Isotopes are variants of an element that have the same number of protons but different numbers of neutrons. The shape of a nucleus at the core of the atom can vary from isotope to isotope for a given element. This is because of the interplay between the nuclei and the motion of the protons and neutrons they contain. In a recent experiment, nuclear scientists found that the nucleus of the radioactive isotope selenium-72 has a football-like shape. This shape is similar to the stable, nonradioactive isotopes of selenium, but different from the disk-like shape of radioactive selenium-70 nuclei. This result helps scientists understand how the complex interaction between protons and neutrons in the nucleus leads to collective behavior.
Atomic nuclei are incredibly complex. Despite this, nuclei have several apparently simple collective variables. One of these is well-defined, non-spherical shapes. Scientists’ basic questions about nuclei include how these shapes emerge from the seemingly chaotic interactions within a nucleus. This new research shows that the properties of deformed heavy nuclei such as selenium-72 are essential to understanding elements whose nuclei change shape suddenly from isotope to isotope.
Nuclear scientists know that lighter radioactive isotopes of selenium undergo a sudden change in shape for nuclei with fewer neutrons than the stable isotopes. However, the exact isotope for which this shape change occurs was not known until a recent experiment at the ReA3 re-accelerator at Michigan State University’s National Superconducting Cyclotron Laboratory (NSCL). Scientists smashed a krypton-78 beam into a beryllium foil at 30 percent of the speed of light to produce radioactive selenium-72 nuclei. These selenium-72 nuclei were then separated, collected, and brought to rest. The scientists could then reaccelerate the selenium-72 nuclei as a pure, high-quality beam traveling at 9 percent of the speed of light before impacting a lead-208 foil. The energy of the selenium-72 beam was sufficiently low that scattering was not caused by nuclear interactions, but only by the longer-range electromagnetic force generated by the protons inside the selenium and lead nuclei. This electromagnetic process excited selenium-72 from its ground state to the first excited state and above, and it then deexcited by emitting gamma rays that were detected using an array of 16 segmented germanium detectors and a pair of double-sided segmented silicon detectors. The probability of this excitation was measured precisely enough to determine the spectroscopic quadrupole moment—a measure of the degree to which the shape deviates from a sphere. The results show that selenium-72 nuclei have a shape similar to stable selenium isotopes. The experiment therefore pinpoints the location of a sudden transition from the prolate, football-like shapes observed in stable selenium isotopes to the oblate, disk-like shape of selenium-70 along the isotopic chain. Present nuclear models cannot adequately describe these results and require modifications to incorporate additional degrees of freedom.
The experiment also demonstrated that the shapes of radioactive nuclei can be determined even with beam intensities that are 10 million times lower than what is typically achievable for stable isotopes. This establishes an experimental procedure to study the many radioactive isotopes produced at the NSCL and the future Facility for Rare Isotope Beams. The experiment was conducted by a team from Lawrence Livermore National Laboratory in collaboration with scientists from the NSCL, the University of Rochester, and the University of Massachusetts Lowell.
Dr. Ching-Yen Wu
Lawrence Livermore National Laboratory
Professor Alexandra Gade
Michigan State University
Funding was provided by the Department of Energy Office of Science, Office of Nuclear Physics, and the National Nuclear Security Administration. Operation of NSCL as a national user facility is supported by the Experimental Nuclear Physics Program of the National Science Foundation.
Henderson, J., et al., “Localizing the Shape Transition in Neutron-Deficient Selenium”, Physical Review Letters 121, 082502 (2018). [DOI: http://dx.doi.org/10.1103/PhysRevLett.121.082502]