Silicon surface barrier detectors are shown to exhibit a "pulse height defect" for a low energy Al 27 and Mg 24 ions. The ions were produced inside the detector as the recoils of the Si 28 (γ, p) and (γ, α) reactions by monochromatic γ-rays of 17.6 MeV. The pulse height defect ranges from 70 to 152 keV for Al 27 ions of 91 keV and 225 keV respectively, while for Mg 24 ions it is 138 keV and 145 keV for energies of 910 keV and 1105 keV. It is temperature independent (from -70°C to +20°C) and does not vary when the electric field is varied by a factor 2. The evidence suggests that the effect is due to a reduction of the efficiency in the electron-hole production when the ions are sufficiently slowed down.