Far from the neighborhoods frequented by everyday elements lay a nuclear region where Alice-in-Wonderland norms rule. A recent experiment performed at NSCL confirms magnesium-36 to be a denizen of this strange place, where the structure of atomic nuclei becomes odd and unexpected.
"Ten years ago, complicated experiments like this one were a dream," said collaborating scientist Kirby Kemper, professor of physics and vice president for research at Florida State University. "Five years ago, we thought that in the next ten years we would be able to carry it out. Now we have done one and so are much farther along in experimental capability than even our wildest hopes."
Protons and neutrons that comprise a nucleus array themselves in shells, much like electrons around an atom. The phenomenon is described by the nuclear shell model. According to the model, specific numbers of protons and neutrons lead to shell structures that are especially stable — except, that is, for nuclei in the Island of Inversion. In this through-the-looking-glass region, nuclei in their lowest energy states that would otherwise have fairly typical shell structures adopt weird and strongly deformed structures. Mapping out which unstable nuclei are within or outside this region helps researchers extend the usefulness of the nuclear shell model, which earned its creators a Nobel Prize in the 1960s for its ability to explain the properties of stable nuclei.
Alexandra Gade, NSCL assistant professor, collaborated with researchers from Michigan State University and Florida State University, the University of Tokyo and RIKEN in Japan, and the University of Surrey in England to study magnesium-36, Mg-36. Contemporary theoretical models suggested that the nucleus, with 12 protons and 24 neutrons, should exist just within the Island of Inversion. But until the team's result, which will appear in Physical Review Letters, experimentalists hadn't made the necessary measurements of the rare magnesium isotope to know for sure.
The experiment was conducted at the NSCL Coupled Cyclotron Facility, where a fast beam of calcium-48 nuclei was generated and directed at a beryllium target. This generated a variety of reaction products, including silicon-38, Si-38. The A1900 fragment separator was tuned to allow Si-38 to pass through and continue down the beam line.
Downstream, these Si-38 isotopes struck a second beryllium target, resulting in a new smattering of nuclei, which included Mg-36. The beam was turned up into the focal plane of the three-story S800 spectrograph, which was set only to accept Mg-36. When analyzed, the spectroscopic data indicated that Mg-36 is within the Island of Inversion.
"Gamma-ray spectroscopy for Mg-36 has never been done because this nucleus is incredibly hard to reach," said Gade, lead author of the paper. "It's not just another nucleus."
For every 400,000 Si-38 nuclei impacting the second target, just one Mg-36 nucleus was produced. The particle identification spectrum, continued Gade, demonstrates the selectivity and sensitivity of the experimental equipment at NSCL. Mg-36, the researchers wrote, "will be the most neutron-rich nucleus with Z=12 (12 protons) accessible to gamma-ray spectroscopy until next-generation rare isotope facilities are operational."
"This is what a fast-beam facility is all about; we can do a reaction and disentangle the mess of hundreds of thousands of particles," Gade said. "We can say 'this is the guy that I want.'"
The research was supported in part by the U.S. National Science Foundation.
NSCL is a world-leading laboratory for rare isotope research and nuclear science education.