A new excited nuclear state

has been seen in unstable, neutron-rich isotopes of tin. Typically, an unexcited atomic nucleus has its constituent neutrons and protons bobbing around in a roughly spherical configuration. When excited, however, the nucleus can be made to spin, vibrate, or otherwise deform. One excited mode, called a giant dipole resonance, has the protons move in one direction while the neutrons go the other way. In neutron-rich nuclei, a tight core of nucleons can be surrounded by a diffuse halo or skin of more loosely bound neutrons. In the new experiment, the LAND collaboration at the GSI lab in Darmstadt, Germany, generated a beam of uranium-238 nuclei that then dissolved by fission into a swarm of daughter nuclei, from which the desired tin isotopes, ¹³⁰Sn and ¹³²Sn, were extracted and excited by passing through a lead target. From the debris of the subsequent decays, the researchers reconstructed the excitation energy spectra of the tin nuclei. The expected dipole resonance was seen at about 16 MeV, and a brand new resonance showed up at 10 MeV. The collective nature of the new “pygmy” dipole resonance is not yet fully clear, although calculations show that excess neutrons are oscillating. (P. Adrich et al. , Phys. Rev. Lett. 95, 132501, 2005)