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

The pulse radiolysis of low density polyethylene (LPDE), using optical absorption detection and emission spectroscopy, has been investigated in the temperature range 6–150 K, the experiments being complemented by a study of the photoluminescence.

It was found that excitons can be autolocalized at temperatures below 20 K, and this process is strongly dependent on the thermal history of the sample.

The influence of high dose levels up to 1 MGy of both γ- and neutron radiation on the epoxy resin system MY750/HY956 has been studied using d.c. conductivity, thermally stimulated discharge current and differential scanning calorimetry measurements. Inter-related post-irradiation changes in electrical and structural properties have been observed, and these are influenced by annealing processes. A qualitative explanation of the observed effects has been made.

A mechanism is proposed for electron transport in inhomogeneous systems in which the dimensionality of space where diffusion or hopping takes place can be reduced. Instead of bringing an electron to a small target (physical trap or a localized hole) by probing three-dimensional space the mechanism reduces the process to a sequence of fast processes in one- or two-dimensional subspaces.

The model is applicable to the inhomogeneous media of fibre-like structures. We assume that the mobile charge carriers occupy the quasi one-dimensional, conducting polymers. The polymer fibres are separated by the non-conducting regions which are accessible for electrons only by quantum-mechanical tunnelling. The three-dimensional tunnelling of the electron to its target is thus reduced to the fast one-dimensional diffusion along the polymer chain coupled with the two-dimensional tunnelling from the polymer to the target.

This mechanism was assumed as a basis for theoretical calculations in two cases. The first case corresponds to the electron reaction with homogeneously-distributed physical traps. The expressions for the trapping rate constant and the electron survival probability as functions of time were derived. The second application of the model provides the description of the electron-hole recombination in pulse-irradiated columnar aggregates of peripherally octa-n-alkoxy-substituted phthalocyanines. The charge recombination occurs via intercolumnar electron tunnelling through the hydrocarbon mantle coupled with free movement of charge carriers along the phthalocyanine cores of the columns. The carrier survival probability was calculated as a function of time.

The polymer electrolyte formed with poly(ethylene oxide), P(EO) and LiClO₄ in the n = 8:1 ratio (n = [EO units]/[Li]) was exposed to γ-radiation at room temperature in aqueous solution, then dried and characterized using thermal and conductivity measurements. Its improved electrical and mechanical properties, in comparison with those of the non-irradiated material, suggest that cross-linked P(EO)₈·LiClO₄ can advantageously replace the usual polymer electrolyte of the same composition in electrochemical devices in the solid state.

The nature of the radicals contributing to the room temperature spectrum of irradiated “dry” DNA, with special reference to the central structure, is discussed, and the thesis of their ionic origin tested by irradiation experiments with intercalators. The mechanism of spin transfer protein→DNA has been investigated through a comparative ESR study on the DNA-histones complex, the structureless random molecular mixture of the DNA-histones and the neat components. The yield of spin transfer is enhanced in the random mixture, presumably because of the greater efficiency of molecular contacts. Evidence of the scavenging of electrons by the thymine and cytosine bases, as a key mechanism for the spin transfer, has been obtained.

Radioluminescence from γ and electron pulse-irradiated polyethylene at low temperatures (77–298 K) was investigated. The influence of sample morphology, alkane solvent soaking and aromatic admixtures (naphthalene, pyrene, difenyl etc.) on isothermal and radiothermoluminescence was examined.

The mechanism of charge trapping and radioluminescence generation in irradiated polyethylene is discussed. Fast exciton migration followed by molecular ion rearrangement and recombination seem to be the main processes responsible for excited state formation.

The simulation of the transient d.c. conductivity in a quasi one-dimensional system of charges produced by a pulse of ionizing radiation in a solid sample has been performed. The simulation is based on the macroscopic conductivity equations and can provide physical insight into d.c. conductivity measurements, particularly for the case of transient currents in samples with internal space charge.

We consider the system of mobile (negative) and immobile (positive) charges produced by a pulse of ionizing radiation in the sample under a fixed external voltage V₀. The presence of space charge results in an electric field which is a function of both the spatial and the time variable: E(z, t). Given the space charge density, the electric field can be calculated from the Poisson equation. However, for an arbitrary space charge distribution, the corresponding equations can only be solved numerically.

The two non-trivial cases for which approximate analytical solutions can be provided are:

• 1.

  (i) The density of the current carriers n(z, t) is negligible in comparison with the density of immobile space charge N(z). A general analytical solution has been found for this case using Green's functions. The solutions for two cases, viz. the homogeneous distribution of space charge N(z) = N, and the non-homogeneous exponential distribution N(z) = A exp(-Bz), have been separately discussed.

• 2.

  (ii) The space charge created in the pulse without any space charge present prior to the irradiation.

A programme of experiments has been undertaken to examine transient charging/discharging currents and steady state currents in low-density polyethylene (LDPE) under the application of direct fields. This has been undertaken for pristine material and for material which has received doses of radiation between 10⁴ and 10⁶ Gy from either a ⁶⁰Co γ-source or a research reactor. The material was irradiated in ambient air or dry nitrogen. Measurements were made for applied fields in the range 6.7 × 10⁵-5.3 × 10⁷ V m ^-1 and temperatures between ambient and 90°C. With pristine material at low fields, transient charging/discharging currents decreased monotonically with time. However, the mechanism changed at higher fields with a peak occurring in the charging transient indicating a space-charge limited process. Substantial charge injection was also in evidence as demonstrated by anomalous discharging currents. This transient response was echoed by the current/voltage characteristics of steady state behaviour. Gamma and neutron irradiation brought about a change in this situation and the charge transport mechanism altered gradually from space-charge-limited conduction to an ohmic process with increasing dose. The role played by charge traps appears to be significant.