Showing papers in "Radiation Physics and Chemistry in 1997"

Improvement of radiation resistance for polytetrafluoroethylene (PTFE) by radiation crosslinking

Abstract: Crosslinked polytetrafluoroethylene(PTFE) was prepared by electron beam irradiation in the molten state of PTFE at 340 ± 3°C in inert gas atmosphere. The crosslinking density was changed by the irradiation dose. Radiation resistance of the crosslinked PTFE was examined on the mechanical properties after γ-ray irradiation at room temperature under vacuum and in air. Dose at a half value of elongation at break was about 1 MGy for 500 kGy-crosslinked PTFE, while dose for non-crosslinked PTFE was only 3.5 kGy. It has been found that the radiation resistance of PTFE can be improved to large extent by crosslinking.

Flavonoids function as antioxidants: By scavenging reactive oxygen species or by chelating iron?

Abstract: Flavonoids have been reported to exhibit strong antioxidative activity. In the present work, a systematic mechanistic study has been performed on five flavonoids (baicalin, hesperidin, naringin, quercetin and rutin) selected according to their structural characteristics. The experimental results reveal that flavonoids function as antioxidant mainly by chelating iron ions and by scavenging peroxyl radicals whereas their OH radical scavenging effect is much less important.

Effective atomic numbers for materials of dosimetric interest

Abstract: Effective atomic numbers ( Z eff ) for different materials of dosimetric interest have been calculated for total photon interaction in the energy region 1 keV–20 MeV. The calculations are made using the mass attenuation coefficients data from Tables of X-ray Mass Attenuation Coefficients and Mass Energy-absorption Coefficients 1 keV to 20 MeV for elements Z = 1 to 92 and 48 Additional Substances of Dosimetric Interest , J. H. Hubbell and S. M. Seltzer (Hubbell and Seltzer, 1995), NISTIR-5632. The variation of Z eff value with energy is discussed.

An overview of the theories used in compton scattering calculations

Abstract: This paper discusses theoretical approaches for investigating Compton scattering from bound atomic electrons. It particularly emphasizes the understanding of methods currently used to obtain the scattered photon energy distribution at fixed scattering angle. This distribution is dominated by three features: the Compton line or peak, the resonant-Raman-Compton scattering and the infrared divergence at soft scattered photon energies. No single common approximation accesses all three regions, though they are all exhibited analytically in the nonrelativistic hydrogenic case. S-matrix calculations describe all of these features. The usual approximations utilized for Compton scattering are discussed and assessed in comparison with the more exact, relativistic S-matrix calculations. Finally, some recent developments in the theory of Compton scattering from bound electrons are noted. These include the effort to describe the double ionization of helium by Compton scattering and the theoretical analysis of the more completely differential Compton scattering experiment in which the ejected electron is also observed.

Inelastic photon scattering : Effects and applications in biomedical science and industry

Abstract: Compton scattering is widely used to analyse electron momentum distributions in solid state systems. Perhaps less well-known is its role as the major cause of image contrast in medical and industrial radiography. This article discusses the principles and applications of a technique, known as Compton scatter imaging (CSI), which is based on direct registration of the Compton scatter radiation. Following a historical survey of the major developments in this field, the strengths and weaknesses of transmission radiography and Compton scatter imaging are compared in order to determine those measurement situations to which the latter is best suited. A description is given of several disturbing effects to which CSI is prone and ways in which these may be accounted for to yield quantitative density data are presented. The most important types of imaging system based on Compton scatter are discussed and compared. The ComScan (an acronym for Compton scatter scanner) is a commercially-available backscatter imaging system which is discussed here in some detail. ComScan images taken from applications of topical and historical interest are presented.

