What exactly is the s-process, and why is it so important for understanding the universe?

The s-process, or slow neutron capture process, is a crucial mechanism within stars responsible for creating elements heavier than iron. It involves the gradual capture of neutrons by atomic nuclei. Understanding the s-process helps us unravel the mystery of the universe's chemical composition and the origins of elements beyond hydrogen and helium, which were formed in the Big Bang.

How do scientists actually create the radioactive targets, like those using 147Pm and 171Tm, needed for s-process experiments?

Scientists create radioactive targets for s-process experiments through a multi-step process. First, stable isotopes like 146Nd and 170Er are irradiated with neutrons at facilities like the Institute Laue-Langevin (ILL). This transforms them into radioactive isotopes, specifically 147Pm and 171Tm. These isotopes then undergo rigorous chemical separation and purification at places like the Paul Scherrer Institute (PSI). Finally, the purified material is electroplated onto thin aluminum backings to create targets suitable for neutron capture measurements.

What methods are used to measure neutron capture cross-sections, especially for radioactive isotopes like 147Pm and 171Tm?

Neutron capture cross-sections for radioactive isotopes like 147Pm and 171Tm are measured using complementary methods. Time-of-flight experiments, such as those conducted at CERN's n_TOF facility, and activation experiments, like those at the SARAF facility's Liquid-Lithium Target (LiLiT), are employed. These experiments help scientists understand how these isotopes interact with neutrons, providing critical data for understanding the s-process and stellar nucleosynthesis. The data from n_TOF, for example, provides detailed information about capture resonances for 171Tm.

What are 'branching points' in the s-process, and why is understanding isotopes like 147Pm and 171Tm at these points significant?

Branching points in the s-process refer to unstable nuclei where the path of nucleosynthesis can diverge based on reaction rates and environmental conditions within a star. The neutron capture cross-sections of isotopes at these branching points, such as 147Pm and 171Tm, are particularly important because they determine which elements are more likely to be produced. If we could also understand other reaction types beyond neutron capture it would provide a more complete picture.

How do experiments with radioactive isotopes, like 147Pm and 171Tm, help us understand the bigger picture of stellar nucleosynthesis and the chemical evolution of the universe?

Experiments focusing on isotopes like 147Pm and 171Tm provide essential data to refine models of stellar nucleosynthesis and the s-process. Preliminary results from facilities such as n_TOF show clearly resolved capture resonances for 171Tm, offering valuable insights into the energy levels within the nucleus and its interaction with neutrons. Analyzing data, including resonance parameters derived with the Hauser-Feshbach statistical model, helps scientists understand how elements are created in stars and how the chemical composition of the universe evolves over time. The next steps could be to examine other heavy elements.

A surreal illustration of a star with radioactive isotopes within, symbolizing stellar nucleosynthesis.

Unlocking the Secrets of the Stars: How Radioactive Isotopes Help Us Understand the Universe

Soraya Malik in Science & Nature December 2025 • 4 min read.

"Groundbreaking experiments with 147Pm and 171Tm offer new insights into stellar nucleosynthesis and the origins of heavy elements."

The universe is a vast and complex place, and one of the most intriguing questions is how the elements that make up everything around us were formed. While light elements like hydrogen and helium were created in the Big Bang, heavier elements are forged in the hearts of stars through nuclear reactions. Among these processes, the s-process (slow neutron capture process) plays a crucial role in creating many elements heavier than iron.

Understanding the s-process requires detailed knowledge of neutron capture rates for various isotopes, especially those at branching points – unstable nuclei where the path of nucleosynthesis can split depending on reaction rates and environmental conditions. Measuring these rates is incredibly challenging due to the difficulty of producing sufficient quantities of radioactive isotopes and the need for high-precision experiments.

Recent research has focused on measuring the neutron capture cross-sections of 147Pm and 171Tm, two key isotopes in the s-process. By creating radioactive targets and using advanced experimental techniques, scientists are unlocking new insights into how these elements are created in stars and how the universe has evolved chemically.

Creating Radioactive Targets: A Journey from Reactor to Experiment

A surreal illustration of a star with radioactive isotopes within, symbolizing stellar nucleosynthesis.

The first step in these experiments is producing the radioactive isotopes themselves. Researchers at the Institute Laue-Langevin (ILL) in Grenoble, France, irradiated stable isotopes of neodymium (146Nd) and erbium (170Er) with neutrons. This process transformed some of the stable atoms into the desired radioactive isotopes, 147Pm and 171Tm, respectively. These were then chemically separated and purified at the Paul Scherrer Institute (PSI).

The purification process is critical to remove unwanted materials and create high-quality targets for subsequent experiments. In the case of 147Pm and 171Tm, the purified material was electroplated onto thin aluminum backings to create targets suitable for neutron capture measurements. The creation of these targets is a significant achievement, paving the way for groundbreaking research.

Irradiation at ILL: Stable isotopes are bombarded with neutrons to create radioactive isotopes.
Chemical Separation: Rigorous purification to isolate the desired radioactive material.
Target Preparation: Electroplating onto thin backings for optimal experimental conditions.

With the radioactive targets in hand, the next step is to measure their neutron capture cross-sections. This was accomplished using two complementary approaches: time-of-flight experiments at CERN's n_TOF facility and activation experiments at the SARAF facility's Liquid-Lithium Target (LiLiT).

Unlocking the Secrets of the Stars: What's Next?

The data collected from these experiments are currently being analyzed, and preliminary results from the n_TOF facility show clearly resolved capture resonances for 171Tm. These resonances provide valuable information about the energy levels in the nucleus and how it interacts with neutrons. The analysis will provide, for the first time, a complete set of resonance parameters and the unresolved resonance region will be derived from these with the help of the Hauser-Feshbach statistical model.

Furthermore, activation measurements at LiLiT are expected to provide Maxwellian Averaged Cross Sections (MACS) at 30 keV with an accuracy of 10%. These MACS values are crucial inputs for stellar models, allowing scientists to simulate the s-process in different stellar environments and predict the abundances of heavy elements that are produced.

By combining the results from these different experimental approaches, researchers are gaining a more complete understanding of the neutron capture cross-sections of 147Pm and 171Tm. This knowledge will not only refine our understanding of the s-process but also shed light on the chemical evolution of the galaxy and the origins of the elements that make up our world.