Effect of neutron irradiation on optical spectra of α-Al2O3 and α-SiO2 single crystals

Using two crystal dielectrics (α-Al₂O₃ and α-SiO₂) as examples, the author studied the effect of large neutron-radiation doses on their optical spectra. The variation in IR spectra in the region of α-SiO₂ stretching and deformation vibrations is investigated; its relation to changes in the structure of the irradiated crystal is discussed. Institute of Nuclear Physics, Academy of Sciences of the Republic of Uzbekistan, Ulugbek Settlement, Tashkent, 702132. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 1, pp. 135–138, January–February, 1999.     

Influence of radiation damage on lattice vibration parameters in crystalline and vitreous SiO2

The influence of radiation damage on a number of lattice vibration parameters in two SiO₂ modifications was investigated using IR reflectance spectroscopy. The radiation kinetics of changes in spectral characteristics of SiO₂ fundamental vibrations in crystalline and glassy states were determined. The reflectivity coefficient and the frequency of degenerate vibrations as functions of dose showed minima, the locations of which were governed by the type of sample. At high neutron irradiation doses (10²¹ cm⁻²), certain characteristics of the bands had the same values for both modified materials. Features of the radiation kinetics of sample dynamic parameters were determined. It was deduced that the specific character of the observed radiation-induced changes in spectral and dynamic parameters of vibrations near degenerate modes was due to both the accumulation of radiation damage and a change in the force field surrounding bridging bonds, which is related to a change in the SiO₂ structure. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 3, pp. 354–358, May–June, 2008.     

Spectroscopic investigation on the lattice dynamics of different cut quartz samples irradiated by neutrons

A spectroscopic study of the effect of neutron radiation on the lattice dynamics of different cut crystalline and glassy quartz (SiO₂) has been performed. The IR spectroscopy method is used to determine the mechanisms by which the spectral characteristics of deformation and valence vibrations vary by radiation-induced alteration of the lattices in two types of modified quartz (α-SiO₂ and ν-SiO₂) as a function of the neutron fluence at low and high frequencies. It has been established that nonlinear changes in the dose dependences between the spectral characteristics of the fundamental modes occur in two types of crystals, with different orientations, and in glass, and the features of the spectrum modifications exist near 300–550 and 700–1400 cm⁻¹ and at higher frequencies. The derived dose dependences are compared to those of the degenerate vibration parameters in ν-SiO₂, and the extreme kinetics of the force coefficients is determined. A conclusion is made that the accumulation of radiation damages, changes in the force field around the bridge bonds, and the phase states of SiO₂ affect the observed specific behavior of the radiation-modified spectral and force parameters of the degenerate and deformation modes in quartz samples of different types.     

Effect of Neutron Irradiation on the Optical Properties of Corundum

The characteristic features of the process of radiation-defect formation and of a change in the structural and optical properties of corundum crystals exposed to reactor radiation have been investigated by spectroscopic methods. The dose dependences of the generation of a number of color and luminescence centers, the regularities of their thermal and photodecoloration, and the energies of activation of the centers have been determined. The influence of the overlapping of the absorption and luminescence centers has been estimated. The radiation-induced change in the structural parameters of the crystal and in the characteristics of the main reflexes has been determined. It has been established that at large reactor radiation fluxes the processes of damage of the crystal are accelerated and a small halo appears at 12.5°. The possible mechanism of damage and disordering of the structure of corundum irradiated in the reactor with a fairly large flux is discussed.     

Vibrational Spectra and Radiation-Stimulated Change in the Structure of Crystalline Silica

A spectroscopic investigation of a radiation-stimulated change in the structure of a phase transition in a crystalline silica (alpha quartz) exposed to a set of fluences of fast neutrons has been carried out using optical methods (IR reflection spectroscopy, Raman scattering) and x-ray methods. The laws governing the change in the spectral characteristics of some bands as a result of structural transformation of the crystal on exposure to radiation and the change in the parameters of the stretching and deformation vibrations of bridge bonds have been established. It is assumed that irradiation of a crystal leads to two processes of destruction of the Si—O—Si bonds and their deformation, causing a change in the initial structure. The x-ray spectroscopy data for individual parameters were compared to the Raman spectrum for a heated quartz. A conclusion has been drawn concerning the correlation between corresponding dependences and the formation of a state similar to the high-temperature modification of quartz at a certain stage of irradiation with fast neutrons.     

