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Gamma-ray spectroscopy provides diagnostics of particle acceleration in solarflares, but care must be taken when interpreting the spectra due to effects ofthe angular distribution of the accelerated particles (such as relativisticbeaming) and Compton reprocessing of the radiation in the solar atmosphere. Inthis paper, we use the GEANT4 Monte Carlo package to simulate the interactionsof accelerated electrons and protons and study these effects on the gamma-raysresulting from electron bremsstrahlung and pion decay. We consider the ratio ofthe 511~keV annihilation-line flux to the continuum at 200~keV and in theenergy band just above the nuclear de-excitation lines (8--15~MeV) as adiagnostic of the accelerated particles and a point of comparison with datafrom the X17 flare of 2003 October 28. We also find that pion secondaries fromaccelerated protons produce a positron annihilation line component at a depthof $\sim$ 10 g cm$^{-2}$, and that the subsequent Compton scattering of the511~keV photons produces a continuum that can mimic the spectrum expected fromthe 3$\gamma$ decay of orthopositronium.

GEANT4 Simulations of Gamma-Ray Emission from Accelerated Particles in Solar Flares