The Gamma Spectroscopy Group at IFIC is devoted to the investigation of several aspects of the structure of atomic nuclei and the applications of Nuclear Physics to related fields.
A central question in modern science is how the elements heavier than iron were formed in the Universe. Nearly all such elements are produced inside stars through neutron-capture processes, which take place in different astrophysical environments and under very different conditions.
The slow neutron-capture process (s-process) occurs in environments such as asymptotic giant branch (AGB) stars, where neutron capture happens more slowly than radioactive beta decay. This process builds many of the stable isotopes we find today. This recent PRL-article describes one of the most challenging (n,g) experiments conducted at CERN n_TOF to assess the origin of the heaviest s-only nucleus Pb-204. However, certain radioactive nuclei act as branching points in the s-process path, where the competition between neutron capture and beta decay determines the final abundance patterns. Studying these nuclei requires extremely precise measurements of their neutron-capture cross sections, which is technically challenging due to the small quantities available, possible contamination, and high gamma-ray backgrounds from their own radioactivity. Advances and new ideas for this type of experiments are reported in this article, and a summary with all the s-process branching nuclei measured at CERN n_TOF is available in this review article.
To overcome these challenges, our group has designed and built innovative detection systems such as the i-TED gamma-ray imager, which greatly suppresses background and allows neutron-capture measurements on highly radioactive and rare samples. With this technology, we have carried out landmark experiments at CERN’s n_TOF facility on isotopes such as ⁷⁹Se and ⁹⁴Nb, providing unique insights into the temperature, neutron density, and timescales of the s-process. These measurements are essential for stellar modelling and for understanding the chemical evolution of our Galaxy.
At CERN n_TOF we are pioneering new detector concepts like STAR, an array of solid-state organic scintillators coupled to silicon photosensors, capable of handling the very high neutron fluxes required for experiments on extremely small, highly radioactive samples. These developments will enable measurements that have previously been beyond experimental reach.
While the s-process explains many heavy isotopes, others—especially in the germanium-to-molybdenum region—cannot be explained by any mix of s- and r-processes. The intermediate neutron-capture process (i-process) has been proposed to account for these cases. It occurs under neutron densities between those of the s- and r-processes, such as during helium-shell flashes in rapidly accreting white dwarfs.
Validating the i-process requires neutron-capture data on short-lived isotopes that can only be produced at rare-isotope beam facilities. We are developing ultra-sensitive activation techniques at the NEAR station of n_TOF, capable of working with nanogram-scale samples. In parallel, we are developing the CYCLING station at CERN n_TOF , a dedicated cyclic activation setup to investigate i-process isotopes with unprecedented sensitivity.
The rapid neutron-capture process (r-process) produces the heaviest neutron-rich nuclei in the Universe and occurs in extreme environments such as neutron-star mergers and possibly certain types of supernovae. The detection in 2017 of gravitational waves and electromagnetic emission from a neutron star merger, revealing freshly synthesized heavy elements, was a watershed moment in the field.
The r-process path runs through nuclei far from stability, where experimental data are scarce. Our group plays a leading role in the BRIKEN collaboration at RIKEN, which has measured hundreds of new beta-decay half-lives and beta-delayed neutron probabilities. These measurements feed directly into r-process simulations, helping to identify the astrophysical sites and conditions where the process occurs. This recent PRL-article describes some of the most exciting recent results in this field.
Alpha-induced neutron production plays a central role in nuclear astrophysics, underground rare-event experiments, nuclear technology, and security applications. The available data for many isotopes are incomplete or inconsistent.
We co-lead the MANY collaboration in Spain, which is building comprehensive datasets for (α, n) reactions using advanced detectors like miniBELEN at accelerator facilities in Madrid (CMAM) and Seville (CNA). These measurements support s- and r-process modelling, background reduction in underground experiments, and the design of advanced nuclear systems.