ANTICOOL: Simulating positron cooling and annihilation in atomic gases

Abstract

The Fortran program ANTICOOL, developed to simulate positron cooling and annihilation in atomic gases below the positronium-formation threshold, is presented. Given positron-atom elastic scattering phase shifts, normalised annihilation rates Z eff , and γ spectra as a function of momentum k, ANTICOOL enables the calculation of the positron momentum distribution f (k, t) as a function of time t, the time-varying normalised annihilation rateZ eff (t), the lifetime spectrum and time-varying annihilation γ spectra. The capability and functionality of the program is demonstrated via a tutorial-style example for positron cooling and annihilation in room temperature helium gas, using accurate scattering and annihilation cross sections and γ spectra calculated using many-body theory as input. PROGRAM SUMMARYProgram Title: ANTICOOL Program Files doi: http://dx.doi.org/10.17632/kjstc6xskg.1 Licensing provisions: GPLv3 Programming language: Fortran 90 Nature of problem: Calculation of dynamics of antiparticle cooling and annihilating in atomic gases. Developed for positron cooling and annihilation in atomic gases, but easily modifiable and applicable to other binary elastic-collision cooling processes governed by quantum-mechanical cross sections. Solution method: Monte Carlo simulation. Restrictions: Elastic scattering and annihilation only: positron energies must be below the positroniumformation threshold. It is a single particle program, i.e., positron-positron interactions are not included and binary collisions with the gas atoms are assumed. $ The associated computer program and corresponding manual will be made available from the CPC library * Corresponding author. Tel: +44 (0)28 9097 1935.

Positron Cooling and Annihilation in Noble Gases

“…It has also been successfully applied to positron scattering and annihilation on hydrogenlike ions [26] and to study the effect of positron-atom correlations on positron molecule γ spectra [27,28].Here, we show that the MBT data enables accurate modeling of positron cooling and annihilation in noble gases. Using the MBT data in Monte-Carlo (MC) simulations [29,30], we calculate the time-evolving positron momentum distribution f (k, τ ) [we work in units where τ is the time (in ns) scaled by the number density of the gas n g (in amagat): τ = n g t] and from this the time-varying annihilation rateZ eff (τ ) = Z eff (k)f (k, τ )dk/ f (k, τ )dk, which can be compared with experiments. The fraction of positrons surviving to thermalisation is shown to be strikingly small.Z eff (τ ) is shown to be sensitive to the amount of particles annihilating before thermalisation, conclusively explaining the long-standing discrepancy between the gas-cell and trap-basedZ eff results in Xe.…”

“…Here, we show that the MBT data enables accurate modeling of positron cooling and annihilation in noble gases. Using the MBT data in Monte-Carlo (MC) simulations [29,30], we calculate the time-evolving positron momentum distribution f (k, τ ) [we work in units where τ is the time (in ns) scaled by the number density of the gas n g (in amagat): τ = n g t] and from this the time-varying annihilation rate Zeff (τ ) = Z eff (k)f (k, τ )dk/ f (k, τ )dk, which can be compared with experiments. The fraction of positrons surviving to thermalisation is shown to be strikingly small.…”

Semiempirical models for low energy positron scattering by Ar, Kr and Xe

“…Understanding positron scattering cross sections is a fundamental requirement for modelling positron tracks [1][2][3] and thermalization times in condensed and gaseous media [4][5][6][7]. Among many different atoms and molecules, noble gases deserve a special place.…”

Maxwellianization of Positrons Cooling in CF4 and N2 Gases

“…Simulation procedure.-PMD fðk; τÞ, where k is the momentum and τ ≡ n g t is the time-density (n g being the gas number density and t the time), normalized as R ∞ 0 fðk; τÞdk ¼ 1, is calculated using the ANTICOOL program [43], modified to include vibrational and rotational inelastic positron-gas collisions and positron-positron collisions. We employ a grid in τ with constant step size Δτ ≡ n g Δt.…”