The $\beta$ decay of ${}^{101,102}\mathrm{Zr}$ and ${}^{109}\mathrm{Tc}$ was studied using the technique of total absorption spectroscopy. The experiment was performed at the National Superconducting Cyclotron Laboratory using the Summing NaI(Tl) (SuN) detector in the first-ever application of total absorption spectroscopy with a fast beam produced via projectile fragmentation. The $\beta$-decay feeding intensity and Gamow-Teller transition strength distributions were extracted for these three decays. The extracted distributions were compared to three different quasiparticle random-phase approximation (QRPA) models based on different mean-field potentials. A comparison with calculations from one of the QRPA models was performed to learn about the ground-state shape of the parent nucleus. For ${}^{101}\mathrm{Zr}$ and ${}^{102}\mathrm{Zr}$, calculations assuming a pure shape configuration (oblate or prolate) were not able to reproduce the extracted distributions. These results may indicate that some type of mixture between oblate and prolate shapes is necessary to reproduce the extracted distributions. For ${}^{109}\mathrm{Tc}$, a comparison of the extracted distributions with QRPA calculations suggests a dominant oblate configuration. The other two QRPA models are commonly used to provide $\beta$-decay properties in $r$-process network calculations. This work shows the importance of making comparisons between the experimental and theoretical $\beta$-decay distributions, rather than just half-lives and $\beta$-delayed neutron emission probabilities, as close to the $r$-process path as possible.

The $\beta$ decays of ${}^{103,104m}\mathrm{Nb}$ were studied with the Summing NaI(Tl) (SuN) detector at the National Superconducting Cyclotron Laboratory. The $\beta$-decay feeding intensity distribution $I_{\beta }\left(E\right)$ for each isotope was extracted by measuring $\gamma$ rays in coincidence with an emitted electron. The $I_{\beta }\left(E\right)$ was extracted via the total absorption spectroscopy technique. The $I_{\beta }\left(E\right)$ for each nucleus was compared to predictions made by the quasiparticle random-phase approximation (QRPA) model which is commonly used to calculate $\beta$-decay properties for astrophysical applications. The main goal was to provide experimental data for neutron-rich nuclei, relevant to the astrophysical $r$ process. In addition, the extracted $\beta$-decay feeding intensity distributions can lead to a better understanding of nuclear structure in a region of rapid structure changes around $A=100$. Finally, experimental data for ${}^{104m}\mathrm{Nb}$ are also of interest to antineutrino studies of nuclear reactors.

Massive stars are a major source of chemical elements in the cosmos, ejecting freshly produced nuclei through winds and core-collapse supernova explosions into the interstellar medium. Among the material ejected, long-lived radioisotopes, such as ⁶⁰Fe (iron) and ²⁶Al (aluminum), offer unique signs of active nucleosynthesis in our galaxy. There is a long-standing discrepancy between the observed ⁶⁰Fe/²⁶Al ratio by γ-ray telescopes and predictions from supernova models. This discrepancy has been attributed to uncertainties in the nuclear reaction networks producing ⁶⁰Fe, and one reaction in particular, the neutron-capture on ⁵⁹Fe. Here we present experimental results that provide a strong constraint on this reaction. We use these results to show that the production of ⁶⁰Fe in massive stars is higher than previously thought, further increasing the discrepancy between observed and predicted ⁶⁰Fe/²⁶Al ratios. The persisting discrepancy can therefore not be attributed to nuclear uncertainties, and points to issues in massive-star models.

The first complete measurement of the $\beta$-decay strength distribution of ${}_{17}{}^{45}{\mathrm{Cl}}_{28}$ was performed at the Facility for Rare Isotope Beams (FRIB) with the FRIB Decay Station Initiator during the second FRIB experiment. The measurement involved the detection of neutrons and $\gamma$ rays in two focal planes of the FRIB Decay Station Initiator in a single experiment for the first time. This enabled an analytical consistency in extracting the $\beta$-decay strength distribution over the large range of excitation energies, including neutron unbound states. We observe a rapid increase in the $\beta$-decay strength distribution above the neutron separation energy in ${}_{18}{}^{45}{\mathrm{Ar}}_{27}$. This was interpreted to be caused by the transitioning of neutrons into protons excited across the $Z=20$ shell gap. The SDPF-MU interaction with reduced shell gap best reproduced the data. The measurement demonstrates a new approach that is sensitive to the proton shell gap in neutron rich nuclei according to SDPF-MU calculations.

One of the biggest questions in nuclear astrophysics is understanding where the elements come from and how they are made. This work focuses on the $p$ process, a nucleosynthesis process that consists of a series of photodisintegration reactions responsible for producing stable isotopes on the proton-rich side of stability. These nuclei, known as the $p$ nuclei, cannot be made through the well-known neutron-capture processes. Currently $p$-process models rely heavily on theory to provide the relevant reaction rates to predict the final $p$-nuclei abundances and more experimental data is needed. The present work reports on an experiment performed with the SuN detector at the National Superconducting Cyclotron Laboratory, NSCL, at Michigan State University using the ReA facility to measure the ${}^{84}\mathrm{Kr}(p,\gamma ){}^{85}\mathrm{Rb}$ reaction cross section in inverse kinematics. The reverse ${}^{85}\mathrm{Rb}(\gamma ,p){}^{84}\mathrm{Kr}$ reaction is a branching point in the $p$-process reaction network that was highlighted as an important reaction in sensitivity studies in the production of the ${}^{78}\mathrm{Kr}$ $p$ nucleus. A new hydrogen gas target was designed and fabricated and a new analysis technique for background subtraction and efficiency calculations of the detector were developed. The experimental cross section is compared to standard statistical model calculations using the non-smoker and talys codes.

New astronomical observations point to a nucleosynthesis picture that goes beyond what was accepted until recently. The intermediate “$i$” process was proposed as a plausible scenario to explain some of the unusual abundance patterns observed in metal-poor stars. The most important nuclear physics properties entering $i$-process calculations are the neutron-capture cross sections and they are almost exclusively not known experimentally. Here we provide the first experimental constraints on the ${}^{139}\mathrm{Ba}(n,\gamma ){}^{140}\mathrm{Ba}$ reaction rate, which is the dominant source of uncertainty for the production of lanthanum, a key indicator of $i$-process conditions. This is an important step towards identifying the exact astrophysical site of stars carrying the $i$-process signature.