Dipole defects in BeO

Thermally Stimulated Depolarization Currents (TSDC) technique was applied to study the effects of thermal treatments and gamma irradiation in the formation, aggregation and destruction processes of dipole defects in beryllium oxide (BeO). In this work, a TSDC dipole peak was observed at 350 K that was assigned to impurity-vacancy complexes. The TSDC peak presents a linear dependence with the Polarization Voltage (V_P), typical behaviour of defects with dipole origin. Thermal treatments of 1 h were performed between 400 and 900 °C and a decay in the peak intensity was observed. After the gamma irradiation of the thermal treated samples a restoration tendency of the TSDC peak was observed.     

Recombination of close frenkel defects in gold

The occurrence of close-vacancy-interstitial-pairs in electron-irradiated gold and the annealing temperature of this type of defect have been investigated by means of “sub-threshold” irradiations. Different samples have been irradiated with 3 MeV electrons at five different temperatures between 7.5°K and 36°K. These samples containing Frenkel defects have then been irradiated at 12°K with 1.2 MeV electrons. This energy is less than the threshold for defect production. During the sub-threshold irradiation, defect annihilation rates have been measured which depend strongly on the temperature at which the defects had been produced. For 7.5°K and 12°K they are of the same order of magnitude as were found in Al, Cu, Pt, and Ta, and were ascribed to recombination of closevacancy-interstitial-pairs. For defects produced at 22°K the annihilation rate is smaller by one order of magnitude and even smaller for defects produced at still higher temperatures. It is concluded from this that close-pairs become thermally unstable in gold between 12 and 22°K.     

Paramagnetic frenkel defects in irradiated alkali-haloid crystals

In the EPR spectra of γ-irradiated NaF,⁶LiF, and LiF crystals with natural content of isotopes (independent of the impurity composition), the hyperfine structure (HFS) is observed against the background of a broad band. Absorption saturation in the system of defects responsible for the HFS and the broadband occurs at widely different power levels of microwave radiation, and broad band suppression takes place at registration in quadrature. The experimentally measured intensity distribution and the number of EPR lines in the⁶LiF crystal correlate with the calculated data when the spin interaction of an unpaired electron with 14 equaivalent fluorine nuclei is taken into account. A model of major radiation-induced paramagnetic defects in the form of Frenkel pairs, in which one component (the negatively charged quasi-molecule consisting of two halogen atoms) can be responsible for the HFS and the other component (F-center) for the broad band in the EPR spectrum, is proposed.     

Stimulated luminescence emission from localized recombination in randomly distributed defects

We present a new kinetic model describing localized electronic recombination through the excited state of the donor (d) to an acceptor (a) centre in luminescent materials. In contrast to the existing models based on the localized transition model (LTM) of Halperin and Braner (1960 Phys. Rev. 117 408-15) which assumes a fixed d?→?a tunnelling probability for the entire crystal, our model is based on nearest-neighbour recombination within randomly distributed centres. Such a random distribution can occur through the entire volume or within the defect complexes of the dosimeter, and implies that the tunnelling probability varies with the donor-acceptor (d-a) separation distance. We first develop an 'exact kinetic model' that incorporates this variation in tunnelling probabilities, and evolves both in spatial as well as temporal domains. We then develop a simplified one-dimensional, semi-analytical model that evolves only in the temporal domain. An excellent agreement is observed between thermally and optically stimulated luminescence (TL and OSL) results produced from the two models. In comparison to the first-order kinetic behaviour of the LTM of Halperin and Braner (1960 Phys. Rev. 117 408-15), our model results in a highly asymmetric TL peak; this peak can be understood to derive from a continuum of several first-order TL peaks. Our model also shows an extended power law behaviour for OSL (or prompt luminescence), which is expected from localized recombination mechanisms in materials with random distribution of centres.     

The production of frenkel defects in fcc metals

Experimental results from polycrystalline and single crystalline samples of aluminum are analyzed in terms of how electron irradiation causes atomic displacements along 〈100〉 and 〈110〉 crystallographic directions. This interpretation is made for atoms that recoil with energies sliihtly greater than the threshold energy. It is noted that definite correlations exist between some of the substages of stage I and the natural required behavior of recoiling atoms. By applying this criterion to polycrystalline samples, one establishes a means for identifying other phenomena; e.g. multiple atomic displacements, spontaneous recombination, etc.     

Magnetooptical studies of defects and recombination luminescence in LiBaF3

Optically detected EPR investigations have been performed on the recombination luminescence (RL-EPR) of LiBaF₃ crystal, X-irradiated at T=4.2 K. RL-EPR lines of V_K- centres were found, as well as further lines of a defect with and an axial g-tensor with its main axis along a [1 0 0] direction of the crystal.

