Anton Wallner

Where are the heavy elements produced in the Universe? While core-collapse supernovae are confirmed sources of ⁶⁰ Fe, the origin of heavier r-process nuclei such as ²⁴⁴ Pu and ²⁴⁷ Cm remains debated. Here we used single-atom counting of live ⁶⁰ Fe, ²⁴⁴ Pu and ²⁴⁷ Cm in a deep-ocean ferromanganese crust to trace such nucleosynthesis events near Earth over millions of years. The observed non-correlation of the ²⁴⁴ Pu and ⁶⁰ Fe profiles supports a continuous r-process influx to Earth that is independent of core-collapse supernovae and constrains the core-collapse supernova r-process yield of actinide isotopes. The non-observation of ²⁴⁷ Cm, produced concurrently with ²⁴⁴ Pu, sets a time limit on the last actinide-producing r-process event near Earth of more than 90–100 Myr ago. Our measured interstellar signatures suggest the occurrence of an old and rare r-process event leading to a diffuse ²⁴⁴ Pu background inside and outside the Local Bubble.

Neutron capture reactions provide essential nuclear physics input for modeling the synthesis of heavy elements in stars. The growing precision of stellar spectroscopy and isotopic measurements in presolar SiC grains now demands cross sections with improved accuracy over the full energy range, and access to unstable nuclei relevant to slow (s-) process branchings and the intermediate (i-) process. This article reviews recent progress in direct neutron capture measurements, focusing on time-of-flight (TOF) experiments at CERN n_TOF and complementary activation techniques. Substantial advances have been achieved for stable s-only and bottleneck isotopes, significantly improving constraints on s-process models. In parallel, the combination of high instantaneous neutron fluxes and advanced detector systems has facilitated first-time neutron capture measurements on several radioactive branching-point nuclei. Feasibility studies, however, reveal current limitations related to sample availability, background conditions, and restricted energy coverage. In this context, the complementarity between TOF and activation emerges as a central strategy. Future developments, including high-flux facilities and novel inverse kinematics experiments in ion storage rings, are expected to extend the boundaries of neutron capture measurements, overcoming current limitations and helping unlock new frontiers in our understanding of stellar nucleosynthesis.

The solar system currently traverses the Local Interstellar Cloud (LIC), one of several warm cloudlets of the Complex of Local Interstellar Clouds (CLIC) in the solar neighborhood. The origin of these cloudlets is unknown and could be related to supernova shock wave dynamics. If supernovae are the source of these cloudlets or influence their properties, the CLIC may act as a cosmic archive for the supernova-produced radionuclide Fe 60 . We detected minuscule traces of interstellar Fe 60 in 295 kg of the European Project for Ice Coring in Antarctica Dronning Maud Land ice core, covering the time period 40–81 kyr ago. We find an Fe 60 / Mn 53 ratio of ( 8 ± 5 ) × 10 − 4 , slightly above the expected cosmogenic production, corresponding to an interstellar deposition rate of ( 0.22 − 0.12 + 0.14 ) Fe 60 at / cm 2 / yr . This influx is significantly lower than that observed in recent Antarctic snow and marine sediment records during the last 40 kyr. These results suggest that the LIC is a cosmic archive for supernova-produced Fe 60 . The imprinted Fe 60 time profile is evidence for a changing local interstellar environment over the last 80 kyr.