Heavy elements such as gold and platinum are born in some of the Universe’s most violent events. These heavy elements form when stars collapse or when neutron stars collide, triggering a rapid chain of nuclear reactions known as the r-process. In this extreme environment, atomic nuclei become more unstable due to rapid neutron absorption.
Eventually, these heavy nuclei decay into more stable forms. This often happens through beta decay, a process that can release one or more neutrons as the nucleus stabilizes. Because the nuclei involved are extremely rare and short-lived, scientists usually rely on theoretical models rather than direct measurements.
Now, scientists from the University of Tennessee have uncovered new details about this mysterious process. In experiments at CERN, the team made three important discoveries about how unstable atomic nuclei break apart during these reactions.
Their findings, published in Physical Review Letters, provide crucial data that could improve models explaining how the Universe produces heavy elements like gold.
Understanding the cosmic origins of heavy elements
To better understand this process, scientists studied the rare isotope indium-134. The experiments took place at the ISOLDE Decay Station at CERN, where advanced laser separation techniques produced a pure beam of the isotope.
Professor Robert Grzywacz from the University of Tennessee at Knoxville stated, “These nuclei are hard to make and require a lot of new technology to synthesize in sufficient quantities.”
When indium-134 decays, it forms excited versions of several tin isotopes, including tin-134, tin-133, and tin-132. These nuclei can release neutrons as they settle into more stable states. Using a highly sensitive neutron detector supported by the National Science Foundation, scientists were able to track these emissions in unprecedented detail.
The team’s most important breakthrough was the first measurement of neutron energies produced during beta-delayed two-neutron emission.
Models reveal the first hints of nuclear fission in cosmos
This rare process occurs in exotic nuclei that exist only briefly. Until now, scientists could detect the emission but had never measured the energy of the released neutrons.
Neutrons are difficult to track because they scatter easily inside detectors. Distinguishing whether one neutron or two were emitted has therefore been a long-standing challenge.
“The reason this is hard is that 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.”
The new measurement opens the door to studying similar nuclear processes along the r-process pathway.
The scientists also detected, for the first time, a long-predicted single-particle neutron state in tin-133.
Scientists discovered that tin nuclei retain a “memory” of their beta decay. It contradicts the earlier belief that they released neutrons and forgot the event. Thanks to advanced neutron detectors, this hidden nuclear state came out from behind a curtain. These findings suggest the need for new models to explain why some decays emit one neutron while others emit two.
“People were searching for it for 20 years, and we found it,” Grzywacz said. “Those two neutrons allowed us to see this state.”
“The newly observed state represents an intermediate stage in the two-neutron emission sequence. It also represents the final elementary excitation of the tin-133 nucleus, helping complete the nuclear structure picture and improving the accuracy of theoretical calculations.”
The third discovery surprised scientists even more.
The newly observed nuclear state did not appear according to the statistical patterns normally expected in nuclear decay. In this experiment, the environment was relatively clean, with nuclear states separated instead of crowded together.
“Yet, in most cases, the system still behaves statistically, like split‑pea soup,” as Grzywacz put it. “Why this happens, and why it doesn’t in this case, remains unclear.”
The findings suggest that as scientists probe more exotic nuclei, such as Tennessine, existing models may no longer be enough. New theories will be needed to explain these extreme systems.
Improved models of the r-process will help scientists better understand the chemical evolution of the Universe and explain how elements such as gold and platinum are forged in stellar explosions and collisions.
The study also highlights the importance of advanced facilities like CERN and international collaboration in exploring some of the rarest forms of matter in the Universe.
Journal Reference:
P. Dyszel, R. Grzywacz et al. First β-Delayed Two-Neutron Spectroscopy of the r-Process Nucleus In134 and Observation of the i13/2 Single-Particle Neutron State in Sn133. Physical Review Letters, 2025; 135 (15) DOI: 10.1103/l24v-5m31