An international team of scientists from the University of Tennessee and the ISOLDE Collaboration at CERN has made a breakthrough in understanding how heavy chemical elements—from silver to gold, platinum, and uranium—are formed in the Universe. The study, published in Physical Review Letters, has for the first time allowed researchers to directly measure an extremely rare type of nuclear decay that plays a crucial role in the so-called r-process.

How heavy elements are formed

Heavy elements (those heavier than iron) cannot form in ordinary stars. Their creation requires extreme environments such as supernova explosions or collisions of neutron stars.

During such events, the rapid neutron capture process (r-process) occurs: atomic nuclei rapidly capture huge numbers of neutrons, becoming extremely heavy and unstable. They then transform into more stable atoms through chains of beta decays.

Many of the intermediate nuclei in this chain exist for only fractions of a second and are extremely rare, which makes their properties almost impossible to study directly. Because of this, significant gaps have remained in our understanding of exactly how elements like gold, platinum, and other precious metals form.

The ISOLDE experiment: indium-134 at the center

Scientists studied the extremely rare isotope Indium‑134, observing its decay using the ISOLDE facility at CERN.

During beta decay, indium-134 transforms into excited states of tin isotopes:

  • Tin‑134
  • Tin‑133
  • Tin‑132

A high-sensitivity neutron detector was used to register the decay products.

The most important result was the first direct measurement of neutron energies in a beta-delayed two-neutron emission process (β2n). This extremely rare decay occurs only in the most unstable nuclei near the neutron drip line.

“The emission of two neutrons is the most important result of our work,” said Professor Robert Grzywocz from the University of Tennessee.

Until now, scientists could only detect the fact that neutrons were emitted, but their energies had never been measured directly. Neutrons scatter easily, making it very difficult to distinguish one neutron from two. The new technique opens the door to precise measurements.

Other key discoveries

First: the team obtained the first experimental confirmation of a previously predicted neutron state in the nucleus of Tin‑133. This state appears to be an intermediate step in the two-neutron emission process.

Previously, scientists believed that after beta decay the nucleus quickly “forgot” its previous state and simply emitted neutrons to lose excess energy. The new data suggest the opposite: the nucleus partially retains memory of how it was formed.

Second: the observed state forms in a way that differs from predictions made by existing statistical nuclear reaction models. This suggests that theoretical descriptions of extremely neutron-rich nuclei may require significant revision.

Why this matters for astrophysics

Precise data about the β2n process and neutron energies will allow astrophysicists to significantly improve models of the r-process. With better data, scientists can more accurately estimate how much gold, platinum, and other heavy elements are produced during neutron-star mergers and supernova explosions.

“The results will help improve r-process models and more accurately describe the formation of heavy elements in cosmic catastrophes,” the authors emphasize.

Further studies of unstable nuclei could dramatically reshape our understanding of the chemical evolution of the Universe and the processes occurring in the most extreme astrophysical events.

In brief

Physicists from the University of Tennessee and CERN have for the first time measured neutron energies in the beta-delayed two-neutron emission of Indium‑134, a key step in the r-process that produces gold, platinum, and other heavy elements. The researchers also observed a predicted neutron state in Tin‑133 and showed that nuclei retain “memory” of their previous states.

The results challenge some existing statistical models and open the path to more accurate astrophysical calculations of how heavy elements form in the Universe.