A.G. Kozakiewicz

Defect clusters and thermoluminescence in LiF crystals

The thermoluminescence of pure and doped LiF crystals has been measured following irradiation at ambient temperature with gamma and x rays. Factors investigated include the effects of dopants (Mg, Mg/Ti, and Fe) and of dose. Results are compared with optical absorption and photoluminescence measurements made previously on the same crystals. The role of F, F3+, Z2, and colloids in the emission process is discussed. Some new effects are reported including step-like features in the glow curves at large doses. These are explained in terms of the clustering of interstitial defects formed during radiolysis.

Thermoluminescence in pure LiF crystals: Glow peaks and their connection with color centers

Nominally pure LiF crystals were irradiated with the same dose (0.85 106 R) of gamma rays at ambient and low temperatures (−60 °C) and the resulting thermoluminescence (TL) is reported. Various optical and thermal treatments were applied in order to change the concentration of color centers (CCs). The effect of such treatments on the glow curves is observed. Knowing the coloration from optical transmission and photoluminescence measurements made on the same samples, we attribute many of the glow peaks (GPs) to the annealing of F center aggregates. For the present conditions of irradiation and dose, TL processes begin with decay of F3+ centers that display a GP at 164 °C. F3(R) centers follow and are responsible for GPs at 193 and 228 °C. A GP at 263 °C is ascribed to F2 centers. Several peaks at temperatures in the range of 280–380 °C are associated with impurity perturbed F centers. A GP at 410 °C is associated with a complex of aggregated F and H centers. These attributions are accomplished by means of ...

Photoluminescence and the thermal stability of color centers in γ-irradiated LiF and LiF(Mg)

Pure and magnesium doped (2000 ppm) LiF crystals have been irradiated with γ rays from 241Am and 60Co sources and the thermal stability of the resulting defects has been studied using photoluminescence (PL) and optical absorption (OA). The composite nature of the M band at 440 nm is highlighted by PL data which reveal emission attributed to F3+ defects otherwise hidden in OA measurements. The following bands appear in excitation spectra; Z3 (222 nm), Z2 (285 nm), R1 (329 nm), R2 (375 nm), F3+ (440 nm), and F2 (440 nm). Notable differences are observed between pure and doped samples during thermal annealing relating to the F2 and F3+ bands in particular. Results support the idea of Z centers as being F and F− centers trapped at impurity ions in the crystal.

Effects of bleaching on the thermoluminescence of gamma-irradiated LiF crystals

Abstract Light emitted as a function of temperature by crystalline materials, which have been exposed to ionizing radiation, is commonly known as thermoluminescence. Its structured shape, which is often resolved in narrow bands called glow peaks, is related to the material, the impurities and the defect centers generated by the irradiation. Several efforts have been made to associate these peaks to the defect centers, but up to now in the majority of the cases and in particular in LiF, there is not any certain attribution. In order to overcome this situation, pure LiF samples have been irradiated with gamma rays and treated thermally and optically in order to produce samples with different known concentrations of color centers. By comparing the glow curves from ambient temperature to 450°C, and optical absorption and emission spectra in the visible region, it has been possible to establish a link between F3+ centers and the low temperature region of the glow curve below about 200°C.

Spectroscopic measurements and thermoluminescence of γ-ray colored LiF crystals

Abstract Light produced when heating materials which have been previously exposed to ionizing radiation is commonly known as thermoluminescence (TL). The structure of TL glow curves is characteristic of the material and is influenced by the impurities it contains as well as the type of defect centers generated by the irradiation. An extensive body of knowledge exists concerning TL. In the case of LiF, several efforts have been made to associate glow peaks with defect centers responsible for optical absorption, but up to now there is no certain attribution. In this work nominally pure LiF samples have been irradiated with γ rays, then treated thermally and optically to change the concentration of electron color centers in a known way, and the resulting TL has been studied in conjunction with optical absorption. By comparing the glow curves from ambient temperature to 450 °C and the optical absorption in the purple-blue spectral region, it has been possible to establish a link between F 3 + color centers and the low temperature region of the glow curve below 200 °C. Accurate spectroscopic measurements and theoretical fittings of the absorption spectrum in the UV–VIS spectral region have also been made in order to ascertain the possible role of other aggregate color centers in TL.

