Engineering topics
Tobin, Z.
Publications and source records attributed to Tobin, Z..
Investigating the time dependence of neutron-proton equilibration using molecular dynamics simulations
Neutron-proton equilibration has previously been studied experimentally in dynamically deformed nuclei in heavy ion collisions [Phys. Rev. Lett. 118, 062501 (2017); Phys. Rev. C 95, 044604 (2017)] using the NIMROD detector array at the Cyclotron Institute at Texas A&M University. Results indicated the composition of the two heaviest fragments originating from the excited projectile-like fragment evolved exponential with respect to their orientation angle to be more similar. Constrained Molecular Dynamics and Antisymmetrized Molecular Dynamics simulations were performed for different formulations of the density dependence of the asymmetry energy term of the nuclear equation of state. The simulations are compared to experimental results, which indicate a better agreement with a softer interaction in the nuclear equation of state.
Proton and neutron exchange as a prelude to fusion at near-barrier energies
Systematic examination of fusion for 39,41,45,47 K + 28 Si and 36,44 Ar + 28 Si provides insight into the impact of neutron and proton exchange on fusion for nuclei at and near the N = 20 and N = 28 shells. Comparison of the reduced excitation functions reveals a marked difference between the behavior of open-shell and closed-shell systems. While coupled channels calculations provide a good description for the closed-shell nuclei, they significantly underpredict the fusion cross section for open-shell nuclei. Furthermore, the observed trends are examined in the context of a potential energy surface, including shell effects, and multinucleon exchange with consideration of Pauli blocking.
Investigation of sCVD diamond detectors for low energy heavy-ion reactions
Here the performance characteristics of single-crystalline diamond detectors at varying rates of heavy-ions below 50 MeV/u have been investigated. When tested with a 228 Th source, the energy resolution of these detectors is better than 1.0% FWHM for 8.8 MeV α-particles. With a slight reduction in the energy resolution, simultaneous timing resolution of 314 ps has been achieved. Isotopic resolution has been achieved up to magnesium using a diamond telescope at low rates with a 20 MeV/u 20 Ne + 12 C reaction. At moderate rates of direct 7.5 MeV/u 78 Kr, polarization effects are observed, though this effect can be diminished by increasing the applied voltage on the detector up to a limit. After an accumulated dose of 6.9 × 10 8 direct 78 Kr particles transmitting through the detector, a degradation in the energy resolution of the 8.8 MeV peak from 1.6% to 5.3% is observed, indicating that permanent damage has been done to the detector.
Impact of shell structure on the fusion of neutron-rich mid-mass nuclei
The influence of shell effects on fusion of mid-mass nuclei is explored using isotopic chains of K and Ar ions on an oxygen target. Comparison of the reduced excitation functions reveals that the fusion cross section for the open neutron-shell projectile nuclei 41 K and 45 K is systematically larger than for the closed neutron-shell projectiles 39 K and 47 K. Furthermore, the São Paulo fusion model using matter densities from systematics fails to describe the measured excitation functions. Use of more realistic densities from a Dirac-Hartree-Bogoliubov (DHB) approach performs significantly better though it still overpredicts the closed-shell nuclei.
Advances in 211 At production at Texas A&M University
Alpha emitting radionuclides with medically relevant half-lives are interesting for treatment of tumors and other diseases because they deposit large amounts of energy close to the location of the radioisotope. Researchers at the Cyclotron Institute at Texas A&M University are developing a program to produce 211 At, an alpha emitter with a medically relevant half-life. Here, the properties of 211 At make it a great candidate for targeted alpha therapy for cancer due to its short half-life (7.2 h). Astatine-211 has now been produced multiple times and reliability of this process is being improved.
Correlation between time and angular alignment in molecular dynamics simulations of heavy ion collisions
Neutron-proton equilibration is a process which has been used to study the density dependence of the symmetry energy term in the nuclear equation-of-state. Here, this study utilizes constrained molecular dynamics (CoMD) simulations of 70 Zn + 70 Zn with collision energies of 35 and 45 MeV/nucleon. An algorithm is used which searches through CoMD events and identifies the PLF* after it separates from the target and determines its lifetime, Δt. It also determines the fragments that the PLF* breaks apart into and determines their angular alignment. This technique gives an opportunity to explore how the average alignment of dynamically produced fragments, $\langle$α $\rangle$ dyn , evolves with PLF* lifetime. An approximately linear relationship was determined with d$\langle$α $\rangle$ dyn /dΔt = 0.98 ± 0.08 rad / zs and 1.06 ± 0.09 rad / zs for the 35 and 45 MeV / nucleon, respectively, indicating a correlation with magnitude consistent with classically determined values which were used for prior experimental studies.