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The Availability of Relatively Copious Sources of Antiprotons Has EXOTIC ATOMS Experiment boosts the idea of a nuclear halo 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. Fig. 1: The more neutrons, the more they tend to live on the nuclear periphery. Left: results from an experiment at CERN's low- energy antiproton ring (LEAR) show that the difference between the radii of nuclear neutron and proton distributions (vertical axis) increases with the bulk neutron excess of the nucleus (horizontal axis). Right: nuclei contain protons (shown in blue) and neutrons (shown in brown). An antiproton grazing the nuclear surface readily annihilates with a subnuclear particle. The results of these annihilations reflect the relative proton-neutron composition of the nuclear periphery. Our everyday world is made up of atoms in which protons and other, heavier particles that pass very close to the nucleus, physi- neutrons are packed together by the strong nuclear force into a tiny cists are able to get a close look at the centre of the atom in the core of positive electric charge, surrounded by a sparse cloud of same way that space probes, such as the SOHO satellite, see a very "orbiting" electrons, these carrying an equal and opposite negative different picture when approaching the Sun's surface, electric charge. Being electrically charged, protons can be mapped by probing The electron orbits are hundreds or thousands of times larger than the nucleus with charged particle beams, like electrons. Neutrons the nucleus. Seen from an orbiting electron, the central nucleus are more difficult to map, especially at the outer, less dense edges looks far away and rather structureless, in the same way that the of the nucleus. Over the years, experiments using a variety of Sun appears to us as a distant homogeneous sphere. techniques have suggested the existence of a neutron "halo" - a By making artificial atoms in which electrons are replaced by sort of uniform nuclear stratosphere, relatively isolated from the 20 CERN Courier November 2001 EXOTIC ATOMS "weather" at the centre of tron and proton densities the nucleus. An experiment at the outermost layer of at CERN has given fresh the nucleus. support to this idea. This was done for 19 Many of the particles medium and heavy nuclei, commonly produced by from calcium to uranium. high-energy beams can The results reveal that the carry negative charge. outer nuclear density ex­ Examples are the muon, tends much further than the pion, the kaon, hyper- the effective charge radius ons and the antiproton. of the nucleus, showing Normally these particles that neutrons populate the travel so fast that they tear nuclear periphery. Also, the past target atoms, ripping more neutrons in a parti­ out electrons in their wake. cular isotope, the more they However, as the particles • tend to settle near the lose energy and slow surface. down, they can eventually In the second approach, reach a point where they physicists look at the knock out an electron for Fig. 2: the neutrons tend to populate an outer nuclear "halo" (right), the detailed spectroscopy of the last time and become neutron density of which increases with distance from the nuclear centre, the antiprotonic atoms. In captured by the electric rather than a nuclear "skin" (left) with a constant thickness. these atoms the intruder field of the neighbouring antiprotons have definite nucleus, thus forming an "exotic" atom. energy levels, analogous to those of the electrons in ordinary atoms. In such an atom the intruder orbital particle is much heavier than When the atoms are nudged, orbital particles, whether electrons or the electron that it has replaced, so its orbit is consequently smaller. antiprotons, can shift from one energy level to another, emitting or A muon, for example, is 200 times as heavy as an electron and is absorbing quanta of radiation. While for the electrons of ordinary able to pass correspondingly closer to the nucleus. However, a atoms these are usually quanta of visible light, for the more compact muon, like an electron, does not feel the strong nuclear force, even antiprotonic atoms the quanta are in the X-ray region. at very close distances. For atomic electrons, the Pauli Exclusion Principle restricts the Strongly-interacting particles such as the pion, the kaon, hyperons atomic seating accommodation. However, single antiprotons see no and the antiproton do feel the strong force of the nucleus. In addi­ such competitor particles and can sit in whatever available energy tion, the strongly interacting particles are heavier still (an antiproton level they like. being 2000 times heavier than an electron) so that they can get The energies (wavelengths) of these spectral lines can be very close to an atomic nucleus. Exotic atoms are therefore a good calculated from atomic quantum mechanics that take into account laboratory for studying the periphery of the nucleus. the electromagnetic attraction between the nucleus and the orbital particle.The antiproton, as it passes by the nucleus, is also affected Nuclear tools by the nuclear force. This can both shift and blur the X-ray signal. The availability of copious sources of antiprotons at the antiproton These deviations from the purely electromagnetic predictions give factories at CERN and Fermilab from the 1980s has opened a new an indication of nuclear effects. In the LEAR experiment, physicists chapter in antiproton physics, and the study of antiprotonic atoms is measured these for 34 different nuclear targets, ranging from one of the beneficiaries. oxygen-16 to uranium-238. One of the experiments at CERN's LEAR low-energy antiproton The key parameter in each case is the difference between neutron ring, by a CERN/Munich/Warsaw collaboration, set out to look at and proton populations at large radii. The two different approaches the neutron-proton distributions in a range of nuclei. LEAR was are in broad agreement, showing that the more neutrons there are, closed in 1996 but the results of the difficult experiment have now the more they tend to live on the nuclear periphery (figure l).The been published. neutrons also tend to populate an outer nuclear halo, the neutron Several techniques could be used. In one approach, the physi­ excess of which increases with distance from the nuclear centre, cists waited for the orbital antiprotons to be swallowed up by the rather than a nuclear skin with a constant neutron excess (figure 2). nuclei. The antiproton then annihilated with a nuclear particle, Another antiproton experiment at CERN (CERN Courier October forming a nucleus one mass lighter than the parent. The appear­ p35) uses antiprotonic atoms as a precision laboratory for meas­ ance of such a nucleus signalled the disappearance of an antipro­ urements of the antiproton itself. • ton.The daughter nuclei could be analysed radiochemical^ and the resultant nuclear yields, which depend on whether the antiproton Reference was annihilated with a proton or with a neutron, measure the neu­ ATrzcinska et al, 2001 Phs Rev Lett 87 082501-1. CERN Courier November 2001 21 .
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