The colouration of CaF 2 crystals by keV AND GeV ions

CaF 2 crystals have been implanted with a variety of ions of widely different energies and mass. Effects have been monitored using optical absorption in the range 120-750 v nm. This includes the vacuum UV region. For 100 v keV ions (Al, Mg, Kr) we observe extrinsic colloid bands in the case of implanted metal ions at high fluences (10 17 v ions v cm 2 ) but no colour centres (F, F 2 etc). For GeV ions (U, Ni) we observe prominent absorption bands in the visible region at fluences of 10 12 v ions v cm m 2 attributed to extrinsic calcium colloids. New optical features are discussed including an absorption band near 185 v nm in the VUV and bands at 604 v nm and 672 v nm in the visible region.

Fast heavy ion induced VUV absorption in LiF crystals

Abstract LiF crystals were irradiated with various heavy ions (Ni, Zn, Au, Pb and U) with a specific energy 2–11 MeV/u and a fluence between 1010 and 1012 ions/cm2. Spectroscopic absorption studies were performed in the wavelength range 100–700 nm. In all investigated crystals, the most dominant absorption bands correspond to F-centers (maximum at 248 nm) and F2 centers (445 nm) and the complementary V3-hole center (114 nm). In crystals irradiated with the heaviest projectiles (Au, Pb, Bi and U ions), a new prominent absorption band appears at 121 nm. This peak is absent for irradiations with lighter ions and with Co-60 gamma-rays. Under optical bleaching in the F-center absorption region (∼250 nm), F- and V3-centers are completely destroyed whereas the 121 nm absorption band is stable. This absorption band is thought to be directly linked to damage creation in the core of the track as evidenced earlier by chemical etching and small-angle X-ray scattering experiments. The damage in this core region is created only if the energy loss of the projectiles exceeds a critical value of 10 keV/nm. The nature of the 121 nm band is discussed.

The effect of pre-irradiation annealing on TL glow curves of LiF (Mg)

Abstract The practice of pre-annealing dosimeter crystals before irradiation is examined in this investigation. LiF crystals with high (2000 ppm) and low (100 ppm) concentrations of Mg impurities are investigated using optical absorption (OA), photoluminescence (PL) and thermoluminescence (TL) techniques. Pre-annealing was at 400°C for 1 h and irradiations were with γ-rays from an Am 241 source in the dose range 0.6–8.0 kGy. In 2000 ppm samples pre-annealing produces noticeable effects at low doses, namely enhanced optical absorption at 310 nm compared with untreated crystals but reduced intensity in the glow curves. At higher doses, the glow curves are similar but pre-annealed samples have a significantly smaller TL response. Pre-annealing 100 ppm samples produces smaller changes. Some implications for thermoluminescences are considered. The present results can be explained by considering the effects of pre-irradiation annealing on the distribution of Mg which acts as a trapping centre for interstitial fluorine atoms.

The colouration by 60 Co-gamma rays of NaCl and KCl crystals doped with europium ions

Pure and europium doped NaCl and KCl crystals have been irradiated with 60 Co gamma rays and with ultraviolet light. The dose was 10 v kGy and temperatures during irradiation were ambient and liquid nitrogen. The effects of irradiation are determined using optical absorption and thermoluminescence. The role of the europium dopant is compared for the two materials. Factors investigated include the temperature of irradiation, the concentration of the dopant and the state of impurity aggregation and precipitation. A link is indicated between impurity precipitates and UV stimulated thermoluminescence.

VUV optical absorption in sapphire crystals following implantation with 100 keV O+, Al+ and Cr+ Ions

Abstract We report optical absorption (OA) measurements in Al2O3 crystals following implantation with O+, Al+ and Cr+ ions of 100 keV energy. Fluences were between 4×1015 and 1×1017 ions/cm2 and the wavelength range was 150–350 nm. The vacuum ultraviolet region is included in our investigation. In addition to the F band near 204 nm (6.08 eV) we observe ion specific effects at shorter wavelengths. This is attributed to a different ability of the projectile ions to form stable F and F+ centres when implanted near ambient temperature. Implantation at a higher temperature of 200 °C results in enhanced and similar OA at 200 nm in the case of Al+ and Cr+ ions.