ESR study on free radicals trapped in crosslinked polytetrafluoroethylene (PTFE)

Abstract: Free radicals in crosslinked PTFE which formed by 60 Co γ-rays irradiaion at 77 K and at room temperature were studied by electron spin resonance (ESR) spectroscopy The crosslinked PTFE specimens with different crosslinking density were prepared by electron beam irradiation in the molten state The ESR spectra observed in the irradiated crosslinked PTFE are much different from those in non-crosslinked PTFE (virgin); a broad singlet component increases with increasing the crosslinking density, G -value of radicals is much higher in crosslinked PTFE than in non-crosslinked one Free radicals related to the broad component are trapped in the non-crystalline region of crosslinked PTFE and rather stable at room temperature, whereas radicals trapped in amorphous non-crosslinked PTFE are unstable at room temperature It is thought that most of free radicals trapped in the crosslinked PTFE are formed in the crosslinked amorphous region The trapped radicals decays around 383 K (110°C) due to the molecular motion of α-relaxation

Hydroxyl radical scavenging ability of bacterioruberin

Abstract: A red carotenoid pigment, bacterioruberin, was extracted from Rubrobacter radiotolerans. The OH scavenging effect of this pigment was studied using a system of thymine degradation and compared with those of cysteine and β-carotene. Thymine solution (5 × 10−4 mol/dm3 with 0.2% SDS buffered at pH 7.0) was irradiated with 60Co γ-rays in the presence and absence of the scavengers. We found that the dose reducing factor (5.3) of bacterioruberin is much higher than that (2.5) of cysteine and β-carotene.

Summary of existing information on the incoherent scattering of photons, particularly on the validity of the use of the incoherent scattering function

Abstract: X-ray scattering by individual atomic electrons is known as Compton scattering if the simplifying approximation is invoked that the target electrons are initially free and rest, in which case the differential and integral scattering cross sections can be computed using the compact analytical expressions given by Klein and Nishina in 1929. In real atoms the electrons are neither free nor at rest, resulting in departures from the Klein-Nishina expressions. These departures have been estimated theoretically in varying degrees of sophistication, with correspondingly varying degrees of ease in applying these corrections to the free electron model. At present the most widely used treatment of these departures is via the incoherent scattering function S(x,Z) in which x is a momentum transfer variable dependent on the incident photon energy and the scattered-photon deflection angle, and Z is the atomic number of the atom associated with the target electron. The incoherent scattering function S(x,Z) also enters into calculations of the cross section for electron-positron pair production in the fields of the atomic electrons, known also as triplet production. This report includes a historical survey of experimental and theoretical work on the Compton effect and on the incoherent scattering function. The validity of the use of the incoherent scattering function is examined and discussed.

Time development of diffusion-limited oxidation profiles in a radiation environment

Abstract: We have previously presented a quantitative model that combines diffusion and kinetic expressions with chemical/mechanical property relationships to predict the time development of diffusion-limited oxidation profiles in polymers. This model was shown to be applicable to thermal aging environments and should be generally applicable to environmental stresses for which total damage to the material is directly proportion to total exposure time. In this work, we use a fluorocarbon elastomer to demonstrate applicability of the model to ionizing-radiation environments.

Temperature effects on radiation induced phenomena in polytetrafluoroetylene (PTFE)—Change of G-value

Abstract: Temperature dependencies on the radiation induced phenomena and G-value of polytetrafluoroethylene (PTFE) have been studied in a temperature range 77–653 K. It is well known that main chain scission occurs very effectively below the melting temperature of PTFE (600 K). We have found in our experiments that G-value of chain scission increases significantly with increasing irradiation temperature, until 600 K. In addition to that, we have realized that crosslinking occurs by irradiation in the molten state at 613 K (Tabata, 1992; Oshima et al. , 1995; Tabata et al. , 1996). In the molten state, G-value of crosslinking was found to be 0.35 (number of crosslinking/100 eV absorption), as a lower limit, and the apparent G-value of chain scission is drastically reduced. Above 633 K, radiation induced crosslinking mainly occurs, however parallel thermal depolymerization or decomposition takes place to some extent.

