A. Shmilevich

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 Ali+ ions. Prolonged UV- illumination with F-light caused a notable increase in the PL yield in the C-doped samples. Irradiation into the Ali+ absorption band at 300 nm had a reverse effect and was also efficient for the optical bleaching of the phototransferred TL (PTTL).

Study of optical and dosimetric properties of doped fluoride crystals

Abstract Optical properties and radiation effects of various fluoride crystals and their possible application to solid state dosimetry were studied. In particular optical absorption, PL, OSL, TL and OSTL of LiKYF 5 and LiKYF 5 :Pr 3+ crystals were measured and compared to those of pure and Eu 3+ -doped LiF. After X- or β-irradiation of LiKYF 5 :Pr 3+ a strong TL was recorded. In samples irradiated at RT also an OSTL could be stimulated at LNT; the stimulation maximum for the OSTL was at 442 nm. The traps responsible for the TL were stable to about 600 K. In LiF: Eu 3+ the TL could also be excited by VUV radiation. The VUV excited TL had excitation maxima near 125 and 190 nm. An OSTL emission was recorded in pure irradiated LiF crystals; and had an excitation maximum at 190 nm, indicating that the TL in the doped samples and the OSTL in the pure crystals are due to the same trapping states. Prolonged illumination into the F-band caused in LiF a decrease of the M-luminescence and an increase of the R + -emission, which is apparently due to an M to R conversion. The dose dependence of the TL in LiKYF 5 :Pr 3+ was found to be linear for β doses to above 2 kGy. The TL sensitivity of the powdered LiF:Eu 3+ samples was an order of magnitude greater than that of pure LiF crystals, but by an order of magnitude lower than that of the known efficient LiF:Mg Ti (TLD-100) phosphor. The sensitivity of LiKYF 5 :Pr 3+ crystals were of the order of the TLD-100 phosphor.

Irradiation effects in LiF:Eu+3

Abstract Effects of X, β and VUV irradiations on LiF:Eu3+ were studied and compared to those induced in nominally pure LiF. In the pure crystals thermoluminescence (TL) could be excited by X- or β-rays; in LiF:Eu3+ the TL could also be excited by VUV radiation with excitation maxima at 190 nm and near 125 nm. The 190 nm maximum coincided with a weak absorption band, recorded in the irradiated crystals. A strong optically stimulated thermoluminescence (OSTL) also appeared in pure crystals, which had previously been X- or β-irradiated at RT and then illuminated at LNT with 190 nm. The fact that the OSTL and the TL showed the same excitation maxima in both crystals, indicates that they are due to the same trapping states. Different emission bands were, however, recorded by the same excitation in LiF:Eu3+ and pure LiF samples. Results have shown that the radiation sensitivity of the Eu3+ doped samples is by an order of magnitude higher than that of pure LiF crystals but by about an order of magnitude lower than that of LiF:Mg,Ti (TLD100).

Luminescence of BaFCI: Eu2+ and SrFCI : Eu2+

Abstract The TL, XL, PTTL and OSL of BaFCI: Eu2+ and SrFCI : Eu2+ were studied. The PTTL stimulation spectra of BaFCI : Eu+ had maxima at the Eu3+, Eu2+ absorption bands and at the long wavelength tail of Fabsorption. The PTTL appeared at the same temperatures and with the same thermal activation energies as the TL peaks, indicating that the same defects are responsible for the trapping states. The TL emission spectra of the BaFCI: Eu2+ crystals showed narrow bands at 315 and 365 nm and a broader band near 435 nm. The OSL and XL showed the same bands as well a 385 nm emission. The 365 nm band also appeared in SrFCI: Eu2+ and is attributed to the emission of Eu2+.

Effects of predose treatment on the thermoluminescence in synthetic quartz

Abstract The effects of thermal annealing and of exposure to high β doses as well as of the pre-dose treatment on the TL sensitivity were studied separately for the main 370 and 460 nm emission bands. These bands were differently influenced by the treatments. The thermal activation energies of the main TL peaks were the same for the two emission bands, indicating that the carriers are thermally released from the same traps. The observed changes in the TL intensities are attributed to phase transitions of the crystal.

Sensitization and desensitization of the luminescence yield of Al2O3: C

Abstract The effects of optical and thermal treatments on the luminescence and thermo-luminescence efficiencies were investigated in α-Al2O3: C crystals. Emission bands appeared at 500, 410 and 330 nm with excitation maxima near 300, 200 and 260 nm respectively. These bands are ascribed to the emission of interstitial Al+ ions, F and F+ centers. Prolonged UV illumination with 200-220 nm light caused a notable increase in the luminescence yield of the 410 and 330 nm luminescence bands, while the illumination with 300 nm had a reverse effect on these bands. Wavelengths of 260 and 300 nm were most efficient for the optical bleaching of the PTTL.

Studies of excitation, optical bleaching and thermal annealing of OSL in natural quartz

Optically stimulated luminescence (OSL) was excited at LNT by monochromatic light in x-irradiated natural quartz crystals. Excitation maxima appeared at 290 and 310 nm. The phototransferred thermoluminescence (PTTL) of these crystals showed excitation maxima at the same wavelengths, indicating that carriers are optically stimulated from the same traps. This is supported by the finding that the same wavelengths are most efficient for optical bleaching caused by prolonged illumination. The effects of thermal annealing to gradually increasing temperatures on the OSL were investigated and a correlation to thermoluminescence (TL) peaks was found. The OSL intensities showed sharp decreases after annealing to 160, 190 and 280 K; these temperatures coincide with the temperatures of the main TL glow peaks. It appears therefore that the OSL and the main TL peaks between liquid nitrogen temperature (LNT) and room temperature (RT) are due to the same donor levels. The emission spectrum of the OSL showed that this emission is composed of some bands, which appear also in PTTL as well as in x- and photoluminescence, indicating that the same luminescence centres are responsible for these emissions.