During the last decade, six new superheavy elements were added the Periodic Table of Chemical Elements. Recently, the United Nations proclaimed 2019 as the International Year of the Periodic Table because it marks the 150th anniversary of the formulation of the table created by Dmitry Mendeleev. A review of the field and future challenges has been published in a recent Reviews of Modern Physics Colloquium.
Nuclear and atomic physics aim to explain the physical world, from the origins of nuclei to their structure. All elements with more than 103 protons are labeled as “superheavy," and are part of a vast, totally unknown territory of these nuclei that scientists are trying to uncover. Questions motivating the search for these systems include:
- What are the heaviest atomic nuclei that can exist?
- How are these nuclei held together?
- How long do they live as superheavy nuclei before decaying into lighter nuclei?
- Are they produced in stellar explosions?
Exploring this uncharted territory provides prospects for discoveries that connect the broad areas of nuclear physics, atomic physics, chemistry and astrophysics.
In 2012 and 2016, six new synthetic elements – nihonium, flerovium, moscovium, livermorium, tennessine and oganesson - joined the periodic table. Their atomic numbers – the number of protons in the nucleus that determines their chemical properties and place in the periodic table – are 113, 114, 115, 116, 117 and 118, respectively. These elements define the current upper limits of mass and atomic numbers. As such, they carry the potential to transform the way we currently understand nuclear and atomic physics, and chemistry. This would in turn significantly affect how the overarching questions are answered.
The heaviest superheavy elements exhibit strange features as compared to their lighter siblings. The currently known superheavy elements belong to seventh and last period of the periodic table. The unusual element oganesson completes that seventh period, and it is the only element of that period that does not naturally occur.
A holy grail for the field is to produce long-lived superheavy elements with about 184 neutrons. Theory and experiment suggest that at this limit, superheavy nuclei will live longer before decaying into lighter nuclei. This increased stability would facilitate chemical studies. However, getting 184 neutrons in a nucleus is not going to be easy. Scientists are still on the lookout for the optimal way to synthetize such systems. Another goal is to understand the role that superheavy elements play in stellar events such as neutron star mergers or supernovae.
How far can the periodic table go? It is still a mystery. As experimental capabilities progress, scientists will be able to pursue heavier elements. The next steps include the search for the next element, with the atomic number 119, in several laboratories worldwide, and to further understand how masses and charges are assigned to these superheavy elements.
“The scientific expedition continues into the uncharted regions of atomic number and nuclear mass,” said Witek Nazarewicz, Hannah Distinguished Professor of Physics at MSU and chief scientist at the Facility for Rare Isotope Beams, currently under construction at MSU. Nazarewicz and several collaborators wrote the article published in the Reviews of Modern PhysicsColloquium. “The prospects for discoveries in the interdisciplinary field of superheavy nuclei and atoms, on the intersection of nuclear physics, atomic physics, chemistry and astrophysics, are outstanding.”
In the future, dedicated new accelerator facilities in Dubna (Russia) and RIKEN (Japan) will be able to produce more of these species, and research conducted at Dubna, RIKEN, the GSI Helmholtz Centre for Heavy Ion Research/Facility for Antiproton and Ion Research in Germany, Oak Ridge National Laboratory and Lawrence Berkeley National Laboratory in the United States, Grand Accèlèrateur National d' Ions Lourds in France, and several others worldwide will lead this effort as new territory is explored beyond the standard periodic table.
FRIB at MSU will also play an important role. It is expected that nuclear reactions using radioactive heavy beams will enable scientists to get closer to the expected region of increased stability. By providing unique data on these neutron-rich nuclei, FRIB will improve our understanding of the cosmic origin of superheavy elements.
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Michigan State University is establishing FRIB as a new scientific user facility for the Office of Nuclear Physics in the U.S. Department of Energy Office of Science. Under construction on campus and operated by MSU, FRIB will enable scientists to make discoveries about the properties of rare isotopes in order to better understand the physics of nuclei, nuclear astrophysics, fundamental interactions, and applications for society, including in medicine, homeland security and industry.