Change of molecular motion of polytetrafluoroethylene (PTFE) by radiation induced crosslinking

Abstract: The mechanical relaxation of radiation crosslinked PTFE was investigated in conjunction with molecular motion by means of dynamic visco-elastic property measurement in a temperature range from −150 to 350°C under vacuum. The molecular motion is very much affected by crosslinking, that is, the logarithmic decrement of β relaxation (19 and 30°C) quickly disappears, the γ relaxation (−97°C) shifts to a higher temperature and α relaxation (130°C) shifts to a lower temperature, as the crosslinking density increases. The γ relaxation is assigned to be a glass transition temperature and the α relaxation to be the transition in molecular motion of inter-polymer chains among either crystallites or crosslinking sites.

Compton scattering and the study of electron momentum density distributions

Abstract: The Doppler-broadening experienced by photons Compton scattered from bound electrons allows the momentum distribution of the target to be probed. The Compton profile so obtained turns out to be sensitive to the wave-mechanical description of the scatterer and has therefore been exploited, for example, to check band theoretical models of solids, to identify electron correlation effects and most recently to extract Fermi surface topography. The technique has suffered from the low photon energy and lack of monochromaticity of X-ray tube sources and the weakness of gamma emitting isotopes. Both these problems have been circumvented with the use of synchrotron radiation which has the added benefit of allowing spin densities to be studied through the extraction of circularly polarised photons. In this paper the interpretation of Compton line-shapes within the Impulse Approximation will be discussed and illustrated with a selection of recent results.

Degradation of monochlorophenols by γ-irradiation

Abstract: The degradation of aqueous 5 × 10 −4 mol dm −3 2-, 3- and 4-chlorophenol (2-CIP, 3-CIP, 4-CIP) under reducing and oxidizing conditions has been studied by steady state γ-radiolysis. The reaction with e aq − leads to reductive dechlorination which equals the degradation of the chlorophenols. Solely phenol is the major organic compound formed. In N 2 O saturated solutions the dehalogenation is much less effective. In the case of 4-CIP 40% and in that of 2- and 3-CIP only 20% of the decomposed substrates are resulting in chlorine free organic compounds. From these pyrocatechol for 2-CIP and hydroquinone for 4-CIP could be identified in minor amounts. Main products are the hydroxylated substrates originating from the primarily formed ortho- and para OH-adducts. These are for 2-CIP: chlorohydroquinone and 3-chlorocatechol, for 3-CIP: chlorohydroquinone, 3- and 4-chlorocatechol and for 4-CIP: 4-chlorocatechol. In the presence of oxygen, where the OH-adducts are converted into peroxyl radicals, the degradation yield of the substrates corresponds to that of OH radicals up to doses of 0.5 kGy. The dechlorination process in this dose range is 50% for 4-CIP, and 33% for 2- and 3-CIP. The position of chlorine on the aromatic ring strongly influences the dehalogenation and degradation process. A pronounced dependency of the dose/yield curves on the oxygen concentration was observed. This indicates that for an efficient degradation process of chlorophenols oxygen supply is necessary during irradiation.

Radiation induced physical and chemical processes in zeolite materials

Abstract: Ionic processes induced by high energy radiation in zeolites, including electron and hole trapping and related chemical reactions, are reviewed in this paper. Electronic structures of electrons localized in clusters of charge balancing cations and those solvated in zeolite confined water clusters are characterized by a combination of spectroscopic techniques such as ESR and transient UV–visible absorption. Reactivities of these trapped electron species confined in a nanometer dimension are understood in correlation with their structural characteristics. Positive charge transfer from the zeolite framework to molecular adsorbates and subsequent oxidation reactions are also examined.

Spin correlation and magnetic field effects in radiolysis

Abstract: When pairs of radicals meet at random the reaction probability is limited to one quarter by spin considerations, unless spin relaxation is exceptionally fast. Conversely, dissociation or ionisation from a singlet state can always be followed by geminate recombination, providing the initial spin correlation is retained. The spin correlation must decay eventually because of spin relaxation but usually a faster process intervenes—spin evolution due to hyperfine coupling. This gives rise to magnetic field effects: so too does electronic fine structure (zero-field splitting) in processes involving pairs of triplet excited states. The role of these processes in radiation chemistry is reviewed and the possible future applications of magnetic studies are discussed.

