Matthew Devlin

Prompt $\ensuremath{\gamma}$-ray production cross sections are important for nuclear reaction and structure calculations and in several applications. In particular, $\ensuremath{\gamma}$-ray production data and calculations are used in material assay and neutron diagnostics. $^{86}\mathrm{Kr}$ is a common, stable fission fragment, and as such can be used to test nuclear reaction models at the edge of stability, in order to improve calculations extending into unstable fission fragments in the region. Partial prompt $\ensuremath{\gamma}$-ray cross sections were measured, produced in neutron-induced reactions for neutron energies $11\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}<{E}_{n}<80\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ on $^{86}\mathrm{Kr}$: three $\ensuremath{\gamma}$ rays of $^{85}\mathrm{Kr}$ and one $\ensuremath{\gamma}$ ray each in $^{82,83,84}\mathrm{Kr}$ and $^{83,85}\mathrm{Br}$. A large part of the total cross section for the $^{86}\mathrm{Kr}(n,3n)^{84}\mathrm{Kr}$ reaction is experimentally observed. The results are compared with predictions from calculations using the ${\mathrm{CoH}}_{3}$ reaction code and are used to check on the consistency of the parameters in the calculations.

A single-crystal chemical-vapor-deposited diamond detector was used at the Los Alamos Neutron Science Center to measure neutron-induced reactions on carbon. Neutrons with a broad energy spectrum were produced via spallation. Relative cross sections are reported from above threshold up to 26 MeV incident neutron energy for $^{12}\mathrm{C}(n,{p}_{2})^{12}\mathrm{B}$, up to 23 MeV for $^{12}\mathrm{C}(n,{p}_{3+4})^{12}\mathrm{B}$, and up to 35 MeV for $^{12}\mathrm{C}(n,{d}_{1})^{11}\mathrm{B}$ and $^{12}\mathrm{C}(n,{d}_{2})^{11}\mathrm{B}$, all of which represent the highest-energy measurements for these reactions to date and the first measurement for $(n,{d}_{2})$. Cross sections for $^{12}\mathrm{C}(n,p){^{12}\mathrm{B}}_{\text{total}}$ and $^{12}\mathrm{C}(n,d){^{11}\mathrm{B}}_{\text{total}}$ are also reported from sums of the partial cross sections. These measurements are compared with recent data from EXFOR as well as talys calculations. These data are useful for future nuclear data evaluations and can be used as cross-section inputs for data-based simulation packages.

The collective structure of low-lying states in the $N=50$ nucleus $^{92}\mathrm{Mo}$ $(Z=42)$ has been investigated through sub-barrier Coulomb excitation using the CHICO2 device coupled to the $\ensuremath{\gamma}$-ray energy tracking in-beam nuclear array (GRETINA). Quadrupole and octupole collective properties are measured for the ${2}_{1}^{+}$, ${2}_{2}^{+}$, ${2}_{3}^{+}$, ${2}_{4}^{+}$, and ${3}_{1}^{\ensuremath{-}}$ states. The spectroscopic quadrupole moment for the ${2}_{1}^{+}$ state is measured to be ${Q}_{s}=0.23(8)\phantom{\rule{0.28em}{0ex}}e\mathrm{b}$, showing a weakly deformed oblate nature in good agreement with recent state-of-the-art Shell-Model calculations. A modest reduced electric-octupole transition strength of $B(E3;{3}_{1}^{\ensuremath{-}}\ensuremath{\rightarrow}{0}_{1}^{+})=15(3)\phantom{\rule{0.28em}{0ex}}\mathrm{W}.\mathrm{u}.$ is obtained, in disagreement with the literature, revealing an emerging tension in the strength of octupole collectivity for the $N=50$ nuclei between historic $(e,{e}^{\ensuremath{'}})$ and $(p,{p}^{\ensuremath{'}})$ experiments and more recent experiments utilizing Coulomb excitation and lifetime plus branching-ratio data.

The electromagnetic properties of low-lying states in $^{70}$Ge were investigated via multi-step Coulomb excitation of a $^{70}$Ge beam impinging on a $^{208}$Pb target at the ATLAS facility of the Argonne National Laboratory. A total of 27 transitional elements and six diagonal matrix elements coupling 11 low-lying states, were extracted from the measured cross sections. These were used to calculate reduced transition probabilities, spectroscopic quadrupole moments, and rotational invariant shape parameters, providing enhanced precision and expanding on previous studies. The experimental data were compared within several theoretical frameworks, including the generalized triaxial rotor model, configuration interaction shell-model calculations, and computations within the combined frameworks of relativistic density functional theory and the five-dimensional collective Hamiltonian. The results demonstrate a good agreement with the experimental data and, in conjunction with calculations using a two-state mixing model, support significant triaxiality and strong mixing between the $0^+_1$ and $0^+_2$ states. This results in the magnitudes of their respective quadrupole deformations $[β_\text{rms}(0^+_1) = 0.228\,(3),\,β_\text{rms}(0^+_2) = 0.273\,(1)]$ being more similar than previously observed. The implications of these results for understanding the complex shape coexistence phenomena, the role of triaxiality, and shape evolution along the Ge isotopic chain are discussed.