Chromium-62 study helps researchers better understand shapes around islands of inversion

In a recent paper in Nature Physics, an international research collaboration used world-class instrumentation at the Facility for Rare Isotope Beams (FRIB) to study the exotic nuclide, or rare isotope, chromium-62. Researchers used a gamma-ray spectroscopy experiment in tandem with theoretical models to identify an unexpected variety of shapes in chromium-62. The finding provides more insight into so-called “islands of inversion,” or regions in the nuclear chart where certain nuclides diverge from traditional viewpoints based on the properties of stable nuclei.

The work involved the joint effort of 23 researchers with 12 different affiliations among them. Led by Alexandra Gade, professor of physics at FRIB and in MSU's Department of Physics and Astronomy and FRIB scientific director, the collaboration also included Robert Janssens, Edward G. Bilpuch Distinguished Professor at the University of North Carolina at Chapel Hill, and Brenden Longfellow, former FRIB graduate researcher and current staff scientist at Lawrence Livermore National Laboratory, as significant contributors.

“One goal of nuclear theory is to develop a model that describes the properties of all nuclei, including rare isotopes that have many more neutrons than protons and that often do not follow the textbook physics established for their stable cousins,” Gade said. “Models must be able to describe the structural change in islands of inversion, otherwise they do not incorporate the correct physics and further extrapolation using them may not be useful. In that sense, nuclei in islands of inversion are some of the best stepping stones for testing nuclear models before extrapolating into the unknown.”  

Unexpected shapes abound in islands of inversion

Using new, powerful particle accelerators that can probe more exotic nuclei, many researchers are focused on understanding the properties of short-lived, neutron-rich nuclei, including their shape. Scientists know that the more familiar side of the nuclear chart abides by magic numbers of both neutrons and protons.

In recent decades, however, researchers have started to notice that isotopes with many more neutrons than protons can break these rules, and that magic numbers are not as immutable as once thought. Consequently, certain neutron-rich nuclei differ markedly in their nuclear structure when compared to their stable counterparts.

“The interesting thing about these islands of inversion is that the nuclei there are expected to be spherical since they have a magic number of neutrons, but instead they have deformed ground states,” Longfellow said. “The way the protons and neutrons are filling their orbitals in the nuclear shell model is different far from stability.”

Janssens and Gade have worked together investigating magic nuclear numbers for over 20 years. Janssens pointed out that the technological and infrastructural investments that grew FRIB out of its predecessor, the National Superconducting Cyclotron Laboratory, enabled the researchers to advance work on the frontier of neutron-heavy exotic matter.

“We’ve done many experiments through the years, but until FRIB came online and we also had access to the GRETINA gamma-ray detector, we were almost at a roadblock in this work,” Janssens said. “This is actually the first experiment at FRIB to use the facility’s fragmentation beams in flight.”

GRETINA boosts collaborative research

To investigate chromium-62, the FRIB fragment separator team first shot a high-energy zinc isotope beam toward a beryllium target. In the process, the researchers produced iron-64 isotopes. By knocking out two protons from these iron isotopes, the team was able to form chromium-62. Even more important to the experiment, however, was access to the Gamma-Ray Energy Tracking In-Beam Nuclear Array (GRETINA). GRETINA was developed by a collaboration led by scientists from Lawrence Berkeley National Laboratory (Berkeley Lab) to serve as a state-of-the-art gamma-ray detection instrument for use at the nation’s leading particle accelerator facilities.

“GRETINA was an integral part of the work,” Gade said. “We tagged the excited states of chromium-62 via their emitted gamma rays. The ways that excited states decay are unique fingerprints, and by selecting them, we can study the properties of individual final states of chromium-62.”

With the help of the FRIB infrastructure and GRETINA, the team found that chromium-62 had a deformed shape in its ground state but was less deformed and with a non-axially symmetric shape at higher excitation energy. The team extrapolated its findings to calcium isotopes near chromium-62 in the nuclear chart and has a line of investigation for future experimental work.

“Using these findings as a springboard, we will continue our work in this region and measure other observables that characterize these nuclei in the island of inversion. And, as FRIB continues to ramp up its capabilities, we will have access to more neutron-rich tenants of this island of inversion,” Gade said.

In addition, GRETINA will soon be transformed into the Gamma-Ray Energy Tracking Array (GRETA). This will increase the number of gamma-ray detectors that are part of the instrument and enable the detection of signals from nuclei produced in even weaker quantities. Berkeley Lab has had a leadership role in the creation of GRETINA and now GRETA.

The researchers emphasized that in addition to FRIB’s infrastructure, their work benefited from collaborations between multiple U.S.-based research institutions and several European facilities. Gade and Janssens both emphasized that advancing the frontier of nuclear physics requires both investment in research infrastructure and a healthy spirit of collaboration and exchange of ideas.

“Experimental nuclear physics is a team sport,” Gade said. “It takes a group of people with diverse skills to conceive and propose the experiment, run the instruments, analyze, and interpret the data in the framework of many-body computations or nuclear structure and nuclear reactions.”

Gade and Janssens said nuclear science advances the most when experiment and theory work in concert. They cited the close collaboration between the researchers as being indispensable. Those collaborations involved nuclear theorists Frédéric Nowacki and Duc D. Dao from IPHC Strasbourg in France, who invented the nuclear model that allowed the interpretation of chromium-62‘s nuclear shapes; Jeffrey A. Tostevin from the University of Surrey in the United Kingdom, whose reaction theory enabled translation of nuclear-reaction observables into nuclear-structure information; Alfredo Poves from the Universidad Autonoma de Madrid in Spain and Silvia M. Lenzi from the Università degli Studi di Padova in Italy, who provided insights into the shell-model interpretation.

Eric Gedenk is a freelance science writer.

Michigan State University (MSU) operates the Facility for Rare Isotope Beams (FRIB) as a user facility for the U.S. Department of Energy Office of Science (DOE-SC), with financial support from and furthering the mission of the DOE-SC Office of Nuclear Physics. Hosting the most powerful heavy-ion accelerator, FRIB enables 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.

The U.S. Department of Energy Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of today’s most pressing challenges. For more information, visit energy.gov/science.