Browsing by Author "Townsend, PD"

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    Comparisons of tin depth profile analysis in float glass
    Townsend, PD; Can, N; Chandler, PJ; Farmery, BW; Lopez-Heredero, R; Peto, A; Salvin, L; Underdown, D; Yang, B
    Data are presented showing the profile of tin diffusion during the production of float glass, by measuring non-destructively the refractive index profiles in the diffused layer. The optical waveguide modes give unequivocal evidence for an anomaly in the tin depth distribution. The results are compared with those from sectioning techniques, used in depth profiles determined by ion beam analyses and cathodoluminescence (CL). There is agreement between these methods which confirm the presence of a maximum in the tin concentration below the surface which had been in contact with the tin bath (this had been linked by Mossbauer data to a rise in the Sn4+ concentration). The ion beam analyses record different depth profiles for Si, Na and Ca. The Sn4+ feature increases the refractive index, as does the diffusion of Sn2+. The index becomes constant at large tin concentrations. We suggest that Sn4+ is linked to CL emission at 2.68 eV and Sn2+ to the 1.97 eV CL emission. Iron impurities give a 1.73 eV signal. Contrary to earlier suggestions, we prc,pose that the luminescence associated with the presence of tin arises from intrinsic defects stabilised by the tin, not from tin acting directly as a luminescence site. (C) 1998 Elsevier Science B.V.
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    Radioluminescence and thermoluminescence of albite at low temperature
    Can, N; Garcia-Guinea, J; Kibar, R; Çetin, A; Ayvacikli, M; Townsend, PD
    Feldspar as an archaeological and geological natural material for dating and retrospective dosimetry is receiving more and more attention because of its useful luminescence properties. In this study, the 25-280 K thermoluminescence (TL) and radioluminescence (RL) spectra in albite, which is a component of the two main feldspar series, the alkali feldspar (Na, K)AlSi(3)O(8) and the plagioclases (NaAlSi(3)O(8)-CaAl(2)Si(2)O(8)) have been presented for aliquots along (001) and (010) crystallographic orientations. There are four main emission bands that are considered to arise from complexes of intrinsic defects linked in larger complexes with impurities such as Na(+), Mn(2+) or Fe(3+) ions. The consequence of their association is to produce different luminescence efficiencies that produce wavelength sensitive TL curves. Radioluminescence data at low temperature for albites is distorted by contributions from the TL sites, even when the RL is run in a cooling cycle. This indicates the potential for a far more general problem for analysis of low temperature RL in insulating materials. (C) 2011 Elsevier Ltd. All rights reserved.
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    TL and EPR studies of Cu, Ag and P doped Li2B4O7 phosphor
    Can, N; Karali, T; Townsend, PD; Yildiz, F
    Key characteristics of a newly prepared tissue-equivalent, highly sensitive thermoluminescence dosimeter, Li2B4O7: Cu, Ag, P, are presented. The material was developed at the Institute of Nuclear Sciences, Belgrade, in the form of sintered pellets. A new preparation procedure has greatly increased the sensitivity of the basic copper activated lithium borate and the glow curve of Li2B4O7 : Cu, Ag, P consists of a well-defined main dosimetric peak situated at about 460-465K with a sensitivity which is about four to five times higher than that of LiF : Mg, Ti (TLD-100). The exceptionally good response features of Li2B4O7 : Cu, Ag, P are attributed to the incorporation of Cu as a dopant. Both low and high temperature emission spectra are presented and the origins of the various emission bands are considered. Additional data are provided from electron paramagnetic resonance measurements.
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    Influence of Li dopants on thermoluminescence spectra of CaSO4 doped with Dy or Tm
    Wang, Y; Can, N; Townsend, PD
    Thermoluminescence emission spectra are presented for lithium doped variants of CaSO4:Dy or CaSO4:Tm dosimetry material. All three dopants (Li, Dy and Tm) variously introduce different changes in both the glow peak temperatures and the luminescence efficiency. In every case the emission signals display the line emission characteristic of the rare earth ions. At temperatures below similar to 50 K the relative peak intensities differ for Dy and Tm doped samples, and there are small temperature shifts between the Dy:Li and Tm:Li co-doped materials. Above room temperature the rare earth ions do not show peak temperature movements when co-doped with lithium. However they do influence the peak temperature by similar to 5 degrees C when they are the sole dopant. Inclusion of lithium dramatically moves the high temperature glow peak from similar to 200 degrees C down to 120 degrees C. All these changes are consistent with a single defect model in which the trapping sites and luminescence occur within the complexes formed of the rare earth ion, an intrinsic sulphate defect and lithium. The evidence and rationale for such a model are presented. There is discussion which suggests that such defect complexes are the norm in thermoluminescence. (C) 2011 Elsevier B.V. All rights reserved.
