International Journal of Radiation Applications and Instrumentation. Part C. Radiation Physics and Chemistry最新文献

Hardened slag cement pastes were made using the initial water/cement ratios of 0.25 and 0.4 to obtain low and high porosity cement pastes, respectively. The neat cement pastes were hydrated for various time intervals of 0.5, 1.5, 6 and 12 h and 1, 3, 7, 28 and 90 days; these pastes were tested for compressive strength and kinetics of hydration. Polymer-impregnated pastes were prepared by impregnation in methyl methacrylate monomer, followed by irradiation using γ-rays with different doses of 1, 3 and 5 Mrad. The results of compressive strength tests could be related to the polymer loading as well as the porosity of the cement pastes.

Unusual “high energy” chemical pathways can result when ionizing radiation is deposited into condensed-phase systems. In photoionization it has often been surmised that all excess energy above that needed for ionization appears as kinetic energy of the ejected electron. Alternatively, some of the excess energy could remain in the radical cation, resulting in new modes of radical cation reactivity.

The multiphoton ionization of aromatic hydrocarbons (AH, e.g. anthracene) in alkane (RH, e.g. cyclohexane) solvents was carried out using intense pulsed 248 or 308 nm irradiation. Photoproducts derived from solvent radicals (e.g. bicyclohexyl) are observed, and the yields increase with increasing photon energy. Flash photolysis and transient conductivity studies were also carried out, and the relative yield of aromatic radical cations was found to decrease with increasing photon energy. Our observations suggest an alternate “high energy” pathway for aromatic radical cations. One possible pathway is proton transfer to the solvent which would lead to aryl radical formation (AH^+∗_· + RH → A + RH⁺₂). Subsequent hydrogen atom abstraction by intermediate aryl radicals (A + RH → AH + R) would then result in the observed increase in solvent-derived radicals.

Infrared absorption spectroscopy has been used to investigate the effect of γ-radiation on copolymer ethylene-propylene (EPR). The IR spectra showed that exposure of EPR samples to gamma doses from 50 kGy to 0.6 MGy results in noticeable changes in their absorption bands, in particular an appearance of a CO band at 1720 cm^-1 and O—H band at 3450 cm^-1. The dependence of the 1720 cm^-1 band as a function of dose and dose rate is reported and discussed in terms of a simple model for the oxidation process.

Blends of gamma irradiated and unirradiated low density polyethylene and polyamide 6 were made in order to study possible modifications induced by this irradiation. Results indicate that blends with gamma irradiated polyethylene have structure and properties significantly different with respect to blends made with the unirradiated one. This difference has been ascribed to the formation of functionalized groups in the polyethylene chains, which interact with polyamide.

The simple dosimeter system: LED-liquid optical waveguide dosimeter-neutral OWG-photodiode shows a sublinear response of absorbance vs absorbed dose due to the polychromaticity of the light source. It is shown that if the sensitivity of measurement is high and the “dosimetric” reaction in unirradiated compound takes place spontaneously with time (ageing), the use of a system with sublinear response can hardly be advised. This has been demonstrated with a solution of hexahydroxyethyl pararosaniline cyanide for which, based on the separately determined G-value and molar absorption coefficient, the theoretical behavior of the system in the gamma radiation field has been derived.

Factors affecting radiation induced defects in lithia aluminosilicate porcelain were investigated. Porcelains with β-eucryptite (Li₂O · Al₂O₃ · 2SiO₂), its solid solution (SS) (Li₂O · Al₂O₃ · 3SiO₂) and β-spodumen (Li₂O · Al₂O₃ · 4SiO₂) were prepared and vitrified. Irradiation was performed at room temperature with three fluences from ²⁵²Cf. X-ray line profile analysis, thermal expansion and scanning electron microscopy were carried out pre- and post-irradiation.

Results showed that when large crystallites were present in strain free body such as β-spodumen (tetragonal) their sizes were greatly influenced. Both high strain and large crystallite size in hexagonal β-eucryptite were severely reduced. In the solid solution (SS) β-eucryptite having minimum crystallite size and strain free level, defects appeared as swelling of the crystallites. Accompanying morphological changes conformed with the above findings. The negativity of thermal expansion increased with low fluences rather than higher ones leading to recovery of induced defects. The optimum neutron fluence, inducing maximum change, proved to be composition dependent. A wave behaviour of the induced change was interpreted in terms of radiation damage followed by saturation and further annihilation.

The optical absorption spectra of the excess electrons solvated in n-propane-h₈ and n-propane-d₈ at 91 K are calculated in terms of a model presented in a previous paper of ours and compared with the experimental data. We have shown that the band shape and the spectral changes due to the isotope substitution can be explained on the basis of the coupling between an excess electron and intramolecular vibrational modes of the solvent in which the electron is stabilized.

Reactions initiated by OH radicals or e^-_aq in aqueous IrCl^3-₆ solutions were studied by electron pulse radiolysis using a 600 keV Febetron electron accelerator. Solutions of IrCl^3-₆ were made basic by adding Na₂CO₃; using the carbonate competition method, we find the rate constant for the reaction of OH with IrCl^3-₆ to be 4.7 × 10⁹ M^-1 s^-1. The product IrCl^2-₆ disappears rapidly in N₂O-saturated basic solution or in neutral N₂-saturated solution (N₂O absent) but is nearly inert in neutral solution with H₂O present. We find that IrCl^2-₆ reacts rapidly with hydrogen peroxide in basic media, as confirmed on the benchtop and by stopped-flow kinetics. We therefore infer that reaction with HO^-₂ may account for the loss of IrCl^2-₆ under basic conditions. Since e^-_aq reduces Ir(III) chloride to the Ir(II) state with a rate constant of 6.1 × 10⁹ M^-1 s^-1, we suggest that loss of Ir(IV) in neutral deaerated solution without added N₂O may involve electron transfer from Ir(II). Loss of Ir(IV) in aerated solution is attributed to reduction by the superoxide ion, O^-₂. Kinetic simulation of the system on the model described gives good agreement with our experimental results.

The mechanism of the radiolytic formation and disappearance of hydrogen peroxide in aerated clay water was studied. The yield of H₂O₂ formation in clay water in an air atmosphere is equal to 0.25 μmol J^-1. When initially present in the solution, hydrogen peroxide disappears with a yield dependent upon the concentration and the dose rate. In both cases a steady state is reached dependent on the dose rate. In order to define more precisely the role of OH free radicals in this process, the reaction of these free radicals in clay water was studied by pulse radiolysis. As expected, OH is scavenged by different solutes, therefore, it cannot react with hydrogen peroxide. A kinetic scheme based upon these results is proposed. Using a minimal equation set (nine equations), it is possible to simulate the [H₂O₂] evolution with a very good accuracy, for doses going up to 20,000 Gy. It is also demonstrated that the reaction of H₂ with OH does not occur in such conditions, which is consistent with the accumulation of dihydrogen during the radiolysis of ground water.

Exposure buildup factors for point isotropic cobalt-60 sources are calculated by the Monte Carlo method with statistical errors ranging from 1.5 to 7% for 1–5 mean free paths (mfp) thick water and iron single slabs and for 1 and 2 mfp iron layers followed by water layers 1–5 mfp thick. The computations take into account Compton scattering. The Monte Carlo data for single slab geometries are approximated by Geometric Progression formula. Kalos's formula using the calculated single slab buildup factors may be applied to reproduce the data for two-layered slabs. The presented results and discussion may help when choosing the manner in which the radiation field gamma irradiation units will be described.