Radiation-dose dependence of E′1 centers in samples of quartz containing uncharged oxygen vacancies

EPR is used to study the generation of E′₁ centers (oxygen vacancies that have trapped one electron) in quartz samples containing uncharged oxygen vacancies as a function of irradiation dose. It is found experimentally that an irradiation dose of order 400 Gy is sufficient to allow every oxygen vacancy to trap two electrons apiece in essentially all such quartz samples. The linear segment of the dose dependences of E′₁ centers in samples annealed at 300 °C for 15 minutes can be used to reconstruct prior radiation doses up to 60–70 Gy. If the concentration of oxygen vacancies in the original sample is larger than 10¹⁸ cm⁻³, the signal intensity from E′₁ centers in the sample can be used to detect radiation doses as low as 1–3 Gy, which is significantly lower than the minimum radiation dose detectable by other paramagnetic centers in quartz. Fiz. Tverd. Tela (St. Petersburg) 40, 651–652 (April 1998)     

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.     

Nuclear relaxation in dynamic nuclear polarization

The behavior of nuclear polarization in a substance, i.e., a solution of the complex HMBA(Cr^V)Na⁺ in 1,2-propylene glycol used in polarized nuclear targets is experimentally investigated by magnetic spectroscopic methods under conditions of dynamic nuclear polarization at hv_S/kT=≈1.5−3.2. Nuclear polarization is measured and analyzed as a function of time at different values of the saturating microwave signal and temperature. It is shown that the process of decreasing the nuclear polarization involving free nuclear relaxation is described by a nonmonoexponential law with two damping decrements, which determine the time of reaching equilibrium between the Zeeman nuclear subsystem, the dipole-dipole pool, and the lattice. Specific features of dynamic processes proceeding in the electronic-nuclear system of the substance investigated are discussed. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 3, pp. 363–366, May–June, 1997.     

Formation of optical color and fluorescence centers in polytetrafluoroethylene under γ-irradiation

Fluorescent properties and colors of polytetrafluoroethylene (PTFE) samples were studied as functions of absorbed dose of γ-radiation at a temperature above the melting temperature of the crystallites (327°C) in the dose range 0.2–0.8 MGy. The samples were irradiated at low pressure (10^–6 Torr) in glass ampuls and at atmospheric pressure in a chamber purged with argon. Samples in ampuls acquired an off-white tint and fluoresced weakly. The fluorescence intensity increased slowly as the radiation dose increased from 0.2 to 0.6 MGy and decreased at 0.8 MGy. Samples irradiated in the chamber fluoresced more strongly and acquired colors that changed from gray (0.2 MGy) to dark-brown (0.8 MGy). Color centers were formed only in the sample surface layer. Their appearance was associated with the adsorption of particles of an unknown nature from the environment. The argon-purged samples turned white upon removing their surface layer (50 μm). The intensity and shape of fluorescence bands emitted by these samples remained essentially unaltered, i.e., depended weakly on the absorbed dose in the range 0.2–0.8 MGy.     

Temperature dependences of spectral and kinetic characteristics of photoluminescence of alkali and alkaline-earth metal chromate crystals

Based on the results of temperature studies of spectral and kinetic characteristics, a study is made to determine the influence of the structure of different types of luminescence centers (LC) formed in chromate crystals on the processes of relaxation and radiationless deactivation of excitation energy. It is shown that "isolated" LC are characterized by simple temperature quenching of luminescence above the barrier. It is established that several excited states take part in the radiation of complex LC. The possibility of radiationless transitions between them with a change in spin is due to spin-lattice relaxation, which, as calculations showed, is described for different types of LC by the "van Vleck" or "defect" mechanisms. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp. 182–187, March–April, 2000.     

Assessment of the therapeutic action of low-intensity laser radiation from the extent of local tissue saturation with oxygen

We propose a method for dosimetry of the biological response to laser irradiation based on determination of the amount of oxygen additionally supplied to the tissue. We show that it is feasible to determine the therapeutic dose from the change in the degree of oxygen saturation of the blood, heart rate, hemoglobin concentration in the blood, and exposure time. __________ 13Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 4, pp. 516–520, July–August, 2006.     

