1. A method for improving the accuracy of the full energy peak measurement during radionuclide spectroscopy analysis of a radiation source exhibiting any EC decay scheme, the analysis based on measurements obtained using a photon detection device, the method steps comprising: obtaining output data from at least one photon detection device; calculating, with a processing device, a gamma-gamma coincidence summing correction factor and an X-ray coincidence summing correction factor for multiple coincident gamma and X-ray detections; and applying the X-ray coincidence summing correction factor to the gamma-gamma coincidence summing correction factor to improve the accuracy of the full energy peak measurement of the radiation source.