Focus: Antiprotons Map Nuclear Surface
Imagine a geologist’s frustration if she couldn’t find the Earth’s surface, and you’ll understand the plight of researchers studying heavy isotope nuclei. The topography of these nuclei is difficult to map because their surfaces consists mostly of neutrons, which evade many methods for detection. But now a research team has mapped the surfaces of over 20 heavy isotope nuclei using two new techniques, both of which employ antiprotons. Their results, which appear in the 20 August print issue of PRL, show that the neutrons form a halo around the nuclei–a finding that may settle a long-standing debate in nuclear physics.
The nuclei of heavy isotopes are rich in neutrons, which dominate their nuclear surface. For decades researchers have debated the topography of these neutron shells. Some theories suggest that they form a uniform skin, while others propose a halo akin to the Earth’s atmosphere. Now that debate seems to have been settled by a collaboration of Polish and German researchers. The team used antiprotons from the Low-Energy Antiproton Ring (LEAR) at CERN, the European particle physics center in Geneva, to probe the surfaces of over 20 heavy nuclei with two different techniques.
The first technique was to bombard a sample of material with antiprotons. The antiprotons–which are negatively charged, like electrons–took up orbits around nuclei. They dropped from high-energy orbitals to close-in orbits near the nuclear surface, where their quarks annihilated with those of either a proton or a neutron. The identity of the particle destroyed by the antiproton determined the type of radioactive isotope left behind. By measuring the decay of isotopes inside a sample over a period of months and feeding that information into nuclear models, the team got a picture of the nuclear surface.
In the second set of experiments, the researchers showered a thin film of material with antiprotons. This time they observed the x-rays emitted as the antiprotons descended through the energy levels. The spectra of x rays from the lowest energy levels changed as a result of antiproton interactions with the nuclear surface. The team put the change in spectra into the same models to determine the neutron distribution on the surface of the nucleus.
“Having all the data in our hands, we now believe that the neutrons are distributed more like a halo than a skin,” says team member Jerzy Jastrzebski of Warsaw University, but not everyone in the field is convinced. Alex Brown of Michigan State University in East Lansing says that the models used to construct the surface map may not have been accurate enough to end the debate. “Still,” Brown says, “this has to be one of the best experiments yet.” And, he believes, it will encourage others to confirm the team’s results.