PsSource: an installable Geant4 extension for transport-coupled positronium annihilation with validated Ore-Powell three-photon physics

Abstract

PsSource is an installable Geant4 extension for configurable positronium-aware terminal positron annihilation. Geant4 transports the positron normally, after which PsSource replaces only terminal at-rest annihilation, preserves the terminal position and global time, resolves a fixed or host-defined environment, samples the annihilation class and delay, and returns ordinary two- or three-photon secondaries. Supported outcomes are direct two-photon, para-positronium two-photon, ortho-positronium two-photon, and ortho-positronium three-photon annihilation. Fixed and exponential delays and approximate phase-space, Geant4 Ore-Powell, and polarized Geant4 Ore-Powell backends are available. Regression tests verified terminal-state preservation, timing decomposition, all four classes, photon multiplicity and parentage, equivalent environment pathways, and unchanged photon physics when PsSource-specific truth recording was disabled. The Ore-Powell backend was evaluated using 100,000 three-photon events. Comparison with an analytic joint-energy distribution yielded chi-square = 1236.91 for 1274 degrees of freedom, a standardized score of -0.735, and a maximum marginal empirical CDF difference of 0.00208. Ordered photon-energy spectra agreed with native Geant4 and GATE references, with no pairwise empirical CDF difference greater than 0.00452. Photon directions and event-plane normals were isotropic with no detectable Cartesian-axis bias. The polarized backend satisfied normalization and transversality to within 1.5 x 10^-12 and preserved the ordinary Ore-Powell energy spectrum. PsSource provides a validated integration layer for positronium-aware terminal annihilation in Geant4.

Major revision following peer review. PsSource was redesigned as an installable transport-coupled Geant4 annihilation extension. Added Ore-Powell and polarized Ore-Powell physics, independent analytic validation, native Geant4 and GATE comparisons, isotropy and polarization tests, and transport-coupled regressions. Previous detector-reconstruction analyses were removed