Birefringence Induced by Light in Potassium Chloride Crystals Coloured by Ionizing Radiation

A new anisotropic effect is oberserved in some potassium chloride crystals coloured by ionizing radiation. The coloured samples exhibit birefringence after illumination with ordinary or polarized light. By using suitable illumination conditions it is possible to obtain both uni‐ and biaxial crystals with optical directions parallel to the cube edges, and possessing either negative or positive optical signs. It is found that only light of wavelength between 625 and 750 nm is effective in producing the optical anisotropy and that the light undergoing double refraction lies in the same range of wavelengths. Some properties of the defects responsible for the birefringence and a possible mechanism of the effect are discussed.