A photon source model for alpha-emitter radionuclides.

A Monte Carlo virtual source model that generates photons emitted in the complex decay chain process of alpha-emitter radionuclides is proposed, typically for use during simulation of SPECT image acquisition. 
Given an alpha-emitter radionuclide, the model extracts from Geant4 databases the photon emission lines from all decaying daughters for both isometric transition and atomic relaxation processes. According to a given time range, abundances and activities in the decay chain are considered thanks to the Bateman equations, taking into account the decay rates and the initial abundances. 
The proposed source model is evaluated versus analog Monte Carlo simulation. It generates photons with the correct energy and temporal distribution, avoiding the costly Monte Carlo simulation of the complete decay chain. Photons emitted from Bremsstrahlung are ignored, but are not significant for SPECT imaging. As an example for 225Ac, comparison of the proposed model versus a simple source with the two main photon emission lines (218 and 440 keV) show that 2.6 times more particles are simulated and 60% more counts are detected in the images. 
The proposed model can simulate any alpha-emitter radionuclide available in the Geant4 database. It is available in GATE and ease investigation of imaging gamma rays from alpha-emitters.