International Journal for Radiation Physics and Chemistry最新文献

Results are presented for the luminescence decay in polyethylene following irradiation at liquid nitrogen temperature and its enhancement on application of an electric field. It is found that both the luminescence enhancement and its subsequent decay may be described by a model involving electron tunnelling from a monoenergetic trap distribution to the parent positive ion. The possible nature of the trap is briefly discussed.

The formation of trapped electrons in γ-irradiated polycrystalline hydrocarbon matrices is examined by E.S.R. at 77 and 4 K. Results indicate that no thermalized electrons are trapped in polycrystalline matrices of linear hydrocarbons (from n-hexane to n-hexadecane). They also strongly suggest that the trapped electrons in polycrystalline hydrocarbons so far reported may have been trapped in glassy regions in the matrices used.

The γ-radiolysis of CO₂ has been studied in glass vessels packed with glass spheres of radii 8 × 10⁻³–2 × 10 ⁻⁵ m. The yields of CO were greater than in unpacked vessesls and an initial G(CO) = 4.1 was recorded for the finest packing. Baking the vessels in oxygen at 500–550°C resulted in a lowering of the CO yields but oxygen was a significant product under these circumstances. It is suggested that the results indicate deactivation, on the walls, of species responsible for CO re-oxidation.

To investigate higher excited states, including the superexcited states proposed by Platzman, a method was developed to analyse the entire emission spectra from the gases electron-bombarded with electrons of energy lower than 300 eV. From several standpoints (energetic relations, cascading processes and kinetic isotopic effects), the existence of the higher excited states could be confirmed in some simple aliphatic and aromatic compounds. On the basis of the results, their fragmentation schemes in radiolysis are discussed. The most extensive conclusions were drawn on comparing the ratio of H and D atoms produced from partially deuterated compounds by measuring the intensity of the corresponding Balmer lines.

The only significant product obtained by gamma irradiation of tyrosine in aqueous solution was 3,4-dihydroxyphenylalanine (dopa). G (dopa) was 1.0 in aerated solution, 0.9 in N₂O-saturated solutions, and less than 0.1 in deaerated solution. Pulse irradiation of a N₂O-saturated solution gave G (dopa) 0.6. The absorption spectrum of the initial OH adduct had λ_max at 300 nm (Gϵ = 16,600) and at 330 nm (Gϵ = 18,000). This adduct decayed to give some phenoxyl radical with λ_max at ≈410 nm. The rate of reaction of OH with tyrosine, measured by following formation of the transient, was 6.5±0.3 × 10⁹mol⁻¹ cm³ s⁻¹. Irradiation of the peptides, gly.tyr, tyr.gly, and gly.tyr.gly, gave analogous results; i.e. yields, reaction rates and spectra were all similar strongly suggesting that tyrosine was the main residue attacked. Radiolytic conversion of phenylalanine to tyrosine was examined for comparison. The mechanism of dopa formation is discussed in the light of available results.

Exposure of thallous formate to ⁶⁰Coλ-rays at 77 K gave a species exhibiting a multiline E.S.R. spectrum spread over about 1600 G which is interpreted in terms of $\text{C}\text{}$O₂^−: radicals undergoing strong interaction with two inequivalent thallous ions, a smaller interaction with another pair and weak interaction with a third pair. No Tl(II) centre was detected.

Thallous acetate also gave a multiline spectrum spread over about 500 G, which is assigned to H₂$\text{C}\text{}$CO₂⁻ raidcals interacting with two inequivalent thallous ions, and weakly with two others. A high-field feature assigned to Tl(II) was detected in this case.

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.