The halo nucleus.  (Source: GSI)
Three scientists recently calculated the 'halo' formed from Flourine-17

We're not talking about the virtuous kind, or the popular video game. Instead, the halo that three scientists were working with was in fact a halo nucleus. Thomas Papenbrock, associate professor at the University of Tennessee, Knoxville, Gaute Hagen from Oak Ridge National Laboratory, and Morten Hjorth-Jenson from the University of Olso recently calculated the 'halo' of such nuclei, in terms of physics. Their report was published earlier this month in Physical Review Letters, titled "Ab-initio computation of the 17F proton-halo state and resonances in A = 17 nuclei."

The atom fluorine-17 was used, because it is in fact a 'mirror nucleus' of oxygen-17. Where fluorine-17 has nine protons and eight neutrons, oxygen-17 has eight protons and nine neutrons. These two atoms surround oxygen-16: the most abundant and stable isotope of oxygen. By exciting a proton found orbiting around a fluorine-17, a halo is formed. This proton is far away from the oxygen-16 core.

A halo nucleus is defined as a nucleus that, because of weak forces, has its nucleons (protons or neutrons, normally found within the nucleus) pushed away from the center. These nucleons then form a sort of halo. One of the challenges the scientists faced was the fact that halo nuclei only live for a few milliseconds and are very fragile. Moreover, one could find halo nuclei at the far end of the nuclear existence spectrum (the dripline), where a slight imbalance between protons and neutrons means that the nucleus will fall apart.

Because there were more than two bodies interacting in this system, calculations proved difficult to pinpoint the exact numbers. This called for sophisticated methods when working with the 17 interacting particles in the isotope, titled many-body problems. The team, at first, employed a nuclear Hamiltonian, which reads the energy of the system regarding its momentum and coordinates. Then, a numerical method, known as the coupled-cluster method, which calculates the proton halo state in fluorine-17 by solving the many-body problems. All this was done on the Oak Ridge National Laboratory's super-computer Jaguar. Impressive, considering the calculations reflect a similar value for the binding energy compared to experimental data, as well as no adjustable parameters.

Scientists hope that the increased knowledge of nuclei will lead them to discover the limits of nuclear existence. By investigating the nucleus, applications can be made to all aspects of life, such as medicine or space travel.  

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