The availability of relatively copious sources of antiprotons has stimulated the study of “exotic” atoms, in which a negatively charged antiproton replaces an orbital atomic electron. When they approach the nucleus these antiprotons feel the nuclear force and can be used to probe nuclear forces and structure. A new result from CERN underlines the existence of an outer nuclear “halo” composed mainly of neutrons.
Our everyday world is made up of atoms in which protons and neutrons are packed together by the strong nuclear force into a tiny core of positive electric charge, surrounded by a sparse cloud of “orbiting” electrons, these carrying an equal and opposite negative electric charge.
The electron orbits are hundreds or thousands of times larger than the nucleus. Seen from an orbiting electron, the central nucleus looks far away and rather structureless, in the same way that the Sun appears to us as a distant homogeneous sphere.
By making artificial atoms in which electrons are replaced by other, heavier particles that pass very close to the nucleus, physicists are able to get a close look at the centre of the atom in the same way that space probes, such as the SOHO satellite, see a very different picture when approaching the Sun’s surface.
Being electrically charged, protons can be mapped by probing the nucleus with charged particle beams, like electrons. Neutrons are more difficult to map, especially at the outer, less dense edges of the nucleus. Over the years, experiments using a variety of techniques have suggested the existence of a neutron “halo” – a sort of uniform nuclear stratosphere, relatively isolated from the “weather” at the centre of the nucleus. An experiment at CERN has given fresh support to this idea.
Many of the particles commonly produced by high-energy beams can carry negative charge. Examples are the muon, the pion, the kaon, hyperons and the antiproton. Normally these particles travel so fast that they tear past target atoms, ripping out electrons in their wake. However, as the particles lose energy and slow down, they can eventually reach a point where they knock out an electron for the last time and become captured by the electric field of the neighbouring nucleus, thus forming an “exotic” atom.
In such an atom the intruder orbital particle is much heavier than the electron that it has replaced, so its orbit is consequently smaller. A muon, for example, is 200 times as heavy as an electron and is able to pass correspondingly closer to the nucleus. However, a muon, like an electron, does not feel the strong nuclear force, even at very close distances.
Strongly-interacting particles such as the pion, the kaon, hyperons and the antiproton do feel the strong force of the nucleus. In addition, the strongly interacting particles are heavier still (an antiproton being 2000 times heavier than an electron) so that they can get very close to an atomic nucleus. Exotic atoms are therefore a good laboratory for studying the periphery of the nucleus.
Further reading
A Trzcinska et al., 2001 Phys. Rev. Lett. 87 082501-1.
