Interlaboratory comparison of positron and positronium lifetimes in polymers

A comparison of the results of a series of positron annihilation lifetime measurements performed in 12 laboratories is presented. The measurements were conducted on three different polymer samples, all prepared in one laboratory under standard conditions. The objective of the work was to gain insight into the variation in derived positron and positronium lifetimes and intensities measured in the different laboratories on identical specimens. Lifetime data were collected at room temperature by each laboratory following their own standard measurement and data evaluation procedures. The polymers used were a semi-crystalline linear polyethylene, an amorphous poly(methyl methacrylate) and an amorphous copolymer of styrene-acrylonitrile. The results show that the ortho-positronium lifetimes and intensities agree fairly well between different laboratories, despite large differences in experimental conditions, while the lifetimes and intensities of short-lived components show appreciable variations between laboratories.     

Positron annihilation and X-ray diffraction studies on tin oxide thin films

Positron annihilation spectroscopy along with glancing incidence X-ray diffraction have been used to investigate tin oxide thin films grown on Si by pulsed laser deposition. The films were prepared at room temperature and at 670 K under oxygen partial pressure. As-grown samples are amorphous and are found to contain large concentration of open volume sites (vacancy defects). Post-deposition annealing of as-grown samples at 970 K is found to drastically reduce the number of open volume sites and the film becomes crystalline. However, film grown under elevated temperature and under partial pressure of oxygen is found to exhibit a lower S-parameter, indicating lower defect concentration. Based on the analysis of experimental positron annihilation results, the defect-sensitive S-parameter and the overlayer thickness of tin oxide thin films are deduced. S-W correlation plots exhibit distinct positron trapping defect states in three samples.     

On features of the radiation from an electron moving along a helix inside a cylindrical hole in a homogeneous dielectric

The radiation from a charge moving along a helical trajectory inside a cylindrical hole in homogeneous dielectric medium is investigated. Prompted by availability of materials with large dielectric permittivity ε and small absorption, we discuss the features of this type of radiation for media with ε?1. It is shown that there are high peaks in the angular distribution of radiation intensity at well-defined harmonics. The conditions are specified for the cavity-to-helix radii ratio, ρ₁/ρ₀, under which the angle-integrated radiation intensity on some harmonics exceeds that in the empty space. Though the amplification of radiation intensity increases with increasing ε, the corresponding “resonant” values of ρ₁/ρ₀ ratio are practically independent of the dielectric permittivity of surrounding medium. It is shown that an analogous amplification of radiation takes place essentially for the same values of ρ₁/ρ₀ also for the radiation in a cylindrical waveguide with conducting walls. An explanation of this phenomenon is given.     

Magnetic texture determination by conversion electron Mössbauer spectroscopy with circularly polarized beam

A new construction of a CEMS detector is presented. The detector is dedicated to resonant polarimetric studies of samples in an external magnetic field. The construction permits investigations at different angles of radiation with respect to the sample surface. A possibility of magnetic texture measurements using circularly polarized radiation is demonstrated. An average square of the cosine 〈(γm)²〉 and an average cosine 〈γm〉 were determined and compared with results of independent measurements of the magnetic field inhomogeneity by a Hall probe. CEMS measurements performed on an isotopically enriched sample show a substantial influence of thickness effects on the measured spectra. An appropriate correction for a saturation effect was proposed and tested.     

Rheological study of polypropylene irradiated with polyfunctional monomers

The aim of this paper is to investigate the rheological properties of polypropylene (PP) modified by ionization radiation (gamma rays) in the presence of two different monomers. The samples were mixed in a twin-screw extruder with ethylene glycol dimethacrylate (EGDMA) or trimethylolpropane trimethacrylate (TMPTMA) with concentration in the range of 0.5-5.0 mmol. After that, they were irradiated with 20 kGy dose of gamma radiation. The structural modification of polypropylene was analyzed in the melt state by measuring melt flow rate (MFR), η* (complex viscosity) and G′ (storage modulus) in the angular frequency range of 10⁻¹ to 3 × 10² rad s⁻¹. From the oscillatory rheology data, one could obtain the values of η₀ (zero shear viscosity) that would be related to the molar mass. All results were discussed with respect to the crosslinking and degradation process that occur in the post-reactor treatment to produce controlled rheology polypropylene.The resulting polymeric materials were submitted the cytotoxicity in vitro test by neutral red uptake methodology with NCTC L 929 cell line from American Type Culture Collection bank. All modified PP samples presented no cytotoxicity.     

