T. Baumann

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.

Background: The recent discovery and spectroscopic measurements of ${}^{27}\mathrm{O}$ and ${}^{28}\mathrm{O}$ suggests the disappearance of the $N=20$ shell structure in these neutron-rich oxygen isotopes.

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.

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 ground state two-proton decay lifetime of ${}^{19}\mathrm{Mg}$, populated by the one-neutron knockout of an intermediate-energy ${}^{20}\mathrm{Mg}$ radioactive beam, was measured utilizing a new experimental technique. A thin silicon detector positioned at varying distances (0.0–1.0 mm) downstream of the reaction target measured the energy loss of ${}^{19}\mathrm{Mg}$ and the two-proton decay product ${}^{17}\mathrm{Ne}$. The lifetime was deduced from fits to the measured energy-loss line shapes and depended upon the contribution of prompt reaction processes to the yield of ${}^{17}\mathrm{Ne}$. For relative ${}^{17}\mathrm{Ne}$ prompt contributions from 82% to 92%, the extracted lifetimes ranged from $1.{75}_{-0.42}^{+0.43}$ to $6.4_{-2.7}^{+2.4}$ ps. The results are consistent with the previously reported ${}^{19}\mathrm{Mg}$ lifetime measurement and serve as both an important complementary study and a validation of this new technique, which can provide lifetime information for short-lived states beyond the proton drip line.