Experimental studies of inelastic X-ray and γ-ray scattering

Abstract: Studies of inelastic scattering of X-rays and γ-rays of energies higher than 30 keV performed with the help of NaI(Tl) and semi-conductor detectors of moderate and good resolution, respectively, are considered in the first part of this review. Theoretical treatments based on the non-relativistic ( e 2 /2 mc 2 ) A 2 interaction term, the impulse approximation, the incoherent scattering function approximation, the (− e / mc ) P·A interaction term, and the relativistic second order S matrix approach have been used to understand a few representative experimental results concerning Compton scattering from K- and L-shells, and from whole atoms for values of the momentum transfer parameter x larger than about 2 A −1 , where x = sin( θ /2)/ λ , θ is the scattering angle and λ is the wavelength of the incident radiation. Techniques of detection of recoil ions have been described in the context of recent measurements of relative probabilities for double and single ionization in Compton scattering from helium. The second part of this review deals with high resolution measurements concerning inelastic scattering of X-rays of energies lower than about 20 keV. A recently demonstrated dependence of K and L X-ray fluorescence spectra on incident photon energy near respective thresholds is highlighted along with its explanation based on crystal momenta and a single step resonant inelastic scattering treatment. Non-resonant inelastic scattering studies involving the excitation of plasmons and phonons in condensed matter are also discussed. Nonlinear photon-density dependent effects interpreted as Compton scattering, and final remarks are then presented.

The effect of additives on the enhancement of methyl methacrylate grafting to cellulose in the presence of UV and ionising radiation

Abstract: The role of various additives in the grafting of methyl methacrylate (MMA) to cellulose using UV and ionising radiation has been investigated. MMA grafting yields are hampered by competing homopolymerisation. The styrene comonomer technique was utilised to overcome this problem. The role of acid in these reactions has been studied as well as additives like, an inclusion compound (urea), thermal initiators and photoinitiators. Methanol was used as the swelling agent in all the experiments. Molecular weight studies with homopolymers indicate that both chemical and physical processes are involved in the mechanism of the reaction. The physical process involves a partitioning phenomenon whereas the chemical process concerns additional radical reactions in the radiation initiation step. This grafting mechanism is shown to be applicable to the simple radiation polymerisation of monomers in solution and also analogous UV fast curing systems.

Temperature effect on radiation induced reactions in ethylene and tetrafluoroethylene copolymer (ETFE)

Abstract: Ethylene and tetrafluoroethylene copolymer (ETFE) was irradiated by γ-rays or electron beam (EB) under oxygen-free atmosphere at various temperatures ranging from 77 to 573 K. Mechanical and thermal properties, and absorption spectra of the irradiated ETFEs were measured. The mechanical properties of the film have been observed to change by irradiation. The modulus and yield strength increase with increasing dose, and these phenomena are clearly distinguished above the melting temperature of ETFE (533 K). Heat of crystallization changes drastically as a function of irradiation dose around the melting temperature, compared with other temperatures. The absorption band around 250 nm of irradiated ETFE shifts to a longer wavelength region with increase of irradiation temperature. Therefore, it was concluded from those experimental results mentioned above that crosslinking takes place and conjugated double bonds formation proceeds in a wide range of irradiation temperatures. Those reactions are enhanced by increasing temperature. The homogeneous crosslinking takes place in the molten state, while the heterogeneous crosslinking does in the crystalline solid state.