Characterization of gamma-stabilized PP with blends of hindered amine/phenolic stabilizers

In this work, the effect of γ radiation at doses of 25 and 50 kGy on polypropylene (PP) stabilized with a binary system of antioxidants of the phenol-hindered amine type (BHT-Chimassorb 994) at different compositions (0.1/0.2, 0.1/0.5, 0.1/0.8) was studied. The results showed that the carbonyl index (I_c) of the stabilized samples drastically decreases when compared to that of the pure PP sample. Nonetheless, the I_c values of the different stabilized samples were very similar after irradiation. On the other hand, the average molecular weights (M_w and M_n) and melt flow index (MFI) values decrease in all the samples after irradiation. This effect is less pronounced in PP with 0.1/0.8 of the binary blend of antioxidants. The melting and crystallization peak temperatures and the crystallinity degree remained almost unchanged and the tensile properties such as Young’s modulus and tensile strength did not vary either, except for the elongation at break, which decreases in less proportion in those blends of PP with the antioxidants, independently of the absorbed dose.     

Degradation of elastomer by heat and/or radiation

This article studied various problems on the degradation of elastomers by heat and/or radiation. Three kinds of elastomers were irradiated and evaluated by the radiation resistant property using the measurement of tensile test. The fluorine containing elastomer, which has excellent heat resistant properties, was found to be less durable for irradiation than ethylene-propylene-diene (EPDM) elastomer. Ten kinds of different compounding formulas of EPDM were prepared to investigate whether the compounding for heat resistant has durability for irradiation. The thermal exposure was performed in an air oven. The duration of thermal exposure at 140 °C was 384 h. The irradiation condition was 5.0 kGy/h at 70 °C, and the total dose was 0.9 MGy. Elongation retained was taken for the evaluation of the stability. It was found that the formulas for improving the thermal stability did not bring radiation resistant of samples in the experiment.The rate constant of the increase in CO concentration by heat and radiation was measured and defined as k_c(h) and k_c(r), respectively. The rate constant of that under the combined addition of the heat and the radiation is expressed as k_c(h + r). Eq. (1) was obtained by the experiment and it was found that there is a synergistic relationship between heat and radiation on the increase in CO concentration

(1)     

Irradiation effect on dielectric properties and electrical conductivity of Au/SiO2/n-Si (MOS) structures

The Au/SiO₂/n-Si (MOS) structures were exposed to beta-ray irradiation to a total dose of 30 kGy at room temperature. Irradiation effect on dielectric properties of MOS structures were investigated using capacitance−voltage (C−V) and conductance−voltage (G/ω−V) characteristics. The C−V and G/ω−V measurements carried out in the frequency range from 1 kHz to 10 MHz and at various radiation doses, while the dc voltage was swept from positive bias to negative bias for MOS structures. The dielectric constant (ε′), dielectric loss (ε″), loss factor (tan δ) and ac electrical conductivity (σ_ac) were calculated from the C−V and G/ω−V measurements and plotted as a function of frequency at various radiation doses. A decrease in the ε′ and ε″ were observed when the irradiation dose increased. The decrease in the ε′ and ε″ of irradiated MOS structures in magnitude is explained on the basis of Maxwell−Wagner interfacial polarization. Also, the σ_ac is found to decrease with increasing radiation dose. In addition, the values of the tan δ decrease with increasing radiation dose and give a peak. From the experimental results, it is confirmed that the peak of loss tangent is due to the interaction between majority carriers and interface states which induced by radiation.     

PIXE and RBS analysis of Tl-1223 superconducting phase substituted by scandium

Ion beam analysis techniques (IBA) were performed to determine the elemental stoichiometry of superconducting samples of type TlBa₂Ca_2−xSc_xCu₃O_9−δ, with 0 ? x ? 0.6, prepared via solid-state reaction technique. By combining particle induced X-ray emission (PIXE) with Rutherford backscattering spectrometry (RBS), the stoichiometry of the samples is determined. However, the oxygen content is obtained by using non-Rutherford backscattering cross-section at 3 MeV proton beam. Furthermore, the prepared samples were also characterized using X-ray powder diffraction (XRD) and electrical resistivity measurements. The X-ray data indicate that the partial substitution of Ca²⁺ by Sc³⁺ ions does not affect the tetragonal structure of Tl-1223 superconducting phase. The superconducting transition temperatures T_c, determined from electrical resistivity measurements, was found to be highly correlated to the Sc-content.     