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    Ionoluminescence of silicates for ceramic uses at cryogenic temperatures
    Garcia-Guinea, J; Hole, D; Finch, A; Correcher, V; Valle-Fuentes, FJ; Can, N; Townsend, PD
    Requirements on accurate physical properties need new characterization techniques for structural defects. The use of light ions such as H+ and He+ with MeV energies allows penetration to a depth of several microns in most materials producing strong luminescence to gain information on the defect structures present. Examples of ionoluminescence spectra of silicates for ceramic uses are included such as Mn2+-Fe2+ coupled pairs in quartz, Dy3+ in zircon, hatch-twinning in microcline and spontaneous strain and sodium in albite. In cryogenic thermal conditions tectosilicates display both, reorganization of Mn2+ and Fe3+ centres plus a large enlargement of the UV-blue emissions by stress mechanisms. Nesosilicates, i.e., zircon, shows: differences such as existence of REE and (Dy3+) and a selective reduction of some UV-blue peaks, e.g., at 340 and 400 nm..
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    Optical propel-ties of Tb implantation into ZnO
    Çetin, A; Kibar, R; Ayvacikli, K; Tuncer, Y; Buchal, C; Townsend, PD; Karali, T; Selvi, S; Can, N
    ZnO [0001] single crystals were implanted at room temperature with 400 keV Tb+ ions at fluences in the range of 1 x 10(16) -2 x 10(17) ions/cm(2). Zinc oxide was chosen because of its potential for photonic applications as a semiconductor with high radiation resistance. After implantation and post-irradiation annealing, optical absorption was measured in a UV-VIS-NIR range and radioluminescence spectra were recorded at room temperature. Emission signals were generated by the Tb+ ion implants and intrinsic emission of the ZnO matrix were observed. The implant signal intensities were comparable with the host radioluminescence, even though the implants modify the surface of the crystal. It is suggested that the presence of Tb at high concentration generates stresses which influence the bulk material and also potentially forms precipitates or nanoparticles in the near surface region. Overall ion implantation of ZnO results in strongly modified luminescence. (C) 2007 Published by Elsevier B.V.
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    Luminescence of levitated Zr-Ba-La-Al-Na fluoride glass
    Yang, B; Townsend, PD; Can, N; Janke, A; Baniel, P; Blanc, O; Granier, J
    Radioluminescence, cathodoluminescence, and thermoluminescence of Zr-Ba-La-Al-Na (ZBLAN) fluoride glass reveal different aspects of the intrinsic imperfections and impurities which control the emission. Except for weak low-temperature thermoluminescence signals which could be attributed to rare-earth impurities, the majority of the emission bands were common to all samples. These are considered to arise from intrinsic electron-hole recombination processes, either during relaxed exciton emission or at intrinsic defect sites in the glass network. Deconvolution of the spectra indicates that six emission bands occur in all the samples. Production of crystallite inclusions or intrinsic defects enhance the signal intensities. The luminescence is shown to be a useful probe of the quality of the ZBLAN glass, both in the preform stage, and after fiber drawing, even when working with the very high grade material produced by gas film levitation growth.
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    Luminescence properties of Tb implanted ZnO
    Cetin, A; Kibar, R; Selvi, S; Townsend, PD; Can, N
    ZnO [0 0 0 1] crystals were irradiated at room temperature with Tb+ ions of 400 keV with fluences from 1 x 10(16) to 2 x 10(17) cm(-2). The implanted layer was examined by several methods, including radioluminescence (RL), Rutherford backscattering spectrometry (RBS) and optical spectroscopy. The optical extinction spectra were simulated using Mie scattering theory. Absorption spectra predicted by Mie theory for particles of decreasing diameter were compared with those obtained experimentally. Some qualitative agreement between theoretical and experimental data was achieved. It was also shown that the intensities of the characteristic green emission bands associated with Tb produced by D-5(4) -> F-7(j=5,4) transitions have increased about 8 times after annealing. Optical spectroscopy and radioluminescence data have revealed that the ion implantation is a promising tool for synthesizing Tb nanoparticles in the ZnO surface. The Tb nanoparticles exhibit a rather weak plasma resonance. (C) 2009 Elsevier B.V. All rights reserved.
