The modelling of non-equilibrium light lepton transport in gases and liquids

Boyle, Gregory (2015) The modelling of non-equilibrium light lepton transport in gases and liquids. PhD thesis, James Cook University.

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An accurate quantitative understanding of the transport of light leptons, such as electrons and positrons, in dilute gaseous and soft condensed mediums is of interest to a number of technological applications, as well as from the perspective of fundamental physics research. In particular, this research has been directly applied to cross section set validation, is motivated by nuclear medicine, and will find application in liquid particle detectors and plasma medicine.

The connection between the microscopic description of matter, such as scattering cross sections, and macroscopic applications is usually made via Monte Carlo simulations or kinetic theory, which are often used in a complementary fashion. The fundamental kinetic equation considered in this work is Boltzmann's equation, which describes the evolution of the swarm particle phase-space distribution in time and space due to the influence of collisions with a background medium and external forces on the system. Following the work of White and co-workers [1] a full, multiterm, space-time Boltzmann equation solver has been developed for the first time for highly nonequilibrium electron and positron transport in dilute gases, dense gases and liquids. By simulating the evolution of the Boltzmann equation Green's function, the result from a single simulation can be used to model a wide variety of experimental configurations and applications including pulsed-Townsend, steady-state Townsend and other practical experimental devices.

Swarm experiments, which operate in the hydrodynamic regime, provide stringent tests on the accuracy and completeness of cross section sets, as well as a benchmark for the energy-dependent component of the numerical code. An investigation of benchmark model and real systems, including electron-neon, positron-helium and positron-molecular-hydrogen, have allowed us to assess and validate various scattering processes, as well as to comment on the consistency and accuracy of established cross sections with experimental measurements in the low-energy regime. A new collision operator for positron impact ionization was developed and systematically benchmarked as part of this.

A major focus of the present work is extending the kinetic theory formalism beyond dilute gases to dense gases, liquids and soft-condensed matter such as biological matter. The study of swarm transport in dense mediums is considerably more complex due to the density effects arising from the small interparticle spacings and highly correlated scattering centres. We have generalized the ab initio method of Lekner and Cohen [2, 3] overcoming several approximations which are no longer necessary in modern day transport and scattering theory. Liquid argon was chosen as the test bed for our calculations, and by including both coherent scattering effects and modifications to the electron-atom potential, a high level of agreement between the calculated and measured transport coefficients was achieved.

An investigation of the full spatio-temporal evolution of electrons in a model hard-sphere liquid successively demonstrated the periodic non-hydrodynamic phenomena expected, and was confirmed by independent Monte Carlo simulation. Finally, the spatio-temporal evolution of electron swarms in gas- and liquid-phase argon were compared. Striking differences were evident in the evolution of the distribution function components, which were a reflection of the reduced momentum-transfer and lack of a Ramsauer minimum in the liquid-phase when compared to the gas-phase cross sections. This highlights the problems with treating liquid systems as gaseous systems with increased density, with implications to various applications.

Item ID: 43770
Item Type: Thesis (PhD)
Keywords: Boltzmann's equation; electron swarms; electron transfer; electron transport; electrons; gases; kinetic theory; leptons; light leptons; liquids; Monte Carlo simulations; positron transfer; positron transport; positrons; swarm simulations
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 4: Tattersall, W.J., Cocks, D.G., Boyle, G.J., Buckman, S.J., and White, R.D. (2015) An improved Monte Carlo study of coherent scattering effects of low energy charged particle transport in Percus-Yevick liquids. Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), 91 (4). pp. 1-11.

Chapter 5: Boyle, G.J., Casey, M.J.E., White, R.D., Cheng, Y., and Mitroy, J. (2014) Transport properties of electron swarms in gaseous neon at low values ofE/N. Journal of Physics D: applied physics, 47 (34). pp. 1-9.

Chapter 5: Boyle, G.J., Casey, M.J.E., White, R.D., and Mitroy, J. (2014) Transport theory for low-energy positron thermalization and annihilation in helium. Physical Review A : Atomic, Molecular and Optical Physics, 89 (2).

Chapter 6: Boyle, G.J., Tattersall, W.J., Cocks, D.G., Dujiko, S., and White, R.D. (2015) Kinetic theory of positron-impact ionization in gases. Physical Review A (Atomic, Molecular and Optical Physics), 91 (5). pp. 1-13.

Chapter 7: Boyle, G.J., McEachran, R.P., Cocks, D.G., and White, R.D. (2015) Electron scattering and transport in liquid argon. Journal of Chemical Physics, 142 (15). pp. 1-12.

Appendix: Boyle, G.J., White, R.D., Robson, R.E., Dujko, S., and Petrovic, Z.Lj. (2012) On the approximation of transport properties in structured materials using momentum-transfer theory. New Journal of Physics, 14. pp. 1-25

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Date Deposited: 17 May 2016 00:04
FoR Codes: 02 PHYSICAL SCIENCES > 0202 Atomic, Molecular, Nuclear, Particle and Plasma Physics > 020201 Atomic and Molecular Physics @ 33%
02 PHYSICAL SCIENCES > 0202 Atomic, Molecular, Nuclear, Particle and Plasma Physics > 020203 Particle Physics @ 34%
02 PHYSICAL SCIENCES > 0202 Atomic, Molecular, Nuclear, Particle and Plasma Physics > 020202 Nuclear Physics @ 33%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970102 Expanding Knowledge in the Physical Sciences @ 100%
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