Srdjan Marjanovic

On the use of Monte Carlo simulations to model transport of positrons in gases and liquids.

In this paper we make a parallel between the swarm method in physics of ionized gases and modeling of positrons in radiation therapy and diagnostics. The basic idea is to take advantage of the experience gained in the past with electron swarms and to use it in establishing procedures of modeling positron diagnostics and therapy based on the well-established experimental binary collision data. In doing so we discuss the application of Monte Carlo technique for positrons in the same manner as used previously for electron swarms, we discuss the role of complete cross section sets (complete in terms of number, momentum and energy balance and tested against measured swarm parameters), we discuss the role of benchmarks and how to choose benchmarks for electrons that may perhaps be a subject to experimental verification. Finally we show some samples of positron trajectories together with secondary electrons that were established solely on the basis of accurate binary cross sections and also how those may be used in modeling of both gas filled traps and living organisms.

Scattering data for modelling positron tracks in gaseous and liquid water

We present in this study a self-consistent set of scattering cross sections for positron collisions with water molecules, in the energy range 0.1-10 000 eV, with the prime motivation being to provide data for modelling purposes. The structure of the database is based on a new model potential calculation, including interference terms, which provides differential and integral elastic as well as integral inelastic positron scattering cross sections for water molecules over the whole energy range considered here. Experimental and theoretical data available in the literature have been integrated into the database after a careful analysis of their uncertainties and their self-consistency. These data have been used as input parameters for a step-by-step Monte Carlo simulation procedure, providing valuable information on energy deposition, positron range, and the relative percentages of specific interactions (e.g. positronium formation, direct ionisation, electronic, vibrational and rotational excitations) in gaseous and liquid water.

Dynamics of a Lightning Channel Corona Sheath Using a Generalized Traveling-Current-Source Return Stroke Model—Theory and Calculations

Abstract-A generalized lightning traveling-current-source return stroke model (GTCS) and the measurements of Miki et al. [J. Geophys. Res., vol. 107, no. D16, pp. ACL2.1-ACL2.11, 2002], are used to calculate the dynamics of a lightning channel corona sheath surrounding a thin channel core during the return stroke stage. The channel corona sheath model that predicts the charge motion in the corona sheath is used to determine the corona sheath dynamics. This model can be viewed as the generalization of the corona sheath model proposed by Maslowski and Rakov [J. Geophys. Res., vol. 111, D14110, pp. 1-16, 2006]. According to this model, the corona sheath surrounding the thin channel core consists of two zones containing charge, zone 1 (inner zone containing net positive charge) and zone 2 (zone containing negative charge surrounding zone 1), respectively, and an outer zone 3 surrounding zone 2 without charge. Theoretical expressions for the corona sheath radii and the velocities of both zones are derived. Using a theoretical expression for the radial electric field in the immediate vicinity of the channel core derived for the GTCS model and the measured electric field waveforms of Miki et al. [J. Geophys. Res., vol. 107, no. D16, pp. ACL2.1-ACL2.11, 2002], the channel discharge function versus time is calculated. Based on this function and the measured channel-base current function, the corona sheath radii of both zones and their velocities versus time in the bottom portion of the channel are calculated. It is shown that the maximum radii of zones 1 and 2 at 2 m above ground are less than 1.5 and 6 cm, respectively. Corresponding maximum radial corona sheath velocities are less than 6 × 104 m/s. Small values of the maximum radii of zones 1 and 2 can be explained by the small value of the channel line charge density of 6.7 μC/m, due to vicinity of perfect ground. Using measured channel-base current and the calculated channel discharge function, the line charge distribution versus height is calculated. The current reflections from the striking point are considered. For the ground current reflection factor R = 1 (the reflection from the perfectly conducting ground) the peak value of the channel line charge density is 0.75 mC/m at the height of about 12 m above ground and for R = 0 (no current reflections) the peak value is 1.3 mC/m, at about 17 m above ground. The corresponding calculated values of the return stroke velocities in the channel bottom are 1.29 × 108 m/s (0.43c) and 1.68 × 108 m/s (0.56c), respectively. The corona sheath expansion velocity is about three orders of magnitude less than the calculated lightning return stroke velocity. The result concerning the channel line charge distribution is in the agreement with the measurements of Crawford et al. [J. Geophys. Res., vol. 106, pp. 14909-14917, 2001], whereas the calculated return stroke velocities are in a good agreement with the optical measurements of Willet et al. [J. Geophys. Res., vol. 93, pp. 3867-3878, 1988] and [J. Geophys. Res., vol. 94, pp. 1327513286,1989].

