arXiv subjects

Teck-Ghee Lee

Publications and source records attributed to Teck-Ghee Lee.

13 recordsLinked to original sources

arXiv2023

Fusion and reactions of $\alpha$+$^8$Be in the Hoyle resonance and associated resonances region

The fusion of $\alpha$ and $^8$Be to produce a $^{12}$C nucleus is a crucial process in nucleosynthesis. In the laboratory, this process can only be studied theoretically as a $^8$Be target or projectile cannot be prepared experimentally. We use the potential scattering theory in the coupled-channel formalism to study such a process in terms of the collision between the $\alpha$ particle on a deformed $^8$Be nucleus, both on resonance and off resonance in the Hoyle resonance and associated resonances region. The experimental $^{12}$C energy levels and widths constrain the nuclear potential to suggest the need to include a parity-dependent surface potential component that is more attractive for even-$L$ positive-parity partial waves than for odd-$L$ negative-parity partial waves. As a consequence, the radial dependence of the total potentials for the set of \{0$^+$, 2$^+$, 4$^+$\} resonances of ${}^{12}$C exhibit a double-hump behavior, possessing two local energy minima and a doublet of each of the ${}^{12}$C \{0$^+$, 2$^+$, 4$^+$\} resonances in the Hoyle and associated resonances region. We examine the approximate agreement of the theoretical results with experiment and suggest the search for the as-yet unobserved lower-energy 2${}^+_2$ and 4${}_1^+$ resonances to test the double-hump potential description. In addition, for practical astrophysical applications, we evaluate and estimate the astrophysical $S(E_{\rm c.m.})$-factor for the $\alpha$+$^8$Be $\to$ $^{12}$C$(0^{+*})$ $\to$ $^{12}$C$(2_1^+)$ + $\gamma$ reaction for $E_{\rm c.m.}$ $<$ 1.0 MeV.

nucl-th↗

arXiv2021

Pocket resonances in low-energy antineutrons reactions with nuclei

Upon investigating whether the variation of the antineutron-nucleus annihilation cross-sections at very low energies satisfy Bethe-Landau's power law of $σ_{\rm ann} (p) \propto 1/p^α$ behavior as a function of the antineutron momentum $p$, we uncover unexpected regular oscillatory structures in the low antineutron energy region from 0.001 to 10 MeV, with small amplitudes and narrow periodicity in the logarithm of the antineutron energies, for large-$A$ nuclei such as Pb and Ag. Subsequent semiclassical analyses of the $S$ matrices reveal that these oscillations are pocket resonances that arise from quasi-bound states inside the pocket and the interference between the waves reflecting inside the optical potential pockets with those from beyond the potential barriers, implicit in the nuclear Ramsauer effect. They are the continuation of bound states in the continuum. Experimental observations of these pocket resonances will provide vital information on the properties of the optical model potentials and the nature of the antineutron annihilation process.

nucl-th↗

arXiv2020

Super-transition-array calculations for synthetic spectra and opacity of high-density, high-temperature germanium plasmas

The synthetic emission spectra and opacity of high-density, high-temperature germanium (Z=32) plasma from super-transition-array (STA) calculations are presented. The viability of the STA model, which is based on a statistical superconfigurations accounting approach for calculating the atomic and radiative properties, is examined by comparing and contrasting its results against the available experimental data and other theoretical calculations. First, we focus on the emission data. To model the data, the Eulerian radiation-hydrodynamics code FastRad3D is used in conjunction with STA to obtain the STA-required inputs, namely, the time-dependent temperature and density profiles of the Ge plasmas. Consequently, we find that STA results fit the experimental spectrum [High Energy Density Phys., 6 (2010) 105] reasonably well. However, careful comparison between experimental and theoretical results in the photon-energy regions of ~ 1.7 keV shows some degrees of disparity between the two. This may be due to the non-LTE effects and the presence of spatial gradients in the sample. Limitations of STA to model the experimental spectrum precisely is expected and underscoring the difficulty of the present attempts as the model assumed local thermodynamics equilibrium population dynamics. Second, we examine the STA calculated multi-frequency opacities, ionized population fraction and average ionization for a broad range of Ge plasma conditions. Comparing with a hybrid LTE opacity code which combines the statistical super-transition-array and fine-structure methods [High Energy Density Phys., 7 (2011) 234], impressively good agreement is found between the two calculations.

physics.plasm-ph↗

arXiv2018

Radiative and atomic properties of C and CH plasmas in the warm-dense matter regime

