We have fabricated a prototype imaging X-ray detector, consisting of Al/AlOx/Al/Nb superconductive tunnel junctions sputtered directly on a high purity Nb single crystal with an RRR≈15000. Such a detector in principle offers a high energy resolution, combined with a high quantum efficiency, a large detection area and imaging capabilities. The quasiparticle collection efficiency and consequently the X-ray response of the detector is still very poor due to the large thickness of the crystal (120 µm), but agrees with proximity theory. By irradiation with a focused electron beam of a Low Temperature Scanning Electron Microscope, the spatial and temporal response of the detector has been determined. The derived diffusion coefficient is two orders of magnitude smaller than expected on the basis of the RRR.