Properties of Matter with Nucleon Degrees of Freedom
A short story: Does size matter?
At the Heavy Metal bar, two nuclei Bilal and Porter, known as Bi and Po to their friends, are unwinding after a long day at the RIB factory.
‘Did you hear about Liam?’, said Bi.
‘Li? No, what? replied Po .
Li was also working at the factory. They would often meet at the exclusive Heavy Metal bar, where only the heavy and large nuclei belonging to the metallic part of the chart of nuclides were allowed.
‘He’s not allowed in here anymore. Apparently he cheated to get into our club!’, revealed Bi.
‘Cheated? But how can that be? He is so big and shiny!’, said Po , struggling to understand.
‘Big yes, but not heavy!’ sneered Bi.
So how could a small nucleus like Li (Lithium, 3 protons) masquerade as a large one, such as Bi (Bismuth, 83 protons) or Po (Polonium, 84 protons)?
The answer lies in the structure of the nucleus, whose size can be related to the number of its constituents A (protons and neutrons) through the simple relation R=R0A1/3, where R is the radius and R0 a constant. However, this relation becomes invalid when looking at exotic nuclei lying far from stability. This is the case for 11Li, which can be represented by an inert core (3 protons and 6 neutrons) and two very weakly bound neutrons, giving the impression of a halo around the nucleus. This has the consequence that a halo system is spatially large compared to its more stable isotope. In the case of 11Li, which is one of the most studied nuclei because of its pronounced halo structure, its halo radius is as large as the radius of 208Pb.
Halo nuclei are short lived and have to be studied using Radioactive Ion Beams (RIB), in which they are formed and then used to initiate nuclear reactions on stable targets. In the 1980s, the total cross-sections for the interaction of Li isotopes with various targets were measured and showed a steep rise for 11Li , which was interpreted as due to a large increase of the nuclear matter radius. In this type of experiments, the nuclear matter radius of the projectile can be simply estimated from the total cross-section of the reaction. More recent techniques use proton-nucleus elastic scattering experiments, which also relate the measured cross-section to the nuclear matter distribution. With the advent of FAIR (Facility for Antiproton and Ion Research) in Darmstadt, the study of halo nuclei will benefit from the production of more exotic ion beams, extending the scope of potential halo candidates to more neutron-rich elements.
The Collaboration of the international NUclear Structure and Astrophysics community at FAIR.