Coincidence Doppler Broadening Research Articles

Positron annihilation study on deformation-induced Au precipitation in Fe–Au and Fe–Au–B–N alloys

We have investigated the potential self-healing of deformation-induced defects by Au precipitation during isothermal aging at 550 °C in Fe–Au and Fe–Au–B–N alloys using positron annihilation spectroscopy. Two different samples with 0 and 24 % pre-strain were used to study the influence of dislocations on the Au precipitation. Dislocations introduced prior to the aging process play an essential role in the formation of Au precipitates. The Coincidence Doppler broadening (CDB) technique shows that Au precipitation in the matrix occurs in the pre-strained samples only. TEM observations confirm the heterogeneous nature of the Au precipitation which occurs exclusively on dislocations and grain boundaries. The evolution of S and W parameters derived from the CDB indicates a three-stage precipitation process for the pre-strained samples. Both the hardness tests and the positron annihilation spectroscopy indicate that the addition of boron and nitrogen to the Fe–Au alloy causes a deceleration of the Au precipitation in the pre-strained samples, but does not alter the defect-induced mechanism of the Au precipitation. The defect-induced Au precipitation provides a promising site-specific autonomous repair mechanism to extend the lifetime of Fe-based alloys for high temperature structural applications.

Positron Annihilation Study of Zirconia Nanopowders and Nanoceramics Stabilized by Magnesia and Ceria

Positron lifetime (LT) and coincidence Doppler broadening (CDB) measurements on nanopowders and ceramics of ceria‐ and magnesia‐stabilized zirconia (CeSZ and MgSZ, respectively) are presented. The nanopowders were prepared by the coprecipitation technique. Effects of nanopowder calcination and sintering at various temperatures were investigated. In the nanopowders, the two kinds of open‐volume defects associated with grain boundaries (GBs) could be identified via positron trapping: (i) vacancy‐like misfit defects situated along GBs and (ii) larger defects at the intersections of at least three GBs (triple points). CDB measurements on CeSZ compacted nanopowders indicated a segregation of Ce ions along GBs. A few percent fractions of positrons were found to form positronium localized in pores of ≈1.8 nm diameter in compacted nanopowders. Sintering of nanopowders at 1500°C appeared to be sufficient for disappearance of pores and triple point defects. In sintered ceramics, contrary to compacted nanopowders, positrons were trapped in zirconium vacancies in grain interiors.

양전자 소멸 측정법으로 양성자 조사에너지 변화에 대한 n, p형 실리콘 구조 특성

동시 계수 도플러 넓어짐 양전자 소멸 분광법으로 n형과 p형 실리콘 시료에 40.0, 3.98 MeV 에너지를 가진 <TEX>$0.0{\sim}20.0{\times}10^{13}protons/cm^2$</TEX> 양성자 빔 조사에 의한 결함을 측정하여 시료 특성을 조사하였다. 양전자와 전자의 쌍소멸로 발생하는 감마선 스펙트럼의 전자 밀도 에너지에 의한 수리적 해석 방법인 S-변수를 사용하여, 시료의 구조 변화를 측정하였다. 본 연구에서 측정된 S-변수는 시료에 조사된 양성자 조사량의 변화에 따라 결함이 증가하였으며, 그리고 40 MeV 양성자 빔의 세기는 n형 실리콘에서 빔의 조사량 <TEX>$20.0{\times}10^{13}protons/cm^2$</TEX>에서 3.98 MeV 보다 결함의 영향이 더 큰 것으로 나타났다. 그 결과 조사에 너지와 조사량의 상관관계를 비교 분석하였다. SRIM 시뮬레이션의 결과는, 양성자의 Bragg 피크 특성 때문에 시료 전체에 대한 결함으로 나타나기 보다는 양성자가 시료의 특정 깊이에 주로 결함을 형성하는 것을 보여 준다. The n-type and p-type silicon samples were exposed by 40.0, 3.98 MeV proton beams ranging between 0 to <TEX>$20.0{\times}10^{13}protons/cm^2$</TEX>. Coincidence Doppler Broadening Positron Annihilation Spectroscopy (CDBPAS) were applied to study of defect characteristics of p type and n type silicon samples. In this investigation the numerical analysis of the 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 gamma spectrum and the total counts of whole gamma spectrum. The S-parameter values strongly depend on the irradiated proton beam that indicated the defects generate more, rather than the energy intensity. 40 MeV irradiated proton beam in the n-type silicon at <TEX>$20.0{\times}10^{13}protons/cm^2$</TEX> was larger defects than 3.98 MeV irradiated proton beam. It was analysis between the proton irradiation beams and the proton intensities of the irradiation. Because of the Bragg peak, SRIM results shows mainly in a certain depth of the sample to form the defect by the proton irradiation, rather than the defects to appear for the entire sample.

