Neutron-rich nuclei can sometimes be regarded as consisting of two units which oscillate resonantly together. Helium-6, for instance, has been previously observed to oscillate in the form of a "giant dipole resonance" (GDR), in which two protons form one unit and four neutrons form the other unit, and a "spin dipole resonance" (SDR) in which nucleons with their spins point up oscillate in opposition to nucleons whose spins point down. Now, for the first time, another resonance has been observed, the "soft dipole resonance" (soft-DR), in which two neutrons oscillate against a unit consisting of two neutrons and two protons.
In the figure to the left, L stands for the change in isospin (a proton changing into a neutron in the collision process in which the Helium-6 was created), while S refers to the net spin of the nucleus. The figure to the right shows the energy levels involved in the various states of the nuclear resonance.
Reported by: Nakyama et al., Physical Review Letters, 10 July 2000