Neutrino oscillation experiments have shown that neutrinos must have a finite rest mass, but are unable to put a number on how massive they are. The COBRA experiment makes use of the expected neutrinoless double beta decay (0-decay) to directly determine the effective Majorana mass of the electron neutrino. This mass can be derived from a measurement of the half-life of the 0-decay using Cadmium Zinc Telluride (CZT) detectors in CoPlanar Grid (CPG) configuration. As a part of the detector investigations the charge collection efficiency (CCE) must be determined carefully. It is known, that the energy resolution and the detection efficiency of high volume CZT detectors is strongly dependent on the interaction depth of the photons within the crystal. This effect becomes more important the closer to the anodes the interaction happens. In order to characterize this position dependent detection efficiency of the COBRA-CZT-CPG detectors, a highly collimated gamma ray source is used to perform linescans along the z-axis. This first experiment describes the basics on the upcoming three dimensional determination of the CCE.