Physicists unlock secrets of stellar alchemy, yielding new insights into gold’s cosmic origins

You can’t have gold until a nucleus decays. The specifics of that process have been hard to pin down, but UT’s nuclear physicists have published three discoveries in one paper explaining key details. The results can help scientists come up with new models to describe the stellar processes that give us heavy elements, as well as make better predictions about the expanding landscape of exotic nuclei.

The work is published in the journal Physical Review Letters.

The physics of bling

Elements like gold and platinum are created under extreme conditions, such as when stars collapse, explode, or collide. In the rapid neutron-capture process (or r-process for short), a nucleus captures a barrage of neutrons in quick succession until it becomes so heavy it decays into lighter, more stable nuclei.

As it crosses the nuclide chart, the r-process path winds through territory where the main decay mode is beta decay of the parent nucleus, followed by the emission of two neutrons.

The nuclei involved are difficult (if not impossible) to study experimentally, so the calculations describing them lean heavily on models that must be validated in the lab.

To get a better picture of how all this happens, researchers including UT Graduate Students Peter Dyszel and Jacob Gouge, Professor Robert Grzywacz, Associate Professor Miguel Madurga, and Research Associate Monika Piersa-Silkowska worked with a host of scientists from other institutions. Building on data analysis methods outlined by Research Assistant Professor Zhengyu Xu, they started with large amounts of indium-134.

“These nuclei are hard to make and require a lot of new technology to synthesize in sufficient quantities,” Grzywacz explained.

The ISOLDE Decay Station at CERN met the challenge by providing plenty of indium-134 nuclei, as well as sophisticated laser separation technology to make sure they were pristine. When indium-134 decays, it populates excited states in tin-134, tin-133, and tin-132. Using a neutron detector built at UT, scientists made three important discoveries. At the top of the list, they made the first measurement of neutron energies for beta-delayed two-neutron emission.

“The two-neutron emission is the biggest deal,” Grzywacz said.

Beta-delayed two-neutron emission occurs only in exotic nuclei, those that are short-lived and unstable. The two-neutron separation energy is very small, but in this experiment it was enough to be measured.

“The reason this is hard is because neutrons like to bounce around. It’s hard to tell if it’s one or two,” Grzywacz explained. In earlier attempts, “no one measured energies,” so this approach “opens a completely new field,” he said.

This is the first study detailing the two-neutron emission for a nucleus that follows the r-process path, opening the door for clearer models about how stellar events can create elements like gold.

Tin never forgets

A second discovery was the first observation of a long-sought single-particle neutron state in tin-133. Grzywacz explained that “tin is in an excited state. (It) has to cool off. It can spit out a neutron, or with enough energy, it can spit out two neutrons. It should always spit two neutrons, but it doesn’t.”

He said the traditional view is that tin “boils off” neutrons to cool down, becoming “an amnesiac nucleus,” with no memory of beta decay.

“We say the tin doesn’t forget,” Grzywacz said. “This ‘shadow’ of indium doesn’t completely disappear. The memory is not erased.”

In this experiment, state-of-the-art neutron detectors identified this elusive state, indicating that a better theoretical framework is needed to understand why sometimes one neutron is emitted and sometimes two are.

“People were searching for it for 20 years and we found it,” Grzywacz said. “Those two neutrons allowed us to see this state.”

He explained that this newly-observed state is an intermediate step in the two-neutron emission process. It’s also the last elementary excitation in the tin-133 nucleus, completing the picture and helping make calculations more accurate.

Better calculations and modeling are tied to the third discovery this research brought to light—the observation of the non-statistical population of this newly-observed state. Grzywacz explained that the decay process is relatively clean, so everything is separate with no neighboring states.

“You’re not making split-pea soup,” he said. “Still, in most cases, it behaves like split-pea soup. Somehow this statistical mechanism happens. Why is it statistical, even though it shouldn’t be, and why, in our case, isn’t it?”

The results indicate that as you travel across the nuclear landscape, farther from stability and into the realm of exotic nuclei like Tennessine, the old models don’t hold and new ones are needed.

The pursuit of curiosity

The need for new models to explain nuclear origins and structure presents a tremendous opportunity for graduate students like Dyszel. He joined Grzywacz’s group in 2022 and was the first author on the Physical Review Letters paper outlining the three discoveries.

His to-do list for this experiment was a long one, from constructing physical pieces to interpreting the results. He built frames for the neutron tracking detectors and assembled them in the experimental setup. He set up the required electronics and made beta detectors. He ran test measurements, helped with software for data acquisition, made corrections for optimal timing resolution, and analyzed the experimental data. With all that, Dyszel’s work was still part of a multi-person effort.

“The success of this work is due in part to my colleagues and collaborators, whose guidance and constructive input were crucial,” he said.