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

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.

Neutrons of 14.7 MeV energy were bombarded on natural zirconium target and cross-section for formation of ^90mZr through the nuclear reactions ⁹⁰Zr(n, n′)^90mZr and ⁹¹Zr(n, 2n)^90mZr was measured. The cross-section value (σ = 470 ± 30 mb) was later on used for estimation of zirconium concentration, varying from a few hundred to a few thousand ppm, in iron and titanium based alloys.

An estimation sensitivity aroung 150 μg, in 1 g of sample weight, was achieved for both iron and titanium matrices which is much higher compared to an earlier reported value for geological samples.

The presented results are the continuation of our work on paramagnetic molecular centers in the model clay compounds. Here it was attempted to incorporate β-alanine in the aluminium hydroxide matrix. A series of precipitates were prepared with the highest ratio of β-alanine/Al = 0.22 obtained at pH = 7.

X-ray diffraction analysis revealed in addition to amorphous precipitates, microcrystalline samples of four different X-ray patterns in nine precipitates, indicating variations in the structure of aluminium hydroxide matrix with incorporated β-alanine. Selected samples were exposed to γ-irradiation at 77 K, and at room temperature (293 K) in the presence and in the absence of air. In all the samples of amorphous structure no stable paramagnetic centers were determined by ESR after γ-irradiation in air.

In all samples of microcrystalline structure similar stable paramagnetic centers were determined by ESR after γ-irradiation in air and in the evacuated tubes. These paramagnetic centers that we were unable to identify due to the poor resolution of the lines in the ESR spectrum can be most probably ascribed to structural defects introduced into the aluminium hydroxide matrix by β-alanine.

In all amorphous samples γ-irradiated in the absence of air, the radical C̊H₂CH₂COOH formed by abstraction of amino group from the β-alanine was determined by ESR. The radical was unstable in air. Thus in the absence of air β-alanine may be considered as a storage site for the γ-irradiation energy in the amorphous aluminium hydroxides.

Solvent effects on the formation of cross-linking microspheres by the radiation-induced polymerization are described. Polymerization of some bi- and tri-vinyl monomers in several organic solvents was tested. Diethylene glycol dimethacrylate (2G) is the unique monomer that gives microspheres. The distance between two vinyl groups of monomer molecules is one of the important factors for the formation of microspheres. The comparison of the solvent properties for the preparation of microspheres suggests that the forms of the monomer molecules in the solutions are also important, that is, the stretched form is favorable. Alcohols, except t-butanol, are not favorable as the solvents for the preparation of microspheres because of not only solubility but also an occurrence of the chain transfer reaction. The chain transfer reactions to the solvents and additives interfere with the formation of microspheres.

Polymerization of spin-casted epoxy oligomer resin-films was initiated by electron beam (EB) or UV irradiation and the conversion to polymer was studied as a function of the thickness of the film. The film thickness was varied from 1 to 25 μm. In the EB-irradiation, the number of oligomer molecules reacted per 100 eV of energy absorption (G-value) increased with increasing thickness up to 10 μm but did not change above this value. In the UV-irradiation, the G-value increased linearly with increasing film thickness. The result was explained by the difference in the penetrating power of the two radiations. The G-values and the quantum yields of the polymerization in the EB- and the UV-irradiations were compared on the basis of the same energy absorbed in the systems; it was found that the EB-irradiation was more effective than the UV-irradiation. Some anomalous polymerization behaviors were observed in thin, spin-casted films.

Optical absorption spectra of radical anions of poly(1-phenyl-1-propyne)(PPPr) and poly(phenylacetylene)(PPA) were studied in 2-methyltetrahydrofuran (MTHF). The radical anions of the polymers were obtained by γ-radiolysis at 77 K (and 4 K) and pulse radiolysis at ambient temperature and 140 K. There were two different trapping sites for electrons in PPPr dissolved in MTHF. The electron produced by ionizing radiation was initially trapped by a site to form the radical anion which gave rise to an absorption spectrum with λ_max at 390 and 560 nm and a shoulder at ca 800 nm. In liquids or warmed matrices, the spectrum relaxed to a new one which was similar to that of the radical anion of trans-β-methylstyrene, a model compound of the polymer. The result suggested that an electron trapped by a shallow site was ultimately localised on a deeper site of the size of a monomer unit. In contrast, the absorption spectrum of PPA^- showed a strong IR band which was attributed to a dimer-type radical anion. It was concluded therefore that the excess electron was delocalized over two monomer units in PPA. The difference was interpreted in terms of the twisted main chain structure of PPPr arising from the repulsive interactions between bulky phenyl groups and methyl groups of the polymer.

Polysilanes (catena-polysilicons) of the structure—SiR¹R²—(R¹, methyl; R², phenyl, n-propyl, n-hexyl, β-phenethyl) and a copolymer (repeating units:—SiMe₂—and—SiPhe₂—) were irradiated with UV light (347 nm) or 15 MeV electrons. The dominant feature in both photolysis and radiolysis is main-chain scission. Typical quantum yields of main-chain scission determined by light scattering (LS) measurements are: φ(S) = 0.2 [poly(methyl-n-hexyl silane)] and φ(S) = 0.5 [poly(methyl-β-phenethyl silane)]. The radius of gyration r_G depends on M_W as r_G ∝ M^v_W; v = 0.5. Flash photolysis and pulse radiolysis studies in conjunction with optical absorption and LS measurements revealed that main-chain cleavage occurs very rapidly (τ<20 ns). The resulting polymer fragments exhibit transient optical absorption probably originating from free radicals. The rate of decrease in LS intensity after the pulse corresponds to the rate of separation of fragments generated by main-chain cleavage. Regarding solutions of polysilane in liquids of bad solvent quality the rate of fragment separation is determined by the lifetime of contact pairs corresponding to physical interactions between polymer segments [at 23°C ca 100 μs in a 66/34 (v/v) mixture of 1,4-dioxane/1-propanol]. Moreover, it turned out that in benzene solution main-chain scission is sensitized by energy transfer from solvent to polysilane.

The usual formula for the elastic modulus E of a crosslinked highly elastic network E = 3 ρRT/M_c involves molecular weight between crosslinks M_c. It has to be modified by several factors such as the fraction not involved in any external stress and the initial molecular weight distribution. Several correction terms have been proposed. In this paper a fuller calculation is made for two initial molecular weight distributions, uniform and random, to see how far these corrections are valid. Even at high degrees of crosslinking a very significant proportion of the network does not participate in this elastic deformation.

It may appear preferable to relate the elastic modulus E to the number of effective segments between crosslinks, independent of their molecular weight M_cE = 3ρkTQ(1 − s²)p where Q is the number of crosslinked monomer units per unit volume, and (1 − s²)p is a correction factor to allow for segments which are ineffective in an elastic deformation.

ESR and electron spin echo measurements of alkyl radicals generated by γ-irradiation of glassy and crystalline branched alkanes C₁₀ ≈ C₁₃ have been carried out to elucidate the effect of molecular structure and solid phase on the selectivity of alkyl radical formation. Alkyl radicals generated and stabilized at 77 K in the glassy alkanes are secondary penultimate radicals. Tertiary radicals and secondary radicals other than the penultimate one are not generated either by hydrogen abstraction or from ionized or exited molecules. In the crystalline alkanes, however, a small amount of secondary internal radicals are generated in addition to the predominant formation of the secondary penultimate radicals. It is concluded that the detachment of C-H hydrogen preferentially takes place at the location where the motion of carbon atoms assisting the detachment of the C-H hydrogen easily occurs.