Radiation effects in pure and doped Al₂O₃ crystals

Effects of β-, X- and UV-radiation on optical properties of nominally pure as well as of variously doped α-Al₂O₃ crystals were studied. Optical absorption, thermoluminescence (TL) and photoluminescence (PL) were measured. The irradiation with wavelengths below 145 nm had essentially the same effects as X-irradiation, indicating that the same defects were formed in both cases. F and F⁺ emission bands at 410 and 330 nm appeared in the TL and in the PL of all samples. In crystals doped with rare-earth ions the narrow bands, characteristic for these impurities, were dominant. In some of the nominally pure samples also a strong Cr³⁺ band appeared at 698 nm. In the C-doped crystals the TL and PL emissions were relatively strong and dominated by the F and F⁺ emission bands. These results support previous suggestions regarding a high anion vacancy concentration in the C-doped crystals. A weak emission band appeared near 500 nm with an excitation maximum near 300 nm and is attributed to interstitial Al⁺_i ions. Prolonged UV-illumination with F-light caused a notable increase in the PL yield in the C-doped samples. Irradiation into the Al⁺_i absorption band at 300 nm had a reverse effect and was also efficient for the optical bleaching of the phototransferred TL (PTTL).