Nanometer-resolution scanning optical microscope with resonance excitation of the fluorescence of the samples from a single-atom excited center

A scanning optical microscope whose active element (the tip) consists of a crystal containing impurity ions or color centers, which are excited by laser radiation, is proposed. The excitation energy from a single impurity center located at the point of the tip is transferred to the fluorescence centers of the experimental sample by the mechanism of resonant dipole-dipole exchange. It is shown that this approach can give nanometer spatial resolution with high sensitivity, which in many cases is much higher than the sensitivity of the “standard” near-field fluorescence microscopy. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 5, 311–315 (10 March 1996)     

Kinetics of nonequilibrium processes excited in broad-band dielectrics by synchrotron radiation

Luminescence spectroscopy with subnanosecond time resolution is used to study features of nonequilibrium processes excited in several broad-zone dielectrics (mainly inorganic scintillators) by pulses of synchrotron radiation (SR). When excitation density exceeds a certain level, which is different for each material, there is an abrupt change in the kinetics of relaxation of the nonequilibrium states. This change is accompanied by nonuniform broadening or shortwave shifting of the luminescence spectrum and a drop in quantum light yield. The decay time for natural luminescence decreases by 1–3 orders, to nanoseconds, and is independent of temperature within the range 80–450 K. The build-up stage disappears in the kinetics of luminescence of Ce³⁺-centers and decay time is reduced by a factor of 2–4. Density effects are found to be independent of the conditions under which the material is exposed to SR. A model is proposed in which density effects are related to nonradiative energy transfer from the upper excited states of the luminescence centers to external quenching centers. The contribution of the space charge induced by SR is also examined. Ural State Technical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 120–135, November, 1996.     

Temperature and frequency dependence of the spin-lattice relaxation times of E′1 centers in neutron-irradiated quartz glass

Results are reported for measurements of the spin-lattice relaxation times of E′₁ centers in quartz glass, produced by neutron irradiation, with the measurements made at two frequencies 9.25 and 24.0 GHz over a wide temperature interval 1.5–300 K. The experimental data are interpreted on the basis of interaction mechanisms of the spins with two-level systems with excitation energies ∼6, ∼26, and ∼420 cm⁻¹. A small modification of the existing theory allows us to explain a number of features of the observed temperature and frequency dependence of the relaxation rate. The results are compared with the data available in the literature on spin-lattice relaxation of irradiation centers in crystalline quartz and quartz glass. Fiz. Tverd. Tela (St. Petersburg) 39, 1335–1337 (August 1997)     

Dipolar broadening and exchange narrowing of EPR lines from the paramagnetic centers distributed on a solid surface

Experiments with char samples provide support for the theory of EPR line shape in a system of surface paramagnetic centers with dipole-dipole interactions. It is also demonstrated that, if a strong Heisenberg exchange dominates over the dipole-dipole interactions, the EPR line of a two-dimensional system narrows and acquires Lorentzian shape, similar to the three-dimensional case.     

Corrections of the influence of irradiation and conservation conditions on thermoluminescence detectors

The method of the correction of the influence of temperature during the conservation and irradiation and of the periods of the conservation and irradiation on thermoluminescence detectors for the determination of the integrated dose of the absorbed radiation which is based on the knowledge of the kinetic parameters of thermoluminescence detectors is proposed in this paper. It is possible, using the proposed relation, to determine the dose of radiation absorbed by detectors on the basis of the knowledge of time and temperature of the conservation and irradiation and of integrated thermoluminescence.     

Study on the quality assurance of diagnostic X-ray machines and assessment of the absorbed dose to patients

Radiation exposure and image quality in X-ray diagnostic radiology provide a clear understanding of the relationship between the radiation dose delivered to a patient and image quality in optimizing medical diagnostic radiology. Because a certain amount of radiation is unavoidably delivered to patients, this should be as low as reasonably achievable. Several X-ray diagnostic machines were used at different medical diagnostic centers in Egypt for studying the beam quality and the dose delivered to the patient. This article studies the factors affecting the beam quality, such as the kilo-volt peak (kVp), exposure time (mSc), tube current (mAs) and the absorbed dose in (μGy) for different examinations. The maximum absorbed dose measured per mAs was 594±239 and 12.5±3.7 μGy for the abdomen and the chest, respectively, while the absorbed dose at the elbow was 18±6 μGy, which was the minimum dose recorded. The compound and expanded uncertainties accompanying these measurements were 4±0.35% and 8±0.7%, respectively. The measurements were done through quality control tests as acceptance procedures.     

