Short-lived primary radiation defects in LiF crystals

The spectral and kinetic parameters of electron-pulse-initiated transient absorption and emission of LiF crystals were studied using pulsed spectrometry with a nanosecond time resolution. The measurements were performed in the spectral region of 6 eV, the temperature range of 11–150 K, and within 10^?8–10 s after the termination of an electron pulse. It is shown that the electron-pulse irradiation not only gives rise to F, V _k , and H centers in the LiF crystal but also to certain short-lived defects of two types that differ in the spectral positions of the absorptive and radiative transitions, the lifetime, and the temperature dependence of the production efficiency. Defects of type I feature absorptive transitions at 5.5 and 5.1 eV and a radiative transition at 5.8 eV, whereas the absorptive transitions at 5.3 and 4.75 eV and a radiative transition at 4.4 eV are characteristic of type-II defects. It is found that a variation in the ratio between the concentrations of the different types of short-lived centers in the range of 11–150 K does not affect the quantum efficiency of the F centers. It is assumed that the observed centers are self-trapped excitons of various types.     

Influence of polyvalent metal ions on formation processes of radiation defects in LiF crystals

Radiation-induced processes of formation of intrinsic and activator defects in LiF crystals doped with oxides of various metals (Li, W, Fe, Ti) have been investigated. The mechanism of influence of the oxygen impurity on accumulation of electron color centers in the context of notions on self-organization the subradiation defect structure in a form of nanodefect conglomerates in the region of polyvalent cations has been proposed.     

Evolution of primary radiation defects in ionic crystals

Using pulsed spectrometry with nanosecond time resolution, we have studied the characteristics of creation of induced defects and the relaxation of such defects in the initial stage in ionic crystals, using as examples ten alkali halide crystals with sc and fcc lattices and an MgF₂ crystal in the temperature range 12.5–500 K. We present the transient absorption spectra, the relaxation kinetics for the induced defects over a broad temperature range, the dependences of the creation and relaxation on the excitation flux density. The results of our investigations are generalized in tables and, in the form of the most characteristic dependences, in figures. We describe the model developed for evolution of primary defects, taking into account the main general characteristics observed experimentally and the results of mathematical modeling based on it, which allowed us to put together a general picture for the series of processes determining the evolution of primary defects. Tomsk Polytechnical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 5–29, November, 1996.     

Relaxation kinetics of primary pairs of radiation defects in ionic crystals

The relaxation kinetics of primary pairs of radiation defects in ionic crystals with a face-centered lattice is investigated using the Monte Carlo method. The dependence of the relaxation kinetics of an F-H pair on the parameters of the interaction potential between the components of the pair is studied. The obtained kinetic dependences are analyzed to determine the factors responsible for the relaxation processes.     

External-field-induced formation of stabilized F 2 + centers in colored LiF crystals

The production of complexes, which are thermally stable at room temperature, with F ₂ ⁺ centers in radiation-colored LiF crystals by the combined action of different fields is investigated. The half-life of these laser centers produced by, specifically, hard UV radiation and a shock wave increased by almost two orders of magnitude. Fiz. Tverd. Tela (St. Petersburg) 40, 2044–2050 (November 1998)     

The efficiency of direct annihilation of resistivity silicon primary radiation defects in high-resistivity silicon

Abstract

The change of charge-carrier concentration and lifetime in high-resistivity float-zone n- and p-Si with equilibrium concentrations of electrons n₀=(2 to 30)× 10¹² cm^?3 and holes p₀=(5 to 30)× 10¹¹ cm^?3 when irradiating by ⁶⁰Co γ-rays (T= 330 K) is studied. There is a drastic increase of the initial charge- carrier removal rate and the coefficient for the radiation variation of their lifetime. The results obtained are explained on the assumption of the dependence of the annihilation probability of Frenkel pairs on the charge state of vacancy (V) and interstitial Si atom (I).The energetic position of the levels separating the states I⁰/I⁺⁺ and V^?/V⁰ which are at E_c ?(0.43?0.45) eV and E_c ?(0.47?0.54) eV, respectively, is estimated.     

Particle track phenomena and radiation power effects at the formation of radiation defects in ionic crystals

The mechanisms of the radiation defect formation in alkali halide crystals are studied in an extremely wide range of the absorbed radiation dose rate (10¹–10¹² Gy/s). It is found that the power dependence of color centers accumulation is described by a curve with a maximum at a dose rate of about 10¹⁰ Gy/s. The electron and proton track parameters for ionic crystals are calculated in the context of the theory of ionization losses of charged-particle energy. Proceeding from the concept of the charged-particle track overlap, the theoretical relations are obtained that explain the radiation power effect in all dielectric materials including alkali halide crystals. The suppression of color center accumulation in these crystals under high-power electron irradiation is due to a more regular topography of the radiation defect formation. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 10–21, February, 2007.     

