Long-lived quantum state points the way to solving a mystery in radioactive nuclei

A research conducted by Timothy Gray at the Oak Ridge National Laboratory of the Department of Energy may have discovered an unanticipated alteration in the structure of an atomic nucleus. Our knowledge of what binds nuclei together, how protons and neutrons interact, and how elements create may be impacted by the unexpected discovery.

"We used radioactive beams of excited sodium-32 nuclei to test our understanding of nuclear shapes far from stability and found an unexpected result that raises questions about how nuclear shapes evolve," stated nuclear physicist Gray. Physical Review Letters reported the findings.

Atomic nuclei can change throughout time into various configurations in terms of their shapes and energy. Usually, nuclei have spherical or distorted forms and exist as quantum entities. The former have a basketball-like appearance, whereas the later are more like American footballs.

For the scientific community, a crucial unanswered question is how forms and energy levels link. Models of nuclear structure have difficulty extending to areas where there are little experimental data.

The forms anticipated by conventional models are the reverse of what is seen for some unusual radioactive nuclei. In contrast to expectations, radioactive nuclei were found to be distorted in their ground states, or lowest-energy configurations.

What has the power to reverse a quantum state?

The spherical shape is the high-energy one because, in theory, the energy of an excited deformed state might fall below that of a spherical ground state. Unexpectedly, when the natural ratio of neutrons to protons falls out of balance, this role reversal appears to be occurring for some unusual nuclei. But the excited spherical post-reversal states have never been discovered. It appears that as soon as the ground state deforms, so do all the stimulated states.

There are several instances of spherical ground states and distorted excited states in nuclei. Similar to this, many nuclei have distorted ground states followed by deformed excited states, sometimes with differing quantities or types of distortion. However, it is considerably harder to find nuclei that have both spherical excited states and distorted ground states.

Gray's team identified a long-lived excited state of radioactive sodium-32 using data gathered in 2022 from the first experiment at the Facility for Rare Isotope Beams, or FRIB, a DOE Office of Science user facility at Michigan State University. The newly discovered excited state has a lifespan of 24 microseconds, which is extraordinarily lengthy compared to the ordinary nuclear excited state and roughly a million times longer.

Isomers are long-lasting enthusiastic moods. A extended lifespan suggests that something unexpected is happening. For instance, a challenge in reverting to a distorted ground state if the excited state is spherical might explain its prolonged existence.

66 people from 20 universities and national laboratories took part in the investigation. The University of Tennessee, Knoxville, Florida State University, Mississippi State University, Ohio Research and Development Laboratory, and ORNL all contributed co-principal investigators.

The FRIB Decay Station initiator, or FDSi, a modular multidetector system that is very sensitive to rare isotope decay signs, was utilized in the 2022 experiment that produced the data necessary to determine the 2023 result.

"FDSi's versatile combination of detectors shows that the long-lived excited state of sodium-32 is delivered within the FRIB beam and that it then decays internally by emitting gamma rays to the ground state of the same nucleus," said Mitch Allmond, an ORNL researcher and co-author of the study.

An implantation detector created by UT Knoxville was placed in the center of FDSi to block the very energetic radioactive beam that is emitted by FRIB and moves at almost 50% the speed of light. A gamma-ray detector array known as DEGAi, which had 15 fast-timing lanthanum bromide detectors and 11 germanium clover-style detectors, was located to the north of the beam line. 88 modules of the NEXTi detector, which measures the duration of flight of neutrons released during radioactive decay, were located south of the beam line.

A stream of excited sodium-32 nuclei halted in the detector and released gamma rays as they decayed to the distorted ground state. How long the excited state lasted was determined by analyzing gamma-ray spectra to determine the interval between beam implantation and gamma-ray emission. The novel isomer has the longest lifespan among gamma-ray emitting isomers with 20 to 28 neutrons, lasting 24 microseconds. In terms of sodium-32 nuclei, the new isomer was found to make up around 1.8% of them.

The energy and lifetimes that we have seen in the experiment may be explained by two separate theories, according to Gray.

To ascertain if the excited state in sodium-32 is spherical, a greater beam strength experiment is required. If so, the state would possess six quantized units of angular momentum, a property of a nucleus related to its whole-body rotation or the orbital motion of its individual protons and/or neutrons around the center of mass. The excited state in sodium-32 would, however, have zero quantized units of angular momentum if it were to be deformed.

More power will be available from a future update of FRIB, increasing the number of nuclei in the beam. An experiment that can discriminate between the two options will be made possible by data from the higher powerful beam.

Gray said that correlations between the angles of two gamma rays that are released in a cascade would be classified as such. The gamma rays' angular correlations in the two scenarios are considerably different. We could separate the pattern that shows a clear answer if we have enough statistics.