International Journal for Radiation Physics and Chemistry最新文献

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.

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 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.

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.

Radiolysis of frozen aqueous or methanolic glasses containing sodium borohydride gave E.S.R. features characteristic of ·BH₃⁻ as the major radical product. These radicals were strongly associated with the solvent lattice in contrast with their behaviour in the parent salt. Additional E.S.R. features separated by ca. 200 G are assigned to BH₄, but since this spectrum could never be obtained in the absence of the intense BH₃⁻ features, this assignment is tentative.

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.

Methods are outlined for compiling optical spectra obtained in pulse radiolysis experiments in a form suitable for detailed synthesis and analysis of composite spectra. The experimental data are processed with a programmable calculator having a cassette recorder for storage of the output data files and a peripheral plotter. The spectra are first smoothed by fitting them parabolically segment by segment. The overall spectrum is then assembled in digital form by interpolating the fitted data on a 1 nm grid and the results are stored on cassette files for further processing. Composite spectra can be readily calculated and plotted from the data on these files or known components can be subtracted from observed spectra to examine underlying contributions. The use of the fairly simple data processing methods described here permits an interactive mode of operation by the investigator which can maximize insight into details of the various contributions to an observed spectrum. Several examples of the use of these methods in conjunction with data obtained with a computer controlled pulse radiolysis data acquisition system are given.