Positron transport in water vapour

Transport properties of positron swarms in water vapour under the influence of electric and magnetic fields are investigated using a Monte Carlo simulation technique and a multi-term theory for solving the Boltzmann equation. Special attention is paid to the correct treatment of the non-conservative nature of positronium (Ps) formation and its explicit and implicit influences on various positron transport properties. Many interesting and atypical phenomena induced by these influences are identified and discussed. Calculated transport properties for positrons are compared with those for electrons, and the most important differences are highlighted. The significant impact of a magnetic field on non-conservative positron transport in a crossed field configuration is also investigated. In general, the mean energy and diffusion coefficients are lowered, while for the measurable drift velocity an unexpected phenomenon arises: for certain values of the reduced electric field, the magnetic field enhances the drift. The variation of transport coefficients with the reduced electric and magnetic fields is addressed using physical arguments with the goal of understanding the synergistic effects of Ps formation and magnetic field on the drift and diffusion of positrons in neutral gases

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

Water relations and photosynthesis as criteria for adequate irrigation management in 'Tahiti' lime trees

Irrigation scheduling based on soil moisture status is one of the most useful methods because of its practicality and low cost. The effects of available soil water depletion on evapotranspiration (ETc), transpiration (E), leaf water potential at predawn (psiP) and midday (psiM), stomatal conductance (gs) and net CO2 assimilation (A) in lime 'Tahiti' trees (Citrus latifolia) were evaluated to improve irrigation schedule and minimize water use without causing water stress. The trees were spaced 7 <FONT FACE=Symbol>&acute;</FONT> 4 m and drip-irrigated by four drippers with the available soil water content (AWC) depleted by suspension of irrigation (40 days). Leaf water potential was measured on a pressure chamber (psiP and psiM) and leaf gas exchange was measured by infrared gas analyzer (E, gs and A). Evapotranspiration was determined with the aid of weighing lysimeter. Water soil content and potential (psiS) were monitored with TDR probes and tensiometers, respectively, installed at 0.3, 0.6 and 0.9 m depths. Meteorological variables were monitored with an automatic weather station in the experimental area. The threshold AWC level for the onset of ETc decline was 43%, and 60% for gs, A, E and Y P. Also, psiP was more sensitive to AWC than psiM, and is therefore a better tool for irrigation. When AWC was around 60%, values of psiP and psis were -0.62 MPa and -48.8 kPa, respectively

Relaxation of low energy positrons in molecular gases

The calculations of thermalization times and spatial relaxation profiles of the positron transport properties in H_2, N_2 and in mixture of N_2 and CF_4 are presented. Recently, the data needed to compile comprehensive set of cross sections for these gases became available which made our calculations possible. The main difference between positron and electron transport is the existence of the effect of positronium (Ps) formation which changes the number of particles and has a strong energy dependence. The relative magnitude of positronium formation to electronic excitations and the relative positions of their thresholds control the efficiency of thermalization and non-conservative transport phenomena. The effect of vibrational and rotational excitations on thermalization has been considered and relative contributions were determined. Our calculated thermalization data are in reasonably good agreement with the experimental data of Al-Qaradawi et al. (2000)

Negative differential conductivity of positrons in gases

This paper reports on a new series of calculations of positron transport properties based on current experimental cross section data. It is found that negative differential conductivity (NDC) occurs in the bulk drift velocity W but not the flux drift velocity w. The origin of the phenomenon lies in the "reactive" nature of positron collisions associated with positronium Ps formation, and is quite different in origin to the better known NDC effect in w arising from certain combinations of inelastic-elastic cross sections. Moreover, while the Ps formation process is qualitatively similar (at least from a kinetic theory perspective) to electron attachment, it is characterized by a cross section several orders of magnitude larger and hence the "reactive" NDC effect is correspondingly more pronounced. In this paper we test both established conditions for NDC, and develop new criteria, using simple mathematics and physical arguments where possible

