Mysterious sixth-order transition hints at enigmatic nuclear process

Friday 5 July 2024 10am

Physicists have seen the only known example of a sixth-order electromagnetic decay process in nature, emitted as a gamma ray from an excited state in iron-53 nuclei.  

The team from the Department of Nuclear Physics and Applications believe it is evidence for an extremely rare and complex multi-particle process that may never be fully understood.

“In the universe, this is the only known E6 transition,” said lead researcher from NPAA, Dr AJ Mitchell.

The ways that excited nuclei lose energy are related to the overall change in angular momentum as they rearrange from one configuration to another. The transition that involves the lowest-possible change of angular momentum is the most common one – one unit of change, known as electric-dipole transitions and denoted E1. 

Higher-order decay, such as quadrupole (E2) and octupole (E3) transitions are less common but well studied and offer important insights into nuclear properties, such as their shape or the individual and collective motion of nucleons within the system. 

However, Dr Mitchell said very little was known about even higher-order processes, such as the E6 observed in this experiment (known as a hexacontatetrapole transition). 

“Our results show that something very different is going on in this transition. We usually associate the familiar E2 transitions with collective, coherent motion of nucleons within a nucleus. That isn’t what we see here with the E6 transition, but more cases need to be studied to verify if this isn’t a one-off example.”

“But, for data, I have no idea where you would go looking for another E6 transition!”

The team published their work in Physical Review Letters on the study of the gamma ray in question, of energy 3.04 MeV. The radiation originated from the nucleus of a radioactive isotope of iron, iron-53, which was made using nuclear reactions with the Heavy Ion Accelerator Facility (HIAF) at ANU.

The observation settles a fifty-year debate. The same gamma ray was observed in an experiment in the 1970s, but its origin was unclear. Theorist Professor Alex Brown from Michigan State University drew attention to the measurement at a conference talk in 2011, which was attended by a contingent of HIAF experimentalists, including Professor Andrew Stuchbery. 

“Professor Brown pointed to it as the only known E6 transition in nature, but we thought, hang on, are you sure it’s not an artefact of the measurement?” said Professor Stuchbery, head of the Nuclear Physics and Accelerator Applications Department.

“A sum peak caused by multiple lower-energy gamma rays hitting the detector at the same time would give the same result. 

“But we thought, ‘We can measure that!’”

When they got back to Canberra, the team used the high-precision facilities at HIAF to create the requisite iron-53 isotope by bombarding a target of vanadium-51 with a lithium-6 beam moving at 10% of the speed of light. This reaction results in four neutrons flying out, which leaves the iron-53 in an excited state that lives, on average, for around 2.5 minutes. There are few pathways available for it to decay by, and so 1 in 2000 times it drops to the ground state in one go, emitting the unique E6 photon.

The flexible nature of the CAESAR gamma-ray detector array at HIAF allowed the team to determine that coincident gamma rays from the stronger multi-step pathway did, in fact, also create a 3-MeV gamma ray peak in the data, as they had initially suspected. 

But once they had subtracted that signal, there was clearly a hexacontatetrapole peak as well – in the haystack they had found the only known transition of its kind.

As the celebrations for a world record died down, more was in store. Detailed analysis of the results showed that a key parameter of the theoretical calculations, the effective charges, had to be modified to around 60% of the value expected from collective E2 transitions. This is a strong indicator that the high-multipolarity transitions (E6 and E4) are fundamentally different from the well-known collective ones (E2). 

But whether any further hexacontatetrapole transitions will be found to help solve this mystery is anyone’s guess.  The first candidate the team checked out was cobalt-53; where iron-53 has 26 protons and 27 neutrons, cobalt-53 has the numbers reversed (27 protons and 26 neutrons). But it undergoes beta decay—a different process altogether—not electromagnetic decay.

It’ll take a bit of luck to find such a transition amongst the huge number of processes that take place across the thousands of known isotopes, says Dr Mitchell.

“Iron-53 really is a special case, due to the way the protons and neutrons line up in f7/2 orbitals, and properties of the states that exist below the state in question, limiting the ways for it to lose energy.”

“It’s just a fortunate coincidence that nature frames it this way in this particular nucleus.”


Dr AJ Mitchell
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