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    Thermoluminescence of Ce and Nd co-doped CaF2 phosphors after beta irradiation
    Yazan, H; Portakal-Uçar, ZG; Akça, S; Topaksu, M; Townsend, PD; Can, N
    In this study, we have synthesized CaF2:Nd3+,Ce3+ phosphors by solid state reaction method. The thermoluminescence (TL) properties of the family of synthesized phosphors (CaF2:Ce3+ and CaF2:Ce3+,Nd3+) were investigated using a combination of a SCHOTT BG39 and HC414/46 filters. This gave the highest TL intensity of each phosphor after 10 Gy beta-irradiation. Two TL glow peaks of CaF2:Nd3+,Ce3+ phosphor are centered at around 90 and 265 degrees C (with a heating rate of 2 degrees Cs-1). The glow peaks displayed a linear dose response in the range of 0.1-50 Gy. Various heating rates (VHR) were performed between 0.5 and 10 degrees Cs-1. It was seen that the TL intensity at 90 degrees C decreases and peak maximum temperature shifts to higher temperatures with an increasing heating rate. The TL intensity of the glow peak located at 265 degrees C increases with heating rate. The initial rise (IR) applied after T-M-T-stop experiment and computerized glow curve deconvolution (CGCD) methods in addition to the VHR method on the glow curves of CaF2:Nd3+,Ce3+ phosphor were used to determine the number of peaks, the order of kinetics (b), activation energy (E) and frequency factor (s). The response of TL glow curves remained constant within +2% deviation from the initial value after 10 cycles of reuse.
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    Radioluminescence and thermoluminescence of rare earth element and phosphorus-doped zircon
    Karali, T; Can, N; Townsend, PD; Rowlands, AP; Hanchar, JM
    The radioluminescence and thermoluminescence spectra of synthetic zircon crystals doped with individual trivalent rare earth element (REE) ions (Pr, Sm, Eu, Gd, Dy, Ho Er, and Yb) and P are reported in the temperature range 25 to 673 K. Although there is some intrinsic UV/blue emission from the host lattice, the dominant signals are from the rare-earth sites, with signals characteristic of the REE3+ states. The shapes of the glow curves are different for each dopant, and there are distinct differences between glow peak temperatures for different rare-earth lines of the same element. Within the overall set of signals there are indications of linear trends in which some glow peak temperatures vary as a function of the ionic size of the rare earth ions. The temperature shifts of the peaks are considerable, up to 200 degrees, and much larger than those cited in other rare-earth-doped crystals of LaF3 and Bi4Ge3O12. The data clearly suggest that the rare-earth ions are active both in the trapping and luminescence steps, and hence the TL occurs within localized defect complexes that include REE3+ ions.
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    Cathodoluminescence spectra recorded from surfaces of solids with hydrous molecules
    Garcia-Guinea, J; Correcher, V; Can, N; Garrido, F; Townsend, PD
    Modern cathodoluminescence detectors coupled to Environmental Scanning Electron Microscopes (ESEM) with X-ray microanalysis are increasingly sensitive and, nowadays, low intense luminescence spectra may be recorded from nominally non-luminescent solid materials. Structural or attached hydroxyl groups are universally present in solid samples kept in environmental conditions being potentially active luminescence emitters. Hence, we collected cathodoluminescence (CL) spectra of sixty solid materials of minerals and synthetic compounds, organics and inorganics, metallic and non-metallic, crystalline and amorphous, including widespread CL bands at circa 315 nm and 620 and 650 nm from surfaces of solids with hydroxyl and hydrous molecules. We observed as iron bearing materials, such as biotite, olivine or native iron, show appreciable cathodoluminescence spectral bands associated with hydroxyl groups (315 nm) and water molecules (620 and 650 mn), being probable precursors of non-bridging oxygen hole centers. The wide set of very different samples analyzed shows spectral similarities among separated sample types but with the only common factor of the hydrous molecules. These conclusions seem crucial for remote sensing detection of water; environmental dosimetry; degradation of phosphor materials, etc. Analyzing spectral luminescence emissions of hydrated materials it is frequent to observe erroneous interpretations of their defect-emission relationships which should be avoided.
