Study of chromium-62 helps researchers better understand the geometry around inversion islands

In a recent paper published in Nature Physics, an international collaboration used world-class instruments at the Rare Isotope Beam Facility (FRIB) to study an unusual nuclide, the rare isotope chromium-62.

Researchers used gamma-ray spectroscopy experiments in parallel with theoretical models to identify an unexpected variety of shapes for chromium-62. The discovery provides more insight into so-called “islands of inversion,” areas on the nuclear map where certain nuclides diverge from the traditional perspective based on stable nuclear properties.

The study involved the collaboration of 23 researchers from 12 different affiliations. The collaboration was led by FRIB and Alexandra Gade, professor of physics and FRIB scientific director in the MSU Department of Physics and Astronomy, and also included Robert Janssens, Edward G. Bilpuch Distinguished Professor at the University of North Carolina at Chapel Hill . Key contributors include Brenden Longfellow, a former FRIB graduate researcher and current staff scientist at Lawrence Livermore National Laboratory;

“One of the goals of nuclear theory is to characterize the properties of all atomic nuclei, including rare isotopes, which have a much larger number of neutrons than protons and often do not follow established textbook physics for stable isotopes. The goal is to develop a model that describes the

“The model must be able to describe the structural changes in the inversion island. If it is not, the model does not incorporate the correct physics and further extrapolation using the model may not be useful.” In a sense, the inversion island nucleus is to test the nuclear model before extrapolating the unknown.”

Inversion Island has many unexpected shapes.

Many researchers are focused on understanding the properties, including the shape, of short-lived, neutron-rich nuclei using new powerful particle accelerators that can probe more exotic nuclei. Scientists know that the more familiar side of the nuclear diagram follows a magic number of both neutrons and protons.

But in recent decades, researchers have discovered that isotopes with more neutrons than protons can break these rules, and that the magic number is not as immutable as once thought. I started noticing. Therefore, a particular neutron-rich nucleus differs significantly in its nuclear structure when compared to a stable nucleus.

“What’s interesting about these inversion islands is that the nuclei there have a magic number of neutrons and would be expected to be spherical, but instead their ground state is deformed,” Long said. Mr. Fellows said. “The way the proton and neutron orbits fill in the nuclear shell model is different and far from stable.”

Janssens and Gade have been working together for more than 20 years to investigate the magical nuclear number. Jansens noted that the technological and infrastructure investments that grew FRIB from its predecessor, the National Superconducting Cyclotron Laboratory, have allowed researchers to advance work at the frontier of neutron-weight exotic materials.

“We have done many experiments over the years, but this research was pretty much at a standstill until FRIB came online and we also had access to the GRETINA gamma-ray detector,” Janssens said. he said. “In fact, this is the first experiment using a fragmentation beam of the facility in flight at FRIB.”

GRETINA fosters collaborative research

To investigate chromium-62, the FRIB fragmentation team first fired a high-energy zinc isotope beam at a beryllium target. In the process, the researchers produced iron-64 isotope. By knocking out two protons from these iron isotopes, the researchers were able to form chromium-62.

But even more important to the experiment was access to the Gamma-Ray Energy Tracking In-Beam Nuclear Array (GRETINA). GRETINA was developed in collaboration with scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) and serves as a state-of-the-art gamma-ray detection instrument used in the nation’s major particle accelerator facilities.

“GRETINA was an integral part of the work,” says Gade. “We tagged the excited states of chromium-62 via emitted gamma rays. The way the excited states decay is a unique fingerprint, and by selecting them we can You can study the properties of the state.”

With the help of the FRIB infrastructure and GRETINA, the research team found that chromium-62 had a deformed shape in the ground state, but at higher excitation energies it became less deformed and non-axisymmetric. I did. The research team extrapolated the results to the calcium isotope near chromium-62 in the nuclear map and created a research plan for future experimental research.

“With these discoveries as a starting point, we plan to continue our research in this area and measure other observables that characterize these cores of inversion islands, and we plan to continue to work with FRIB as it continues to strengthen its capabilities. “As we move forward, we will have access to more neutrons,” said Gade, “the inhabitants of this inversion island.”

Additionally, GRETINA will soon be converted into a Gamma-Ray Energy Tracking Array (GRETA). This increases the number of gamma ray detectors included in the device, making it possible to detect signals from even minute amounts of atomic nuclei. Berkeley Lab has played a leading role in the creation of GRETINA and now GRETA.

In addition to FRIB’s infrastructure, the researchers highlighted research results from collaborations with multiple U.S.-based research institutions and several European facilities. Both Gade and Janssens emphasized that advancing the frontiers of nuclear physics requires both investment in research infrastructure and a spirit of healthy collaboration and exchange of ideas.

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