J. M. Berkman

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.

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.

We provide the first experimental cross section of the ^{73}As(p,γ)^{74}Se reaction to constrain one of the main destruction mechanisms of the p nucleus ^{74}Se in explosive stellar environments. The measurement was done using a radioactive ^{73}As beam at effective center-of-mass energies of 2.9 and 2.3 MeV/nucleon. Along with the total cross-section measurement, statistical properties of the ^{74}Se compound nucleus were extracted, constraining the reaction cross section in the upper Gamow window of the γ process. The impact of the experimentally constrained reaction rate on ^{74}Se production in Type II supernovae was investigated through Monte Carlo one-zone network simulations. The results indicate that the overproduction of ^{74}Se by Type II supernova models cannot be resolved by nuclear physics alone and point toward the need for a more detailed understanding of the astrophysical conditions of relevance for the γ process.

The 247-keV state in ^{54}Sc, populated in the β decay of ^{54}Ca, is reported here as a nanosecond isomer with a half-life of 26.0(22) ns. The state is interpreted as the 1^{+} member of the πf_{7/2}⊗νf_{5/2} spin-coupled multiplet, which decays to the 3^{+},πf_{7/2}⊗νp_{1/2} ground state. The new half-life corresponds to a pure E2 transition with a strength of 1.93(16) W.u., providing the most precise, unambiguous B(E2) value in the neutron-rich fp region to date for a nucleus with valence protons above Z=20. Notably, it is roughly 4 times larger than the B(E2;1/2^{-}→5/2^{-}) value in ^{55}Ca. The results, as compared to semiempirical and ab initio shell-model calculations, indicate (1) a weak N=34 subshell gap relative to N=32, (2) a large E2 enhancement in Sc as compared to Ca due to 1p-1h proton excitations across Z=28, and (3) empirical effective proton and neutron charges e_{π}=1.30(8)e and e_{ν}=0.452(7)e, respectively, that are in contrast to reports of e_{π}≈1.1-1.15e and e_{ν}≈0.6-0.8e for fp-shell nuclei near N=Z. We demonstrate that these reports are erroneous and that, in fact, a universal set of effective charges can be used across the sd and fp shells.