On Explanation of the Double-Valued Paschen-Like Curve for RF Breakdown in Argon

This paper represents an investigation of the dependence of the breakdown voltage on the gas pressure in radio-frequency argon discharges under conditions when ion-induced secondary electron production is negligible. Calculations were performed by using a Monte Carlo collision code including electrons only. Our simulation results clearly show a region, occurring at low pressure, where multiple values of the breakdown voltage exist at a given pressure, in agreement with previous experimental observations. The two different regimes of operation, each satisfying the breakdown condition, may be best analyzed in contour plots of electron density, ionization rate, and mean energy.

Numerical Modeling of Thermalization of Positrons in Gas-Filled Surko Traps

In this paper, we present the results of our Monte Carlo-based numerical simulation of a Penning-Malmberg-Surko positron trap. The results of simulations show the effect that various processes (such as positronium (Ps) formation, annihilation, losses on walls, etc.) have on trapping efficiency. The thermalization profile is shown, along with the evolution of the energy distribution that morphs from a particle beam to a broad swarm-type distribution.

On new developments in the physics of positron swarms

Recently a new wave of swarm studies of positrons was initiated based on more complete scattering cross section sets. Initially some interesting and new physics was discovered, most importantly negative differential conductivity (NDC) that occurs only for the bulk drift velocity while it does not exist for the flux property. However the ultimate goal was to develop tools to model positron transport in realistic applications and the work that is progressing along these lines is reviewed here. It includes studies of positron transport in molecular gases, thermalization in generic swarm situations and in realistic gas filled traps and transport of positrons in crossed electric and magnetic fields. Finally we have extended the same technique of simulation (Monte Carlo) to studies of thermalization of positronium molecule. In addition, recently published first steps towards including effects of dense media on positron transport are summarized here.

A CF4 based positron trap

All buffer-gas positron traps in use today rely on N2 as the primary trapping gas due to its conveniently placed

Data for modeling of positron collisions and transport in gases

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Positrons in gas filled traps and their transport in molecular gases

electronic excitation cross-section. The energy loss per excitation in this process is 8.5 eV, which is sufficient to capture positrons from low-energy moderated beams into a Penning-trap configuration of electric and magnetic fields. However, the energy range over which this cross-section is accessible overlaps with that for positronium (Ps) formation, resulting in inevitable losses and setting an intrinsic upper limit on the overall trapping efficiency of ~25%. In this paper we present a numerical simulation of a device that uses CF4 as the primary trapping gas, exploiting vibrational excitation as the main inelastic capture process. The threshold for such excitations is far below that for Ps formation and hence, in principle, a CF4 trap can be highly efficient; our simulations indicate that it may be possible to achieve trapping efficiencies as high as 90%. We also report the results of an attempt to re-purpose an existing two-stage N2-based buffer-gas positron trap. Operating the device using CF4 proved unsuccessful, which we attribute to back scattering and expansion of the positron beam following interactions with the CF4 gas, and an unfavourably broad longitudinal beam energy spread arising from the magnetic field differential between the source and trap regions. The observed performance was broadly consistent with subsequent simulations that included parameters specific to the test system, and we outline the modifications that would be required to realise efficient positron trapping with CF4. However, additional losses appear to be present which require further investigation through both simulation and experiment.

Monte Carlo modelling of positron transport in real world applications

This work is supported by MNPRS Projects ON171037 and III41011 and the Australian Research Council’s Centre of Excellence Program.