Physicists Unlock Secrets of Tarar Alchemy, Gaining New Insights into Gold’s Cosmic Origin

Until you can sleep until the end of the nucleus. The specifics of the process have been difficult to pin down, but UT nuclear physicists published three discoveries in a paper that describe key details. The findings could help scientists come up with new models to explain the amazing processes that give us the heavy elements, as well as better predictions about the expanding landscape of exotic nuclei.

The work has been published in the journal Physical review letters.

Physics of bowling

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 rapid succession until it becomes so heavy that it moves to lighter, more stable nuclei.

As it crosses the nuclide chart, the R process has taken place in the region where the primary 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 calculations based on these models are highly constrained and must be validated in the lab.

To get a better picture of how it all happens, researchers including UT graduate students Peter Diesel and Jacob Gouge, Professor Robert Grzewacz, Associate Professor Miguel Madurga, and Research Associate Monika Piersa Silkoska worked with scientists from other institutions. Based on data analysis methods described by research assistant professor Zhengyu Xu, they started with large amounts of indium 134.

“A lot of new technology is needed and required to synthesize these nuclei in sufficient quantities.”

The Isolate Deck Station at CERN met this challenge by providing a large number of indium-134 nuclei, as well as sophisticated laser separation technology to ensure they were pristine. When INDIUM-134 decides, it populates excited states in Tn-134, Tn-133, and Tn-132. Using a neutron detector built at UT, scientists made three important discoveries. At the top of the list, they made the first measurements of neutron energies for the emission of two neutrons with a beta delay.

“The emission of two neutrons is the biggest deal,” Griezweis said.

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

“The reason it’s hard is because neutrons like to bounce around. It’s hard to tell if it’s one or two,” Griezvaz explained. In previous attempts, “nobody measured the energies,” so this approach “opens up a whole new field.”

This is the first study to describe the emission of two neutrons for a nucleus that follows the Ar process, opening the door to clear models of how stellar events can produce elements such as gold.

Tin never forgets

The second discovery was the first observation of the long-sought single-particle neutron state in Tn-133. “Tin is in an excited state,” explained Grzywacz.

He said the traditional theory is that tin “boils” to cool, becoming “an amnesic nucleus”, with no memory of beta decay.

“We say that tin forgets not.” “This ‘shadow’ of indium does not completely disappear. The memory is not erased.”

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

“People had been looking for this for 20 years and we found it,” Griezweis said. “Those two neutrons allowed us to see this state.”

They explained that this newly observed state is an intermediate step in the two-neutron emission process. It is also the last initial excitation in the Tn 133 nucleus, completing the picture and helping to make the calculations more accurate.

Better calculations and modeling are linked to the third discovery that this research has brought to light. The decay process is relatively clean, so everything is isolated with no neighboring states, Grzewacz explained.

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

The results show that as you travel through the nuclear landscape away from stability and into the realm of exotic nuclei like the Tennessean, the old models no longer hold and new ones are needed.

Pursuit of curiosity

The need for new models to explain nuclear origin and structure presents a tremendous opportunity for graduate students like DeCell. He joined Grizoz’s group in 2022 and was the first author Physical review letters paper outlining all three findings.

His to-do list for this experiment was a long one, from building the physical pieces to interpreting the results. He built the frames for the neutron tracking detector and assembled them into the experimental setup. He set up the required electronics and built the beta detector. He performed test measurements, assisted with software for data acquisition, optimized for optimal time resolution, and analyzed experimental data. Through it all, Diesel’s work was still part of a multidisciplinary effort.

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