When fast ions are incident on a silicon nuclear radiation detector, the current pulses observed with high bandwidth electronics can show marked features which depend on materials inhomogeneities in the silicon from which the detector is made. This paper describes how an attempt to develop an atomic number pulse shape discrimination technique for fission fragments led instead to an investigation of the materials effects in the surface barrier detectors which were being used. Results obtained with alpha particles and accelerator-produced heavy ions scanned across a detector face are correlated with each other and with laser photoresponse studies which were also made. The effects in such measurements which spatial resistivity and thickness changes can produce are analysed in particular detail. For the two main detectors studied the evidence all indicates that in their cases resistivity inhomogeneities were responsible for the results observed.