Antimatter is notoriously volatile, but physicists have learned to control it so well that they are now starting to harness it as a tool for the first time. In a project that began last month, researchers will transport antimatter by truck and then use it to study the strange behaviour of rare radioactive nuclei. The work aims to provide a better understanding of fundamental processes inside atomic nuclei and to help astrophysicists to learn about the interiors of neutron stars, which contain the densest form of matter in the Universe.
“Antimatter has long been studied for itself, but now it is mastered well enough that people can start to use it as a probe for matter,” says Alexandre Obertelli, a physicist at the Technical University of Darmstadt in Germany, who leads the project, known as PUMA (anti-Proton Unstable Matter Annihilation), which will take place at CERN, Europe’s particle physics laboratory near Geneva, Switzerland.
CERN’s antimatter factory makes antiprotons — the rare mirror image of protons — by slamming a proton beam into a metal target, then dramatically slowing the emerging antiparticles so they can be used in experiments. Obertelli and his colleagues plan to use magnetic and electric fields to trap a cloud of antiprotons within a vacuum (see ‘Antimatter to go’). Then they will load the trap into a van and drive it a few hundred metres to the site of a neighbouring experiment, known as ISOLDE, that produces rare, radioactive atomic nuclei that decay too quickly to be transported anywhere themselves. “It’s almost science fiction to be driving around antimatter in a truck,” says Charles Horowitz, a theoretical nuclear physicist at Indiana University, Bloomington. “It’s a wonderful idea.”
Because antiprotons annihilate so readily, both with protons and with neutrons, they present a unique way to study the unusual configurations of radioactive nuclei. While everyday atomic hearts host protons and neutrons in roughly equal measure, radioactive isotopes are stuffed with extra neutrons. This imbalance can cause exotic behaviour, including a surface ‘skin’ that is richer in neutrons than protons, or an extended halo in which neutrons orbit alone, as in lithium-11 (see ‘Probing a halo’). By observing how often antiprotons annihilate with a proton versus a neutron, the team will be able to understand the relative densities of these particles at the very edge of the nucleus. And because annihilation happens so rapidly, the test will be fast enough to probe even short-lived nuclei. “It’s a kind of test we haven’t been able to do before on these new, more exotic nuclei, which may have very interesting structures,” says Horowitz.