Special features of the EPR spectroscopy of a system of centers with different relaxation times

We found that the interaction of paramagnetic centers that have different relaxation times differs fundamentally from the interaction of centers having close relaxation times. Simulation showed that in this case there is an anomalous redistribution of the spectral-line intensity from the center to the wings with a virtually preserved distance between extremal points (super-Lorentzian shape of the line), which leads to underestimation of the total intensity recorded. The results obtained make it possible to explain a number of aspects of the radiospectroscopy of carbon materials of practical importance such as the nature of the generally accepted maximum on the curve for the dependence of the total intensity of an EPR signal on the temperature of the heat treatment of organic compounds and the degree of metamorphism of natural coals, the specific features of the effect of oxygen molecules and paramagnetic ions of metals on the EPR spectra of carbon materials, etc. Belarusian State University, 4, F. Skorina Ave., Minsk, 220080 Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 2, pp. 224–229, March–April, 1998.     

Low-voltage low-temperature electroluminescence of CdF2-RE thin films

Different electroluminescence spectrums of the blueλ _max =420 nm wide-banded low-temperature low-voltage radiation of CdF₂-RE films at different levels of excitation in the region of temperatures of 77–300K are investigated. The kinetic characteristics and temperature dependence of this radiation are explored. An effect of the resonance interaction of the centers of luminescence of the wide-band and line radiation of the RE-centers is found. A model of the center of the wide-band radiation is suggested. It is shown that the “blue” radiation is caused by recombination of the carriers. Institute of Physics of Semiconductors of the National Academy of Sciences of Ukraine, Nauka Ave., 45, 252028, Kiev. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 1, pp. 127–132, January–February, 1999.     

EPR and UV spectrometry investigation of sucrose irradiated with carbon particles

Solid state/EPR (SS/EPR) dosimeters of carbon ions irradiated sucrose are studied with EPR, and their water solutions – with UV spectroscopy. Doses between 20 and 200 Gy are used with linear energy transfer (LET) values for carbon ions of 63, 77, 96 and 230 keV μm^?1. After irradiation all samples show typical for irradiated sucrose EPR and UV spectra. The obtained data are compared with those previously reported for nitrogen particles and gamma rays irradiated sucrose. The identical shape of both the EPR and UV spectra of irradiated with various type radiation samples suggests that generated free radicals are not influenced by the nature of radiation. The lack of difference in the line width of the separate lines or the whole EPR spectrum, obtained for gamma and heavy particles irradiation, suggests negligible spin–spin interaction among the radiation-generated free radicals in the samples. The linear dependence of the EPR response on the absorbed dose radiation is found to be higher when generated by gamma rays, than by the same absorbed dose of heavy particles. In addition, the EPR response for carbon ions is higher than that for nitrogen ions. Water solutions of irradiated sucrose exhibit UV spectrum with absorption maximum at 267 nm, attributed to the recombination products of free radicals. The UV band intensity depends on the absorbed dose radiation. The UV spectra obtained for carbon, nitrogen and gamma rays irradiated sucrose are also compared.     

Theory of the line shape of gamma-photon transitions

There is a discussion of the line shape for the nuclear two-quantum transition due to the simultaneous absorption of a Mossbauer quantum and a quantum from a classical photon source. The problem is solved through the use of density-matrix equations, and an equation is derived for the intensity of absorbed gamma rays. In contrast with the case of the ordinary probabilistic approach, the equation for the intensity of gamma absorption reveals a dependence on the relaxation times within the ground and excited states. The line shape for gamma-photon transitions for low intensities of the incident radiation usually differs little from the shape of the Mossbauer line. However, at large irradiation intensities, a shift and a broadening of the resonant lines occur, proportional to the radiation power. These corrections must be taken into account in, e.g., gamma-magnetic resonance.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 12, No. 7, pp. 24–30, July, 1969.     