Thin film annealing and alloying of a Au/Cu two-layer system studied by positron annihilation spectroscopy

Abstract The high temperature behavior of a Au(d = 180 nm)/Cu(d = 480 nm) two-layer system was studied by Positron Annihilation Spectroscopy (PAS). Therefor defect sensitive and element specific Coincident Doppler Broadening Spectroscopy (CDBS) with a monoenergetic positron beam was applied. In situ CDBS during isothermal tempering at 633 K for 7 h enabled the observation of both annealing of the Au film and alloying of Au and Cu. The changes in the recorded spectra could be separated into two stages: In the first stage, annealing in the Au film was detected, whereas in the second stage intermixing of Au and Cu atoms was observed. The intermixing zone was found to be homogeneous up to a depth of more than 180 nm. In addition, ab initio calculations were performed in order to determine the composition of the alloyed film, which was found to be Au0.7Cu0.3.

양전자 소멸 측정을 이용한 발광 박막 구조 결함 특성

양전자 소멸 분광법으로 발광 박막 시료에 3.0 MeV 에너지를 가진 양성자 빔을 <TEX>$0.0{\sim}20.0{\times}10^{13}$</TEX> protons/<TEX>$cm^2$</TEX>의 조사에 의해 생성된 결함을 측정하여 박막구조 특성에 대하여 실험하였다. 동시 계수 도플러 넓어짐 양전자 소멸법 스펙트럼의 수리적 해석 방법인 S-변수를 사용하고, 양전자 수명 측정 방법에 의한 양전자 수명 <TEX>${\tau}_1$</TEX>과 <TEX>${\tau}_2$</TEX>, 이에 따른 세기 <TEX>$I_1$</TEX>과 <TEX>$I_2$</TEX>를 사용하여, 박막구조에 대한 결함 특성 변화를 측정하였다. 측정된 S-변수는 박막에 조사된 양성자의 빔 조사량에 따라 양성자가 빈자리에 포획되어 감소하는 값을 보였다. 양전자 수명 <TEX>${\tau}_1$</TEX>은 증가하고, <TEX>${\tau}_2$</TEX>은 일정한 값을 나타내었으나, 반면에 세기 <TEX>$I_1$</TEX>과 <TEX>$I_2$</TEX>는 큰 변화가 없었다. 그 이유는 양성자 조사 빔의 변화에 따라서 단일 빈자리의 크기는 증가하고, 다 결정체 알갱이 빈자리 때문에 양성자에 의한 다수의 빈자리 결함의 양은 큰 차이가 없기 때문이다. 그리고 Bragg 피크로 인하여 박막 시료의 특정 깊이에 결함을 형성하여 박막 전체의 결함으로 잘 나타나지 않기 때문으로 판단된다. It is described that the proton beam induces micro-size defects and electronic deep levels in luminescence Thin Film. Coincidence Doppler Broadening Positron Annihilation Spectroscopy (CDBPAS) and Positron lifetime Spectroscopy were applied to study of characteristics of a poly crystal samples. In this investigation the numerical analysis of the Doppler spectra was employed to the determination of the shape parameter, S-parameter value. The samples were exposed by 3.0 MeV proton beams with the intensities ranging between 0 to <TEX>${\sim}10^{14}$</TEX> particles. The S-parameter values decreased as increased the proton beam, that indicates the protons trapped in vacancies. Lifetime <TEX>${\tau}_1$</TEX> shows that positrons are trapped in mono vacancies. Lifetime <TEX>${\tau}_2$</TEX> is not changed according to proton irradiation that indicate the cluster vacancies of the grain structure.

