Nuclear Seismology

Physicists at the GSI lab in Darmstadt, Germany, have discovered a new excited nuclear state, one in which a tide of neutrons swells away from the rest of the nucleus. Ordinarily, in its unexcited state, a typical atomic nucleus consists of a number of constituent neutrons and protons (collectively known as nucleons) bobbing around inside a roughly spherical shape. However, if struck by a projectile from outside, such as a beam particle supplied by an accelerator, the nucleus can be set to spinning, or it might distend. In one kind of excited mode called a dipole resonance, the protons can move slightly in one direction while the neutrons go the other way. In another type of excitation, a nucleus might consist of a stable core blob of nucleons surrounded by a surplus complement of one or two neutrons, which constitute a sort of halo around the core (see PNU 702). In the new GSI experiment, yet another nuclear mode has been observed. The nuclei used, two isotopes of tin, are the most neutron-rich among the heavier nuclei that can be produced at this time. Sn-130 and Sn-132 are so top-heavy with neutrons that they are quite unstable and must be made artificially in the lab. At GSI this is done by shooting a uranium beam at a beryllium target. The U-238 nuclei, agitated by the collision, eventually fission in flight, creating a swarm of more than 1,000 types of daughter nuclei, from which the desired tin isotopes can be extracted for study. The tin nuclei are excited when they pass through a secondary target, made of lead. The excited tin states later disintegrate; the debris coming out allows the researchers to reconstruct the turbulent nature of the tin nuclei. The dipole resonance was seen, as expected, but also a new resonance: an excess of neutrons pushing off from the core nucleus. Furthermore, the neutron resonance appears at a lower excitation energy than does the dipole resonance. Team leader Hans Emling (h.emling@gsi.de) says that there was some previous evidence for the existence for the neutron mode in work with lighter nuclei, but not the actual oscillation observed in the present work.

Adrich et al., Physical Review Letters, 23 September 2005.)
The GSI lab