A theoretical model based on the method of super transition arrays (STA) is used to compute the emissivities, opacities and average ionization states of carbon (C) and polystyrene (CH) plasmas in the warm-dense matter regime in which the coupling constant varies between 0.02 to 2.0. The accuracy of results of STA calculations is assessed by benchmarking against the available experimental data and results obtained using other theoretical methods, assuming that a state of local thermodynamic equilibrium exists in the plasma. In the case of a carbon plasma, the STA method yields spectral features that are in reasonably-good agreement with Dirac-Fock and Hartree-Fock-Slater theories; in the case of CH, we find that STA-derived opacities are very similar to those derived using quantum-molecular-dynamics density-functional theory and Hartree-Fock method down to plasma temperature of about 20 eV. Our calculations also compare favorably with available experimental measurements of Gamboa {\it et al} [High Energy Density Phys. {\bf 11}, 75 (2014)] of the plasma temperature and average ionization state behind a blast wave in a pure carbon foam. Although the STA-computed average-ionization charge state in the rarefaction region appears to be lower than the experimental data, it is within the experimental uncertainty and the discrepancy is nevertheless consistent with results reported using an atomic kinetic model. In addition, we further predict the temperature dependence of average ionization states of CH plasma in the same temperature range as for the carbon plasma.

physics.plasm-ph↗

arXiv2018

Optical model potential analysis of $\bar nA$ and $nA$ interactions

We use a momentum-dependent optical model potential to analyze the annihilation cross sections of antineutron $\bar n$ on C, Al, Fe, Cu, Ag, Sn, and Pb nuclei for projectile momenta $p_{\rm lab}$ $\lesssim$ 500 MeV/$c$. We obtain good description of annihilation cross sections data of Barbina {\it et al.} [Nucl.~Phys.~A {\bf 612}, ~346~(1997)] and of Astrua {\it et al.} [Nucl.~Phys.~A {\bf 697},~209~(2002)] which exhibit an interesting dependence of the cross sections on the $p_{\rm lab}$ as well as on the target atomic mass number $A$. We also obtain the neutron ($n$) non-elastic reaction cross sections for the same targets. Contrasting the $nA$ reaction cross sections $σ^{nA}_{\rm rec}$ to the $\bar nA$ annihilation cross sections $σ^{\bar nA}_{\rm ann}$, we find the $σ^{\bar nA}_{\rm ann}$ is significantly larger than the $σ^{nA}_{\rm rec}$, that is, the $σ^{\bar nA}_{\rm ann}$/$σ^{nA}_{\rm rec}$ cross section ratio lies between the values of order 1.8 and 3.8 in the momentum region where comparison is possible. The dependence of the annihilation cross section on the projectile charge is also examined in comparison with antiproton $\bar p$. Here we predict the $\bar pA$ annihilation cross section on the simplest assumption that both $\bar pA$ and $\bar nA$ interactions have the same nuclear part of the optical model potential but differs only on the electrostatic Coulomb interaction. Deviation from such simple model extrapolation in measurements will provide new information on the difference between $\bar nA$ and $\bar pA$ potentials.

nucl-th↗

arXiv2016

Nuclear annihilation by antinucleons

We examine the momentum dependence of $\bar p$$p$ and $\bar n$$p$ annihilation cross sections by considering the transmission through a nuclear potential and the $\bar p p$ Coulomb interaction. Compared to the $\bar n p$ annihilation cross section, the $\bar p p$ annihilation cross section is significantly enhanced by the Coulomb interaction for projectile momenta below $p_{\rm lab} <$ 500 MeV/$c$, and the two annihilation cross sections approach the Pomeranchuk's equality limit [JETP Lett. {\bb 30}, 423 (1956)] at $p_{\rm lab}\sim 500$ MeV/$c$. Using these elementary cross sections as the basic input data, the extended Glauber model is employed to evaluate the annihilation cross sections for $\bar n$ and $\bar p$ interaction with nuclei and the results compare well with experimental data.

nucl-th↗

arXiv2015

Investigating the static dipole polarisability of noble gas atoms confined in impenetrable spheres and shells

The static dipole polarisability of noble gas atoms confined by impenetrable spheres and spherical shells is studied using the B-spline random phase with exchange approximation. The general trend in dipole polarisabilities across the noble gas sequence shows a decrease in the dipole polarisability as the volume of the confining impenetrable sphere is reduced and a large increase in the dipole polarisability for confinement by impenetrable spherical shells as the inner shell radius is increased.

physics.atom-ph↗

arXiv2014

Extended Glauber Model of Antiproton-Nucleus Annihilation for All Energies and Mass Numbers

Previous analytical formulas in the Glauber model for high-energy nucleus-nucleus collisions developed by Wong are utilized and extended to study antiproton-nucleus annihilations for both high and low energies, after taking into account the effects of Coulomb and nuclear interactions, and the change of the antiproton momentum inside a nucleus. The extended analytical formulas capture the main features of the experimental antiproton-nucleus annihilation cross sections for all energies and mass numbers. At high antiproton energies, they exhibit the granular property for the lightest nuclei and the black-disk limit for the heavy nuclei. At low antiproton energies, they display the effect of the antiproton momentum increase due to the nuclear interaction for the light nuclei, and the effect of the magnification due to the attractive Coulomb interaction for the heavy nuclei.