Electrical properties changes induced by electron radiation at TiO2/Si interface

TiO₂/Si structures were fabricated by electron beam evaporation, and exposed to electron beam irradiation to investigate their electrical properties using the high frequency capacitance-voltage measurements. It was found that samples annealed in oxygen became more radiation resistant than un-annealed samples, which can be explained by the Ti valence variations induced by radiation. The samples were characterized by X-ray diffraction to show the Ti₂O₃ crystalline phase transformed to anatase-crystalline phase after oxygen annealing.     

Monte Carlo simulation study of the positron contribution to displacements per atom production in YBCO superconductors

The contribution from positrons to the displacements per atom (dpa) distribution induced by the gamma irradiation on YBCO superconducting slabs is presented. The procedure implemented previously by the authors was adapted to take into account the contribution from positrons to dpa induced by the gamma radiation. The results show that, when positrons are considered in the atom displacement process, the total dpa almost doubles at 10 MeV of incident gamma radiation. At that energy positrons contribute 7% more to the total dpa than electrons, although electrons maintain having the highest contribution up to about 8 MeV.     

Rayleigh to Compton ratio with monochromatic radiation from an X-ray tube (preliminary results)

Results on the Rayleigh to Compton ratio (R/C) for elements and compounds with low atomic number (5 ? Z ? 12) are presented. These materials are difficult to identify and characterize with other radiological techniques because of their very close linear attenuation coefficients. A transportable setup for R/C measurements was assembled and tested. This comprises an X-ray tube, in which the output radiation is partially “converted” to monochromatic radiation emitted by a secondary target. The experimental results are compared with theory, determined through coherent and incoherent scattering cross sections.     

Interference and shadow effects in the production of light by charged particles in optical fibers

A charged particle passing through or near a narrow optical fiber induces, by polarisation, coherent light guided by the fiber. In the limit of zero crossing angle, the radiation tends towards a Cherenkov radiation with a discrete spectrum, studied by different authors. If the particle crosses a bent fiber at regularly spaced points, interference gives quasi-monochromatic lines. If the particle passes near an end of the fiber, light is produced by the capture of virtual photons through the end face. An alternative way consists in sticking a metallic ball to the fiber: the passing particle induces plasmons which are then evacuated as light in the fiber. Interferences can occur between lights from several ends or balls. Applications of these various light signals to beam diagnostics are discussed. The shadow effect, which reduces the photon yield when the particle runs parallel to a row of balls, is pointed out and an upper bound -dE/dz≤C(Ze/b)² for the particle energy loss is conjectured (Ze is the particle charge, b the impact parameter and C a numerical constant). This bound should also apply to other kinds of light sources, in particular to Smith-Purcell radiation.     

Gamma-ray shielding and structural properties of PbO-SiO2 glasses

Gamma-ray attenuation coefficients have been determined experimentally using a narrow beam transmission method for the xPbO(1−x)SiO₂ (x = 0.45-0.70) glass system at 662, 1173 and 1332 keV photon energies. These values have also been obtained theoretically using the ‘mixture rule’ and the ‘XCOM’ computer software. The results have been used to calculate half value layer parameters. Gamma-ray shielding properties of PbO-SiO₂ glass samples have been compared with standard radiation shielding concretes. The molar volume, FTIR and acoustic investigations have been used to study the structural properties of the prepared glass system.     

Use of Al2O3 layer as a dielectric in MOS based radiation sensors fabricated on a Si substrate

The use of Al₂O₃ dielectric in MOS based radiation sensors has been investigated. Their response has been compared with conventional MOS capacitors with a SiO₂ dielectric. The study includes gamma radiation effects with dose up to 4 Gy. The effect of radiation has been determined from the valance band shift in C-V curves. The amount of charge induced by the radiation has been calculated and compared with the response of MOS capacitors with SiO₂ with the same and different thicknesses. Fading properties have been studied and compared. Results show that MOS capacitors with Al₂O₃ dielectric exhibit sensitivity greater than that obtained from MOS capacitors with SiO₂. This higher sensitivity is attributed to higher trapping efficiency in the Al₂O₃ layer.     

