A Monte Carlo positronium decay source model with multiple annihilation channels in GATE.

Positronium-based imaging, including Positronium Lifetime Imaging (PLI) and multi-photon Positron Emission Tomography (PET), requires realistic modelling of positronium (Ps) decay in matter. Existing Monte Carlo (MC) implementations in Geant4 Application for Tomographic Emission (GATE) are limited to simplified single-or two-channel descriptions, preventing accurate representation of the multi-component lifetime structure observed in biological and material systems. The objective of this work is to develop and validate a flexible MC Ps decay source model supporting multiple annihilation channels within the GATE framework. Approach. We introduce a modular Ps decay model implemented in GATE 9.4 and GATE 10, enabling the definition of an arbitrary number of decay channels characterised by lifetime, branching fraction, annihilation multiplicity (2γ/3γ), and optional prompt photon emission. The model is validated through analytical and numerical benchmarks, including lifetime distributions, branching fraction consistency, photon kinematics, and prompt photon emission. Its practical applicability is demonstrated using simulations of mixed annihilation scenarios and the NEMA IEC phantom with a large field-of-view PET system. Results. The proposed model accurately reproduces input lifetime distributions as weighted sums of exponential components and correctly samples decay channel fractions within statistical uncertainties. Simulated two-and three-photon annihilation kinematics are consistent with theoretical expectations. Complex mixtures of decay channels, including varying 3γ-to-2γ ratios and multi-component ortho-positronium lifetimes, are correctly modelled, with observable signatures reflected in both temporal and energy distributions. Phantom simulations demonstrate the capability to generate realistic positronium-sensitive datasets. Significance. This work provides the first general-purpose, multi-channel positronium decay model integrated into GATE, enabling realistic simulations of positronium behaviour in complex media. The model supports the development and optimisation of positronium-based imaging techniques, and applies to medical imaging, industrial tomography, and fundamental physics studies. Its public availability and compatibility with standard GATE workflows make it a valuable tool for the broader research community.