Slow Positron Annihilation Spectroscopy Research Articles

Synchrotron GIXRD and slow positron beam characterisation of Ar ion irradiated pure V and V-4Cr-4Ti alloy: Candidate structural material for Fusion reactor application

Pure V and V-4Cr-4Ti has been irradiated with Ar9+ion at 0.4, 2.3, 8.4 and 23 dpa. GIXRD was carried at two different incident angles i.e. at 0.2° and 0.45° using Synchrotron radiation. The XRD data were analyzed to determine the variation of microstructural parameters like domain size and microstrain as a function of dose by modified Rietveld technique. The dislocation density calculated from these parameters show contrasting behaviour in pure V and V-4Cr-4Ti as a function of dose. In pure V the dislocation density increases at the highest dose, whereas in V-4Cr-4Ti it saturates after 8.4 dpa. However, in V-4Cr-4Ti at the highest dpa clear evidence of segregation of the alloying elements is observed. Slow positron annihilation spectroscopy has been used to characterize the irradiation induced depth dependent changes in the microstructure with dose. The positron diffusion lengths in pure V (202 nm) and V-4Cr-4Ti (370 nm) indicate a defect free bulk state. An increase in S-parameter (∼1.07Sbulk) confirms the creation of vacancy type open volume defects in the damaged region (0−300 nm from surface) of the ion-irradiated samples. It was observed that that there is no appreciable change in open volume defect density and type of defects up to 8.4 dpa for pure V except at the highest dose. However, in contrary to pure V, the increase in S-parameter at highest dpa clearly indicates that the type of defects at highest dose is significantly different from the other samples.

He and O ion implantation induced defects in Si crystal studied using slow positron annihilation spectroscopy

Open volume defect profiles have been obtained in He, O and (He + O) ion implanted silicon crystal 〈1 0 0〉 using slow positron beam measurements. The p-type Si samples have been implanted with He (30 keV), O (120 keV) and both the ions [He (30 keV) + O(120 keV)] with the dose of 1 × 1015 and 1 × 1017 ions/cm2. The average range of these ions in Si is estimated ∼280 nm. The depth dependent defect profiles have also been studied after isochronal annealing (100–900 °C) of the ion implanted Si crystals. On annealing at temperatures (>500 °C), an increase (decrease) in S-parameter was observed in the He (O) ion implanted samples. The variations observed in the S-E profiles indicate that large size open volume defects are created in He irradiated Si followed by annealing at 700 °C. In the case of O and (He + O) implanted samples, variations in S-E and W-E profiles indicate the formation of oxygen-vacancy clusters, a buried silicon oxide layer or oxygen decorated cavities. The S-E (W-E) profiles have been fitted using a program ‘Variable Energy Positron fit’ to evaluate the characteristic S- and W- parameter values of different regions. It is observed through S-W correlation plots that He ions as well as vacancy defects diffuse out of the silicon crystal at higher annealing temperatures. Oxygen-vacancy cluster defects are identified in O implanted (1 × 1015 ions/cm2) and annealed at 700 and 900 °C. In case of O implanted (1 × 1017 ions/cm2) and co-implanted samples (1 × 1015 ions/cm2 and 1 × 1017 ions/cm2) annealed at 700 and 900 °C, formation of oxygen decorated cavities is proposed.

Study of damage in binary Fe85Cr15 alloys irradiated by ions and the effect of an external magnetic field during irradiation

