Positron Cooling and Annihilation in Noble Gases

Area of Science:

• Atomic and Molecular Physics
• Plasma Physics
• Computational Physics

Background:

• Positron interactions with matter are crucial for various applications.
• Accurate cross-sections are needed for reliable simulations.
• Discrepancies exist between theoretical and experimental annihilation rates in gases.

Purpose of the Study:

• To simulate positron cooling and annihilation in noble gases.
• To investigate the energy dependence of scattering and annihilation cross sections.
• To resolve experimental discrepancies in positron annihilation measurements.

Main Methods:

• Utilizing accurate scattering and annihilation cross sections derived from many-body theory.
• Performing simulations of positron behavior in room-temperature noble gases (He, Ne, Ar, Kr, Xe).
• Analyzing the time-varying annihilation rate Z[over ¯]_{eff}(τ) and its sensitivity to momentum distribution.

Main Results:

• A small fraction of positrons survive to thermalization, varying significantly across noble gases (e.g., ~0.1% in He, ~0.01% in Xe).
• Simulations for Xenon demonstrate high sensitivity of Z[over ¯]_{eff}(τ) to momentum depletion, explaining experimental anomalies.
• Calculated Z[over ¯]_{eff}(τ) generally agree with experiments for all noble gases except Neon.

Conclusions:

• The study provides a unified explanation for positron annihilation measurements in noble gases.
• Neon experiments may require re-evaluation due to potential underestimation of surviving positrons or impurities.
• Accurate atomic data from many-body theory is essential for precise modeling of positron-gas interactions.