  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. The 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.
