239 research outputs found
Electron scattering and transport in liquid argon
The transport of excess electrons in liquid argon driven out of equilibrium
by an applied electric field is revisited using a multi-term solution of
Boltzmann's equation together with ab initio liquid phase cross-sections
calculated using the Dirac-Fock scattering equations. The calculation of liquid
phase cross-sections extends previous treatments to consider multipole
polarisabilities and a non-local treatment of exchange while the accuracy of
the electron-argon potential is validated through comparison of the calculated
gas phase cross-section with experiment. The results presented highlight the
inadequacy of local treatments of exchange that are commonly used in liquid and
cluster phase cross-section calculations. The multi-term Boltzmann equation
framework accounting for coherent scattering enables the inclusion of the full
anisotropy in the differential cross-section arising from the interaction and
the structure factor, without an a priori assumption of quasi-isotropy in the
velocity distribution function. The model, which contains no free parameters
and accounts for both coherent scattering and liquid phase screening effects,
was found to reproduce well the experimental drift velocities and
characteristic energies.Comment: 32 pages, 16 figures; minor corrections, added 1 figur
Resonances in low-energy positron-alkali scattering
Close-coupling calculations were performed with up to five target states at energies in the excitation threshold region for positron scattering from Li, Na and K. Resonances were discovered in the L = 0, 1 and 2 channels in the vicinity of the atomic excitation thresholds. The widths of these resonances vary between 0.2 and 130 MeV. Evidence was found for the existence of positron-alkali bound states in all cases
Positron-alkali atom scattering
Positron-alkali atom scattering was recently investigated both theoretically and experimentally in the energy range from a few eV up to 100 eV. On the theoretical side calculations of the integrated elastic and excitation cross sections as well as total cross sections for Li, Na and K were based upon either the close-coupling method or the modified Glauber approximation. These theoretical results are in good agreement with experimental measurements of the total cross section for both Na and K. Resonance structures were also found in the L = 0, 1 and 2 partial waves for positron scattering from the alkalis. The structure of these resonances appears to be quite complex and, as expected, they occur in conjunction with the atomic excitation thresholds. Currently both theoretical and experimental work is in progress on positron-Rb scattering in the same energy range
Fine-structure effect for (e,2e) collisions
For the case of inelastic electron-atom scattering, it has been known for some time that significant spin effects may be observed even if spin-dependent forces on the projectile can be ignored. These spin effects result from the Pauli principle and this phenomenon has become known as the fine-structure effect. Recently, the question of whether or not these same types of effects should be observed for atomic ionization has been considered and the initial indications are that significant spin asymmetries can also be expected for atomic ionization if the final ion satisfies LS coupling and the final J state of the ion can be resolved. In this paper, we consider this problem for electron-impact ionization of inert gases. The theory of the fine-structure effect is presented for ionization and first-order distorted-wave results are compared with very recent experimental data
Positron scattering from the group IIB metals zinc and cadmium: recommended cross sections and transport simulations
Results from the application of our optical potential and relativistic optical potential models to positron scattering from gas-phase zinc (Zn) and cadmium (Cd) are presented. In particular, integral cross sections (ICSs) for elastic scattering, positronium formation, summed discrete electronic-state excitation, and ionization scattering processes are reported for both species and over an extended incident positron energy range. From those ICSs, the total cross section is subsequently constructed by taking their sum. We note that there are currently no experimental data available for any of these scattering processes for either species, with earlier computational results being limited to the elastic channel and restricted to relatively narrow incident positron energy regimes. Nonetheless, we construct recommended positron cross section datasets for both zinc and cadmium over the incident positron energy range of 0-10 000 eV. The recommended positron cross section data are subsequently employed in a multi-term Boltzmann equation analysis to simulate the transport of positrons, under the influence of an applied (external) electric field, through the background Zn and Cd gases. Qualitatively similar behavior in the calculated transport coefficients was observed between both species. Finally, for the case of zinc, the present positron transport coefficients are compared against corresponding results from electron transport with some significant differences now being observed
Longitudinal changes in wellbeing amongst breastfeeding women in Australia and New Zealand during the COVID-19 pandemic
The COVID-19 pandemic has impacted new mothers’ wellbeing and breastfeeding experience. Women have experienced changes in birth and postnatal care and restricted access to their support network. It is unclear how these impacts may have changed over time with shifting rates of infection and policies restricting movement and access to services in Australia and New Zealand. This study investigated the longitudinal effect of the COVID-19 pandemic on breastfeeding and maternal wellbeing in Australia and New Zealand. Mothers (n = 246) completed an online survey every 4 weeks for 6 months that examined feeding methods, maternal mental wellbeing, worries, challenges, and positive experiences during the pandemic. Mothers maintained high full breastfeeding rates at 4 months (81%) which decreased to 37% at 6 months. Perceived low milk supply contributed to the earlier cessation of full breastfeeding. Poor infant sleep was associated with stress, perinatal anxiety, mental wellbeing, and breastfeeding status. Although mothers initially reported that lockdowns helped with family bonding and less pressure, prolonged lockdowns appeared to have adverse effects on access to social networks and extended family support.
Conclusion: The results highlight the changing dynamic of the pandemic and the need for adaptable perinatal services which allow mothers access to in-person services and their support network even in lockdowns. Similarly, access to continuous education and clinical care remains critical for women experiencing concerns about their milk supply, infant sleep, and their own wellbeing
Joint theoretical and experimental study on elastic electron scattering from bismuth
We report on experimental elastic differential and integral cross sections for electron scattering from bismuth. The energy range of those measurements is 10-100 eV, while the scattered electron angular range in the differential cross section data is 10 degrees-150 degrees. We also supplement our experimental results with theoretical data from our optical potential approach, in this case applying two different sets of scattering potentials to the problem. Good overall qualitative accord is typically observed between our measured and calculated results, particularly when allowance is made for the dimer contribution in the experimental atomic beam
Configuration-interaction calculations of positron binding to zinc and cadmium
The configuration-interaction method is applied to the study of positronic zinc (e+Zn) and positronic cadmium (e+Cd). The estimated binding energies and annihilation rates were 0.00373 hartree and 0.42×109 sec-1 for e+Zn and 0.006 10 hartree and 0.56×109 sec-1 for e+Cd. The low-energy elastic cross section and Zeff were estimated from a model potential that was tuned to the binding energies and annihilation rates. Since the scattering lengths were positive (14.5a0 for Zn and 11.6a0 for Cd) the differential cross sections are larger at backward angles than at forward angles just above threshold. The possibilities of measuring differential cross sections to confirm positron binding to these atoms is discussed
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