F-center Luminescence Research Articles

The high-dose and high-temperature monitors of reactor irradiation based on insulators

Radiation-induced changes of the structure and properties have been investigated for oxide and nitride materials, and the use of high-temperature insulators as temperature/dose monitors for in-reactor irradiation of materials test assemblies has been validated.It has been experimentally shown that the use of Al2O3 single crystals and BN ceramics provides means of monitoring the temperature of irradiation from 370 to 1900 K. The temperature is derived from measurements of the optical absorption or X-ray diffraction line shifts after post-radiation annealing of the monitors. We discuss the applicability of (a) the optical absorption and F-center luminescence spectroscopies of irradiated Al2O3 single crystals for gamma dose evaluation and (b) the isotopic analysis of irradiated BN ceramics for neutron dose evaluation.

Development of a 2D dosimetry system based on the optically stimulated luminescence of Al2O3

A laser-scanning 2D dosimetry system based on the Optically Stimulated Luminescence (OSL) signal from Al2O3 films was built and demonstrated. The main challenge of using the OSL from Al2O3 for 2D dosimetry by laser scanning is the long lifetime (∼35 ms) of the main luminescence centers in this material (F-centers). In this work, we demonstrated the possibility of performing 2D dosimetry by laser scanning using a combination of the fast UV emission of F+-centers (lifetime <7 ns) and the slow F-center emission of Al2O3:C, and an algorithm to correct for the slow F-center luminescence lifetime. We also investigated the possibility of using Al2O3:C,Mg, to take advantage of its greater F+-center emission compared to Al2O3:C. Results from 6 MV photon beam irradiations from a clinical linear accelerator were compared to radiographic and radiochromic film profiles showing a good qualitative agreement.

Spectroscopy and optically stimulated luminescence of Al2O3:C using time-resolved measurements

This paper reports the observation of ultraviolet (UV) emission at 335nm in the optically stimulated luminescence (OSL) of carbon-doped aluminum oxide (Al2O3:C) and presents results on the investigation of the OSL properties of this band, including its dose response, time dependence after irradiation, and dependence of the OSL signal on the type of radiation. Time-resolved OSL measurements were used to separate the UV emission band from the dominant OSL emission band of Al2O3:C, namely, the F-center luminescence at 420nm. A comparison of the OSL properties of the UV and F-center emission bands is important for various dosimetric applications because the relative contribution of the UV and F-center emissions to the OSL signal varies with readout technique and optical filters used in the readout equipment. The UV emission band is found to show an ionization density dependence that is different from the dependence observed for the F-center emission, and an increase in intensity with time elapsed after beta irradiation. These results are relevant for OSL dosimetry of radiation fields containing heavy charged particles, such as the space radiation field and the secondary fields created by interactions of matter with energetic neutrons, as well as for understanding of the basic OSL mechanism in Al2O3:C.

Cathodic electrochemiluminescence at double barrier Al/Al 2O 3/Al/Al 2O 3 tunnel emission electrodes

Double insulating barrier tunnel emission electrodes were fabricated by adding a new pure aluminum layer upon oxidized aluminum electrodes by vacuum evaporation and thermally oxidizing the new aluminum layer in air at room temperature. Resulting Al/Al 2O 3/Al/Al 2O 3 electrodes allow the use of various aluminum alloys in the electrode body necessary for hardness or shaping ability of the electrode while obtaining the luminescence properties of pure aluminum oxide. During electrical excitation of luminescent labels by cathodic hot electron injection into aqueous electrolyte solution, the background noise is mainly based on high-field-induced solid-state electroluminescence and F-center luminescence of the outer aluminum oxide film. The more defect states and/or impurity centers the outer oxide film contains, the higher is the background emission intensity. The present electrode fabrication method provides a considerable improvement in signal-to-noise ratio for time-resolved electrochemiluminescence (TR-ECL) measurements when the original native oxide film of the electrode body contains luminescence centers displaying long-lived luminescence. The excellent performance of the present electrodes is demonstrated by extremely low-level detection of Tb(III) chelates, luminol, Pt(II) coproporphyrin and Tb(III) labels in an immunometric immunoassay by time-resolved electrochemiluminescence.

Mechanism of F-center luminescence in anion-defective aluminum oxide single crystals

New experimental data illustrating the effect of deep traps on the luminescence properties of anion-defective α-Al2O3 single crystals are presented. It was established that deep traps have electronic nature and their filling occurs through photoionization of F centers and is accompanied by F → F+-center conversion. Model concepts were developed that describe the luminescence mechanism in anion-defective aluminum oxide single crystals with inclusion of thermal ionization of the excited F-center states. The validity of the model was supported by experimental data obtained in a study of thermoluminescence, thermally stimulated exoelectron emission, and thermally stimulated electrical conductivity.