Understanding radiation-induced damage in iron-chromium alloys is critical for the use of advanced steels in future fusion reactors. However, the role of the strong magnetic fields present in such magnetically confined plasma devices on material damage, though considered important, has not been thoroughly explored to date. In this work, irradiation experiments of Fe85Cr15 alloy (15% Cr content) by heavy (Fe+) and light (He+) ions have been carried out in order to characterize the damage and, additionally, the influence of an external magnetic field (B = 0.4 T) has been taken into account. The analysis of the data has been done with different techniques, the distribution of the Cr and/or vacancies around probe nuclei has been explored by Mössbauer spectroscopy and the vacancy profile by Slow Positron Annihilation Spectroscopy (SPAS). Comparison between samples irradiated by He+ and Fe+, in absence of B, reveals significant differences in the average hyperfine magnetic field (<ΔH> = 0.8 T). This can be attributed to changes in the local environment around the probe nuclei (57Fe) where either vacancies or the Cr distribution play a role. From depth-profiling by SPAS in Fe+ irradiated samples, a large concentration of vacancy-type defects is found with a profile that extends deeper than that predicted by computer simulations. Furthermore, sequential irradiation by Fe+ and He+ ions indicates that the He atoms tend to fill the vacancy clusters just in the depth where the He+ is implanted. On the other hand, small but significant differences in vacancy-type defect profiles are found between the samples irradiated both in the presence and absence of a magnetic field (B = 0.4 T) both independently of single and sequential irradiation. Detailed Mössbauer spectroscopy and SPAS analysis seem to point out that external magnetic fields could be a non-negligible parameter in the material damage due to ion irradiation.

Helium release and lattice swelling in nickel foil irradiated by multiply-energy helium ions

Pure nickel foil was irradiated with multiple-energy helium ions at room temperature. Using elastic recoil detection (ERD), the depth profile of helium concentration in nickel foil after the irradiation was investigated. Result showed that very little helium atoms distributed in the regions near two free surfaces, indicating the occurrence of helium release. Interestingly, the release clues of open cracks probably resulted from the generated surface stress were observed by scanning electron microscopy (SEM). Furthermore, the shifting of X-ray diffraction (XRD) diffraction patters towards the lower diffraction angles revealed the occurrence of lattice swelling. The results of slow positron annihilation spectroscopy (PAS) experiments suggest that abundant vacancy-type defects were introduced in the helium-irradiated nickel foil. The elastic strains caused by the formation of helium-vacancy clusters in the lattice were assumed to be responsible for the lattice swelling.

Effects of Implantation Sequence on the Micro-defects in H and O Implanted Silicon

Cz n-type Si (100) wafers are implanted with 190 keV O and 40 keV H ions in different implantation sequences. Cross sectional transmission electron microscopy and slow positron annihilation spectroscopy are used to study the formation and evolution of micro-defects. Our results clearly show that the defect morphology depends strongly on the implantation sequence. Large cavities are observed in O preimplanted samples, while only platelets are observed in H preimplanted samples. The change regularity of the S parameter is the same for the Si samples co-implanted at different sequences. But in H preimplanted samples, the S parameter is a little higher. The effects of the implantation sequence on the micro-defects have been discussed in combination with H and O implantation-induced defects as well as their interactions upon annealing.

Development of pore interconnectivity/morphology in porous silica films investigated by cyclic voltammetry and slow positron annihilation spectroscopy

Cyclic voltammetry and positronium (Ps) 3γ-annihilation spectroscopy were applied to investigate pore interconnectivity/morphology of porous silica films fabricated with various loading of cetyltrimethyl ammonium bromide (CTAB). With increasing the ratio of CTAB up to 15wt.%, the total charge Q, resulted from I− diffusion across the silica films, increased remarkably, indicative of formation of highly interconnected pores in the films prepared with more porogen. However, it decreased dramatically with further loading CTAB of 25wt.%. Interestingly, 3γ-annihilation fraction I3γ due to a triplet-state Ps (ortho-positronium, o-Ps) emission from the silica films showed a similar behavior as a function of CTAB loading. The abnormal decrement in Q and I3γ in the film fabricated with 25wt.% CTAB was well explained by formation of long nanochannels aligning parallel to the film surface. The results indicated that the total charge Q and Ps 3γ-annihilation fraction were closely associated with I− and Ps diffusion governed by the pore interconnectivity/morphology of the silica films, which made cyclic voltammetry possible to be a feasible tool to characterize pore interconnectivity/morphology of porous thin films.