Determination of electron binding energies in two-dimensional Coincident Doppler Broadening spectra

In Coincident Doppler Broadening Spectroscopy (CDBS) the 1022 keV sum energy of the annihilation photons is utilized to validate an undisturbed detection of the two-gamma electron-positron decay. Due to conservation of energy the sum energy is lowered if the annihilating particles, in particular the electron, are in a bound state. Usually this effect is neglected because the binding energies of annihilated electrons are small compared to the energy of the annihilation photons. A novel data evaluation algorithm allows to clearly identify the influence of binding energies in the CDBS spectrum. Exemplary measurements are presented and compared to calculated spectra.

Positron annihilation study of the Mg-Zn -Y alloys with long period stacking ordered (LPSO) structures

The Mg-Zn-Y alloys with long period stacking ordered (LPSO) structures have been studied by positron annihilation lifetime (PAL), coincidence Doppler broadening (CDB) and atom probe tomography (APT). The positron lifetime for all the Mg-Zn-Y alloys is in a range of 221~225 ps, very close to the positron lifetime for pure Mg bulk, 222 ps. Low temperature measurements of the positron lifetime also give no evidence for shallow positron trapping sites in the LPSO phases. The CDB shows that most of the positrons are annihilated with electrons of Mg. These results suggest that sub-nano scale open volumes, which were expected to exist in the Zn/Y enriched layers synchronized with stacking faults of the LPSO phases by the first principles calculations, are not present.

Effect of carbon on copper precipitation in deformed Fe-based alloys studied by positron annihilation spectroscopy

The role of deformation-induced defects and carbon addition on the copper precipitation during aging at 550 °C is investigated in high-purity Fe-Cu-B-N-C alloy samples by Coincidence Doppler Broadening. In samples with 0% and 8% cold pre-strain, the influence of tensile pre-deformation on the precipitation kinetics of copper is studied. The deformation-induced defects are found to enhance the Cu precipitation kinetics. A sharp reduction in open volume defects is accompanied with a strong increase of Cu signature during the initial stage of aging, implying that the open defects (mainly dislocations) act as nucleation sites for Cu precipitates. A comparison of the time evolution of S-W plots between Fe-Cu, Fe-Cu-B-N, and Fe-Cu-B-N-C alloys indicates that the addition of carbon does not alter the Cu precipitation mechanism but decelerates the kinetics.

Coincidence Doppler Broadening of Positron Annihilation Radiation in Fe

We measured the Doppler broadening annihilation radiation spectrum in Fe, using 22NaCl as a positron source, and two Ge detectors in coincidence arrangement. The two-dimensional coincidence energy spectrum was fitted using a model function that included positron annihilation with the conduction band and 3d electrons, 3s and 3p electrons, and in-flight positron annihilation. Detectors response functions included backscattering and a combination of Compton and pulse pileup, ballistic deficit and shaping effects. The core electrons annihilation intensity was measured as 16.4(3) %, with almost all the remainder assigned to the less bound electrons. The obtained results are in agreement with published theoretical values.

Infiltration and Selective Interactions at the Interface in Polymer-Oxide Hybrid Solar Cells

Positron annihilation spectroscopy was used to characterize polymer-based hybrid solar cells formed by poly(3-hexylthiophene) (P3HT) finely infiltrated in a porous TiO2 skeleton. A step-change improvement in the device performance is enabled by engineering the hybrid interface by the insertion of a proper molecular interlayer namely 4-mercaptopyridine (4-MP). In order to obtain depth-resolved data, positrons were implanted in the sample using a variable-energy positron beam. The characteristics of the partially filled nanoporous structures were evaluated in terms of the depth profile of the positronium yield and the S-parameter. A quantitative evaluation of the pore filling in the deep region is given from the analysis of Coincidence Doppler Broadening taken at fixed implantation energy. We note a remarkable difference in terms of the positronium yield when the 4-MP interlayer is introduced, which means a better covering of P3HT on the porous surface.

