# Why would antiprotons be particularly sensitive to “halo neutrons” as described in this Nature news item?

In the Nature news item Physicists plan antimatter’s first outing — in a van an experiment to study the proposed "neutron halo" around the neutron-rich and short lived isotope 11Li is outlines:

Because antiprotons annihilate so readily, both with protons and with neutrons, they present a unique way to study the unusual configurations of radioactive nuclei. Whereas everyday atomic hearts host protons and neutrons in roughly equal measure, radioactive isotopes are stuffed with extra neutrons. This imbalance can give rise to exotic characteristics, 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.

Has this interaction been modeled yet? What is it that makes researchers fairly certain that this interaction would be particularly sensitive to the last two, weakly bound "halo neutrons" as opposed to equally sensitive to all neutrons?

If it works (several years from now), perhaps it can finally answer the question Has a nuclear “neutron halo” been measured directly? Anything other than “breaks easily”?

A CERN Letter of Intent (Obertelli et al., 2017) gives some more details, including references. The antiprotons annihilate on a nucleon in the nucleus of interest, producing an easily-detectable shower of four to six pions. The pions from each event can be collected and their total charge determined: total charge zero for $p$-$\bar p$ annihilations, and total charge $-1$ for $n$-$\bar p$ annihilations. The plan seems to be for the radioisotopes to pass through the cold antiprotons slowly enough that the antiprotons can capture into orbitals around the nucleus before annihilating. The relaxing of the bound antiproton towards its ground state emits x-rays, whose precise energy levels require modeling of the proton and neutron distributions.

This experiment was apparently done in the early 1970s with an antiproton beam on stable nuclei by Bugg and collaborators. (I had Bugg for my modern physics course as an undergraduate; always fun to run across a familiar name.) What's new here is the ability to use stopped antiprotons, rather than a beam, and therefore to study shorter-lived isotopes.

• Thanks for the great explanation, answer and references! The Letter of Intent is well written and chock-full-o-references as well, this will keep me busy for a while. – uhoh Apr 3 '18 at 1:20