Scientists crack a 20-year nuclear mystery behind the creation of gold

Scientists crack a 20-year nuclear mystery behind the creation of gold

Gold and other heavy elements are born in some of the universe's most violent events—but scientists still struggle to understand the nuclear steps that create them. Now, nuclear physicists have uncovered three key discoveries about how unstable atomic nuclei decay during the rapid neutron-capture process, the chain reaction responsible for forging elements like gold and platinum.

Gold cannot form until certain unstable atomic nuclei break apart. Exactly how those nuclear transformations unfold has long been difficult to determine. Now, nuclear physicists at the University of Tennessee (UT) report three discoveries in a single study that clarify important parts of this process. Their findings could help researchers build improved models of the stellar events that create heavy elements and better predict the behavior of exotic atomic nuclei.

Heavy elements such as gold and platinum are forged under extraordinary conditions, including when stars collapse, explode, or collide. These events trigger the rapid neutron capture process (or r-process for short). During this process, an atomic nucleus absorbs neutrons in rapid succession. As the nucleus grows heavier and more unstable, it eventually breaks down into lighter and more stable forms.

Along this pathway across the nuclide chart, a common sequence involves beta decay of the parent nucleus followed by the release of two neutrons. The atomic nuclei involved in these reactions are extremely rare and unstable, making them difficult or even impossible to study directly in experiments. Because of this, scientists rely heavily on theoretical models, which must be tested and refined using laboratory data.

Studying Rare Nuclei With CERN's ISOLDE Facility

To investigate the process more closely, UT researchers collaborated with scientists from several institutions. The team carried out the experiments at the ISOLDE Decay Station at CERN, which produced abundant indium-134 nuclei and used advanced laser separation techniques to ensure their purity. When indium-134 undergoes decay, it generates excited forms of tin-134, tin-133, and tin-132.

Using a neutron detector funded through the National Science Foundation Major Research Instrumentation program and constructed at UT, the scientists uncovered three major findings. The most significant result was the first measurement of neutron energies associated with 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, which are unstable and exist only briefly. "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."

A Long-Sought Neutron State in Tin

The team's second major discovery was the first observation of a long predicted single particle neutron state in tin-133. Traditionally, scientists believed the tin nucleus simply released neutrons to cool down, effectively losing any trace of the earlier beta decay event (an "amnesiac nucleus").

"We say the tin doesn't forget," Grzywacz said. "This 'shadow' of indium doesn't completely disappear. The memory is not erased." Advanced neutron detectors allowed researchers to detect this elusive nuclear state, suggesting that current theoretical explanations are incomplete.

A Third Discovery Challenges Existing Models

The study also revealed a third important result: a non-statistical population of this newly identified state. In simple terms, the way the state is populated during decay does not follow the patterns that scientists typically expect. The findings suggest that as scientists explore regions of the nuclear landscape farther from stability, existing models may no longer apply, requiring new theoretical approaches.