Low energy positron interactions - trapping, transport and scattering

Recent experiments, theory and modelling of positron interactions with atoms and molecules are discussed. The first half of the paper is devoted to binary collisions between positrons and crossed beams of atoms or molecules (in this case neon) and the second half deals with ensembles of non-interacting positrons, otherwise known as swarms which are transported through the background gas. We review the recent results on measurements of the cross sections based on obtained from collisional positron traps and subsequent calculations of transport properties of positron swarms that may be used to model thermalization experiments, collisional traps and possible applications of positrons in materials science and biomedicine. It was found that kinetic phenomena occur in positron transport that are mainly the result of the positronium (Ps) formation which has a larger cross section than elastic scattering in most gases and at the same time is a non-conservative process. Most importantly negative differential conductivity (NDC) occurs only for the bulk drift velocity while it does not exist for the flux property, a phenomenon that has not been observed for electrons

Electron and positron swarms: Collision and transport data and kinetic phenomena

A broad review of electron swarm studies completed recently is presented with a common thread of both being motivated by major applications which use swarm physics as part of their phenomenological foundation and also with a strong presence of nonconservative (electron number changing) collisions. The review is mainly based on the activities of Gaseous Electronics Laboratory Belgrade and it cannot cover all recent and ongoing activities in swarm physics but it attempts to cover the majority of topics covered by swarm physicists in general. Thus we start with recent determinations of the cross sections from the transport data and calculations of the transport data from the cross sections from other sources in gases such as NO, N2O and mixtures of Ar and N2. We proceed to show how the presence of radicals affects the transport coefficients in CF4, a gas with great potential for applications. The basic features of the transport are discussed for dc and rf electric and magnetic fields. In those two chapters we mainly focus on kinetic phenomena such as negative absolute mobility, non-conservative effects in particle transport and how angle between magnetic and electric field affects the transport coefficients. We also discuss application of semi empirical formulas. Finally we analyze positron transport and its difference from the transport of electrons. The Positronium formation cross section is significantly larger than that for analogous electron non-conservative processes (i.e. electron attachment). Thus transport of positrons gives a much stronger nonconservative effects including a new effect of the negative differential conductivity (NDC) in the bulk (WB - velocity of the center of the swarm that is relevant for the real space diffusion equation) drift velocity while the conditions required for NDC do not exist for the flux drift velocity (w F - mean velocity of particles in the swarm that is relevant for the calculations of flux when using continuity relation)

Electron and positron swarms: Collision and transport data and kinetic phenomena

A broad review of electron swarm studies completed recently is presented with a common thread of both being motivated by major applications which use swarm physics as part of their phenomenological foundation and also with a strong presence of nonconservative (electron number changing) collisions. The review is mainly based on the activities of Gaseous Electronics Laboratory Belgrade and it cannot cover all recent and ongoing activities in swarm physics but it attempts to cover the majority of topics covered by swarm physicists in general. Thus we start with recent determinations of the cross sections from the transport data and calculations of the transport data from the cross sections from other sources in gases such as NO, N2O and mixtures of Ar and N2. We proceed to show how the presence of radicals affects the transport coefficients in CF4, a gas with great potential for applications. The basic features of the transport are discussed for dc and rf electric and magnetic fields. In those two chapters we mainly focus on kinetic phenomena such as negative absolute mobility, non-conservative effects in particle transport and how angle between magnetic and electric field affects the transport coefficients. We also discuss application of semi empirical formulas. Finally we analyze positron transport and its difference from the transport of electrons. The Positronium formation cross section is significantly larger than that for analogous electron non-conservative processes (i.e. electron attachment). Thus transport of positrons gives a much stronger nonconservative effects including a new effect of the negative differential conductivity (NDC) in the bulk (WB - velocity of the center of the swarm that is relevant for the real space diffusion equation) drift velocity while the conditions required for NDC do not exist for the flux drift velocity (w F - mean velocity of particles in the swarm that is relevant for the calculations of flux when using continuity relation)

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