The nature of radiative centers formed by nonbridge oxygen in especially pure sol-gel glasses

We carry out measurements of the optical absorption of sol-gel quartz glasses that have been exposed to γ-radiation. The dependence of optical density on irradiation dose is analyzed, and two types of centers formed by nonbridge oxygen are distinguished. We revealed a difference in the peak wavelengths of absorption bands with an energy of 2.0 eV for various types of centers. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 4, pp. 600–603, July–August, 1998.     

Suppression of superluminescence in a dye Dfb-laser with a dynamic grating

We propose a theoretical model which describes the dynamics of a dye laser with light-induced distributed feedback and takes into account both the radiation at the lasing frequency and the superluminescence developed in the amplification band. We show the possibility of suppressing the superluminescence and enhancing the contrast of radiation generated at a frequency distant from the amplification-line center to the luminescent background by using a mixture of dyes with active and absorbing centers. It has been established that this method is most effective in the case where pump pulses shorter than the relaxation time of active-medium inversion are used. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 4, pp. 469–473, July–August, 2000.     

Antimicrobial photodynamic therapy: The impact of laser radiation on mucous tissue stained with photosensitizer methylene blue

We have calculated the spatial distribution of absorbed light energy within layers of a mucous tissue that contains photosensitizer methylene blue. Under irradiation regimes that are typical of antimicrobial photodynamic therapy (laser radiation wavelength, 670 nm; power density, 150–300 mW/cm²; dose, 9–18 J/cm²), we have analyzed conditions that are necessary for the light penetration and delivery of methylene blue to a nidus of infection on a mucosal surface. We have performed a computer simulation of thermal fields, estimated maximal heating temperatures, and considered the degree at which the heating affects the trans-mission of light through a stained tissue.     

Effect of polarization and coherence of low-intensity optical radiation on fish embryos

  We have shown that brief exposure of sturgeon embryos (fertilized roe) in the organogenesis stage to low-intensity radiation in the visible region of the spectrum can have a long-term effect on embryonic and post-embryonic development of the fish, detectable 50 days after the irradiation procedure. The biological effects (size-weight characteristics and hardiness parameters of the fish relative to unfavorable habitat conditions) induced by linearly polarized emission from a monochromatic laser source (helium-neon laser, λ = 632.8 nm, Δλ ≈ 0.02 nm) and a quasi-monochromatic light-emitting diode (LED) source (maximum in emission spectrum λ = 631 nm, Δ λ = 15 nm) are practically the same. Going to broadband linearly polarized radiation (λ = 420–800 nm) is accompanied by a decrease in the biological effect. From the results of studies of the effect on embryos from linearly polarized and unpolarized radiation from an LED source and also the effect of linearly polarized, circularly polarized, and unpolarized radiation from a helium-neon laser, we concluded that the type of polarization is of critical importance in realization of the biological effect of radiation. In this case, the maximum stimulating effect (on the size×weight characteristics and the hardiness parameters for juvenile fish) is observed on exposure to linearly polarized radiation; the photobiological effect induced in the same dose range by light with natural polarization (i.e., unpolarized) is significantly less pronounced; the stimulating effect of circularly polarized radiation occupies an intermediate position. Based on the presented data and also on data obtained previously, we conclude that among the resonant and nonresonant photophysical processes (orientational effect of light, effect of gradient forces, dipole-dipole interactions, thermooptic processes) capable of inducing photobiological effects dependent on such laser-specific characteristics as polarization and coherence, the determining influence in the processes studied in this work comes from the orientational effect of light and dipole×dipole interactions. And the orientational effect can appear for anisotropic media with liquid-crystal type ordering (especially domains in membranes and multiple-enzyme complexes) both under conditions when there is no resonant absorption and for weakly absorbing structures, and can initiate a change in their conformations and accordingly their functional characteristics. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 6, pp. 843–858, November–December, 2008.