Just as atoms can link up to form molecules, clusters of protons and neutrons can form inside nuclei and connect with one another. Theories that predict such molecular states in nuclei have now received a boost from an experiment described in the 15 February PRL. A beryllium nucleus with eight neutrons and four protons has been found to arrange itself into a molecule-like structure rather than assuming a spherical shape that more naive theories might suggest. The experiments also open up the possibility of finding even more exotic molecular states in other nuclei that contain more protons and neutrons.
In a molecule, two atoms are held together by an intricate bonding between their electrons. Remove the electrons, and the atoms will fall apart. In nuclei, alpha particles–which consist of two neutrons and two protons–often play the role of atoms, and any spare neutrons have the electrons’ job of holding the alphas together. This is the case in the beryllium nucleus, where nuclei with one, two and three excess neutrons have all neatly fit this picture. “The question arises,” says Martin Freer of the University of Birmingham in England, “as to how far you can go in adding neutrons and still keep forming molecular structures.” To find out, Freer and his colleagues devised a clever experiment to peer into the innards of a beryllium nucleus with twelve nucleons–two alpha particles and four excess neutrons.
“If there is a molecular structure in beryllium-12, then one of the best ways of looking for it is by looking to see what it falls apart into,” says Freer. Beryllium-12 particles were created with the help of the GANIL cyclotron in France, which smashed a beam of oxygen nuclei into a solid beryllium target, so that some target nuclei gained enough neutrons to form beryllium-12. The beryllium-12 particles were then separated from other nuclear debris and induced to bump into another target made of carbon and hydrogen. This second collision pumped energy into the nuclei and coaxed them into highly excited states. The team then measured the energy of the excited beryllium’s decay products as they burrowed into solid-state detectors. The researchers believe that beryllium-12 nuclei were most likely to decay into two helium-6 nuclei, and by measuring the angle at which these emerged from the beryllium, they could deduce its angular momentum. They then calculated the beryllium’s moment of inertia and found that it matched the predicted value for a nucleus made up of two alpha particles in contact with one another and sharing four neutrons.
Witold Nazarewicz, a physicist at the University of Tennessee in Knoxville points out that although molecular structures have been observed before, they have always been associated with stable clusters, like two alpha particles with not more than three neutrons. He is looking forward to seeing experiments performed on nuclei containing even more neutrons, such as beryllium-14, and heavier ones, like carbon-19.