The interaction of deep traps in anion-defective α-Al 2O 3

This paper deals with the examination of thermoluminescence (TL) optical spectra (200–800 nm) for deep traps (DT) observed near 730 K (DT1) and 785 K (DT2) at β=2 K/s in dosimetric α-Al 2O 3 crystals. The DT1 spectrum consisted of two luminescence bands near 3.56 and 2.42 eV. The DT2 spectrum had one band near 2.95 eV. The 3.56 eV emission band was assigned to the F +-center luminescence and the 2.95 eV band to the F-center luminescence. The excitation spectra of the 2.42 eV emission band measured at RT had one peak at 4.11 eV. The intensity correlated with filling of deep traps. It was supposed that anion-defective corundum contained complex F-type centers. The state of those centers changed upon exposure to UV irradiation at a high temperature.

Thermal quenching of F-center luminescence in Al2O3:C

Using time-resolved photoluminescence spectroscopy we have measured the lifetime of the F-center luminescence from α-Al2O3:C. The measurements reveal a lifetime of 35–36 ms at room temperature, decreasing to &amp;lt;2 ms over the temperature range from 370 to 500 K. The decrease in the lifetime is shown to follow a classical Mott-Seitz dependence for thermal quenching of luminescence, with an activation energy W of ∼1.08±0.03 eV and a corresponding frequency factor ν of ∼1014 s−1. Similar values for the energy and frequency factor were also obtained from an analysis of thermoluminescence (TL) glow curves measured at different heating rates, when the TL is measured over a wavelength range corresponding to the F-center luminescence emission (centered at 420 nm). Furthermore, the parameters obtained were independent of the glow curve shape, the degree of trap filling, or the specific conditions under which the crystals were grown. This is interpreted as a demonstration that the well-known heating rate dependence of the TL from this material is a result of thermal quenching of the F-center emission. Whereas the thermal quenching parameters obtained from measurement of the luminescence lifetime and from the heating rate analysis of the TL glow curves are independent of the sample type, the degree of trap filling, and the heating or cooling rate, measurements of the photoluminescence intensity, induced by absorption of F-band light, were found to be dependent upon all of the above conditions. This difference in behavior is attributed to a phosphorescence signal from traps associated with the ∼265, ∼310, and ∼450 K TL peaks.

Quenching of cathodic electrogenerated F-center luminescence of aluminium oxide by lanthanide cations at the electrode/electrolyte interface

Energy transfer between aluminium oxide F-center and lanthanide cations at an oxide-covered aluminium electrode during the cathodic pulse-polarization of the electrode is investigated by means of Stern-Volmer luminescence quenching kinetics. TerbiumIII-specific extrinsic luminescence is observed while some other lanthanides are observed to quench the F-center luminescence. Different quenching efficiencies of the lanthanides are discussed to be dependent on the different energy acceptor characteristics of the tri- or divalent lanthanides.

Luminescence of sapphire and ruby induced by He and Ar ion irradiation

The luminescence spectra of sapphire(α-Al 2O 3) and ruby (α-Al 2O 3:Cr, Cr 2O 3 content 0.02%) induced by 200 keV He + and Ar + ion irradiation were measured at room temperature in the wavelength range of 250 to 450 nm, as a function of irradiation dose. The analysis of the measured spectra gave the existence of three main luminescence features in this wavelength region, namely F-center, F +-center and a band observed around 315 nm. α-Al 2O 3 under He + irradiation showed the growth and decay of F-center and F +-center with increase of irradiation dose. F-center appeared earlier than F +-center and reached its maximum intensity around 1 × 10 14 He +/cm 2 while F +-center reached its maximum around 1 × 10 15 He +/cm 2. It suggests that F-center is easier to be produced by ion irradiation than F +-center, and a concentration quenching on F-center luminescence occurred above 1 × 10 14 He +/cm 2. Furthermore, the F-center luminescence was strongly suppressed by the existence of impurity Cr. On the other hand, the luminescence band around 315 nm appeared in the early stage of irradiation and showed only its decay part. The dose dependent behavior was similar to that of Cr 3+ emission (R-line) in ruby in both He + and Ar + irradiation.

Relationship between OH- defect reorientation rates and the quenching of the F-center luminescence in alkali halides.

