International Journal for Radiation Physics and Chemistry最新文献

In this paper, we describe a new experimental method which is used to determine the effect of temperature on the mechanisms related to the detrapping of electrons trapped in a glass. The studied samples are organic vitreous solutions of an aromatic molecule (TMPD) inside a non-polar glass (3-MP or MCH). The intensity of the isothermal luminescence (ITL) following the photoionization of the sample at 77K is increased when applying thermal jumps ΔT≲2K (the rise time is ⋍s).

A general kinetical theory is used to explain the shapes of the luminescence curves perturbed by thermal jumps. It is shown that the experimental observations can be explained in terms of a slow diffusion of the trapped electrons towards a tunneling detrapping zone. When applying a thermal jump ΔT, the intensity of luminescence is multiplied by X such as: $\text{X =}\text{exp}\text{ΔT}\text{T}\text{E}\text{kT}\text{+ Y}\text{1+Y}$.

This relationship is in good agreement with experience. The thermal detrapping activation energy E and the tunnelling effect ratio Y can be determined through this formula. The shapes of the kinetic curves at T = 77K and T = 77·50K are compared in the case of 3-MP glassy samples (near the glass transition, T_g = 77K). It is concluded that there is a slow diffusion of trapped electrons (as it was already shown); the diffusion activation energy (E_d = 0·65eV) is found to be very close to viscosity activation energy (E = 0·65eV) as given by Willard. This last result seems to support the hypothesis according to which the diffusion of trapped electrons is the consequence of the diffusion of the trapping cavities (at T_g).

The e.p.r. method has been used to study the radical stages of low-temperature photolysis and radiolysis of polystyrene and to demonstrate the difference in them due to the photoconversion of the cyclohexadienyl radical. The authors have determined the quantum yield of the reaction of radical formation (ø ≅ 10⁻⁵) which weakly depends on the wavelength of the absorbed light at 230<λ<340 nm and the mean depth (l) at which radicals form (l≅9 ohms m for λ = 254 nm, i.e. in the region of inherent absorption of PS, and l ≅350 ohms m at λ≅300 nm). A shift is observed in the absorption band of polystyrene to the long-wave region owing to the secondary photoreactions of the cyclohexadienyl and peroxide radicals.

1-Choloropropane has been irradiated in the presence of cyclohexene and the formation of hydrogen chloride and of products derived from cyclohexene has been studied as a function of the concentration of this additive. The irradiations, which were performed with accelerated electrons, were characterized by very high absorbed dose rates. Under such experimental conditions, transfer of excitation energy and radical addition to cyclohexene appear to be only of minor importance. The formation of products derived from cyclohexe is initiated predominantly by ion-molecule reactions between cyclohexene and the positive 1-chloropropane ions. The mechanisms of the subsequent reactions are discussed.

In this paper our experiments on isothermal decay of thermoluminescence of the main peak of CaSO₄ : Dy phosphors at various temperatures and ⁶⁰Co γ doses are described. The decays are not monoexponential; however, as the shape of the decay curves at a same temperature is dose independent, first-order kinetics may be assumed. Consequently, the decay curves at 100 rad are broken down in exponential terms and for each term the decay constant and the related partial light sum are obtained. These results are interpreted, at first, by trying to associate with the peak a multilevel distribution of trap depths; however, the activation energies and the frequency factors for these levels are physically unacceptable. Subsequently, on the hypothesis of a continuous trap distribution, the decay curves are best fitted as a whole; in this way a frequency factor of 3 × 10¹² s⁻¹ and a trap distribution with an approximately gaussian main peak centred at about 1 > ṡ37 eV and three other minor peaks are obtained. To check the consistency of the results, a theoretical glow curve is synthesized and a single peak fairly similar to the experimental one emerges.

A study of the radiolysis of monocrystalline, liquid and aqueous solutions of methane sulphonic acid is reported. E.S.R.-measurements on γ-irradiated single crystals show that two radicals ·CH₂SO₂(OH) and CH₃ŻSO₂(OH)⁻ are formed. Sulphonic acid-water mixtures were investigated by pulse radiolysis. A short-lived transient absorption with maximum at 450 nm is assigned to the radical ion CH₃ŻSO₂(OH)⁻ (ϵ = 1740 dm³ mol⁻¹ cm⁻¹). Two longer-lived transient absorptions at 263 and 347 nm in concentrated solution were shifted to 242 and 332 nm respectively with increasing water concentration.

The 242/263 nm absorption is ascribed to the ·CH₂SO₃⁻ radical and its protonated form and the 332/347 nm absorption to the CH₃SO₂· radical formed by the decomposition of the CH₃ŻSO₂(OH)⁻ radical ion. The interpretation of E.S.R. and optical spectra is based on the assumption of a hydrogen bonding structure of the sulphonic acid.

Gas phase radiation chemistry yield data and electron impact cross-section data are used to derive excitation mechanisms and to discuss the role of excited states in the radiation chemistry of O₂, N₂, N₂O, CO, CO₂, H₂S, H₂O and NH₃. For each of these systems available cross-sections for ionization and neutral excitation are listed, together with relevant reaction rate data and a summary of the radiation chemistry studies at both high and low dose rates. In general, fairly complete mechanisms are derived and further tested by energy balance calculations. In order to present as complete a picture as possible, a summary of rates and products of ion-neutralization reactions is given at the end of the paper.

Various picosecond and nanosecond pulse radiolysis studies of excited states in alkane and erene liquids are described. In particular, the origin of the excited states is sought after. In alkanes ion neutralization gives rise to excited triplet states of solutes. Singlet energy transfer from excited alkanes to solutes gives rise to the excited singlet states of the solutes. In aromatic liquids both the excited singlet and triplet state of the solvent are observed, and energy transfer to solutes is readily established. The role of ion neutralization in the formation of excited states is proportionally less pronounced as the observed yield of ions is low. However, indirect experiments suggest that the ion neutralization reaction is too rapid for conventional observation, and even in arenes large yields of excited states are formed by ion neutralization.