Measurements of the magnetic circular dichroism of the absorption (MCDA) have been performed on LiBaF₃ crystals X-irradiated at two temperatures (4.2 K and RT). After irradiation at T=4.2 K, the main MCDA bands peak at 453 and 500 nm, but after irradiation at T=300 K, the main bands peak at 444 and 390 nm, there is a change of the sign between the peaks in both cases. The MCDA-detected EPR (MCDA-EPR) consists of one broad EPR line in both cases and belong to electron trap centres. Analysis of half-widths of MCDA-EPR lines showed that both defects should have g-tensors with their axes along the [1 0 0] direction. This symmetry has to be expected for F-type centres in LiBaF₃ crystals. The low-temperature electron centre has a more perturbed ground state as the RT centre.     

Low-temperature luminescence and thermally stimulated luminescence of BeO: Mg single crystals

Luminescence and thermally stimulated luminescence (TL) of BeO: Mg crystals are studied at T = 6–380 K. The TL glow curves and the spectra of luminescence (1.2–6.5 eV), luminescence excitation, and reflection (3.7–20 eV) are obtained. It is found that the introduction of an isovalent magnesium impurity into BeO leads to the appearance of three new broad luminescence bands at 6.2–6.3, 4.3–4.4, and 1.9–2.6 eV. The first two are attributed to the radiative annihilation of a relaxed near-impurity (Mg) exciton, the excited state of which is formed as a result of energy transfer by free excitons. The impurity VUV and UV bands are compared with those for the intrinsic luminescence of BeO caused by the radiative annihilation of self-trapped excitons (STE) of two kinds: the band at 6.2–6.3 eV of BeO: Mg is compared with the band at 6.7 eV (STE₁) of BeO, and the band at 4.3–4.4 eV is compared with the band at 4.9 eV (STE²) of BeO. In the visible region, the luminescence spectrum is due to a superposition of intracenter transitions in an impurity complex including a magnesium ion. The manifestation of X-ray-induced luminescence bands at T = 6 K in BeO: Mg indicates their excitation during band-to-band transitions and in recombination processes. The energy characteristics of the impurity states in BeO: Mg are determined; the effect of the isovalent impurity on the fluctuation rearrangement of the BeO: Mg structure in the thermal transformation region of STE₁ → STE₂ is revealed.     

Short-wavelenght luminescence and thermostimulated processes in single crystals of BeO

The results of a study of thermostimulated luminescence and recombination processes in pure and impure single crystals of BeO are reported. The origin of the trapping and recombination centers, their parameters (activation energies, frequency factors), the recombination schemes and the TL spectra were determined. It was shown that, in the BeO single crystals during electron and hole recombinations, the excitation of the intrinsic luminescence band at 4.9 eV proceeds and its characteristics inherit no properties from the recombination centers.     

Intrinsic luminescence in oriented BeO crystals under VUV and inner-shell excitation

Beryllium oxide (BeO) crystals were investigated by time-resolved low temperature VUV-spectroscopy at the SUPERLUMI station and BW3 beam line of HASYLAB (DESY, Hamburg). Photoluminescence spectra (3–10.5 eV), luminescence decay kinetics upon selective photoexcitation, as well as luminescence excitation (50–650 eV) and reflectivity (9–35 eV) spectra were measured and analyzed for oriented BeO crystals. It was shown that study of oriented crystals makes the traditional time-resolved spectroscopy method essentially more informative. Formation of the self-trapped exciton excited states of different multiplicity was found to sensitively depend on excitation energy and mutual orientation of the crystal's C optical axis and electric vector E of exciting polarized synchrotron radiation.     

Luminescence properties of BeO optically stimulated luminescence (OSL) detectors

The objective of this work is to investigate basic luminescence properties of BeO optically stimulated luminescence (OSL) detectors, including the OSL emission and stimulation spectrum, the lifetime of the luminescence centers contributing to the OSL signal, and the temperature dependence of the luminescence lifetime and of the luminescence efficiency. The OSL stimulation spectrum shows a continuous increase in OSL intensity with decreasing stimulation wavelength. The emission spectrum indicates two OSL emission bands at ～310 nm and ～370 nm, the latter being the dominant OSL emission band. We also observed that the luminescence centers associated with the OSL signal are strongly quenched above room temperature, resulting in a reduction in luminescence lifetime from ～27 μs at room temperature down to ～800 ns at 140 °C. The activation energy for non-radiative decay of the luminescence center was determined to be E = (0.568 ± 0.023) eV. The ～27 μs luminescence lifetime observed for BeO indicate that POSL technique may be used to improve the signal-to-noise ratio using stimulation pulses of the order of microseconds. The information obtained in this study may help further optimize the BeO dosimetry systems and provide guidance on the timing parameters to be used for POSL measurements of this material.     

Cross section for formation of frenkel defects by Co60 gamma rays

Cross sections are calculated for Z from 6 to 32 for E_d of 15, 25, and 35 eV; a o = f(Z, E_d) relation is drawn up.     

Trapping of frenkel excitons

We calculate the trapping rate of a Frenkel exciton in the presence of a low concentration of traps. The analysis is based on a second-order perturbation calculation which is applicable to three-dimensional systems when the exciton bandwidth is large in comparison with the maximum single donor-single trap transfer rate. The variation of the transfer rate with exciton wave vector is commented on. Possible experimental tests of the theory are discussed.