A position sensitive germanium detector for the measurement of angular deviation of annihilation radiation

To improve electron momentum sensitivity in Coincidence Doppler Broadening Spectroscopy (CDBS) measurements it is envisaged to measure the angular correlation of annihilation radiation along with the energy of both annihilation photons. For this purpose two position sensitive 36-fold pixelated, planar germanium detectors will be utilized. The position sensitivity of one of those detectors has been tested with a collimated gamma source. A data acquisition system consisting of 37 sampling analogue-to-digital converters with PC based online/off-line processing has been installed. A position sensitivity of 1.6 mm has been achieved.

Positron annihilation study on repeated deformation/precipitation aging in Fe–Cu–B–N alloys

We have investigated the influence of deformation-induced defects on the isothermal Cu precipitation at 550 °C in an as-quenched (solute supersaturated) and annealed (solute depleted) Fe–Cu–B–N alloy by positron annihilation spectroscopy. Preferred precipitation of supersaturated Cu at defects sites was recently proposed as a self-healing mechanism for deformation damage. Repeated deformation/aging cycles result in a significant increase in tensile strength for as-quenched samples. Positron Annihilation Lifetime Spectroscopy revealed a larger positron lifetime for the annealed samples compared to the as-quenched samples caused by the additional open-volume defects at incoherent interfaces between Cu precipitates and the matrix. For both as-quenched and annealed samples deformation generates new defects (mainly dislocations), while subsequent aging leads to a reduction in the density of these defects. Simultaneous Coincidence Doppler Broadening measurements monitored the evolution of local environment at the positron annihilation sites (defects, Cu precipitates and matrix) during the repeated deformation/aging cycles. For the as-quenched samples, aging after deformation results in a sharp decrease in the defect contribution combined with a strong increase in the copper signature, which can be attributed to preferential copper precipitation at the defects introduced by plastic deformation. For the annealed samples, aging after deformation leads to a much weaker evolution of the Cu signature.

Revealing the Nano‐Level Molecular Packing in Chitosan–NiO Nanocomposite by Using Positron Annihilation Spectroscopy and Small‐Angle X‐ray Scattering

Chitosan-NiO nanocomposite (CNC) is shown to be a potential dielectric material with promising properties. CNCs containing NiO nanoparticles (0.2, 0.6, 1, 2, 5 wt %) are prepared through chemical methods. The inclusion of NiO nanoparticles in the chitosan matrix is confirmed by scanning electron microscopy (SEM) and X-ray diffraction. The morphology of the NiO nanoparticles and the nanocomposites is investigated by transmission electron microscopy and SEM, respectively. Positron annihilation lifetime spectroscopy (PALS) and the coincidence Doppler broadening (CDB) technique are used to quantify the free volume and molecular packing in the nanocomposites. The triplet-state positronium lifetime and the corresponding intensity show the changes in nanohole size, density, and size distribution as a function of NiO loading. Small-angle X-ray scattering indicates that the NiO aggregates are identical in all the CNCs. The momentum density distribution obtained from CDB measurements excludes the possibility of a contribution of vacant spaces (pores) available in NiO aggregates to the free volume of nanocomposites upon determination by using PALS. The results show systematic variation in free-volume properties and nano-level molecular packing as a function of NiO loading, which is presumed to play a vital role in determining the various properties of the nanocomposites.

Positron Annihilation Study of ZnO Nanoparticles Grown Under Folic Acid Template

Positron lifetime spectroscopy (PAL), Doppler broadening (DB) as well as coincidence Doppler broadening (CDB) spectroscopy of a new variety of Folic acid (FA) capped zinc oxide nano-particle samples has been performed at room temperature. The results show interesting patterns of observation, hither-to unobserved in ZnO wurtzite crystalline samples, such as predominance of positronium formation, as reflected in the (PAL) analysis, phase transition in the nano crystalline samples at ~0.8-1.0 % FA concentration as depicted from DB results. Also, the chemical environment of the samples has been analysed from the ratio curves of CDB studies. Beside these, other independent results from X-ray diffraction (XRD) data and Debye- Scherrer method, Transmission electron microscopic (TEM) observations and as well as Fourier transformed infrared spectroscopic (FT-IR) analysis are reported for comparison.