Ga-vacancy induced room temperature ferromagnetism observed in N-irradiated GaN films

GaN films prepared on sapphire substrates with thickness of 30μm, were implanted by nitrogen ions with energy of 80keV at doses of 5×1016cm−2 and 2×1017cm−2, respectively. An obvious ferromagnetic loop was obtained in the higher dose irradiated GaN film at room temperature, indicating magnetic defects were induced into this film. After irradiation, the films contained lots of Ga vacancies were investigated by slow positron annihilation spectroscopy. With first-principle calculations, we demonstrated that Ga vacancies could lead to an enhancement of magnetic moment for 3μB in GaN crystal and form ferromagnetic coupling at room temperature between two close range Ga vacancies.

Evolution of microstructure of epoxy coating during UV degradation progress studied by slow positron annihilation spectroscopy and electrochemical impedance spectroscopy

Evolution of chemical functional groups, microstructure and water barrier properties of a polyamide-cured epoxy (diglycidyl ether of bisphenol-A epoxy resin, DGEBA) coating during ultraviolet A (UV-A) photo-oxidative aging is systematically investigated. At the early stage of aging, decrements of S parameter and water uptake coefficient indicate the formation of a more compact structure induced by the post-curing process. After 208hours (h) of UV irradiation, a novel time constant at relatively high frequency (3.5×102Hz) appears in the electrochemical impedance spectroscopy (EIS) spectra suggesting that a microporous layer generates near the surface of DGEBA film. With the increase in irradiation time, overlap of two time constants at frequencies around 18Hz and 3.7×103Hz is observed after 1.33h of immersion, indicating that the micropores grow towards the bulk and form more characteristic layers with microporous structures. After irradiation for 399h, a low S parameter region near the sample surface is observed, which implies that a surface layer with low free volume may have formed. With longer exposure, EIS results also reveal that the water barrier property of the coating can be improved, which confirms the formation of the denser surface layer near the surface as a result of radical recombination during UV-A treatment.

Helium-Implantation-Induced Damage in NHS Steel Investigated by Slow-Positron Annihilation Spectroscopy

Evolutions of defects and helium contained defects produced by atomic displacement and helium deposition with helium implantation at different temperatures in novel high silicon (NHS) steel are investigated by a slow positron beam. Differences of the defect information among samples implanted by helium to a fluence of 1 × 1017 ions/cm2 at room temperature, 300°C, 450°C and 750°C are discussed. It is found that the mobility of vacancies and vacancy clusters, a recombination of vacancy-type defects and the formation of the He-V complex lead to the occurrence of these differences. At high temperature irradiations, a change of the diffusion mechanism of He atoms/He bubbles might be one of the reasons for the change of the S-parameter.

Novel organic–inorganic thin film composite membranes with separation performance surpassing ceramic membranes for isopropanol dehydration

Novel organic–inorganic thin film composite (TFC) membranes have been developed in this work, with an introduction of an inorganic component 3-glycidyloxypropyltrimethoxy-silane (GOTMS) in the chemical structure of the in situ synthesized polyamide layer. These membranes exhibit a pervaporation separation performance surpassing most prior polymeric membranes and inorganic ceramic membranes for isopropanol dehydration. Three different modification methods are demonstrated to include GOTMS in the polyamide structure via suitable molecular design, and all resultant organic–inorganic membranes show improved separation performance as compared to the original TFC membrane without GOTMS. The TFC membrane prepared exhibits an optimized flux of 3.5kg/m2h with a separation factor of 278 for a feed composition of 85/15wt% isopropanol (IPA)/water at 50°C. This high performance can be attributed to the excellent solvent resistance of the inorganic component included and the formation of inorganic cavities in the selective layer. The changes of chemical structures and surface morphology are confirmed and characterized by Fourier Transform Infrared (FTIR) and X-ray energy dispersive spectrometry (EDX). Slow beam positron annihilation spectroscopy (PAS) technique is used to analyze the variation in the fractional free volume of the organic–inorganic TFC membranes. The newly designed and fabricated TFC membranes show a great potential to compete with those commercial inorganic membranes in pervaporation applications.

