Explicit positronium source modeling for Geant4 PET pipelines: controlled 2-gamma and 3-gamma generation and validation

TL;DR

This paper introduces a new Geant4-based positronium source model that allows precise control over annihilation channels, decay timing, and photon emission, with validation demonstrating accurate and efficient simulation capabilities.

Contribution

The authors developed and validated an explicit positronium source model in Geant4 supporting customizable decay modes, timing, and routing, enhancing PET simulation flexibility.

Findings

Accurately controlled 2-gamma and 3-gamma fractions following input parameters.

Reproduced prescribed positronium lifetimes with near-perfect accuracy.

Maintained modest computational overhead with linear scaling.

Abstract

An explicit positronium (Ps) source model was implemented in Geant4 to provide direct event-level control over annihilation channel selection, decay timing, and photon emission topology. The implementation supports direct annihilation, para-positronium (p-Ps), and ortho-positronium (o-Ps) branches with user-defined fractions, explicit routing of o-Ps events to two-photon (2-gamma) or three-photon (3-gamma) decay, exponential or fixed delay sampling, optional prompt-photon emission, and optional positron-range displacement. Event-level truth metadata were retained to support downstream validation and analysis. The implementation was evaluated in controlled Geant4 studies using native reference configurations, explicit branch-fraction sweeps, lifetime sweeps, timing benchmarks, and a frozen point-source downstream test harness. Observed 2-gamma and 3-gamma fractions followed the requested control parameters with the expected linear behavior, and measured mean delays reproduced the prescribed lifetime settings with near one-to-one agreement. Computational cost scaled linearly with event count, with modest overhead relative to native Geant4 operation. A minimal downstream validation framework was used to verify branch-consistent handling of pure and mixed datasets, including expected method-source compatibility and recovery of valid events under unified 2-gamma and 3-gamma routing. These results establish a practical and internally consistent code framework for explicit positronium modeling in Geant4 and provide a simple pathway for PET researchers to incorporate controlled Ps generation into existing simulation pipelines.