Radiation-induced Paramagnetic Centers Research Articles

Magnetic Resonance-Based Analytical Tools to Study Polyvinylpyrrolidone-Hydroxyapatite Composites.

The synthesis of biocompatible and bioresorbable composite materials, such as a "polymer matrix-mineral constituent," stimulating the natural growth of living tissues and the restoration of damaged parts of the body, is one of the challenging problems in regenerative medicine and materials science. Composite films of bioresorbable polymer of polyvinylpyrrolidone (PVP) and hydroxyapatite (HA) were obtained. HA was synthesized in situ in the polymer solution. We applied electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) approaches to study the composite films' properties. The application of EPR in two frequency ranges allowed us to derive spectroscopic parameters of the nitrogen-based light and radiation-induced paramagnetic centers in HA, PVP and PVP-HA with high accuracy. It was shown that PVP did not significantly affect the EPR spectral and relaxation parameters of the radiation-induced paramagnetic centers in HA, while light-induced centers were detected only in PVP. Magic angle spinning (MAS) 1H NMR showed the presence of two signals at 4.7 ppm and -2.15 ppm, attributed to "free" water and hydroxyl groups, while the single line was attributed to 31P. NMR relaxation measurements for 1H and 31P showed that the relaxation decays were multicomponent processes that can be described by three components of the transverse relaxation times. The obtained results demonstrated that the applied magnetic resonance methods can be used for the quality control of PVP-HA composites and, potentially, for the development of analytical tools to follow the processes of sample treatment, resorption, and degradation.

Electron paramagnetic resonance study of radiation-induced paramagnetic centers in succinic anhydride single crystal

Succinic anhydride single crystals were exposed to 60Co-gamma irradiation at room temperature. The irradiated single crystals were investigated at 125 K by Electron Paramagnetic Resonance (EPR) Spectroscopy. The investigation of EPR spectra of irradiated single crystals of succinic anhydride showed the presence of two succinic anhydride anion radicals. The anion radicals observed in gamma-irradiated succinic anhydride single crystal were created by the scission of the carbon-oxygen double bond. The structure of EPR spectra demonstrated that the hyperfine splittings arise from the same radical species. The reduction of succinic anhydride was identified which is formed by the addition of an electron to oxygen of the CO bond. The g values, the hyperfine structure constants and direction cosines of the radiation damage centers observed in succinic anhydride single crystal were obtained.

EPR of radiation-induced paramagnetic centers in irradiated sodium silicate glass for immobilization of solid radioactive wastes synthesized in various conditions

The nature of radiation-induced paramagnetic centers (RPC) in glassy materials produced at vitrification of solid radioactive waste (SRW) surrogates with sodium silicate fluxes at the silicate modulus values of 2, 3, and 4 depending on the SRW oxide loading and the preparation method was studied. The connection between the type of RPC formed at silicon-oxygen units of the glass network and chemical durability of the glassy materials was established.

The effect of thermal treatment of radiation-induced EPR signals of different polymorphic forms of trehalose

Electron paramagnetic resonance (EPR) signals induced by γ-radiation in different polymorphic forms of trehalose were studied with dosimetry applications in view. Dose response of trehalose in terms of the concentration of induced paramagnetic centers was studied in the dose range from 0.5 to 50kGy. The dependences of the dose responses of anhydrous β-crystalline trehalose (TREβ) and glassy trehalose (TREg) on dose are linear up to 15kGy, whereas the linearity of the dependence for trehalose dihydrate (TREh) is limited to about 10kGy. At doses above 15kGy, the dependences get saturated for all three forms. The relative radiation sensitivities pointed to the following order of decreasing concentrations of radiation-induced paramagnetic centers in the forms: TREg>TREβ>TREh. The results showed that at all three trehalose polymorphic forms are suitable for dosimetry, especially for retrospective dose measurements. Also, thermal stability and decay kinetics of the EPR signals of the different forms of trehalose were studied in isothermal annealing experiments. The kinetic parameters, which had been derived by fitting the Arrhenius function to the measured decay rate constants, indicated that the fading of the EPR signals varied from one polymorphic form of trehalose to another. This emphasizes the impact of the molecular packing in the vicinity of the radiation-induced paramagnetic centers on their stability.

Electron paramagnetic resonance of radiation-induced paramagnetic centers in an aerogel

Silicon oxide aerogel samples irradiated with x rays at room temperature have been analyzed using the electron paramagnetic resonance method. It has been found that three types of paramagnetic centers appear: paramagnetic centers with a g factor of 2.0035, centers associated with the presence of protons in SiO2 globules, and centers in the adsorbed film on the aerogel surface. The fast (Tfast = 30 h) and slow (Tslow = 70 d) processes have been revealed in the recombination of these centers.