On the interplay of point defects and Cd in non-polar ZnCdO films

Non-polar ZnCdO films, grown over m- and r-sapphire with a Cd concentration ranging between 0.8% and 5%, have been studied by means of slow positron annihilation spectroscopy (PAS) combined with chemical depth profiling by secondary ion mass spectroscopy and Rutherford back-scattering. Vacancy clusters and Zn vacancies with concentrations up to 1017 cm−3 and 1018 cm−3, respectively, have been measured inside the films. Secondary ion mass spectroscopy results show that most Cd stays inside the ZnCdO film but the diffused atoms can penetrate up to 1.3 μm inside the ZnO buffer. PAS results give an insight to the structure of the meta-stable ZnCdO above the thermodynamical solubility limit of 2%. A correlation between the concentration of vacancy clusters and Cd has been measured. The concentration of Zn vacancies is one order of magnitude larger than in as-grown non-polar ZnO films and the vacancy cluster are, at least partly, created by the aggregation of smaller Zn vacancy related defects. The Zn vacancy related defects and the vacancy clusters accumulate around the Cd atoms as a way to release the strain induced by the substitutional CdZn in the ZnO crystal.

Molecular design of thin film composite (TFC) hollow fiber membranes for isopropanol dehydration via pervaporation

Abstract High performance hollow fiber membranes consisting of a thin film composite (TFC) polyamide layer and a porous Torlon® polyamide-imide substrate have been fabricated via interfacial polymerization for pervaporation dehydration of isopropanol. The effects of substrate characteristics on the formation of the thin polyamide layer and the pervaporation performance are investigated. It is found that the resultant TFC membranes show superior separation performance if the substrates have a fine surface pore size and desirable sub-structural morphology. The effects of amine monomers, i.e., m-phenylenediamine (MPD) and hyperbranched polyethyleneimine (HPEI), for interfacial polymerization with trimesoyl chloride (TMC) on separation performance are also compared. The TFC membrane prepared from HPEI exhibits a higher pervaporation flux than that from MPD possibly due to the fact that the former has higher amine content than the latter. As a consequence, the polyamide selective layer of the former has a greater hydrophilicity than the latter. X-ray photoelectron spectroscopy (XPS) and contact angle measurements have confirmed the hypothesis. The molecular weight of HPEI also affects the pervaporation performance of the resultant TFC membranes. With the optimal interfacial polymerization conditions, the TFC membrane fabricated from HPEI with a molecular weight of 2 kg/mol shows the best separation factor of 624 with a flux of 1282 g/m2-h and a permeate water concentration of 99.1 wt% using a feed composition of 85/15 wt% IPA/water at 50 °C. After post methanol treatment, the flux can be enhanced to 1920 g/m2-h with a slightly decrease in permeate water concentration to 98.4 wt%. The mechanism of flux enhancement has been examined by slow beam positron annihilation spectroscopy (PAS). It is found that the removal of the excess amine from the TFC layer during the methanol treatment is the main cause.

Effect of deuterium ion beam bombarding time oncharacteristic of deuteride titanium target

Effect of the deuterium ion beam bombarding time on characteristic of the deuteride titanium target is studied by measuring the neutron yield as a function of bombarding time with the associated-particles method in ZF-300 neutron generation. Slow positron annihilation spectroscopy for defect structure analysis and scanning electron microscopy for the surface morphology analysis are used to characterize the targets. The results show that the beam bombardment cause the changes of the defect structure and the surface morpholoy of the deuteride titanium target, but the different bombarding times do not lead to the changes of the neutron yield, the defect structure, and the surface morphology of the deuteride titanium target under the work condition. The prominent physical mechanism of the interaction between deuterium ion beam and deuteride titanium target is discussed in detail based on the numerical study of the interaction between beam and solid.