Effect of neutron irradiation of colorless sapphire (leucosapphire) single crystals on their absorption and luminescence spectra

Features of the thermoradiative changes in the optical absorption and luminescence spectra of leucosapphire (colorless sapphire) crystals irradiated by neutron fluences within the range 5·10¹⁵ to 5·10¹⁹ cm⁻² have been studied in the visible region. The stepwise character of these processes was established as well as the basic steps involved in bleaching the induced color of the wafers as a result of isochronal annealing. Some anomalies have been observed in the temperature dependence of the optical density of the 460–620 nm bands, the activation energies for the color centers as well as the color center concentrations have been calculated, and the nature of the radiation-induced centers has been analyzed. An analytical expression is proposed to describe the accumulation kinetics for the centers during irradiation of the crystals by reactor neutrons. It is concluded that there is an interconnection between, for example, the 460 nm color centers and the 540 nm luminescence centers, and that there is a common mechanism for the process of radiation-induced defect formation, initially responsible for their formation in the crystal. The possible effect of reabsorption of radiation on the behavior of the Y(Φ) curve in the irradiated material is discussed. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 2, pp. 247–251, March–April, 2007.     

Dependence of the spectral characteristics of quartz on the fluence of neutrons

The influence of radiation on the change in the optical properties of a crystalline quartz is investigated. The results of the measurement of the optical spectra of luminescence, reflection, and scattering in specimens irradiated with different fluences of neutron radiation are presented. It is shown that the corresponding spectral characteristics experience substantial changes in the region of fluences that are associated with rearrangement of the crystalline structure. The results of approximation of some radiation kinetics are given, and the trend in the process of accumulation of disordered zones is established. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 4, pp. 543–545, July–August, 2000.     

Effect of fast neutrons on structural rearrangement in crystalline quartz

IR spectroscopy has been used to study the effect of reactor radiation on the structural state of different specimens of synthetic quartz crystals. The patterns have been established for modification of the spectral characteristics of the bending vibrations of the crystal lattice as a function of the orientation of the crystals and the fast neutron fluence. It is hypothesized that the identified features of radiation-induced modification of the parameters of the bending modes for the different quartz specimens are due mainly to structural rearrangement occurring in the crystals. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 1, pp. 122–125, January–February, 2009.     

Influence of Radiative Irradiation on the Optical and Electrical Properties of Crystalline Quartz

The influence of radiation on the optical properties of quartz crystals has been investigated. Using the methods of IR and Raman spectroscopy, the lattice dynamics associated with a change in the spectral characteristics of a number of the stretching and deformation vibrations of the crystal lattice upon irradiation of samples by gamma quanta and neutrons to high doses has been determined. The regularities of radiative modification of the spectral characteristics of certain fully symmetrical and degenerate modes of the stretching vibrations of hydroxide groups are determined. To elucidate the reasons for the changes in the optical spectra, the latter were compared with the corresponding spectra obtained upon heating of wafers and with the change in the conductivity of crystals caused by irradiation. It is assumed that the changes in the optical spectra and conductivity of irradiated crystals are mainly due to the rearrangement of the quartz structure and to the stimulated migration of impurities. A possible mechanism of the process of conduction in the material is discussed in the framework of the existing models.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 2, pp. 224–229, March–April, 2005.     

Spectral characteristics and force constants of irradiated α-SiO<Subscript>2</Subscript> single crystals

The IR-radiation reflectance spectra of α-SiO₂ single crystals exposed, in a reactor, to fast neutrons with fluences changing in a wide range have been investigated. The regularities of the change in spectral characteristics of the antisymmetric and degenerate vibrations of the bridge bonds of the crystals studied and in the dynamic parameters of these crystals in the process of their irradiation have been determined. The critical dynamics of the crystal lattice in the region of antisymmetric stretching and degenerate modes and the extremum radiation kinetics of the ion-polarizability parameters and the force constants of the indicated crystals were detected. The changes in some characteristics of these crystals, arising on their irradiation as a result of the damage of the substance, the rupture and extension of some silicon—oxygen bonds, and the transformation of the structure of α-SiO₂ single crystals by two mechanisms, have been considered. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 773–777, November–December, 2005.