Gamma and UV Radiation-Induced Color Centers in Optical Fibers

Radiation-induced losses and paramagnetic centers were investigated in phosphorusdoped and P-free multimode germanosilicate optical fibers after g-rays (~1 MeV) and ultraviolet (5 eV) exposures. After both types of irradiation, the same defects seem to be responsible of the fiber absorption in the spectral range 400 to 1650 nm. In particular, the P1 centers and the Phosphorus Oxygen Hole centers are created in both cases in the phosphorus-doped fibers and explain the high permanent radiation-induced attenuation levels observed in this fiber type. Luminescence and electron spin resonance measurements (77 K, ~9.38 GHz) on irradiated samples confirm that the GeE’, SiE’ and NBOHC defects are also generated in the different irradiated samples. From this study, it seems that the pertinence of a multimode fiber for nuclear space or civil applications could be estimated through low-cost ultraviolet measurements.

Optical transitions of paramagnetic Ge sites created by x-ray irradiation of oxygen-defect-free Ge-dopedSiO2by the sol-gel method

Optical transitions of radiation-induced paramagnetic Ge centers have been investigated in Ge-doped ${\mathrm{SiO}}_{2}$ samples containing a negligible amount of native oxygen coordination defects, whose optical absorption usually dominates the UV spectral range. The analysis of optical and electron paramagnetic resonance (EPR) spectra following different irradiation and thermal annealing treatments shows that two optical absorptions (at 4.4 and 5.7 eV) are related to an orthorhombic EPR signal [Ge(1) signal in literature], while a band at about 6.3 eV follows the evolution of an axial signal [Ge(3) signal] already attributed to axial ${E}^{\ensuremath{'}}\ensuremath{-}\mathrm{Ge}$ center. No evidence of another orthorhombic EPR signal observed by other researchers [Ge(2) signal] has been found in our samples. Our results suggest that the previous assignments of the 4.4- and 5.7-eV bands to Ge(1) and Ge(2) centers, respectively, be changed. Instead, both electronic transitions are likely to be ascribed to the sites responsible for the Ge(1) signal. This conclusion has been checked by comparing anisotropy and inhomogeneous dispersion of the principal g-tensor values with energy separation and the relative bandwidth and intensity of the optical bands. Structural models of the Ge site responsible for the Ge(1) signal have also been discussed. A structure like the one proposed for the ${E}_{\ensuremath{\alpha}}^{\ensuremath{'}}$ center is suggested, attributing the orthorhombic distortion to a nearby oxygen excess group.

Localization of radiation-induced paramagnetic centres of tooth enamel

Stable radiation induced paramagnetic centres (PC) of tooth enamel are usually supposed to be localized in hydroxyapatite crystallite bulk, but in some publications they are considered as surface radicals, though direct experimental proof was not yet fulfilled. This work represents a direct experimental proof of stableradiation PC localization in crystallite bulk based on a study of ambient atmosphere (i.e., atmospheric oxygen) influence upon PC relaxation characteristics.

Radiation-induced paramagnetic centers in Ca3Ga2Ge4O14 crystals

Studies of EPR spectra, additional optical absorption, and the kinetics of buildup and decay of radiation-induced paramagnetic centers in compositionally disordered crystals of trigonal Ca-gallogermanate are reported. The parameters of the spin-Hamiltonian describing the observed room-temperature EPR spectra are determined. It is established that the radiation-induced paramagnetic centers in Ca3Ga2Ge4O14 crystals are O− centers in common 6g sites stabilized by the Ge4+ vacancy in tetrahedral 2d sites.

Estimation of accumulated dose of radiation by method of ESR-spectrometry of dental enamel of mammals

ESR-spectrometry was used to investigate radiation-induced paramagnetic centers in enamel of mammals: carnivores (polar bear and fox), ungulates (reindeer, European bison, moose), and man. Values at half the microwave power saturation of the radiation signal, P 1 2 , evaluated at room temperature, was found to range from 16 to 26 mW for animals and man. A new approach to discrimination of the radiation induced signal from the total ESR spectrum of reindeer enamel is proposed. ‘Dose-response’ dependencies of enamel of different species mammals were measured within the dose range from 0.48 up to 10.08 Gy. Estimations of ‘radiosensitivity’ enamel of carnivores and ungulates showed good agreement with radiosensitivity enamel of man by ESR method.

