Session LE: Nuclear Reactions: Heavy-Ions/Rare isotope Beams II

Dynamics of quantum equilibration, dissipation and fluctuation in nuclear collision

The equlibration processes are studied in collisions of atomic nuclei. We provide a general, systematic method to investigate the timescales of equilibration, dissipation, and fluctuation mechanisms through the use of a time-dependent, fully microscopic theory. Despite direct comparisons between wildly different systems spanning light and heavy nuclei at a range of collision energies, common timescales were found for each of the studied quantities. The longest process by far is mass equilibration, with a general equilibration time of $2\times 10^{-20}$s. Timescales for neutron-to-proton equilibration, mass fluctuation build up, kinetic energy dissipation, and angular momentum dissipation however are found to be on the order of $10^{-21}$s, an order of magnitude faster than that of mass equilibration. This vast separation implies the relative independence of dissipation mechanisms on mass equilibration, and that the primary generator of dissipative effects is fast nucleon exchange between interacting fragments. 1. C. Simenel, K. Godbey, and A. S. Umar, Phys. Rev. Lett. 124, 212504 (2020).   

Time Dependence of Angular Alignment in Heavy Ion Collisions

This study provides theoretical input regarding the time dependency of the angular alignment, $\alpha$, an angle used to assess the amount of rotation of a projectile-like fragment (PLF*) after separating from a target nucleus. A set of Constrained Molecular Dynamics (CoMD) simulations of ${^{70}}$Zn+${^{70}}$Zn nuclear collisions with collision energies of 35 and 45 MeV/nucleon was used to study this correlation. An algorithm is proposed which searches through CoMD events and identifies the excited PLF* after it separates from the target and determines its lifetime, $\Delta t$. It also determines the alignment angle $\alpha$, of the PLF*. The dynamic yield is extracted, and its evolution with PLF* lifetime is studied. The correlation of the average alignment angle in the dynamic contribution, $\left<\alpha\right>_{dyn}$, is approximately linearly correlated with PLF* lifetime with $d\left<\alpha\right>_{dyn}/d\Delta t = 2.0\pm 0.2$ rad/zs for both collision energies studied, consistent with values utilized in prior experiment   

The Search for $\alpha$-Clustered Toroidal Nuclei in ${}^{28}$Si

Ground state stable nuclei typically have spherical geometries, however given excitation energy and/or angular momentum, they may exhibit exotic shapes and form $\alpha$-particle clusters within their bulk. It is predicted that such clustering can promote the production of angular-momentum stabilized toroidal nuclei. By studying the 7-$\alpha$ particle disassembly of ${}^{28}$Si + ${}^{12}$C at 35 MeV/nucleon, an experiment performed on the NIMROD detector array observed evidence of high excitation energy peaks in the same range as predicted toroidal high-spin isomer states [1]. However, the peaks were not well resolved due to the angular resolution of NIMROD. The FAUST detector array uses dual-axis duo-lateral (DADL) position sensitive silicon detectors capable of sub-1mm position resolution. The mechanism of charge splitting in the DADL detector gives position-dependent characteristics in the pulse shape that lead to distortions in position reconstruction. A new waveform analysis method has been developed that reliably extracts charge values, yielding improved linearity in position reconstruction and energy resolution. A precision measurement using FAUST can provide insight to the $\alpha$-clustered structure and exotic deformation of nuclei. [1]Phys. Rev. C 99, 014606 (2019)   

Determining Neutron-Induced Reaction Cross Sections on Pt

For many stable isotopes, neutron capture has been studied in detail. However, the stable platinum isotopes have only been investigated in a few measurements, all of which provide no information below 1~keV, and no resolved resonance information. Photon strength functions in Pt isotopes have exhibited surprising behavior, and neutron capture data across the Pt isotopic chain will allow more detailed study of these effects. To address this at LANSCE, DANCE has been used to measure neutron-capture reaction cross sections on $^{192,194,195,196,198}$Pt, all stable isotopes of Pt with abundance \textgreater0.1\% at the Lujan Center. DANCE is a highly segmented BaF$_2$ detector array with approximately 3.6$\pi$ solid angle coverage for calorimetric measurements of prompt $\gamma$-ray emission following neutron capture. Data have been collected and the status of ongoing analysis will be presented. Experimental determination of the capture cross sections and resonance properties on Pt isotopes will reduce uncertainties in existing nuclear data evaluations and improve confidence in simulations using these evaluations. This will provide improved nuclear data to validate models in this mass region.   

Gamma Ray Detection in DAPPER Array for (d,p$\gamma )$ Reactions

The photon strength function (PSF) describes the bulk quantum mechanical component of photon emission probabilities and thus it is important in describing neutron capture reactions. For some nuclei, experiments have shown an enhancement in the PSF at low energy. Experiments have shown an up-bend in the $^{\mathrm{56}}$Fe and $^{\mathrm{57}}$Fe nuclei. An up-bend in $^{\mathrm{60}}$Fe would illustrate that this feature may be more widespread. The PSF of $^{\mathrm{60}}$Fe will be probed using a $^{\mathrm{59}}$Fe(d,p$\gamma )^{\mathrm{60}}$Fe reaction. A new detector array is currently being constructed called DAPPER (Detector Array for Photons, Protons, and Exotic Residues). In this array the proton emitted from the reaction will be detected by an annular silicon detector, while the gamma arrays from the excited residue will be detected by 128 BaF$_{\mathrm{2}}$ detectors. Two $^{\mathrm{57}}$Fe(d,p$\gamma )^{\mathrm{58}}$Fe test reactions were done using a few of the detectors in the DAPPER array. The gamma ray detector characterization from the test runs will be presented, along with gamma ray simulations on DAPPER.   

Silicon Detector Characterization for the DAPPER Array

An accurate description of neutron capture is important for modeling stellar lifecycles and can give insight into the production of the elements. The gamma rays from the decay of $^{\mathrm{60}}$Fe, an s-process isotope, are seen in the interstellar medium and can constrain stellar evolution models. The photon strength function (PSF), which describes the likelihood of certain nuclear transitions, is necessary in understanding the neutron capture process. An observed enhancement in the PSF at low excitation energies for iron isotopes has implications for the reaction rates of neutron rich isotope production in nucleosynthesis. For the purpose of measuring the PSF of $^{\mathrm{60}}$Fe, the Detector Array for Photons, Protons, and Exotic Residues (DAPPER) is being developed. Design and testing of the array using $^{\mathrm{57}}$Fe(d,p$\gamma )^{\mathrm{58}}$Fe will be discussed, with focus on the proton detector.   

Sub Coulomb barrier d+$^{208}$Pb scattering in the time-dependent basis function approach

We develop and apply a non-perturbative time-dependent basis function (tBF) approach for low-energy nuclear reactions. We successfully reproduce the experimental ratios of elastic cross sections for d+$^{208}$Pb scattering at $E_d=3-7$ MeV employing the tBF method without an explicit optical potential. In this tBF application, we consider all possible electric dipole (E1) transitions among deuteron ground and breakup states that are induced by the Coulomb field of $^{208}$Pb. We obtain the deuteron ground and breakup states, which form the basis representation of the tBF method, by diagonalizing a realistic Hamiltonian in a suitably large harmonic oscillator basis. We also investigate the scattering dynamics, the role of the polarization potential and the sensitivity to microscopic inputs such as the deuteron dipole polarizability.