International Journal for Radiation Physics and Chemistry最新文献

Electron scavenging efficiencies have been measured at 77 and 4 K in ethylene glycol-water glass for the following scavengers which span a 250-fold range of scavenger efficiencies at 77 K: HCl, NaNO₃ and K₂CrO₄. The range of scavenging efficiencies decreases to 62 at 4 K with the largest relative change occurring for the less efficient scavengers. These results are suggested to be most consistent with a model in which scavenging occurs by tunneling from shallowly and deeply trapped electrons at 4 and 77 K, respectively.

Exposure of aqueous alkaline glasses to ⁶⁰Co γ-rays followed by photobleaching to remove e_t⁻ resulted in E.S.R. spectra dominated by O_t^- and H·_t^-, but at ca. 110 K H·_t was lost and e_t⁻ reappeared.

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.

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).

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.

This paper reviews the work of the author and his co-workers on the radiation-induced formation of excited states of aromatic compounds in solution. The experimental methods used are surveyed and in particular the method of measuring the yields of triplet and singlet excited states of the solute are described. The problems discussed are: (1) the effect of solvent on the yields of excited states, (2) formation of excited states in cyclohexane and other alicyclic hydrocarbons, (3) the formation of excited states in benzene and (4) the identification of T-T absorption spectra.

Previous observations on the spectral changes due to electrons in 1-propanol have been extended to 77 K. It is found that down to 118 K the decay in the I.R. and growth in the visible are first order and that from 118 to 313 K the rate constants fit the modified Arrhenius expression of Tamman and Hesse, as do the relaxation times obtained from dielectric dispersion measurements and the alcohol viscosity. These changes are considered to arise from molecular reorientation around the electrons. Below 118 K, the kinetics are no longer first order, the changes are much more extended in time and are also much faster than expected from an extrapolation of the observations at the higher temperatures. Electron loss, presumed due to ion recombination, also occurs. It is concluded that the mechanism responsible for the spectral shifts is different at these lower temperatures and that here thermal excitation from shallow traps and retrapping by deeper ones occurs.

The role of excited molecules and excitation transfer in the radiolysis of solid alkanes at 77 K is described. Studies of the effect of electron scavengers in the radiolysis of solid alkanes show that hydrogen formation is mainly due to a nonionic process. Though the yield of hydrogen in the radiolysis of solid isobutane at 77 K is not affected by the addition of N₂O or SF₆, it decreases sharply upon the addition of CCl₄. Similarly, the yield of C₄H₉ radical in the radiolysis of isobutane at 77 K is not changed by the presence of conventional electron scavengers; however, it decreases remarkably upon the addition of CCl₄ or toluene. When 2,3-dimethylbutane (23DMB) containing a small amount of toluene (To) is irradiated with γ-rays or nanosecond pulses of X-rays in the solid phase at 77 K, luminescence from excited toluene is observed. The emission spectrum from 23DMB-To (2 mol%)-N₂O(2 mol%) during γ-irradiation at 77 K is similar to that from 23DMB-To (2 mol%) during U.V.-illumination. The formation of the singlet-excited toluene is not affected by the addition of electron scavengers and hole scavengers. These results are explained by excitation transfer from alkane to additives. The possibility of exciton migration in the radiolysis of solid alkanes is discussed by a simple theoretical treatment.

The effects of γ-radiation on aqueous solutions of myo-inositol hexasulphate in the free acid form have been studied. The results have been explained on the basis of radical initiated chain processes which lead to the observed high G-values for the ultimate inorganic product namely the sulphate ion and to low hydrogen peroxide yields. In addition, steady-state competition procedures and pulse radiolysis experiments show that the reaction between the OH and the substrate is slow.