MgF2单晶的THz光谱研究

利用THz时域光谱技术对MgF2晶体(样品1)和MgF2:Co晶体(样品2)在0.5-2.5THz的吸收特性进行了研究.在0.5-2.5 THz波段,样品1吸收系数α(ν)随频率ν增加而增大,最大值为24 cm-1.样品2的吸收系数比样品1大得多,Co掺杂使晶格吸收带边向低频移动,而且样品2在1.9THz有吸收峰,吸收系数达到70 cm-1,由此求出F--Co2+离子键伸缩振动的键力常数K为3.40×10-2 N/cm.这一结果表明,THz光谱分析有可能成为研究晶体化学键的一种重要手段.利用光学常数之间的关系计算了两个样品在0.5-2.5 THz的介电函数的实部ε1(ν),得到样品1的ε1(ν)值在4.67至4.73之间,样品2的ε1(ν)值在4.62至5.01之间.     

Heavy ion induced damage in MgF 2 crystals

MgF 2 single crystals were irradiated with 22 Ne and 238 U ions in the energy range 31-2642 v MeV. The ion-induced damage was studied by optical spectroscopy, scanning force microscopy, and volume expansion measurements. The efficiency of damage creation depends on the stopping power (d E /d x ) and the fluence of the ions. Out-of-plane swelling and the formation of hillocks on the crystal surface require a critical d E /d x of approximately 3 v keV/nm. The efficiency for creation of simple defects (color centers) and of more complex damage (swelling, hillock formation) in MgF 2 is compared with the corresponding efficiency in LiF.     

Luminescence in the LiF–MgF2 System Activated by Rare Earths

Journal of Applied Spectroscopy - In recent years, several reports have appeared on luminescence in LiMgF3. Important applications have also been claimed. There is no record of LiMgF3 in the ICDD...     

Energy of Frenkel' defect formation in MgF2

Russian Physics Journal -     

Metastable F2 center states in LiF crystals

Using time-resolved pulsed absorption and luminescence spectroscopy we have studied the creation ofF ₂ centers during electron pulses of nanosecond duration in the temperature range 80 to 300 K. We show that the formation ofF ₂ centers in long-lived metastable states is thermally activated and competes withF ₂ center relaxation to the singlet state. It is shown that the S-T crossing in the set of states which describe theFC₂ center lies above the¹ _u level. We propose that when the crystal is excited with the lowest energy quanta which produce a photocurrent, the formation of tripletF ₂ centers takes place by localization of a conduction electron on anF ₂ ⁺ center.State Academy of Architecture and Construction, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 17–22, November, 1994.     

Some peculiarities of the EPR spectra of radiation-induced defects in LiF and NaF crystals

To refine the structure of radiation-induced defects, a comparison is made of the EPR spectra of LiF and NaF crystals with various constituent impurities γ-irradiated with different doses, as well as the experimentally measured and calculated intensity distributions of HFS lines in the EPR spectra of these crystals. It is shown that the F-centers, whose model represents an ionized quasimolecule consisting of two metal atoms on each side of a halide vacancy, are responsible for the broad band observed independently of the impurity composition of the crystals and irradiation dose, while the HFS is caused by defects based on interstitial halide atoms. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 3, pp. 455–459, May–June, 1998.     

Effect of a magnetic field on radiation strengthening of LiF crystals

The variations in the yield stress and optical absorption spectra of LiF crystals subjected to simultaneous action of a weak magnetic field (B = 0.7 T) and X-ray radiation have been studied. At very low (up to 15 min) exposure doses, the weakening effect of the magnetic field has been observed. No noticeable change in the electronic spectra, in particular, in F-center generation, is caused by a magnetic field.     

EPR of radiation defects in LiBaF 3 crystals

EPR spectra of LiBaF 3 crystals have been investigated after X-irradiation at RT. A spectrum consisting of approximately 35 nearly equidistant EPR lines has a strong angular dependence on the line intensities. The spectrum is caused by a hyperfine interaction (hfs) of a spin S =1/2 with neighbouring groups of nuclei. The observed large number of hfs lines required Li nuclei being in the first shell and fluorine nuclei in the more distant second shell. We analysed the spectrum in the F -centre model, taking reduced hfs values of the F -centre in LiF and found qualitative explanation of the number of hfs lines. The angular dependence of the line intensities could be explained by an anisotropy of the g -tensor with its main axis along the [1 v 0 v 0] axis of the crystal.