arXiv subjects

R. D. White

Publications and source records attributed to R. D. White.

9 recordsLinked to original sources

arXiv2025

Review of the experimental and theoretical landscape of electron transport in noble liquids

We present a review of the current experimental and theoretical understanding of electron transport in noble liquids. Special attention is given to recent measurements that coincide with the development of time projection chambers (TPCs) using liquid xenon and argon as detector media. To enable transparent benchmarking of simulations and to facilitate the comparison between early studies and modern TPC data, we introduce a new open-access database of electron mobility and diffusion measurements. In particular, we emphasize the transition to large-scale detector designs which incorporate extended drift distances alongside improved purity control and field uniformity. On the theoretical side, we contrast empirical transport models with ab initio approaches, highlighting our recent efforts to incorporate low-energy, liquid-specific scattering phenomena, including coherent scattering, polarization screening, and bulk potential modifications. While elastic transport has seen substantial theoretical progress, inelastic processes in liquids, including ionization, exciton formation and interband transitions, remain poorly understood due to the lack of experimental cross sections and validated models. We also discuss the applications and challenges of modeling scintillation, doped and mixture-liquid targets, and gas-liquid interface behavior, all of which are critical for the design and optimization of next-generation detectors.

physics.app-ph↗

arXiv2020

Positron scattering and transport in liquid helium

In previous papers we have proposed a method for the ab initio calculation of fully differential cross-sections for electron scattering in liquids and applied it to liquid argon, xenon and krypton. In this paper, we extend the procedure to the consideration of positron scattering in liquid helium, which is complicated by the annihilation process as well as the fact that the electron definition for the region "owned" by a target atom used previously does not have a positron analogue. We explore several physically motivated definitions to obtain effective positron scattering in the dense fluid. We find that our calculations of a pure helium system cannot precisely match experimental measurements, however by including a small admixture (<0.1%) of an impurity, we can obtain reasonable agreement in the dense gas phase. In contrast, our calculations do not match well to the liquid phase measurements. This provides motivation to explore further multiple scattering effects in the theory.

physics.comp-ph↗

arXiv2016

Framework for solutions of the Boltzmann equation for ions of arbitrary mass

We present a framework for the solution of Boltzmann's equation in the swarm limit for arbitrary mass ratio, allowing for solutions of electron or ion transport. An arbitrary basis set can be used in the framework, which is achieved by using appropriate quadratures to obtain the required matrix elements. We demonstrate an implementation using Burnett functions and benchmark the calculations using Monte-Carlo simulations. Even though the convergence in transport quantities is always good, the particle distributions did not always converge, highlighting that simple benchmarks can give misleading confidence in a choice of basis. We postulate a different basis, which avoids a spherical harmonic expansion, which is better suited to strong electric fields or sharp features such as low-energy attachment processes.

physics.comp-ph↗

arXiv2016

Fluctuation capture in non-polar gases and liquids

We present a new model to identify natural fluctuations in fluids, allowing us to describe localization phenomena in the transport of electrons, positrons and positronium through non-polar fluids. The theory contains no free parameters and allows for the calculation of capture cross sections $σ_{cap}(ε)$ of light-particles in any non-polar fluid, required for non-equilibrium transport simulations. We postulate that localization occurs through large shallow traps before stable bound states are formed. Our results allow us to explain most of the experimental observations of changes in mobility and annihilation rates in the noble gases and liquids as well as make predictions for future experiments. Quantities which are currently inaccessible to experiment, such as positron mobilities, can be obtained from our theory. Unlike other theoretical approaches to localization, the outputs of our theory can be applied in non-equilibrium transport simulations and an extension to the determination of waiting time distributions for localized states is straight forward.

physics.comp-ph↗

arXiv2015

A multi-term solution of the space-time Boltzmann equation for electrons in gaseous and liquid Argon

