An automated, computer-controlled scanning device to map the sensitivity of large-volume CdZnTe gamma detectors is described. It is of modular design and fully based on moderately priced commercial components. The collimated radiation source is moved in X- Y direction over the radiation detector under test using stepping motors. Synchronously, gamma spectra are taken for each X- Y position. Different regions of interest in a spectrum can be evaluated or re-evaluated and the measured gamma countrate (peak area) is displayed and documented in contour maps as a function of the X- Y coordinate. The system is meant for evaluation of new, large-volume room temperature semiconductor detectors and for quality-control measurements on detectors which are used in the Department of Safeguards for the verification of nuclear material.