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    Optical properties and luminescence of metallic nanoclusters in ZnO:Cu
    Karali, T; Can, N; Valberg, L; Stepanov, AL; Townsend, PD; Buchal, C; Ganeev, RA; Ryasnyansky, AI; Belik, HG; Jessett, ML; Ong, C
    Copper nanoparticles have been formed in a zinc oxide (ZnO) substrate by Cu implantation at 160 keV. The implant layer shows amorphised behaviour as monitored by the RBS data and optical absorption indicates the surface plasmon resonance peak characteristic of copper nanoparticles. There is nonlinear optical absorption from the Cu nanoparticles. There are numerous changes in the luminescence responses which include a variety of green and yellow emission bands. During excitation there were changes in the cathodoluminescence intensity and lifetime influenced by the surface events, including some unusual features, which may be considered in terms of interactions with surface water and ice. Radioluminescence signals oscillate between the normal emission bands and the wide red line emission from free Zn. Overall ZnO is, therefore, an unexpectedly interesting host for dispersed Cu nanoparticles, not least because the optical features may be compatible with the semiconductor aspects of ZnO. (c) 2005 Elsevier B.V. All rights reserved.
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    On the spectra luminescence properties of charoite silicate
    Garcia-Guinea, J; Townsend, PD; Can, N; Correcher, V; Sanchez-Muñoz, L; Finch, AA; Hole, D; Avella, M; Jimenez, J; Khanlary, M
    Charoite is a hydrous alkali calcium silicate mineral [K4NaCa7Ba0.75Mn0.2Fe0.05(Si6O15)(2)(Si2O7)Si4O9(OH) center dot 3(H2O)] exhibiting an intense lilac colour related to Mn2+ and Fe3+ colour centres. These ions also contribute to a strong luminescence at similar to 585 and 705 nm. This work studies the thermal dependence of these luminescent centres by (i) thermoluminescence (TL) of pre-heated and pre-irradiated charoite aliquots, (ii) by time-resolved cathodoluminescence (TRS-CL) at room and cryogenic temperatures (RT and CT), (iii) by spatially resolved spectra CL under scanning electron microscopy (SRS-CL-SEM) and (iv) by ion beam spectra luminescence (IBL) with H+, H-2(+) and He-4(+) ions at RT and LT. The main peak, similar to 585 nm, is linked to a transition T-4(1,2)(G)->(6)A(7)(S) in Mn2+ ions in distorted six-fold coordination and the emission at similar to 705 nm with Fe2+ -> Fe3+ oxidation in Si4+ lattice sites. Less intense UV-blue emissions at 340 and 390 nm show multi-order kinetic TL glow curves involving continuous processes of electron trapping and de-trapping along with an irreversible phase transition of charoite by de-hydroxylation and lattice shortening of Delta a = 0.219 angstrom, Delta b = 0.182 angstrom; Delta c = 0.739 angstrom. The Si-O stressed lattice of charoite has non-bridging oxygen or silicon vacancy-hole centres, and Si-O bonding defects which seem to be responsible for the 340 nm emission. Extrinsic defects such as the alkali (or hydrogen)-compensated [AlO4/M+] centres could be linked with the 390 nm emission. Large variations in 585 and 705 nm intensities are strongly temperature dependent, modifying local Fe-O and Mn-O bond distances, short-range-order luminescence centres being very resistant under the action of the heavy ion beam of He-4(+). The SRS-CL demonstrates strong spatial heterogeneity in the luminescence of the charoite. (C) 2007 Elsevier B.V. All rights reserved.
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    Room and Low Temperature Luminescence Properties of CaSO4:Dy,Tm Codoped with Li
    Can, N; Karali, T; Wang, Y; Townsend, PD; Prokic, M; Canimoglu, A
    Rare earths, especially Dy or Tm doped CaSO4 phosphors are actively studied. They have high sensitivity, a large dynamic range, thermal stability and ease of preparation. Nevertheless, they can be enhanced by inclusion of lithium and this study reports some effects of lithium co-dopant on the TL and radioluminescence (RL) emissions of two TL phosphors. Addition of Li as a co-dopant ion was made either during chemical preparation of the phosphors, or as a binder component mixed with the basic phosphors matrix during the process of pressing and sintering the TLD pellets.
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    Optical properties of Cu implanted ZnO
    Cetin, A; Kibar, R; Ayvaclkh, M; Can, N; Buchal, C; Townsend, PD; Stepanov, AL; Karah, T; Selvi, S
    Nanoparticles of Cu have been made in zinc oxide crystals by ion implantation. The Cu ions were implanted at 400 keV into the (0001) face of a single crystal. After implantation and after post-irradiation annealing there are numerous changes in the luminescence responses which include a variety of green and yellow emission bands. Following annealing at temperatures up to 1000 degrees C a green luminescence near 525 nm was observed which has been associated with the isolated Cu ions. The changes between as implanted and annealed luminescence signals suggests that the implants generate clustering or nanoparticle formation of Cu but anneals dissociate them. (c) 2006 Elsevier B.V. All rights reserved.