Quantitative Analysis of EPR Spectra of Powdered Samples Containing a Mixture of Various Paramagnetic Particles

The prevailing majority of EPR spectra in solid samples concern the randomly oriented paramagnetic particles in solids (polycrystalline samples, powders, glasses, ceramics, frozen solutions). In these cases it is necessary to provide numerical simulation or nonlinear optimization of the spin-Hamiltonian parameters to obtain the relevant physical results. The presence of various paramagnetic particles in the same sample often complicates the situation. This paper deals with the numerical solution to these problems. Illustrative examples of EPR spectroscopy of radiation-induced paramagnetic centers in glasses and EPR of Cu(II) as a spin label in the study of the thermal evolution of SiO2gel preparation are also presented.

The influence of glass composition on the properties of silver-doped radiophotoluminescent phosphate glasses

The influence of glass composition on the sensitivity to ionizing radiation as well as on the radiophotoluminescence ‘build-up’ kinetics is studied in silver doped glass system XNaPO3, (100 - X)Ca(PO3)2, where X varied from 0 to 100 mol%. Concentration of Ag2O was 0.1 wt%. All glass compositions were prepared in a globar-heated furnace at 1000–1200°C. Gamma-irradiated glasses, containing radiation-induced paramagnetic centers of photoluminescence, were investigated by electron spin resonance and luminescence spectroscopic methods. The variation of NaPO3 concentration from 100 to 40 mol% have decreased the glass sensitivity to gamma-radiation more than 800 times and simultaneously have increased the ‘build-up’ time of radiophotoluminescent centers. Room temperature stable Ag0 atoms were detected in the glass, containing 40 mol% NaPO3 and less. Results are discussed based on the model of post-radiational relaxation. It is shown that all effects found in this work are caused by the decrease of Ag+ ion mobilities with decrease of alkali content in the glass.

EPR spectroscopy of γ-irradiated 15 Na2O · 10(MgO, CaO, TiO2, ZrO2) · 75SiO2 glasses

The radiation-induced paramagnetic centres in 60Co γ-irradiated 15Na2O · 10(MgO, CaO, TiO2, ZrO2) · 75SiO2 glasses, with various amounts of MgO, CaO, TiO2 and ZrO2 (from zero to 10 mol%), were investigated by electron paramagnetic resonance (EPR) spectroscopy. The EPR spectra at 77 K showed HC1 or HC2 γ-radiation-induced paramagnetic centres for the glass samples containing 10 mol% MgO, CaO, TiO2 or ZrO2, depending on the glass composition. However, the EPR spectra of the TiO2; or ZrO2-containing glasses also exhibited an additional wide line resonance.

Radiation-induced paramagnetic silver centers in LiNaSO 4

The results of ESR investigations carried out on γ-irradiated Ag-doped lithium sodium sulphate (LiNaSO 4) at LNT (77 K) are reported here. Two different centres of Ag (Ag 0, Ag 2+) have been observed at 77 K. In the case of Ag 2+ the parameters are g xx =2.071±0.001, g yy =2.102±0.001, g zz =2.402±0.001, A xx =18±1 G, A yy =20± 1 G,and A zz = 26 ± 1 G. The ground-state wave function has been found to be consist primarily of d x 2− y 2 with a reasonable admixture of other orbitals. The extent of delocalization of the electron in the Ag 2+ has been estimated. For Ag 0 the parameters g and A are found to be isotropic.

ESR of Sugar as a personnel monitor for radiation emergencies

In the case of radiation emergency, a sugar method, with readout by electron spin resonance analysis as personnel monitor for the affected population, is proposed. Critical characteristics of the method are economics, universal availability and portability, with superior quantitative monitoring characteristics for ionizing radiation and relatively simplicity of the procedure for evaluating dose to individuals or to households after irradiation and readout using this widely available, compact monitor. The dose linearity of the sugar/ESR monitor is experimentally verified from about 3 cGy to 6 × 10 5 cGy and the dose detection limit is 0.5 mGy, with the limit of error of about 80% at the confidence level of 95%. A decrease in the number of the radiation-induced paramagnetic centers (free radicals) is not observed for irradiated sugar held at temperature up to 100°C over about 90 min. It is observed that the free radicals cannot be detected from water solutions of the irradiated sugar. The sugar/ESR method is proposed as one of the most useful regional personal monitors for major radiation emergencies.

A radiation accident at an industrial accelerator facility.

