Atomic Shape-Shifters

The Three Shapes of Stable Nickel-64 Nuclei

By Maddy Stratton

The atom remains a mysterious component of reality which modern scientists continue working to understand. Strange phenomena start to occur when analyzing these tiny units of matter—only certain amounts of energy are allowed, one electron can be found in several places at once, and overall, chaos ensues. This leaves scientists pondering how they can approach learning more about the strange world of the atom. For starters, researchers can model the atom in different ways to investigate its properties. The models themselves evolve over time as new information is discovered. J. J. Thomson’s 1904 representation argued the atom consisted of a positively-charged nucleus “soup” with drops of negative charge interspersed to make it look like a plum pudding (the model was appropriately named the “Plum Pudding Model”).1 As researchers continued to learn more about the structure of the atom—namely, that it resembles a sphere, consists of a positively-charged nucleus made up of two types of tiny particles (protons and neutrons), which is orbited by even tinier, negatively-charged particles called electrons—the atomic model changed even further (Figure 1). Today, it is changing again, right now, for nickel-64 (Ni-64). Robert Janssens, working alongside his team at UNC’s Triangle Universities Nuclear Laboratory (TUNL) and three other nuclear laboratories, has established the existence of three distinct shapes of the stable Ni-64 isotope.2 Janssens’s interest in nuclear physics stems from his experience as an undergraduate at the Catholic University of Louvain in Belgium. During his time there, the university was overseeing the construction of a cyclotron, a particle accelerator. Always showing interest in nuclear structure and reactions, Janssens aided in the construction of the accelerator and was inspired to write his thesis in the same domain of nuclear physics during his senior year.3 He gained a PhD in experimental nuclear physics in Louvain before taking a postdoctoral position in the Netherlands. Then, he moved to the United States. Today, he and his team of researchers are studying Ni-64 nuclei. Atoms of the same element (which is determined by their number of protons) with differing amounts of neutrons are isotopes of that element. Usually, the nickel atom has 30 neutrons inside its nucleus. Ni-64, however, has 36. This particular isotope of nickel, though less abundant in nature, has stable properties which makes it easier to study in the lab. Janssens’s team and collaborating labs have identified three distinct shapes of stable Ni-64 nuclei, which appear as energy is added to the atom. Specifically, as energy increases, the nuclear shapes resemble the typical sphere (the ground state), a flattened shape, and an elongated football shape [Figure 2].4 These changes in

“There seems to be an aspect of the nuclear force that isn’t yet understood which shows up in neutron-rich nuclei.”

Figure 1: Adding or removing neutrons to/from an atom creates an isotope of that atom. Adding/removing protons creates a new element.

Figure 2: Shape coexistence through the chain of Nickel isotopes. This shows the relationship between nuclear shape and excitation energy of the nickel nucleus. shape are due to the complex ways the protons and neutrons in the nucleus of the atom arrange themselves as the energy that the nucleus is subjected to increases. The process of finding the nuclear shapes is a matter of adding energy to the atom. After several failed attempts at adding energy to the nuclei, the team found a method that worked: gamma radiation. Gamma rays, the most energetic wavelength of light on the electromagnetic spectrum, are introduced to the top of the nucleus (at high excitation energy) in a “cascade”, causing some energy to be released from the nucleus. The shape of the nucleus can then be interpreted from the energy released. Specifically, Janssens and his team measure the structure of the nuclei as a function of angular momentum, or the atom’s total energetic motion from each proton and neutron present.2 Nuclei with more neutrons than normal, like Ni-64, react differently to the changes in energy. As Janssens stated in our interview, there seems to be an aspect of the nuclear force that isn’t yet understood which shows up in neutron-rich nuclei, and specifically in ones that contain more neutrons than Ni-64.3 In order to further examine this phenomenon, Janssens and his team needed a new accelerator. The concept for the new accelerator was designed by a large team of scientists, including Janssens, and is currently under construction at Michigan State University. It is set to be operational in 2022. In the meantime, Janssens and the team are asking themselves, “is it really true the same phenomenon appears in stable nuclei [as opposed to only the neutron-rich ones]?”3 The answer: most likely, yes. However, the other stable nuclei would require much more energy to be introduced into the atom for the strange shapes to appear and become detectable, whereas one can see the shapes in neutron-rich nuclei at lower energies in the lab. The three shapes should appear in all nickel nuclei, no matter the isotope of nickel. Janssens and his team found it experimentally challenging to find the shapes in Ni-64, but the signature for them was very clear once found. The same thing is expected for other stable nickel nuclei. Now, Janssens and his team are looking ahead to analyzing the Ni-62 stable isotope and seeing if it expresses similar shapes at different energies. This nucleus may prove tougher to analyze, given it will require more energy to bring out the nuclear shapes. Congruently, the team is also looking forward to future experiments to determine if the shapes are specific to nickel nuclei (also known as 28-proton nuclei) or if the shapes can be generalized for other atoms. Janssens’ team will begin experiments in the coming weeks with zinc nuclei, which have two more protons than nickel. One more exciting component of the experiments is the potential for Janssens and his team to measure how long the football-shaped nucleus lasts. This could be doable given the relatively longer time the nickel nucleus remains in this specific state. However, due to the weak nature of the signal, this could prove to be a difficult task.3 Janssens and his team are working hard through setbacks due to the pandemic, which include delayed shipments of materials and the inability to safely work together to build detectors. Their work has big implications for the future knowledge of atomic shapes and structures, since the field is still trying to comprehend the structural results of underlying forces between protons and neutrons inside the nucleus.4 The team’s results will be tied with understanding aspects of the nuclear force, the force that holds the atom together, and will advance our understanding of how the structure of atoms can change. On a final note, learning more about the nuclei means learning more about the stars that produce them, since excited atomic states, neutron-rich nuclei, and many types of reactions are present inside stars extremely more frequently than they are here on earth [Figure 3]. Janssens himself states, “the stars shine only because of nuclear physics”.3

Figure 3: The nuclear reactions Janssens and his team study take place in stars continuously. Dr. Robert Janssens

References

1. Libretexts. “3.4: Rutherford’s Experiment- The Nuclear Model of the Atom.” Chemistry LibreTexts, Libretexts, 13 Aug. 2020, chem. libretexts.org/Bookshelves/Introductory_Chemistry/Map%3A_ Chemistry_for_Changing_Times_(Hill_and_McCreary)/03%3A_ Atomic_Structure/304%3A_Rutherfords_Experiment-_The_Nuclear_ Model_of_the_Atom. 2.“The Map of Nuclear Deformation Takes the Form of a Mountain Landscape.” Phys.org, Phys.org, 30 Dec. 2020, phys.org/news/2020-12nuclear-deformation-mountain-landscape.html (accessed February 8th , 2021). 3. Interview with Robert Janssens, Ph.D. 02/12/21. 4.“Stable Nickel-64 Nuclei Take Three Distinct Shapes.” Https:// www.energy.gov/Science/Listings/Science-Highlight, Department of Energy Office of Science, Nov. 2020 (accessed February 8th , 2021).