Radiation‐induced apparent optical absorption in the wavelength range 0.5–2.5 μ of thin silicon crystals, which were irradiated with 1.8‐MeV He+ ions, is interpreted in terms of scattering from defect zones. Fitting the absorption and refractive index data to the predictions of Mie scattering theory allows determination of the average size, number, and refractive index of the defect zones. For 1015–1016 ions cm−2 incident on a crystal 2.8 μ thick, this analysis indicated a zone radius of about 2200 Å, a refractive index for the zone of about 1.04 times that of undamaged silicon independent of wavelength, and ∼ 10−7 zones produced per incident ion. At larger fluence, the zone size increased and the number of zones decreased. Transmission‐electron‐microscopy (TEM) observations confirm the presence of defective zones whose geometry, size, and number density are in reasonable agreement with the theoretical predictions.