In a recent paper [1] the scattering and transport of excess electrons in liquid argon in the hydrodynamic regime was investigated, generalizing the seminal works of Lekner and Cohen [2,3] with modern scattering theory techniques and kinetic theory. In this paper, the discussion is extended to the non-hydrodynamic regime through the development of a full multi-term space-time solution of Boltzmann's equation for electron transport in gases and liquids using a novel operator-splitting method. A Green's function formalism is considered that enables flexible adaptation to various experimental systems. The spatio-temporal evolution of electrons in liquids in the hydrodynamic regime is studied for a benchmark model Percus-Yevick liquid as well as for liquid argon. The temporal evolution of Franck-Hertz oscillations are observed for liquids, with striking differences in the spatio-temporal development of the velocity distribution function components between the uncorrelated gas and true liquid approximations in argon. Transport properties calculated from the non-hydrodynamic theory in the long time limit, and under steady-state Townsend conditions, are benchmarked against hydrodynamic transport coefficients.

physics.comp-ph↗

arXiv2015

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.

physics.chem-ph↗

arXiv2015

Kinetic Theory of Positron-Impact Ionization in Gases

A kinetic theory model is developed for positron-impact ionization (PII) with neutral, rarefied gases. Particular attention is given to the sharing of available energy between the post-ionization constituents. A simple model for the energy-partition function that qualitatively captures the physics of high-energy and near-threshold ionization is developed for PII, with free parameters that can be used to fit the model to experimental data. By applying the model to the measurements of Kover and Laricchia [1] for positrons in H2, the role of energy-partitioning in PII for positron thermalisation is studied. Although the overall thermalisation time is found to be relatively insensitive to the energy-partitioning, the mean energy profiles at certain times can differ by more than an order of magnitude for the various treatments of energy-partitioning. This can significantly impact the number and energy distribution of secondary electrons.

physics.plasm-ph↗

arXiv2015

An improved Monte Carlo study of coherent scattering effects of low energy charged particle transport in Percus-Yevick liquids

We generalize a simple Monte Carlo (MC) model for dilute gases to consider the transport behavior of positrons and electrons in Percus-Yevick model liquids under highly non-equilibrium conditions, accounting rigorously for coherent scattering processes. The procedure extends an existing technique [Wojcik and Tachiya, Chem. Phys. Lett. 363, 3--4 (1992)], using the static structure factor to account for the altered anisotropy of coherent scattering in structured material. We identify the effects of the approximation used in the original method, and develop a modified method that does not require that approximation. We also present an enhanced MC technique that has been designed to improve the accuracy and flexibility of simulations in spatially-varying electric fields. All of the results are found to be in excellent agreement with an independent multi-term Boltzmann equation solution, providing benchmarks for future transport models in liquids and structured systems.

physics.comp-ph↗

arXiv2013

High order fluid model for streamer discharges: I. Derivation of model and transport data

Streamer discharges pose basic problems in plasma physics, as they are very transient, far from equilibrium and have high ionization density gradients; they appear in diverse areas of science and technology. The present paper focuses on the derivation of a high order fluid model for streamers. Using momentum transfer theory, the fluid equations are obtained as velocity moments of the Boltzmann equation; they are closed in the local mean energy approximation and coupled to the Poisson equation for the space charge generated electric field. The high order tensor in the energy flux equation is approximated by the product of two lower order moments to close the system. The average collision frequencies for momentum and energy transfer in elastic and inelastic collisions for electrons in molecular nitrogen are calculated from a multi term Boltzmann equation solution. We then discuss, in particular, (1) the correct implementation of transport data in streamer models; (2) the accuracy of the two term approximation for solving Boltzmann's equation in the context of streamer studies; and (3) the evaluation of the mean-energy-dependent collision rates for electrons required as an input in the high order fluid model. In the second paper in this sequence, we will discuss the solutions of the high order fluid model for streamers, based on model and input data derived in the present paper.

physics.plasm-ph↗