Numerical Simulation of Positron Diffusion in the Heavy Noble Gases Ar, Kr and Xe

Abstract

Numerical simulation has been carried out of positron annihilation and diffusion under the influence of an external temperature and electric field in heavy noble gases. The electric field was varied over the range 0-200 V cm -1 amagat -1 (1 amagat ... 2.687 xl 0 19 atoms cm -3), whereas the temperature was varied from 300 to 3000 K. It is observed that the decay constant ZeIT increases with increasing atomic mass of the interacting gases He, Ne, Ar, Kr and Xe, while the diffusion coefficient decreases except in the case of neon.

Computer Simulation of Positron Annihilation and Diffusion Characteristics in Kr and Xe

“…When the magnetic field is applied to the gas assembly the value of Ec increases: (EdKr '" 40 V cm-1 and (Ec}xe '" 60 V cm-1 at B = 10 kG, whereas at B = 50 kG, Ec is ",70 V cm-1 for Kr and ",80 V cm-1 for Xe. The magnetic field has more effect in Kr than in Xe because Kr is a lighter gas (see Singh and Grover 1986). The theoretical and experimental annihilation rates by different workers are presented in Table 1.…”

“…a , and we assume that Fo(v,t) =F(v)e-A,t. Using this assumption in equation (1) and after integrating over velocity, we get a linear integro-differential equation (see Singh and Grover 1987), the solution of which yields the equilibrium positron velocity distribution y(v). After calculating y(v), we obtain the annihilation rate…”

“…Now we can also determine the drift and diffusion characteristics of positrons in the presence of electric and magnetic fields (Singh and Grover 1987). In the absence of a magnetic field, the diffusion and drift velocity of the positron can be evaluated as in Huxley and Crompton (1974):…”

“…Studies have been mostly concerned with helium, neon and argon. Among these, helium and argon have been extensively studied by Singh and Grover (1986). Recently, attention has also been given to the study of the behaviour of slow positrons (with an energy less than the positronium formation threshold of 7·2 eV for Kr and 5·3 eV for Xe) in the heavy gases Kr and Xe.…”

“…These gases have been studied experimentally by Canter (1972), Griffith et al (1979), Heyland et al (1982) and Charlton (1985), and theoretically by Schrader and Svetic (1982) and Campeanu (1982). Schrader and Svetic (1982) investigated the equilibrium behaviour of positrons as a function of temperature, while Campeanu (1982) calculated the time-dependent characteristics of positrons, but there has been no investigation of the effect of electric and magnetic fields on the annihilation of positrons in these gases, though it is well known that these fields appreciably influence the positron lifetime and other properties in gases, as investigated by Singh and Grover (1986).…”