of 3.5 billion euros.
Neutrons offer advantages over X-rays and electrons, such as their non-destructive nature. This makes it a valuable tool for probing fragile objects. In 1991, researchers at Oak Ridge National Laboratory in Tennessee used neutrons to study hair samples from Zachary Taylor, the 12th U.S. president, to disprove theories that he was assassinated by drug poisoning. arsenic.
Neutrons can also “see” small atoms such as hydrogen, making them useful for studying samples such as fuel cells. Their magnetic spin can be exploited to probe magnetic materials. One planned application, for example, is developing more sensitive magnetic resonance imaging (MRI) scanners used in cancer detection.
However, clustering neutrons is not an easy task; this requires the division of atomic nuclei. This fracturing can be accomplished via nuclear reactors or, as in the case of ESS, through a process known as nuclear spallation. The latter consists of accelerating protons to a speed close to that of light, then crushing them on a heavy metal target (the ESS target is a rotating disk containing three tons of tungsten). This collision causes the neutrons to “burst” or be ejected. The released neutrons are then slowed down, cooled and directed towards scientific use. Due to reactor limitations, spallation is considered the future of neutron science.
The ESS facility will initially operate at 2 MW, Santoro says, double the power of existing sources; it will then increase to 5 MW, producing 10 billion billion neutrons per year. A more intense neutron beam should provide higher resolution results and speed up experiments; the facility is expected to accelerate the development of more efficient batteries and greener plastics. Particle physicists around the world are also integrating the ESS into their future research plans, with the “big science” facility seen as complementary to CERN in Geneva.
Santoro and his colleagues only need one of these countless neutrons to transform into an anti-neutron, producing a distinctive high-energy signature. “It’s like tossing a crazy number of coins, but we only need one signal,” she says, hoping that three or four years of mining could yield the jackpot, as well as distinct information on phenomena such as dark matter.
Amid a coming feast of neutron science – in biology, chemistry, materials science, drug development, archeology – we may one day learn how our universe began as a cosmic remnant.