We extend in this work the previously obtained results that the presence of sufficient amounts of statistically distributed ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ defects in KCl drastically quench the F-center luminescence, photoconductivity, and excited state (${\mathit{F}}^{\mathrm{*}}$) lifetime \ensuremath{\tau}(${\mathit{F}}^{\mathrm{*}}$). The extension to three new host materials (KBr, RbCl, RbBr) will test whether the ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ reorientation rate (${\mathrm{\ensuremath{\tau}}}_{\mathrm{reor}}^{\mathrm{\ensuremath{-}}1}$) plays the decisive role in this strong ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ dipole ${\mathit{F}}^{\mathrm{*}}$-center interaction effect. While for the earlier studied host KCl above 10 K, the ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ reorientation rate is extremely high [${\mathrm{\ensuremath{\tau}}}_{\mathrm{reor}}^{\mathrm{\ensuremath{-}}1}$(${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$)g${10}^{10}$ ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$], KBr, RbCl, and RbBr have at 10 K dipolar reorientation times long compared to the radiative F-center lifetime, ${\mathrm{\ensuremath{\tau}}}_{\mathrm{reor}}$(${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$)\ensuremath{\gg}\ensuremath{\tau}(${\mathit{F}}^{\mathrm{*}}$). Under temperature increase, however, the ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ dipolar reorientation rates speed up drastically, so that one reaches the opposite condition ${\mathrm{\ensuremath{\tau}}}_{\mathrm{reor}}$(${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$)\ensuremath{\ll}\ensuremath{\tau}(${\mathit{F}}^{\mathrm{*}}$). We find indeed that in these materials, the F luminescence is nearly fully efficient and of long lifetime below 15 K but becomes quenched drastically and short in lifetime when increasing the ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ reorientation rate with the temperature. Quantitative analysis shows that the ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ reorientation time must be about four orders of magnitude shorter than the radiative ${\mathit{F}}^{\mathrm{*}}$ lifetime, in order to fully quench the F luminescence. In contrast to these statistically distributed F-center and ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ defect systems, aggregated pairs of both defects [${\mathit{F}}_{\mathit{H}}$(${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$) defects] show in all hosts, nearly independent of temperature, very strong luminescence quenching. All these results are analyzed and discussed within dynamical ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ reorientational models.

Picosecond ground state recovery of the F-center in RbCl, KCl and KBr

Abstract Optical pump-probe measurements on the F-center in RbCl, KCl and KBr show a ps ground state recovery, when a substantial number of F-aggregates is present. The observed de-excitation process becomes faster for a larger concentration of aggregate centers and is related to energy transfer from the excited state of the F-center to aggregate centers. This energy transfer occurs towards F2- and N-centers in KCl. Our results are in accordance with measurements of the quantum efficiency of the F-center luminescence and with transient absorption measurements.

Luminescence decay and electron traps in thermochemically reduced MgO.

We report an analysis of the decay curves for 2.3-eV F-center luminescence in several samples of thermochemically reduced MgO. Previous measurements have shown that the concentration of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ions in a sample determines the lifetime of the luminescence at 260 K. We show here that the decay tends to become approximately second order in a characteristic time \ensuremath{\tau}, which depends on the concentrations of traps in the crystal, especially ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ions. At room temperature \ensuremath{\tau} is smallest for the sample containing the largest relative ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$-ion concentration, whereas at 40 K the converse is found to be the case. These results are explained semiquantitatively in terms of a general theoretical model involving two kinds of electron traps with different thermal activation energies.

Effect of the spin-orbit coupling on the temperature dependence of a magnetic circular polarization of F-center luminescence

The temperature dependence of a magnetic circular polarization of F-center luminescence is calculated by using a simple vibronic model with the spin-orbit interaction. The effect of the spin-orbit coupling on the polarization is discussed. A smaller value for λ than estimated so far by us is preferable to account for the observed temperature variation of the polarization.

On the luminescence and absence of luminescence of F centers

A model, proposed originally by Dexter, Klick and Russell, is used to explain the occurence or non-occurence of F-center luminescence in ionic crystals, in terms of a simple two-state configuration coordinate diagram. In this model, which works for all known cases, luminescence is quenched by a competing non-radiative process whenever the intersection of the ground and excited state curves lies below the energy reached in absorption in a vertical (Franck-Condon) transition. The criterion for the occurence of luminescence is expressed as Λ < 14, where Λ (=excited-state lattice-relaxation energy/optical-absorption energy) is a parameter, related to the relative displacement of the two curves, which can be inferred from data on the temperature dependence of the F-band line width. Thus the possibility of observing luminescence can be predicted from optical absorption data alone. It is found emperically that Λ for alkali halides with rocksalt structure is independent of lattice parameter, and the observed dependence of Λ on the ratio of ionic radii in terms of ion-size effects. Values of Λ range from 0.009 for CsF to 0.831 for LiI; NaCl with Λ = 0.260 is a marginal case for luminescence.

Luminescence spectrum of the f-center in CaO

Measurements of the F-center luminescence spectrum in additively colored CaO show that previously reported bands at 1.8 and 2.5 eV are not related to transitions of this two-electron defect.

Temperature dependence of the decay time and quantum efficiency of F-center luminescence in KCl

Improved measurements of τ and η in the range 4.2–170°K reveal a temperature dependence of the radiative lifetime, τ R = τ η , consistent with recent Stark effect measurements.