nucl-th↗

arXiv2011

A large-scale R-matrix calculation for electron-impact excitation of the Ne$^{2+}$ O-like ion

The five J$Π$ levels within a $np^2$ or $np^4$ ground state complex provide an excellent testing ground for the comparison of theoretical line ratios with astrophysically observed values, in addition to providing valuable electron temperature and density diagnostics. The low temperature nature of the line ratios ensure that the theoretically derived values are sensitive to the underlying atomic structure and electron-impact excitation rates. Previous R-matrix calculations for the Ne$^{2+}$ O-like ion exhibit large spurious structure in the cross sections at higher electron energies, which may affect Maxwellian averaged rates even at low temperatures. Furthermore, there is an absence of comprehensive excitation data between the excited states that may provide newer diagnostics to compliment the more established lines discussed in this paper. To resolve these issues, we present both a small scale 56-level Breit-Pauli (BP) calculation and a large-scale 554 levels R-matrix Intermediate Coupling Frame Transformation (ICFT) calculation that extends the scope and validity of earlier JAJOM calculations both in terms of the atomic structure and scattering cross sections. Our results provide a comprehensive electron-impact excitation data set for all transitions to higher $n$ shells. The fundamental atomic data for this O-like ion is subsequently used within a collisional radiative framework to provide the line ratios across a range of electron temperatures and densities of interest in astrophysical observations.

astro-ph.EP↗

arXiv2011

Stability of Matter-Antimatter Molecules

We examine the stability of matter-antimatter molecules by reducing the four-body problem into a simpler two-body problem with residual interactions. We find that matter-antimatter molecules with constituents (m1+, m2-, m2bar+, m1bar-) possess bound states if their constituent mass ratio m1/m2 is greater than about 4. This stability condition suggests that the binding of matter-antimatter molecules is a rather common phenomenon. We evaluate the binding energies and eigenstates of matter-antimatter molecules (mu+ e-)-(e+ mu-), (pi+ e-)-(e+ pi-), (K+ e-)-(e+ K-), (p e-)-(e+ pbar), (p mu-)-(mu+ pbar), and (K+ mu-)-(mu+ K-), which satisfy the stability condition. We estimate the molecular annihilation lifetimes in their s states.

physics.chem-ph↗

arXiv2007

Peculiar Features of the Interaction Potential between Hydrogen and Antihydrogen at Intermediate Separations

We evaluate the interaction potential between a hydrogen and an antihydrogen using the second-order perturbation theory within the framework of the four-body system in a separable two-body basis. We find that the H-Hbar interaction potential possesses the peculiar features of a shallow local minimum located around interatomic separations of r ~ 6 a.u. and a barrier rising at r~5 a.u. Additional theoretical and experimental investigations on the nature of these peculiar features will be of great interest.

physics.atom-ph↗

arXiv2007

Quantum Treatment of Continuum Electrons in the Fields of Moving Charges

An ab initio, three-dimensional quantum mechanical calculation has been performed for the time-evolution of continuum electrons in the fields of moving charges. Here the essential singularity associated with the diverging phase factor in the continuum wave function is identified and removed analytically. As a result, the continuum components of the regularized wave function are slowly varying with time. Therefore, one can propagate continuum electrons to asymptotically large times and obtain numerically stable, well-converged ejected electron momentum spectra with very low numerical noise. As a consequence, our approach resolves outstanding controversies concerning structures in electron momentum distributions. The main conclusions are general and are illustrated here for ionization of atomic hydrogen by proton impact. Our results show that in order to obtain correct long-time free-particle propagation, the essential singularity identified here should be removed from the continuum components of solutions to the time-dependent Schrodinger equation.

physics.atom-ph↗

arXiv2006

State-to-state rotational transitions in H$_2$+H$_2$ collisions at low temperatures

We present quantum mechanical close-coupling calculations of collisions between two hydrogen molecules over a wide range of energies, extending from the ultracold limit to the super-thermal region. The two most recently published potential energy surfaces for the H$_2$-H$_2$ complex, the so-called DJ (Diep and Johnson, 2000) and BMKP (Boothroyd et al., 2002) surfaces, are quantitatively evaluated and compared through the investigation of rotational transitions in H$_2$+H$_2$ collisions within rigid rotor approximation. The BMKP surface is expected to be an improvement, approaching chemical accuracy, over all conformations of the potential energy surface compared to previous calculations of H$_2$-H$_2$ interaction. We found significant differences in rotational excitation/de-excitation cross sections computed on the two surfaces in collisions between two para-H$_2$ molecules. The discrepancy persists over a large range of energies from the ultracold regime to thermal energies and occurs for several low-lying initial rotational levels. Good agreement is found with experiment (Maté et al., 2005) for the lowest rotational excitation process, but only with the use of the DJ potential. Rate coefficients computed with the BMKP potential are an order of magnitude smaller.

physics.atom-ph↗