Electron spin resonance study of oxidation in X-irradiated poly(ester urethane) containing nitroplasticizer

The effect of oxidation on X-irradiated Estane^®5703 containing nitroplasticizer (NP) has been examined by electron spin resonance (ESR) spectroscopy, and the results are compared to similar data previously obtained on pristine Estane^®5703. Although both specimens exhibit similar spectra immediately following X-irradiation, their decay upon exposure to air is quite different. The free radical concentration of the pristine specimen continuously decreases with time whereas the NP sample exhibits an initial decrease followed by a significant increase due to the growth of a newly-formed radical. Terminal species of the pristine and NP-Estane^®5703 samples are identified as peroxy and nitroxide radicals, respectively. Hyperfine coupling constants and g-values are extracted for the nitroxide radical and a tentative model is proposed to explain the reaction pathway leading to its production.     

Modification of a shell model for the study of the radiation effects in BaTiO3

In order to study the radiation effects in BaTiO₃ ferroelectric crystal, a previously developed shell model is modified. The modifications include adding the ZBL universal potentials at short distances and distance-dependent spring constants for core-shell interactions. The phase transition sequences in BaTiO₃ were correctly reproduced using molecular dynamics simulations with this modified shell model. Also, the calculated Frenkel pair formation energies agree well with results obtained by first principles calculations, which suggests that this model is suitable for the simulation of the radiation effects in BaTiO₃. The dependence of polarization on the number of oxygen vacancies was also studied.     

Neutron radiation effects on metal oxide semiconductor (MOS) devices

The main purpose of this study is to provide the knowledge and data on Deuterium-Tritium (D-T) fusion neutron induced damage in MOS devices. Silicon metal oxide semiconductor (MOS) devices are currently the cornerstone of the modern microelectronics industry. However, when a MOS device is exposed to a flux of energetic radiation or particles, the resulting effects from this radiation can cause several degradation of the device performance and of its operating life. The part of MOS structure (metal oxide semiconductor) most sensitive to neutron radiation is the oxide insulating layer (SiO₂). When ionizing radiation passes through the oxide, the energy deposited creates electron-hole pairs. These electron-hole pairs have been seriously hazardous to the performance of these electronic components. The degradation of the current gain of the dual n-channel depletion mode MOS caused by neutron displacement defects, was measured using in situ method during neutron irradiation. The average degradation of the gain of the current is about 35 mA, and the change in channel current gain increased proportionally with neutron fluence. The total fusion neutron displacement damage was found to be 4.8 × 10⁻²¹ dpa per n/cm², while the average fraction of damage in the crystal of silicon was found to be 1.24 × 10⁻¹². All the MOS devices tested were found to be controllable after neutron irradiation and no permanent damage was caused by neutron fluence irradiation below 10¹⁰n/cm². The calculation results shows that (n,α) reaction induced soft-error cross-section about 8.7 × 10⁻¹⁴ cm², and for recoil atoms about 2.9 × 10⁻¹⁵ cm², respectively.     

Structural studies of silicon oxynitride layers formed by low energy ion implantation

Silicon oxynitride (Si_xO_yN_z) layers were synthesized by implanting ¹⁶O₂⁺ and ¹⁴N₂⁺ 30 keV ions in 1:1 ratio with fluences ranging from 5 × 10¹⁶ to 1 × 10¹⁸ ions cm⁻² into single crystal silicon at room temperature. Rapid thermal annealing (RTA) of the samples was carried out at different temperatures in nitrogen ambient for 5 min. The FTIR studies show that the structures of ion-beam synthesized oxynitride layers are strongly dependent on total ion-fluence and annealing temperature. It is found that the structures formed at lower ion fluences (∼1 × 10¹⁷ ions cm⁻²) are homogenous oxygen-rich silicon oxynitride. However, at higher fluence levels (∼1 × 10¹⁸ ions cm⁻²) formation of homogenous nitrogen rich silicon oxynitride is observed due to ion-beam induced surface sputtering effects. The Micro-Raman studies on 1173 K annealed samples show formation of partially amorphous oxygen and nitrogen rich silicon oxynitride structures with crystalline silicon beneath it for lower and higher ion fluences, respectively. The Ellipsometry studies on 1173 K annealed samples show an increase in the thickness of silicon oxynitride layer with increasing ion fluence. The refractive index of the ion-beam synthesized layers is found to be in the range 1.54-1.96.