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    Structural and temperature-dependent luminescence of Terbium doped YAl3(BO3)4 phosphor synthesized by the combustion method
    Hakami, J; Kaynar, UH; Ayvacikli, M; Coban, MB; Garcia-Guinea, J; Townsend, PD; Oglakci, M; Can, N
    A series of Y1-xAl3(BO3)4:x Tb3+ (x = 0.5 to 7 wt%) phosphors synthesized by a gel combustion method have been systemically investigated by X-ray diffraction (XRD), Fourier transform infrared (FTIR), energy dispersive spectroscopy (EDS), and photoluminescence (PL) as a function of temperature from 300 K to 10 K and 300 K-550 K. An XRD analysis confirms that the phosphors crystallized, and its crystal structure was analysed. The synthesized phosphor matches the XRD pattern provided in the ICSD File No 96-152-6006. The FTIR analysis indicates that nitrates and organic matter have been completely removed and the BO3 groups are present. The broad PL band peaked at 420 nm with a shoulder circa 460 nm of YAl3(BO3)4 is associated with hydrous components which attached to the sample in environmental conditions after synthesis. The PL spectra of YAl3(BO3):Tb3+ phosphors exhibit a bright and narrow green main emission peak at 543 nm corresponding to the 5D4 -> 7F5 transition under 359 nm excitation. The PL intensity increases with increasing Tb3+ ion concentration up to 5 wt %, followed by evidence for concentration quenching. There is a possibility that higher concentration quenching could be from confinement effects of localised resonant energy transfer. PL data revealed that acti-vation energies for thermal quenching at 485 nm and 543 nm were found to be 0.659 and 0.092 eV, and 0.585 and 0.087 eV, respectively.
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    CATHODOLUMINESCENCE RESPONSE FROM RARE EARTH DOPED Bi4Ge3O12
    Kibar, R; Çetin, A; Tuncer, Y; Uysal, S; Townsend, PD; Canimoglu, A; Karali, T; Can, N
    Room and low temperature cathodoluminescence (CL) of rare earth doped Bi4Ge3O12 (BGO) has been recorded. Luminescence signals noted in the wavelength range (300 - 800 nm) include intrinsic broad emission bands and signals related to Eu3+, Nd3+, and Tm3+. CL measurements made on Bi4Ge3O12 (BGO) doped with rare earth ions are reported for the temperature range 40 to 300 K with different CL excitation modulation frequencies. Dopant levels used in the present study are 1.1, 0.4, and 0.3 wt% Nd, 0.4 wt% Tm and 3 ppm Eu. All dopant levels exhibited different CL spectra with evidence for lines due to the rare-earth dopants intra-4f transitions. The temperature dependence of the intensity of the emission band is discussed.
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    Luminescence characterisation of defects in pld alumina and copper implanted silica
    Wu, Z; Türkler, A; Brooks, R; Hole, DE; Townsend, PD; Köster, SF; Kurt, K; Gonzalo, J; Suarez-Garcia, A
    Luminescence is reported for alumina and Al2O3:Cu films grown by pulsed laser deposition and is contrasted with luminescence from Cu ion implanted silica. The implanted samples display numerous emission bands with at least two associated to charge states or the Cu ions. The relative band intensities are altered by thermal treatments. In the case of the thin films the signals are sensitive to the growth conditions and show evidence for trapped Ar nanoparticles, from argon used as a background gas during film growth. Thus in both thin film and ion implanted material the luminescence offers a route to monitor the state of the defects and the copper impurity ions. (C) 2002 Published by Elsevier Science B.V.
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    Indications of bulk property changes from surface ion implantation
    Wang, Y; Yang, B; Can, N; Townsend, PD
    In the majority of cases, the effects of ion implantation are confined close to the implant zone but, potentially, the resultant distortions and chemical modifications could catalyse relaxations extending into the bulk substrate. Such possibilities are rarely considered but the present data suggest that high dose ion implantation of ZnO has induced bulk changes. Surface implants with Cu and Tb strongly modified the low temperature bulk thermoluminescence properties generated by X-ray irradiation. Suggestions are proposed for the possible mechanisms for bulk relaxations and structural characteristics, which may indicate where such instability may occur in other lattice structures.