On 11 December 1991, a radiation overexposure occurred at an industrial radiation facility in Maryland. The radiation source was a 3-MV potential drop accelerator designed to produce high electron beam currents for materials-processing applications. This accelerator is capable of producing a 25 milliampere swept electron beam that is scanned over a width of 112.5 cm and which emerges from the accelerator vacuum system through a titanium double window assembly. During maintenance on the lower window pressure plate, an operator placed his hands, head, and feet in the beam. This was done with the filament voltage of the electron source turned "off," but with the full accelerating potential on the high voltage terminal. The operator's body, especially his extremities and head, were exposed to electron dark current. In an attempt to reconstruct the accident, radiochromic film and alanine measurements were made with the accelerator operated at two beam currents. Measured dose rates ranged from approximately 40 cGy s-1 inside the victim's shoe to 1,300 cGy s-1 at the hand position. Approximately 3 mo after the accident, it was necessary to amputate the four digits of the victim's right hand and most of the four digits of his left hand. Electron paramagnetic resonance spectrometry, which measures the concentration of radiation-induced paramagnetic centers in calcified tissues, was used to estimate the dose to the victim's extremities. A mean dose estimate of 55.0 +/- 3.5 Gy (95% confidence level) averaged over the mass of the bone was obtained for the victim's left middle finger (middle phalanx).

ESR Dosimetry

Abstract The demonstration 30 years ago that the amino acid, alanine, could serve as a solid dosemeter by electron spin resonance (ESR) spectrometry, has led to a fascinating success story. Radiation-induced paramagnetic centres, the de-aminated free radicals associated with unpaired electrons, are stable during and after irradiation. The free radical concentration proportional to the absorbed dose can be measured as the main peak-to-peak amplitude of the first derivative of the ESR absorption spectrum. In addition to alanine, other solids (e.g. certain sugars and polymers, quartz, bone) may be used to cover a broad range of absorbed dose, 10-108 Gy. Indeed, efforts proceed to improve the sensitivity, accuracy, and precision of ESR dosimetry. Advantages include the ability to simulate biological tissues in terms of radiation absorption properties, small size (e.g. thin films and small pellets), the possibility of in vivo dosimetry, ruggedness, stability, and resistance to environmental and dose rate effects.

EPR dosimetry sensitivity enhancement by detection of rapid passage signal of the tooth enamel at low temperature

The four-fold improvement of EPR signal-to-noise ratio for radiation-induced paramagnetic centres in comparison with a conventional way is achieved by detection of the tooth enamel EPR signal at 77 K and rapid passage conditions.

Electron spin resonance characterization of self-trapped holes in amorphous silicon dioxide

The electron spin resonance spectra of radiation-induced self-trapped holes (STHs) in amorphous silicon dioxide are isolated by isochromal annealing experiments and computer simulation analyses. Two distinct components, denoted STH 1 and STH 2 (plus a third component intermediate between the two), have been identified. The g matrices and 29Si and 17O hyperfine matrices of STH 1 and STH 2 are determined and form the bases for proposing the following models for these defects: STH 1 comprises a hole trapped in the 2p orbital of a normal bridging oxygen in the glass network; it is a small polaron. STH 2 probably consists of a hole rapidly tunneling between degenerate valence-band-edge states on a pair of adjacent oxygens; it is argued to be a peculiar type of Anderson localized state. Among the various radiation-induced paramagnetic centers in silica, the STHs exhibit unique isochronal anneal characteristics; these characteristics closely mimic the corresponding behavior of a component of trapped positive charge in thermally grown SiO 2 films.

The function of paramagnetic iron species in amorphous materials formed by sol-gel method and conventional melting techniques

A 9 and 35 GHz electron paramagnetic resonance investigation of conventional Na 2OAl 2O 3SiO 2 and CaOAl 2O 3SiO 2 glasses has shown that: (a) a fraction of Fe 3+ acts as a network former, (b) precursors of non-stoichiometric FeO x phases are formed, (c) the ratio of the concentration of Fe 2+ ions to the concentration of Fe 3+ ions controls the formation and stabilization of radiation-induced paramagnetic hole centres. 9 GHz EPR of SiO 2 and xNa 2O·0.1Fe 2O 3·100SiO 2 ( x = 0, 0.1, 1.0) sols and gels showed that (i) Fe 3+ sites in the sols are less rhombicly distorted than in glasses (in the presence of a chelating agent such as acac an effect of changing Fe 3+/Na + ratio is not detectable) and (ii) in alcogels, ferric ions are observed in axially distorted sites; (iii) UV irradiation of frozen sols and gels leads to the formation of ·C 2H 5 and CH 3ĊHOH radicals, which provide information about the degree of hydrolyses of the samples; (iv) if sodium is added as NaNO 3, the UV irradiation produces ·NO 2 radicals; (v) the addition of 1 mol% nitrate is sufficient to remove residues of carbon compounds on tempering the gels.