Accurate measurements of the three-dimensional directional distribution of low energy ions in space require a careful calibration of the silicon surface barrier detectors mostly employed. This paper describes a systematic search for the pulse-height defects in the detector response to protons in the 25-150 keV energy range. The results ensured a proper pre-fiight calibration of the ISEE-3 proton spectrometer and also allow some conclusions on the ratio of the average energies % and %_ expended for electron-hole pair generation in silicon.