Hardness measurements of silicone rubber and polyurethane rubber cured by ionizing radiation

Abstract: This work investigates the hardness of both silicone rubber and polyurethane rubber cured by ionizing radiation. Shore A hardness is used to characterize the subject elastomers in relation to the crosslinking process. Various formulations of both materials have been investigated in order to achieve the optimum cure conditions. A small amount of a chemical curing agent has been incorporated in some formulations in order to reduce the required dose to achieve full cure conditions. Silicone rubber improved in hardness with increasing absorbed dose, whereas hardness remained constant over a broad range of absorbed doses for polyurethane rubber.

Radiolytic dechlorination of chlorinated organics

Abstract: The radiolytic dechlorination of 12 low molecular weight chlorinated organic compounds present in pulp mill effluent was investigated. For most of these chloro-organic compounds more than 90% dechlorination was obtained for gamma doses up to 20 kGy. Parameters such as the number of chlorine atoms and aqueous solution concentration were found to affect the dechlorination rate. A reaction set was also created to model the behavior of irradiated 0.49–49 mol m −3 chloroform solutions, giving good agreement with experimental results.

Effects of gamma irradiation on physicochemical properties of Korean red ginseng powder

Abstract: Gamma irradiation was applied to Korean red ginseng powder to improve its quality. Major physicochemical properties (approximate composition, pH, acidity, browning pigment, hydrogen donating activity, fatty acids, minerals and saponin) were not significantly changed by gamma irradiation up to 10 kGy. The TBA value was increased depending on the increment of irradiation dose level. In free amino acids, threonine was increased while, serine and glutamic acid were decreased by gamma irradiation. In total amino acids, total contents were not significantly changed by gamma irradiation though tyrosine was slightly decreased P ⩽ 0.05. In free sugar, glucose, sucrose and maltose were significantly increased by 7.5 and 10 kGy gamma irradiation P ⩽ 0.05

Measurement of radiative vacancy distributions for the L2, L3 subshell and M shell of some elements with atomic range 69≤Z≤92

Abstract: K to L 2 , L 3 subshell and M shell a radiative transitions were measured using an energy disersive X-ray spectroscopy method. It was observed that these values are decreased with increasing atomic number. Present results were compared with previous theoretical results.

Study on jute material with urethane acrylate by U.V. Curing

Abstract: Twenty different formulations were developed with urethane acrylate in the presence of different additives and co-additives. These formulations were applied to prepare different polymer films under u.v. radiation. Physical and mechanical properties of these cured films were studied. Jute material (hessian cloth) was coated with these formulations and cured under u.v. radiation. Different properties such as degree of polymer uptake, tensile properties, water absorption ability and degradability were studied. It was observed that the treated jute materials gained up to 97% polymer uptake, 115% tensile strength and 60% elongation over the untreated jute materials. The material behaved as a synthetic polymer with respect to durability in rain, water and soil. However, it was completely degraded by burying the materials in mud that contains 30% water.

Heat shrinkage of electron beam modified EVA

Abstract: Heat shrinkage of electron beam modified ethylene vinyl acetate copolymer (EVA) has been investigated over a range of times, temperatures, stretching, irradiation doses and trimethylolpropane trimethacrylate (TMPTMA) levels. The irradiated (radiation dose 50 kGy and TMPTMA level 1%) and stretched (100% elongation) sample shrinks to a maximum level when kept at 453K temperature for 60 s. The heat shrinkage of samples irradiated with radiation doses of 20, 50, 100 and 150 kGy increases sharply with increasing stretching in the initial stage. Amnesia rating decreases with increasing radiation dose and TMPTMA level as well as gel content. The high radiation dose and TMPTMA level lower the heat shrinkage due to the chain scission. The effect of temperature at which extension is carried out on heat shrinkage is marginal. The irradiated (radiation dose 50 kGy and TMPTMA level 1%) EVA tubes of different dimensions expanded in a laboratory grade tube expander show similar behaviour at 453K and 60 s. The X-ray and DSC studies reveal that the crystallinity increases on stretching due to orientation of chains and it decreases to a considerable extent on heat shrinking. The theoretical and experimental values of heat shrinkage for tubes and rectangular strips are in good accord, when the radiation dose is 50 kGy and TMPTMA level 1%.

