Long-lived quantum states point the way to solving the mystery of radioactive nuclei ......

A study led by Timothy Gray of the U.S. Department of Energy's Oak Ridge National Laboratory may reveal changes in the shape of atomic nuclei. The unexpected discovery could affect our understanding of the composition of atomic nuclei, how protons and neutrons interact, and how elements are formed.

Gray, a nuclear physicist, said, "We used a radioactive beam of excited sodium-32 nuclei to test our understanding of the shape of nuclei far from stable, and found an unexpected result that raises questions about how the shape of nuclei evolves."

The results were published June 13 in Physical Review Letters.

The shape and energy of atomic nuclei change between configurations over time. Typically, atomic nuclei, as quantum entities, have either a spherical or deformed shape; the former looks like a basketball and the latter like an American soccer.

The relationship between shape and energy level is one of the great open questions in science, and models of nuclear structure are difficult to extrapolate to regions where there is little experimental data.

For some exotic radioactive nuclei, conventional models predict shapes that are the opposite of those observed. The ground state or lowest energy configuration of a radioactive nucleus is thought to be spherical, but turns out to be deformed.

In principle, the energy of the excited deformed form can be reduced to an energy lower than that of the spherical ground state, thus making the spherical shape a high-energy state. Unexpectedly, this role reversal seems to be occurring in some exotic nuclei when the natural ratio of neutrons to protons is out of whack. However, the reversed excited sphere forms (spherical states) have never been found. It is as if once the ground state is deformed, all excited states are deformed as well.

Many nuclei have spherical ground states and deformed excited states. Similarly, many nuclei have deformed ground states and subsequently deformed excited states - sometimes to different degrees or types. However, nuclei with both deformed ground states and spherical excited states are much more elusive.

Using data collected from the first experiments conducted in 2022 at the Rare Isotope Beam Installation (FRIB), a U.S. Department of Energy Office of Science user facility located at Michigan State University, Gray's research group has discovered a long-lived excited state of radioactive sodium-32. The newly observed excited state has an extremely long lifetime of 24 microseconds: about a million times the lifetime of a typical nuclear excited state.

Long-lived excited states are known as isomers, and the long lifetimes indicate that something unexpected is happening. For example, if the excited state is spherical, it is difficult to return to the deformed ground state, which may account for its long lifetime.

Schematic of the two types of isomers

The study involved 66 people from 20 universities and national laboratories. Co-Principal Investigators were from Lawrence Berkeley National Laboratory, Florida State University, Mississippi State University, the University of Tennessee at Knoxville, and ORNL.

A beam of excited sodium-32 nuclei was implanted into the FRIB decay station initiator, which is used to probe the decay signature of the isotope.

The resulting data were generated from the 2022 experiment's use of the FRIB Decay Station Initiator (FDSi) - a modular multi-detector system that is extremely sensitive to rare isotope decay signatures.

"FDSi's versatile combination of detectors showed that the long-lived excited state of sodium-32 was delivered within the FRIB beam and then decayed internally to the ground state of the same nucleus by emitting gamma rays." said Mitch Allmond of ORNL, who co-authored the paper and manages the FDSi program.

To stop FRIB's high-energy radioactive beams, which travel at about 50 percent of the speed of light, UT Knoxville has installed an implanted detector at the FDSi center. North of the beamline is an array of gamma-ray detectors called DEGAi, consisting of 11 germanium cloverleaf-type detectors and 15 fast-timed lanthanum bromide detectors. On the south side of the beamline are 88 detector modules called NEXTi, which measure the time of flight of neutrons emitted in radioactive decay.

An excited beam of sodium-32 nuclei stops in the detectors and decays to a deformed ground state by emitting gamma rays. How long the excited state has existed can be found by analyzing the gamma-ray spectrum to resolve the time difference between beam implantation and gamma-ray emission. The new isomer exists for 24 microseconds, which is the longest of the isomers with 20 to 28 neutrons decaying by gamma-ray emission. About 1.8 percent of the sodium-32 nucleus was observed to be the new isomer.

Gray says, "We can come up with two different models that explain equally well the energies and lifetimes we observe experimentally."

Experiments with higher beam powers will be needed to determine whether the excited state of sodium-32 is spherical. If it is, then the state will have six quantized units of angular momentum, which is a quality of the nucleus associated with the overall rotation of the nucleus or the orbital motion of individual protons and/or neutrons around the center of mass. However, if the excited state of sodium-32 were to deform, the units of angular momentum for that state would be zero.

In the future, the team's planned upgrades to the FRIB will provide more energy and increase the number of nuclei in the beam. Data from the stronger beam will allow the experiment to distinguish between the two possibilities.

We will characterize the correlation between the angles of the two gamma-ray beams emitted in the cascade," Gray explains. The angular correlation between the gamma rays of these two possibilities is very different. If we have enough statistical data, we will be able to discern patterns that reveal clear answers."

Reference link:

[1] https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.242501

[2] https://phys.org/news/2023-08-long-lived-quantum-state-mystery-radioactive.html