Software for Digital Coincidence Doppler Broadening Setup

An advanced digital measurement system for Coincidence Doppler broadening (CDB) technique is presented. Traditional CDB setups are based on shaping amplifiers and multi parameter multi-channel analyzers (MCA). We replaced them by 14-bit data acquisition card (DAQ). Replacing multi-parameter MCA by DAQ was realized also by several groups before, but in our setup the signal shaping tasks are performed digitally by software in a personal computer. Signals from high purity (HP) Ge detectors are sampled at preamplifier outputs. Pulse shaping is realized by a trapezoidal filter.

Positron and Positronium Annihilation as Probes to Identify Cation Substitution Effects in CoCr&lt;sub&gt;x&lt;/sub&gt;Fe&lt;sub&gt;2-x&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;

CoCrxFe2-xO4 samples with x = 0.0 to 2.0 were prepared by air oxidation of aqueous suspension containing Co2+, Cr3+ and Fe3+ ions and characterized by x-ray diffraction. Positron lifetime and coincidence Doppler broadening (CDB) measurements indicated three distinct stages of positron trapping in defects. Initially the vacancy-type defects located at the A-sites (tetrahedral) trapped positrons but, with the substitution of Fe by Cr in low concentrations (x &lt;= 0.7), positrons are trapped by defects at the B-sites (octahedral). Mossbauer spectroscopic results indicated the cationic distribution at B-sites to be stoichiometry-dependent and, till x = 0.7, the deficiency of Fe3+(B) ions was compensated by interchange of Fe3+(A) ions with Co2+(B) ions. Between x = 0.9 and 1.7, the substitution resulted in continued decrease of Fe3+(B) ions and the structure got fully transformed into a normal spinel configuration during x = 1.8 to 2.0.

Identification of Vacancy Formation Sites in LaNi5Cu During Hydrogenation Using in Situ Coincidence Doppler Broadening Technique

To study the formation mechanism of lattice defects introduced during hydrogenation, we developed equipment that can measure in situ positron lifetime and coincidence Doppler broadening (CDB) simultaneously with pressure–composition (P–C) isotherms at room temperature. The equipment, which consisted of a high-pressure sample holder, reduced the intensity of the background signals in the relatively high momentum region of the Doppler broadening spectrum by more than 2 orders of magnitude in comparison to the background shown by conventional Doppler broadening equipment. Further, the quality of data obtained by our instrument was similar to that obtained from ex situ CDB equipment without a high-pressure sample holder. The substitution effect on the positron annihilation behavior was observed from the ratio curves of CDB spectra for LaNi5Cu and LaNi5-xAlx. In addition, using the equipment we developed, we attempted to identify the vacancy formation sites introduced in LaNi5Cu during hydrogenation. Changes i...

양전자 소멸 측정에 의한 n, p형 실리콘 구조 특성

수명 측정법과 동시 계수 도플러 넓어짐 양전자 소멸 분광법으로 p형과 n형 실리콘 시료에 3.98 MeV 에너지를 가진 <TEX>$0.0{\sim}20.0{\times}10^{13}\;protons/cm^2$</TEX> 양성자 빔 조사에 의한 결함을 측정하여 실리콘 결함 특성에 대하여 조사하였다. 양전자와 전자의 쌍소멸로 발생하는 감마선 스펙트럼의 전자 밀도 에너지를 수리적 해석 방법인 S-변수와 열린 부피 결함에 대한 측정법으로서 양전자 수명 <TEX>${\tau}_1$</TEX>과 <TEX>${\tau}_2$</TEX>, 이에 따른 밀도 <TEX>$I_1$</TEX>과 <TEX>$I_2$</TEX>를 사용하여, 시료의 구조 변화를 측정하였다. 본 연구에서 측정된 S-변수와 양전자 수명은 시료에 조사된 양성자 조사량의 변화에 따라 결함이 증가하였으며, 양전자 수명 측정과 같은 경향을 보여준다. SRIM의 결과로써, 양성자 조사 에너지에 따른 Bragg 피크 때문에 양성자는 시료의 특정 깊이에 주로 결함을 형성하여 시료 전체에는 결함으로 잘 나타나지 않기 때문이다. 빔의 조사량에 따른 결함의 영향이 더 큰 것으로 나타났다. It is described that the proton beam induceds micro-size defects and electronic deep levels in n or p type single crystal silicon. Positron lifetime and Coincidence Doppler Broadening Positron Annihilation Spectroscopy were applied to study of characteristics of p type and n type silicon samples. 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 samples were exposed by 3.98 MeV proton beams ranging between 0 to <TEX>${\sim}10^{14}$</TEX> particles. The S-parameter values strongly depend on the irradiated proton beam, that indicated the defects generate more. Positron lifetime shows that positrons trapped in vacancies and lifetime <TEX>${\tau}_2$</TEX> increased according to proton irradiation.

