Artemis Spyrou

The transition of an atomic nucleus from an excited state to a state of lower energy is typically accompanied by the emission of photons. The likelihood of photon emission is a function of the photon energy and of properties of the initial and final nuclear states and is known as the photon (or γ-ray) strength function (γSF)^1,2. Of the features present in the γSF, an enhancement in the low-energy region has been observed in some nuclei, but the electromagnetic nature of this enhancement remains an open question^3,4. This low-energy enhancement in the γSF significantly impacts our understanding of how elements are created in stars and the structure of the nucleus^5,6. Here we present the results from an experiment on the γSF of ⁷⁰Zn that show that the enhancement in the γSF of this nucleus is of magnetic nature. This observation helps address a long-open question about γSFs in atomic nuclei and directly impacts our predictive capabilities in nuclear science.

Short-Lived Radionuclides (SLRs) with million-year lifetimes serve as chronometers of Early Solar System (ESS) formation and can reveal active nucleosynthesis in our Galaxy. Among the nuclides whose decay signatures are preserved in meteorites, the extinct radionuclide ⁹² Nb is one of the few confirmed proton-rich SLRs, and the ESS ⁹² Nb/ ⁹² Mo abundance ratio serves as a sensitive probe of proton-rich nucleosynthesis. However, the interpretation of this ratio is complicated by uncertainties in both nuclear reaction rates and the astrophysical environments. Because ⁹² Nb is shielded by the stable isobars ⁹² Zr and ⁹² Mo, it cannot be produced through β-decay chains. Instead, it must be synthesized through direct reactions in explosive supernova environments, where the photoneutron reaction on ⁹² Nb is a key regulator and currently dominates the uncertainty in the nuclear reaction networks producing ⁹² Nb. Direct measurements of this reaction are presently not feasible. Therefore, recommended values have relied on theoretical calculations. Here, we present the first experimental constraint on this reaction. This newly constrained reaction rate significantly increases the predicted ⁹² Nb abundance in relevant supernova environments, thereby shifting model predictions in the direction needed to ease the long-standing discrepancy in the ESS ⁹² Nb/ ⁹² Mo ratio by removing one of the dominant nuclear uncertainties in the ⁹² Nb production network.

In an experiment performed at the Facility for Rare Isotope Beams (FRIB) using the FRIB Decay Station initiator, 15 new half-lives of isotopes near ^{54}Ca were measured. A new method of extracting lifetimes from experimental data, taking into account the unknown β-delayed neutron emission branches of very neutron-rich nuclei, was developed to enable systematic uncertainty analysis. The experiment observed a dramatic change in the half-life systematics for the isotopes with neutron number N=34. Beyond N=34, the decline of nuclear lifetime is much slower, leading to longer than anticipated lifetimes for near-dripline nuclei. State-of-the-art shell-model calculations can explain the experimental results, revealing the imprint of shell effects and the need for modification of single-particle neutron states. The results from a newly developed quasirandom-phase approximation model with potential for making global predictions were also tested against the experimental results and good agreement was found.