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

An energy spectrum of “subexcitation electrons” produced in liquid water by electrons with initial energies of a few keV is obtained by using a Monte Carlo transport simulation calculation. It is found that the introduction of vibrational-excitation cross sections leads to the appearance of a sharp peak in the probability density function near the electronic-excitation threshold. Electrons contributing to this peak are shown to be more naturally described if a novel energy spectrum, that we propose to name “vibrationally-relaxing electron” spectrum, is introduced. The corresponding distribution function is presented, and an empirical expression of it is given.

Pulse radiolysis has been carried out on aqueous solutions of silver ions complexed with polycrylate (PA). Over the time scale 10⁻⁸−10 s, only a few kinetic steps have been observed which have been identified through spectroscopic and kinetic analysis. The reaction rate of e^-_aq with Ag⁺ is lowered in the presence of PA (3.6×10⁹lmol⁻¹s⁻¹ when [PA]/[Ag⁺]=100). The first products (Ag⁰ λ_max≈360 nm, Ag⁺₂ λ_max=310 nm) are similar to those obtained in the absence of PA, except for an additional absorption band (350–450 nm) which indicates that Ag⁺₂ is itself complexed. Then through dimerization (2k=3×10⁹lmol⁻¹s⁻¹) and reaction with the alcoholic radical (in the presence of 2-PrOH, 1.5×10⁹lmol⁻¹s⁻¹), Ag⁺₂ leads to an “Ag₂” species peaking at 300, 320 and 360 nm, noticeably different from the reported Ag²⁺₄ species. Two further steps are observed which are assigned to a ligand reorganization of “Ag₂” in the millisecond range (10³s⁻¹), followed by a coalescence to “Ag₄” in the range of seconds (10⁵lmol⁻¹s⁻¹). This last product is thought to be the stable Ag_a cluster known to be formed by γ-radiolysis. A kinetic simulation accounts for the whole mechanism which is much lower than in the absence of PA.

Protection effect of aromatic (naphthalene, phenanthrene) and hydroaromatic (tetralin, octahydrophenanthrene) compounds in irradiated liquid alkane has been investigated as a model experiment for the radiolysis of polymers. Liquid n-hexadecane has been selected as the model linear amorphous polymer and the protection effect has been studied by analyzing the radiolysis products. In the presence of additives, degradation of n-hexadecane, reduction of radiolysis products, including H₂ evolution, and selective degradation of the additive have been observed. Energy transfer (excitation and charge) to the additive reduces the solvent excitation which is the precursor of the radiolysis products. The additive reacts with the solvent molecules according to its reactivity to form adduct compounds. Mass analysis shows that these adduct compounds have a smaller number of double bonds in their aromatic rings due to hydrogenation. Aromatic compounds are more reactive than hydroaromatic compounds towards n-hexadecane and their adducts consume more hydrogen than those of the hydroaromatic compounds, and therefore aromatic compounds reduce gas formation more effectively than hydroaromatic compounds. From the effect of aromatic compounds, it is concluded that hexadecene is produced neither by H₂ elimination from the lowest excited state of n-hexadecane nor by disproportionation of n-hexadecyl radicals, but is mainly produced directly from the decomposition of the excited cations of n-hexadecane.

Radiation-induced radicals in sulfite-doped calcite and aragonite were characterized by ESR. Among the many ESR signals produced, only four signals are dependent on the SO^2-₃ concentration; an axial signal (g_⊥=2.0038 and g|=2.0024) in calcite, and an orthorhombic signal (g₁=2.0042, g_2=2.0038 and g₃=2.0025), an isotropic signal (g=2.0034) and an isotropic signal (g=2.0060) in aragonite. The former two signals exhibit hyperfine structures arising from ^33S$\text{(I=}\text{3}\text{2}\text{, 0.75\%}$ natural abundance), and are attributed to SO^-₃ radicals. The isotropic signal at g=2.0034 in aragonite is considered to be due to rotating SO^-₃ radicals. Rotating or tumbling molecules (probably SO^-₂) produced by thermal decomposition of SO^2-₃ seem to be responsible for the isotropic signal at f=2.0060.

Wood-plastic composite (WPC) has been prepared with simul using acrylamide (AM), a water soluble monomer, in place of styrene (ST) and butylmethacrylate (BA). The highest polymer loading (PL) is achieved with AM along with the highest tensile strengths (TS) among the three bulk monomers studied. Effect of urea, NVP and TMPTA has been investigated in these systems. Co-additive (urea) has played a significant role in presence of NVP and TMPTA with AM compared to ST and BA systems. Methanol, water and water/methanol mixtures have been used as swelling agents in order to study their effect on PL and T_f values. TS loss due to the weathering effect is minimum with the WPC, particularly if prepared with swelling agent methanol.

Additive effects of hydroaromatics towards radiation degradation and post-oxidation of high density polyethylene and isotactic polypropylene were investigated. At the same time, the radical scavenging effects of hydroaromatics were examined using DPPH(N,N-diphenyl-N'-picrylhydrazyl). It was found that:

• 1.

  (1) the addition of HHAP (multi-component type hydroaromatics, produced by the hydrogenation of highly aromatic heavy fraction from petroleum) was obviously effective in inhibiting both radiation (electron beam in air) degradation and post-oxidation of polyolefins,

• 2.

  (2) hydroaromatics (tetralin, octahydrophenanthrene and HHAP) have evident radical scavenging abilities from the tests using DPPH and

• 3.

  (3) the inhibiting effects of HHAP can be attributed to its radical scavenging ability.

Wood-plastic composite (WPC) formation has been studied with simul+styrene system at various compositions of styrene with methanol as the swelling solvent. Effect of additives, e.g. multifunctional monomers (MFM) and oligomers used in very low quantity (1% v/v) on the polymer loading (PL) and tensile strength (TS) of the WPC has been elaborately investigated. Enhanced PL and TS values are observed. Inorganic co-additives like Lithium (Li⁺), Copper (Cu²⁺) and acid (H⁺) and urea (U) used in combinations with additives (MFM or oligomers) have influenced the results of PL and TS in these systems. Li⁺ ion has been a good replacement for H⁺ ion; U has substantially enhanced the PL values with retention of the TS values of WPC. Co-additive Cu²⁺ used in these system can act as a preservative and protective agent for WPC.

The bimolecular rate constants for the reaction of e^-_aq with chlorotoluenes, chloronitrobenzenes and chloronitrotoluenes have been determined from the decay of e^-_aq. With chlorotoluenes, e^-_aq reacts by dissociative electron capture without any transient absorption band in 270–500 nm region. In case of chloronitrobenzenes and chloronitrotoluenes, e^-_aq forms a transient band with λ_max in 290–310 nm region without any Cl^- formation. Hydroxyl alkyl radicals, derived from alcohols, do not seem to react with chlorotoluenes whereas a transient species with λ_max=305–315 nm region are formed on addition of hydroxy alkyl radical to —NO₂ group of chloronitrobenzenes.

Evidence is presented for the radiation-induced transmethylation and transsulfuration in a DNA-methionine model system. The extent of such alkylation of DNA is found to be comparable with that of alkylating agents. Therefore, both processes could be initial steps in radiation carcinogenesis.

The protective effect of methionine on DNA strand breaks, due to scavenging of OH radicals, causes the formation of methyl and thiyl radicals.

The kinetics of the conductivity decay in polyethylene film induced by 6 MeV electron pulses was studied using d.c. conductivity techniques. The influence of external field and radiation dose on the transient current has been examined, and the drift mobility of charge carriers was found to be 3.1 × 10^-2 cm²V^-1s^-1.