Sivahami Uthayakumaar

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.

Recent models of the rapid proton (rp) capture process indicate that a competition between the ^{59}Cu(p,γ)^{60}Zn and ^{59}Cu(p,α)^{56}Ni reactions may result in the formation of a nickel-copper (NiCu) cycle that traps the flux of material between ^{56}Ni and ^{60}Zn. Here, we report the identification of 15 proton-unbound levels in ^{60}Zn, populated via ^{59}Cu(d,n) transfer, which govern the rate of the ^{59}Cu(p,γ)^{60}Zn reaction in XRBs. Precise excitation energies for levels in ^{60}Zn were obtained from observed γ decays, and spectroscopic factors were determined from angle-integrated cross sections. Incorporating these results into stellar-model calculations, we find that with experimentally constrained uncertainties a NiCu cycle in XRBs is indeed possible, though we limit its branching strength to less than 38%. While modest, such a branching has significant impact on the light curve, motivating further studies of the relevant rates. Our calculations also indicate that a significant NiCu cycle leads to an increase in the amount of odd-A nuclei in the burst ashes, which may affect Urca cooling processes in neutron star crusts.

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.

Excited states in the proton drip line nucleus Ga 61 were populated via the fusion-evaporation reaction Mg 24 ( Ca 40 , p 2 n ) Ga 61 . The experimental setup at Argonne National Laboratory comprised a novel combination of the Gammasphere array with two CD-shaped double-sided Si-strip detectors inside the Microball CsI(Tl) charged-particle detection array, as well as the Neutron-Shell liquid scintillators and the Fragment Mass Analyzer. Owing to the setup’s unprecedented in-beam proton spectroscopy and tracking capabilities, a coincidence between a 957.6(5)-keV γ ray and a 1.876(24)-MeV proton line was observed, which identifies the quasibound proton π g 9 / 2 single-particle state in Ga 61 at E x = 2150 ( 34 ) keV . This probes isospin symmetry at the limit of nuclear binding by providing a unique challenge for the shell-model interpretation of mirror nuclei beyond doubly magic Ni 56 .

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.