Conceptual Design of a Cosmic Ray Detector Operating Between 10¹7 and 10¹9 eV for the Study of the Galactic Center

For our understanding of the origin of ultra high energy cosmic rays, the energy region between 10^17 and 10^19 eV is of crucial importance. Previous experiments have found indirect evidence that at these energies, the origin of cosmic rays changes from predominantly Galactic to extragalactic. In addition, weak evidence for an excess of cosmic rays from the direction of the Galactic center in a narrow energy band around 10^18 eV has been claimed. However, so far there is no direct evidence supporting this scenario. Neither Galactic nor extragalactic sources have been unambiguously established. Given the importance of this energy range, there is a strong case for a dedicated experiment to study the EeV energy region with high precision. We present the conceptual design of GRaNDScan, a mobile stereo air fluorescence detector optimized to study the spectrum, composition, and arrival direction of cosmic rays in this important energy range. If located at a site on the southern hemisphere with good exposure to the Galactic center, this type of experiment will provide an accurate map of the Galactic center region, long suspected to harbor one or several sources of ultra high energy cosmic rays.