Nuclear Dripline Droops

Several new heavy isotopes have been discovered, at least one of which pushes beyond the neutron dripline. Driplines are the outer edges defining the zone of observed or expected bound nuclei on a map whose horizontal axis is the number of neutrons in a nucleus (denoted by the letter N) and whose vertical axis corresponds to the number of protons (Z). Unlike the Coulomb force which holds atoms together, and where electron behavior and the expected chemical properties of that element can be predicted pretty accurately, with nuclei it's different.

The nuclear force holding neutrons and protons together (even as the like-charged protons repel each other electrostatically) is so strong that no theory (not even the so called nuclear shell model, fashioned in analogy to the atomic model) can confidently predict whether a particular combination of neutrons and protons will form a bound nucleus. Instead experimenters must help theorists by going out and finding or making each nuclide in the lab.

In an experiment conducted recently at the National Superconducting Cyclotron Lab (NSCL) at Michigan State University, a beam of calcium ions was smashed into a tungsten target. A myriad of different nuclides emerged and streamed into a sensitive detector for identification. Two newly found nuclides-Mg-40 and Al-43-came as no surprise. But another, Al-42, was more unusual since it violated the provisional prohibition against nuclei of this size having an odd number of protons and neutrons.

The new nuclides are not stable since they decay within a few milliseconds. But this is pretty long by nuclear standards. Why study such fleeting nuclei? Even though they might not exist naturally, the new nuclides still might play a role inside stars or novas where heavy elements, including those that make up our planet and our bodies, are created. Thomas Baumann (baumann@nscl.msu.edu) suggests that even heavier aluminum-isotopes might exist, and that it is worth exploring any possible islands of stability, not just those at the very edge of the periodic table. (Baumann et al., Nature 25 October 2007; http://www.nscl.msu.edu/magnesium40)