International Journal for Radiation Physics and Chemistry最新文献

A pulse generator is described which produces the trigger pulses required by a linear accelarator and also the pre-pulses required for pulse radiolysis experiments. The latter are adjustable in 1 μs increments from 0 to 9999 μs and in wider steps to 2 s.

The effect of phosphate buffer components on the radiation induced oxidation of chloride to Cl₂⁻ is studied by pulse radiolysis. The yield of Cl₂⁻ in neutral phoshate-free 0.13 mol dm⁻³ NaCl solution is much lower than in the corresponding acidic solution (pH < 3), but increases rapidly with increasing chloride concentration. Addition of 0.2 mol dm⁻³ phosphate buffer (pH 6.8) to 0.13 mol dm⁻³ NaCl results in about a five-fold increased yield of Cl₂⁻ in N₂-saturated solution, and about a 10-fold increase in N₂O. At higher chloride concentrations, the total yield both in N₂ and N₂O is near that at pH 2 when G(Cl₂⁻) is suggested to be equal to G(OH). The phosphate mediated enhancement of G(Cl₂⁻) in neutral solution is suggested to be caused by a proton transfer from H₂PO₄⁻ to the intermediate chloride oxidation product ClOH⁻. From the effect of H⁺ and phosphate on G(Cl₂⁻ we conclude that the rate of the proton donation is about 100 times slower than that for the direct reaction between protons and ClOH⁻.

The results are discussed in the light of previously suggested mechanisms for radiation-induced oxidation of chloride.

The reactions of radiation-generated free radicals with complexes of cobalt(III) and cobalt(II) containing the tetradentate 14-membered macrocyclic ligands 5,7,7,12,14,14 - hexamethyl - 1,4,8,11 - tetraazacyclotetradeca - 4,11 - diene (4,11 - dieneN₄), 5,7,7,12,12,14 - hexamethyl - 1,4,8,11 - tetraazacyclotetradeca - 4,14 - diene (4,14 - dieneN₄), and 2,3,9,10 - tetramethyl - 1,4,8,11 - tetraazacyclotetradeca - 1,3,8,10 - tetraene (1,3,8,10 - tetraeneN₄) have been studied in aqueous solution using pulse radiolysis. The cobalt(III) complexes can be reduced to cobalt(II) by e_aq⁻, CO₂⁻ and (CH₃)₂$\text{Z.}\text{̇}$COH with the rate constants of the reactions depending upon the nature of the macrocycle and the axial ligand coordinated to the metal center. The rate constants and transient spectra arising from the reactions of the cobalt(II) species with H atoms, CH₃ radicals, and Br₂⁻ radicals have also been determined.

The results and conclusions of the work of the author and co-workers on excited states proudced by pulse radiolysis of gaseous systems are summarized. Detailed consideration is given to systems containing naphthalene, for which the following subjects are discussed: (1) ionic and non-ionic mechanisms of excited state formation and their relative importance, (2) G-values of excited state formation and (3) energy partitioning in the non-ionic formation of excited states of naphthalene. Rate constants for reactions of excited states of naphthalene are summarized. The use of emission from excited states produced in ion-recombination reactions to determine rate constants for ion-recombination is also discussed.

The main kinetic patterns of the radiophotoluminescence (RPL) of organic substances at 77 K have been studied. It was established that the kinetics of the long-lived RPL component is determined by the sum of two processes: monomolecular, corresponding to the spontaneous deactivation of the charges of the triplet molecules formed on recombination of charges, and quasi-monomolecular the kinetics of which is determined by the recombination of electroncation pairs. For a number of aliphatic substances at radiation doses of more than 1 Mrad it was possible to find quenching of RPL manifest in an appreciable change in the luminescent characteristics. From the experimental findings it may be concluded that the quenching observed is due to the free radicals formed in the course of radiolysis and which are effective electron acceptors. A theoretical model of RPL is proposed based on the concepts of the tunnelling mechanism of charge migration in the irradiated frozen organic substances. In examining the processes of paired recombination of spatially separated charges consideration was given to the role of the processes competing with them for capture of one of the components of the pair (electron) by free radicals. The model satisfactorily explains the experimentally observed hyperbolic character of the decay of the long-lived RPL component, the shortening of the duration of luminescence with a rise in the radiation dose and an absence of the effect of temperature on the kinetics of RPL.

The broad line N.M.R. spectra of irradiated and unirradiated high density bulk (HD) and single crystals (SC) of polyethylene were found to give rise to two components, e.g. broad and narrow lines, attributed to crystalline and amorphous regions, respectively. An increase in the line width of the narrow component with radiation dose is more marked in bulk HD specimens, and is ascribed to crosslinking in the radiation-sensitive amorphous region; whereas no such increase is apparent for the SC specimen. Though there seems to be a slight increase in the second moment of the total absorption curve with radiation up to about 200 Mrad, both the second moment and the line width of the broad component was found to decrease with radiation dose for both the samples. This indicates an increased chain mobility in the crystalline regions and is considered to be due to the destruction of regularity in the crystal lattices by the combined processes of crosslinking (or endlinking) in the amorphous regions and the formation of lattice defects and imperfections by the ionizing radiation. A relatively faster decrease of line width with radiation dose above about 1000 Mrad may be attributed to the predominance of the latter process at high doses.