Reaction of long-lived radicals and vitamin C in γ-irradiated mammalian cells and their model system at 295 K. Tunneling reaction in biological system

Abstract: When golden hamster embryo (GHE) cells or concentrated albumin solution (0.1 kg dm −3 ) that is a model system of cells is irradiated with γ-rays at 295 K, organic radicals produced can be observed by ESR. The organic radicals survive at both 295 and 310 K for such a long time as 20 h. The long-lived radicals in GHE cells and the albumin solution react with vitamin C by the rate constants of 0.007 dm 3 mol −1 s −1 and 0.014 dm 3 mol −1 s −1 , respectively. The long-lived radicals in human cells cause gene mutation, which is suppressed by addition of vitamin C. The isotope effect on the rate constant ( k ) for the reaction of the long-lived radicals and vitamin C has been studied in the albumin solution by use of protonated vitamin C and deuterated vitamin C. The isotope effect ( k H / k D ) was more than 20 ≈ 50 and was interpreted in terms of tunneling reaction.

Laser-generated silenes and their gas-phase polymerization

Abstract: Infrared and excimer laser-powered decompositions of silacyclobutanes (RH, CH 3 , CH 2 CH and HCC), 4-silaspiro[3,4]octane and 4-silaspiro[3,4]octane and 4-silaspiro[3,3]heptane in the gas phase are shown to be dominated by elimination of ethene to yield transient silenes which efficiently polymerize into solid polycarbosilane deposits, potential precursors to silicon carbide. IR laser powered decomposition of tetramethylcyclotetrasiloxane affords transient hydroxy(methyl)silylene. The organosilicon transients were identified by chemical trapping experiments.

An atomic force microscopic study of the surfaces of polyethylene and polycarbonate films irradiated with gamma rays

Abstract: The changes in the surface topology of polyethylene and polycarbonate films irradiated with gamma rays up to 500 kGy doses have been investigated with Atomic Force Microscopy. The cumulative effect of irradiation has been followed by analyzing exactly the same region of the surfaces after every irradiation step. This has been achieved by applying an addressing operation on the surfaces of virgin polymers. The increase in surface roughness showed a sudden increase up to 100 kGy dose and remained almost unchanged thereafter. The radiation-induced topological changes are found to correlate with the chemical effects of these rays.

Determination of a fraction of spin-correlated radical ion pairs in irradiated alkanes by quantum oscillation technique

Abstract: Amplitudes of quantum oscillations observed in the recombination fluorescence of the (diphenylsulfide-d10)+/(p-terphenyll-d14)− radical ion pairs in alkanene solutions have been used to estimate a fraction of spin-correlated singlet pairs Θ in the radiation track. Θ values of 0.36 – 0.54 for the solvent series studied were found. Values for a fraction of singlet recombinations fs (0.52–0.66), calculated from Θ values, are in satisfactory agreement with those available from literature. It has been observed that Θ and fs values decrease with increasing solvent viscosity (from n-hexane to cis-decalin).

Electron beam degradation of polychlorinated biphenyls

Abstract: A gradual degradation of polychlorinated biphenyls (PCBs) was found under the influence of accelerated electrons (4.5 MeV) absorbed in alkaline 2-propanol solutions containing 0.3–1.0 vol.% of PCBs. A degree of dechlorination grows with increasing dose of irradiation from 2 to 100 kGy. The inhibiting effects of acetone as well as of oxygen were observed.

Analysis of some products from the irradiation of solid chloramphenicol

Abstract: After radiation sterilization, it is always necessary to demonstrate that any products formed in the irradiation are not harmful. The amounts of products formed in the gamma-irradiation of solid samples may be so small, that standard toxicity tests could be ineffective. Hence, analysis of the final products in the radiosterilized solid samples might be required. In this work, some chloramphenicol degradation products that are unique to radiolysis, i.e. different from the normal degradation products, were detected.