Vacancy-type defects near Al surface studied by slow positron annihilation spectroscopy before and after He + implantation

The vacancy-type defects He n V m near Al surface before and after He + implantation and their evolutions with annealing temperatures and aging time have been investigated by mono-energy slow positron annihilation spectroscopy (SPAS) with S parameters. The results show that many vacancies are produced during the sample preparation process, which can be re-occupied by Al atoms during annealing, Al + and MeV He + implantation. S parameters denote the concentration and size of He n V m clusters induced by He + implantation in Al. The higher fluence of He implanted, the larger S parameters will be, indicating more He n V m clusters produced. S parameters decrease with the increase of annealing temperatures until the fastest change temperature, and then an opposite or minor change occurs depending on the fluence of He implanted in Al, showing that the concentration and size of He n V m clusters will vary with the annealing temperatures. Aged at RT for some time, the concentration and mean size of He n V m clusters in Al will get smaller and larger, respectively, resulting in the decrease of S parameters with the aging time. In conclusions, the evolution of vacancy-type defects He n V m near Al surface after He + implantation depends on the annealing temperatures, He concentration and aging time.

The formation and evolution of vacancy-type defects in Ar-implanted silicon studied by slow-positron annihilation spectroscopy

The Doppler broadening spectrum of a silicon wafer was measured using a variable-energy positron beam to investigate the effects of vacancy-type defects induced by 180 keV Ar ion implantation. The S-parameter in the damaged layer decreases with annealing temperature up to 673 K, and then increases with annealing temperature from 673 to 1373 K. At low annealing temperatures ranging from room temperature to 673 K, argon-decorated vacancies are formed by argon atoms combining with open-volume defects at inactive positron sites. With further increase of annealing temperature, argon-decorated vacancies dissociate and subsequently migrate and coalesce, leading to an increase of S-parameter. Furthermore, the buried vacancy-layer becomes narrow with increasing annealing temperature. At 1373 K, the buried vacancy-layer moved towards the sample surface.

저에너지 양전자 소멸 분광법을 이용한 MgB2박막 구조 특성

저속 에너지 도플러 넓어짐 양전자 소멸 분광법으로 <TEX>$MgB_2$</TEX> 박막내의 원자 크기 정도 고체 구조 특성에 대하여 조사하였다. 양전자와 전자의 쌍소멸로 발생하는 511keV 감마선 스펙트럼의 수리적 해석 방법인 S-변수를 사용하여, 상전이 근처 온도에서 박막의 구조 변화를 측정하였다. 비등방성 구조로 된 <TEX>$MgB_2$</TEX> 박막에서 초전도 특성을 갖는 상전이 온도 근처에서 S-변수를 측정하였다. 양전자의 입사 에너지 10keV에서 측정된 S-변수의 최고치는 박막의 온도가 30K에서 0.567이고, 50 K에서는 0.570로 큰 변화는 없었다. 이 결과로부터 양전자가 Boron 층의 초 전자와 소멸하기 보다는 Mg층 근처의 상 전자와 소멸하는 것으로 판단된다. <TEX>$MgB_2$</TEX>의 박막의 외층은 Mg층으로 이루어졌다고 할 수 있다. The Characterization of <TEX>$MgB_2$</TEX> Thin Film by Slow Positron Annihilation Spectroscopy Enhance signal-to-noise ratio, slow positron coincidence Doppler Broadening method has been applied to study of characteristics of <TEX>$MgB_2$</TEX> superconductor film, which were performed at 30 K and 50 K sample temperature near Tc of it. In this investigation the numerical analysis of the Doppler spectra was employed to the determination of the shape parameter, S, defined as the ratio between the amount of counts in a central portion of the spectrum and the total counts of whole spectrum. The S-parameter values were increased then decreased while the positron implantation energies were increasing, that indicated the diffusion into the samples. The S-parameters of the anisotropic 1 <TEX>${\mu}m$</TEX> <TEX>$MgB_2$</TEX> thin film which were implanted by positrons at 10 keV are 0.567 at 30 K and 0.570 at 50 K. It is believed that the positrons annihilate with normal-electrons instead of super-electrons in the <TEX>$MgB_2$</TEX> superconductor.