Defect Behaviour in Yttria-Stabilised Zirconia Nanomaterials Studied by Positron Annihilation Techniques

Recent experimental and theoretical investigations on a variety of yttria-stabilised zirconia (YSZ) nanomaterials are reviewed. The investigations were conducted within the frame of a collaboration of three institutions: (i) Charles University in Prague, (ii) Helmholtz Centre Dresden-Rossendorf and (iii) Donetsk Institute for Physics and Engineering of the NAS of Ukraine, Materials studied involved pressure-compacted nanopowders of binary and ternary (with Cr2O3 additive) YSZ and YSZ ceramics obtained by sintering the nanopowders. The nanopowders were prepared by the co-precipitation technique. Positron annihilation spectroscopy including the conventional positron lifetime (LT) and coincidence Doppler broadening (CDB) techniques was employed as the main experimental tool. Slow positron implantation spectroscopy (SPIS) was used in investigation of commercial YSZ single crystals for reference purposes. Extended state-of-art theoretical ab-initio calculations of positron response in the ZrO2 lattice were carried out for various vacancy-like defect configurations. It was suggested by these calculations that none of the oxygen-vacancy related defects are capable to trap positrons. On the other hand, zirconium vacancy was demonstrated by the calculations to be a deep positron trap, even in the case that a hydrogen atom is attached to the vacancy. The measured positron LT data clearly indicated that positrons annihilate in nanopowders predominantly from trapped states at defects of two kinds: (a) the vacancy-like misfit defects concentrated in layers along the grain boundaries and characterised with lifetimes of 0.180 ns, and (b) the larger defects of open volume comparable to clusters of a few vacancies which are situated at intersections of three (or more) grain boundaries (characteristic lifetimes of 0.380 ns). The intensity ratio of LT components corresponding to these two kinds of defects was found to be correlated with the mean particle size. This correlation reconfirms the above interpretation of LT components and, moreover, the measured ratios could be used to estimate changes of the mean particle size with chromia content or sintering temperature. It was shown in this way that chromia addition to the YSZ nanopowder leads to a smaller particle size compared to the binary YSZ. Similarly, grain growth during sintering could be monitored via this intensity ratio. A portion of 10 % of positrons was found to form positronium (Ps) in compacted binary YSZ nanopowders. The observed ortho-Ps lifetimes correspond to Ps pick-off annihilation in cavities of 3 nm size which may be expected to occur between the primary nanoparticles. On the other hand, an addition of chromia at a concentration as low as 0.3 mol.% appeared to be sufficient to suppress Ps formation below the detection limit. Similarly, Ps formation could not be detected in binary YSZ sintered for 1 hour at a temperature of 1000 °C or higher. The former effect indicates an enhanced concentration of Cr cations at the particle surfaces, while the latter one appears to be due to a decrease of cavity concentration induced by sintering. The measured CDB data supported the idea that vacancy-like trapping centres are similar to zirconium vacancies and gave further evidence of a strong segregation of Cr segregation at particle interfaces. SPIS was further involved in a trial experiment on binary YSZ nanopowders and sintered ceramics. This experiment clearly demonstrated that SPIS may reveal valuable information about changes of depth profiles of microstructure during sintering, e.g. a sintering induced diffusion of defects from sample interior to its surface.

Study of Coincidence Doppler Broadening in Carbon Nanotubes

Coincidence Doppler broadening measurements of positron annihilation for multi-walled carbon nanotubes, double-walled carbon nanotubes, single-walled carbon nanotubes and graphite were performed. The ratio curves of the Doppler broadening for these samples to silicon were obtained. It is shown that there are distinct peaks at the position of 10×10−3m0c for both carbon nanotubes and graphite, however the amplitudes of the peaks are not the same. We have the opinion that these peaks arise from the annihilation of positron with the 2s and 2p electron of carbon element.