Light nuclei that contain more neutrons than protons have proved to be a rich source of new physics over the past 15 years. Now physicists in Japan have observed a so-called soft dipole resonance in such a nucleus for the first time (S Nakayama et al. 2000 Phys. Rev. Lett. 85 262). The resonance was observed in helium-6, which contains two protons and four neutrons.
Helium-6 is a typical neutron-skin nucleus, with two of the neutrons forming a “skin” around the alpha-particle core. Different types of dynamics are possible in such nuclei. In the soft dipole resonance observed by the Japanese group, the alpha particle and neutrons oscillate in opposite directions. In a so-called giant dipole resonance, on the other hand, the protons and neutrons oscillate in opposite directions. This type of behaviour has been observed in lithium-6, which contains three protons and three neutrons. And in a spin-dipole resonance, neutrons and protons with spin “up” move in one direction, while those with spin “down” move in the opposite direction.
S Nakayama and colleagues collided a beam of lithium-7 nuclei from the Ring Cyclotron of the Research Centre for Nuclear Physics at Osaka University onto a lithium-6 target. Some of the collisions resulted in the creation of helium-6 and beryllium-7, with the latter being ejected from the target. By analyzing the beryllium nuclei, Nakayama and colleagues identified a strong candidate for a soft dipole resonance with its energy, cross-section and other characteristics in agreement with predictions.
