Shape is a fundamental property of an atom’s nucleus. The majority of nuclei have shapes like an American football (that is, two or three of the principal axes are equal). However, scientists predicted a squashed football shape for a neutron-rich ruthenium isotope (110Ru) (see figure). To determine the shape, a novel technique was required. At a new facility at the Argonne National Laboratory accelerator, scientists created the needed technique. They re-accelerated a subset of unstable nuclei including 110Ru. Using two world-class detectors, they performed the first measurement on 110Ru. The results confirm that the nucleus exhibits a shape reminiscent of a squashed football shape, or a triaxial shape.
The measurement shows that the new technique works. Ruthenium and other unstable elements can be re-accelerated with enough intensity and energy to measure their shapes. This study contributes to experimental efforts to confirm that triaxial nuclear shapes can exist at low temperatures. The technique and study open up opportunities for shape studies across the periodic table.
The research team devised a new way to study the shape of atomic nuclei using a particle accelerator and pure electromagnetic interactions. At Argonne National Laboratory, a re-accelerated beam of the unstable refractory isotope 110Ru was delivered by the Californium Rare Isotope Breeder Upgrade (CARIBU) facility located at the Argonne Tandem Linac Accelerator System (ATLAS) accelerator. The shape of 110Ru was probed using two world-class detectors, Compact Heavy Ion Counter (CHICO2) and Gamma-Ray Energy Tracking In-beam Nuclear Array (GRETINA). The measurement was undertaken by nuclear physicists from the United States, France, Norway, Australia, Poland, and Mexico. The experimental results confirmed several theoretical calculations, which predict that the shape of the 110Ru nucleus should be similar to a squashed football (that is, triaxial where all three principal axes are unequal). The success of the measurement represents a technical milestone. Also, it paves the way for future studies involving neutron-rich unstable refractory isotopes, where non-axial shapes and other intriguing shape phenomena are predicted to occur.
U.S. Department of Energy (DOE), Office of Science, Office of Nuclear Physics. The researchers used resources at Argonne Tandem Linac Accelerator System (ATLAS), a DOE Office of Science scientific user facility located at Argonne National Laboratory.
D.T. Doherty, J.M. Allmond, R.V.F. Janssens, W. Korten, S. Zhu, M. Zielinska, D.C. Radford, A.D. Ayangeakaa, B. Bucher, J.C. Batchelder, C.W. Beausang, C. Campbell, M.P. Carpenter, D. Cline, H.L. Crawford, H.M. David, J.P. Delaroche, C. Dickerson, P. Fallon, A. Galindo-Uribarri, F.G. Kondev, J.L. Harker, A.B. Hayes, M. Hendricks, P. Humby, M. Girod, C.J. Gross, M. Klintefjord, K. Kolos, G.J. Lane, T. Lauritsen, J. Libert, A.O. Macchiavelli, P.J. Napiorkowski, E. Padilla-Rodal, R.C. Pardo, W. Reviol, D.G. Sarantites, G. Savard, D. Seweryniak, J. Srebrny, R. Varner, R. Vondrasek, A. Wiens, E. Wilson, J.L. Wood, and C.Y. Wu, “Triaxiality near the 110Ru ground state from Coulomb excitation.” Physics Letters B 766, 334-338 (2017). [DOI: 10.1016/j.physletb.2017.01.031]