Experiment Reveals Competing Nuclear Shapes in the Rare Isotope Chromium-62
Successfully modeling chromium-62 hints at an interesting structure for neutron-laden calcium-60.
The Science
The nuclear chart is a two-dimensional landscape of the elements and their various isotopes. The location of individual atomic nuclei on the chart reflects how many neutrons they have on one axis and the number of protons they have on the other axis. Nuclear scientists are working to model key isotopes on this landscape so that they can ultimately predict the properties of isotopes out of reach for measurements. They are particularly interested in short-lived rare isotopes that have far more neutrons than protons in regions of the chart where the properties of neighboring isotopes differ a lot from each other. Researchers refer to such parts of the nuclear as “islands of inversion” because the atomic nuclei in those areas break the textbook rules that guide more common isotopes. One important property is the shape of an atomic nucleus. In this study, researchers found first evidence that nuclei of the isotope chromium-62 can have two different shapes at low excitation energy.
The Impact
Chromium-62, like other nuclei in islands of inversion, is a benchmark for nuclear models designed to explain what drives the structure of nuclei away from stability. This research presented a novel model that allows for intricate configurations of protons and neutrons in nuclei. The model describes the new results on energies and shapes in chromium-62 very well. The researchers used the model to extrapolate to the structure of calcium-60. This isotope’s structure is important for understanding how many neutrons the element of calcium can bind in its nucleus. The results will lead to new experiments at the Facility for Rare Isotope Beams (FRIB), a Department of Energy user facility, and at other laboratories.
Summary
From the first in-beam gamma-ray spectroscopy experiment performed with GRETINA (Gamma-Ray Energy Tracking In-beam Nuclear Array) at FRIB, researchers obtained strong experimental and theoretical evidence for the observation of the excited 0+2, 2+2 and 2+3 states in chromium-62, a member of the N = 40 island of inversion. They interpreted the results using large-scale shell-model computations, including the recently developed Discrete Non-Orthogonal Shell-Model approach. The latter enables an analysis in terms of nuclear shapes. The scientists argue that the shape-coexisting structure calculated to be built on the new 0+2 level is dominated by so-called 2p2h intruder configurations straddling the N = 40 shell gap and is associated with a non-axial shape. This “triaxial” shape differs from the axially symmetric, strong prolate deformation calculated for the collective band built on the ground state, predicted to be dominated by significantly different (4p4h) intruder configurations.
Furthermore, the researchers proposed another newly reported level as the 2+3 state predicted as the head of yet another band structure. For this band, the researchers predicted sizable mixing with the 0+2 band structure and a similar non-axial shape. Based on the excellent experiment-theory agreement, the scientists attempted to extrapolate towards the key nucleus calcium-60, predicting a 2+1 excitation energy of the order of ∼1 MeV.
Contact
Alexandra GadeFacility for Rare Isotope Beams
[email protected]
Robert V. F. Janssens
University of North Carolina at Chapel Hill
[email protected]
Funding
This material is based on work supported by the Department of Energy (DOE) Office of Science, Nuclear Physics and High Energy Physics offices; the U.S. National Science Foundation; the DOE National Nuclear Security Administration; the Lawrence Livermore National Laboratory’s Laboratory Directed Research and Development Program; the Science and Technology Facilities Council of the United Kingdom; and grants from Spain.
Publications
Gade, A., et al., In-beam spectroscopy reveals competing nuclear shapes in the rare isotope 62Cr. Nature Physics 21, 37 (2025). [DOI: 10.1038/s41567-024-02680-0]
Related Links
Nature Physics “News and Views:” Into the islands of inversion
Michigan State University press release: Chromium-62 study helps researchers better understand shapes around islands of inversion
University of North Carolina at Chapel Hill Department of Physics and Astronomy: Prof. Janssens’ Research on Chromium-62 Featured in Nature Physics
Highlight Categories
Program: NP
Performer: University , FRIB
