This paper details preliminary photon measurements with the monolithic silicon detector ATLASPix, a pixel detector built and optimized for the CERN experiment ATLAS. The goal of this paper is to determine the promise of pixelated silicon in future space-based gamma-ray experiments. With this goal in mind, radioactive photon sources were used to determine the energy resolution and detector response of ATLASPix; these are novel measurements for ATLASPix, a detector built for a ground-based particle accelerator. As part of this project a new iteration of monolithic Si pixels, named AstroPix, have been created based on ATLASPix, and the eventual goal is to further optimize AstroPix for gamma-ray detection by constructing a prototype Compton telescope.The energy resolution of both the digital and analog output of ATLASPix is the focus of this paper, as it is a critical metric for Compton telescopes. It was found that with the analog output of the detector, the energyresolution of a single pixel was 7.69 +/- 0.13% at 5.89 keV and 7.27 +/- 1.18% at 30.1 keV, which exceeds the conservative baseline requirements of 10% resolution at 60 keV and is an encouraging start to an optimistic goal of<2% resolution at 60 keV. ... mehrThe digital output of the entire detector consistently yielded energy resolutions that exceeded 100% for different sources. The analog output of the monolithic silicon pixels indicates that thisis a promising technology for future gamma-ray missions, while the analysis of the digital output points to the need for a redesign of future photon-sensitive monolithic silicon pixel detectors.