Nuclear tools
The availability of copious sources of antiprotons at the antiproton factories at CERN and Fermilab from the 1980s has opened a new chapter in antiproton physics, and the study of antiprotonic atoms is one of the beneficiaries.
One of the experiments at CERN's LEAR low-energy antiproton ring, by a CERN/Munich/Warsaw collaboration, set out to look at the neutron_proton distributions in a range of nuclei. LEAR was closed in 1996 but the results of the difficult experiment have now been published.
Several techniques could be used. In one approach, the physicists waited for the orbital antiprotons to be swallowed up by the nuclei. The antiproton then annihilated with a nuclear particle, forming a nucleus one mass lighter than the parent. The appearance of such a nucleus signalled the disappearance of an antiproton. The daughter nuclei could be analysed radiochemically and the resultant nuclear yields, which depend on whether the antiproton was annihilated with a proton or with a neutron, measure the neutron and proton densities at the outermost layer of the nucleus.
This was done for 19 medium and heavy nuclei, from calcium to uranium. The results reveal that the outer nuclear density extends much further than the effective charge radius of the nucleus, showing that neutrons populate the nuclear periphery. Also, the more neutrons in a particular isotope, the more they tend to settle near the surface.
In the second approach, physicists look at the detailed spectroscopy of the antiprotonic atoms. In these atoms the intruder antiprotons have definite energy levels, analogous to those of the electrons in ordinary atoms. When the atoms are nudged, orbital particles, whether electrons or antiprotons, can shift from one energy level to another, emitting or absorbing quanta of radiation. While for the electrons of ordinary atoms these are usually quanta of visible light, for the more compact antiprotonic atoms the quanta are in the X-ray region.
For atomic electrons, the Pauli Exclusion Principle restricts the atomic seating accommodation. However, single antiprotons see no such competitor particles and can sit in whatever available energy level they like.
The energies (wavelengths) of these spectral lines can be calculated from atomic quantum mechanics that take into account the electromagnetic attraction between the nucleus and the orbital particle. The antiproton, as it passes by the nucleus, is also affected by the nuclear force. This can both shift and blur the X-ray signal.
These deviations from the purely electromagnetic predictions give an indication of nuclear effects. In the LEAR experiment, physicists measured these for 34 different nuclear targets, ranging from oxygen-16 to uranium-238.
The key parameter in each case is the difference between neutron and proton populations at large radii. The two different approaches are in broad agreement, showing that the more neutrons there are, the more they tend to live on the nuclear periphery. The neutrons also tend to populate an outer nuclear halo, the neutron excess of which increases with distance from the nuclear centre, rather than a nuclear skin with a constant neutron excess.
Another antiproton experiment at CERN (CERN Courier October p35) uses antiprotonic atoms as a precision laboratory for measurements of the antiproton itself.