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Crowns of neutrons: reactions and decays to penetrate the halos of Be-14 and He-8

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Diving into the particle adventure

EU-funded nuclear physicists are delving into an intriguing type of atomic nuclei. These nuclei contain particles that shear off from the central core to create a halo containing one or two of the excess neutrons.

Climate Change and Environment

Stable nuclei, smaller than the calcium nucleus that has 20 protons, usually have equal numbers of neutrons and protons, bound by the short-range strong nuclear force. The number of protons, which carry a positive charge, determines the chemical nature of the nucleus; hydrogen has one proton, helium has two, and so on. The neutrons, which are uncharged, provide extra strong force to overcome the electrostatic repulsion of protons. However, there are exceptions to this picture. Researchers are looking into these exceptions through the 'Crowns of neutrons: Reactions and decays to penetrate the halos of Be-14 and He-8' (LIGHTHALOS) project. Specifically, they have focused on the halos made up of four neutrons that surround beryllium-14 (Be-14) and helium-8 (He-8). By studying their neutron halos, scientists hope to gain better understanding of the forces within the atomic nucleus that bind atoms together. The neutrons in the halo are weakly bound to the nucleus, with only one tenth of the usual binding force of a neutron inside the nucleus. Furthermore, the displacement of halo neutrons from the atomic nuclear core is incompatible with the concepts of classical nuclear physics. However, the experiments conducted within the framework of the LIGHTHALOS project are expected to provide data necessary to develop an accurate description of the Be-14 and He-8 systems. A beam of the Be-12 isotope was accelerated at the Radioactive beam EXperiment (REX) - Isotope mass Separator On-line facility (ISOLDE) at CERN. Due to the closing of the REX-ISOLDE facility until 2015 for the HIE-ISOLDE upgrade, the disintegration of the Be-14 and He-8 was delayed pending the location of an alternative facility for the experiments. Proposals have been submitted to the Grand Accélérateur National d'Ions Lourds (GANIL) in France and the RIKEN facilities in Japan. For the disintegration of Be-14 and He-8, LIGHTHALOS has evaluated the performance of new experimental methods, including beta-delayed emissions light ions such as alpha and triton particles. The data collected and analysed so far have led to multiple presentations in scientific meetings as well as publications being drafted for submission to peer-reviewed journals. The project's findings expected on its completion should shed valuable light on the riddle of how halo neutrons can exist at such great distances from the core nucleus.


Particle, atomic nuclei, excess neutrons, neutrons, protons, nuclear force, electrostatic repulsion, beryllium-14, helium-8, neutron halos, atomic nuclear core, nuclear physics, isotope

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