Hydrogen‐damaged defects near the surface in heavily deformed iron and steels investigated by slow positron annihilation spectroscopy

AbstractHydrogen‐damaged defects of heavily deformed iron, AISI 304‐, and 316‐stainless steels were investigated by measuring Doppler broadening energy spectra (DBES) of positron annihilation radiation. It is interestingly found that S‐parameter as a function of energy on three kinds of deformed samples falls into the similar variation, independent of chemical elements and impurities. A larger S value near the surface in deformed and hydrogen‐damaged samples than in samples without hydrogen‐damaged samples is because of the additional larger‐sized defects (e.g. voids) induced due to hydrogen damage. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Applications of slow positrons to cancer research: Search for selectivity of positron annihilation to skin cancer

Slow positrons and positron annihilation spectroscopy (PAS) have been applied to medical research in searching for positron annihilation selectivity to cancer cells. We report the results of positron lifetime and Doppler broadening energy spectroscopies in human skin samples with and without cancer as a function of positron incident energy (up to 8 μm depth) and found that the positronium annihilates at a significantly lower rate and forms at a lower probability in the samples having either basal cell carcinoma (BCC) or squamous cell carcinoma (SCC) than in the normal skin. The significant selectivity of positron annihilation to skin cancer may open a new research area of developing positron annihilation spectroscopy as a novel medical tool to detect cancer formation externally and non-invasively at the early stages.

Study of the photodegradation of epoxy polymers with slow positron annihilation spectroscopy

AbstractThe photodegradation of an amine‐cured epoxy coating after exposure to accelerated UV‐340 and UV‐313 irradiation was investigated with an atomic‐level technique, positron annihilation spectroscopy (PAS), which detected and characterized the free volumes and defects as a function of the depth. Significant changes in the subnanometer defect parameters S and W were observed as a function of the exposure time near the surface. This was interpreted as due to a loss of the free volume and hole fraction resulting from photodegradation. A dead layer near the surface, resulting from UV irradiation from the surface up to a thickness of 0.4 μm, at which there was nearly no positronium formation, was observed. Correlations between physical defects from PAS in terms of the free volumes and chemical defects from electron spin resonance spectroscopy in terms of free radicals and chemical structural changes measured by ultraviolet–visible and Fourier transform infrared spectroscopy were established. A high sensitivity of PAS for detecting the early stage of degradation, on the order of hours for UV‐313 and on the order of days for UV‐340 irradiation, was observed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2441–2459, 2004

Studies on the aqueous corrosion behavior and microstructure of zircaloy4 which has been subjected to high-dose Ar ion bombardment

In order to investigate the ion bombardment effect on the corrosion behavior of zircaloy4, the specimen surfaces were bombarded by Ar ion using an accelerator at an energy of 150 keV with a dose range from 3×10 15 to 1×10 17 ions/cm 2 at liquid nitrogen temperature. Post-bombardment corrosion tests were conducted to rank the corrosion resistance of the resulting specimens by potentiodynamic polarization curve measurements in a 0.5 M H 2SO 4 water solution at room temperature. The vacancy profile induced by Ar ions bombardment was calculated using Monte Carlo simulation program SRIM 96, and measured by means of the slow positron annihilation spectroscopy (SPAS). The surface microstructures were examined using glancing angle X-ray diffraction (GAXRD). The potentiodynamic tests showed that with the bombardment dose increasing, the passive current density, closely related to the surface corrosion resistance, decreased firstly and increased subsequently. The mechanism of the corrosion behavior transformation was due to the amorphous phase formation firstly and the amorphous phase destruction and the polycrystalline structure formation in the bombarded surface subsequently.