Annihilation Sites Research Articles

A Positron Study of Early Clustering in Al-Mg-Si Alloys

Early stages of clustering in quenched Al-Mg-Si alloys during natural ageing were studied by positron annihilation lifetime spectroscopy utilizing its unique sensitivity to electron density differences in various atomic defects. Two different positron trapping sites could be identified, one related to a vacancy-type defect, the other to solute clusters. The first trap is deep, i.e. irreversibly traps positrons, the second shallow, from which positrons can escape, which creates the signature of a temperature-dependent positron lifetime. During the first 80 min of NA, the vacancy-related contribution decreases, while the solute clusters increasingly trap positrons, thus reflecting their continuous growth and power to trap positrons. Coincident Doppler broadening spectroscopy of the annihilation radiation shows that the annihilation sites are Si-rich after quenching but contain more Mg after 70 min.

Positron annihilation characteristics in mesostructural silica films with various porosities

Porous silica films with various porosities were prepared via a sol-gel method using a nonionic amphiphilic triblock copolymer F127 as the structure-directing agent. Doppler broadening of positron annihilation radiation (DBAR) spectra were collected for the prepared films using a variable energy slow positron beam. Different linear relationships between positron annihilation line shape parameters S and W are found for the as-deposited films and calcined ones, indicative of the decomposition of the copolymer porogen in the as-deposited films upon calcination. This also reveals the variation of positron annihilation sites as a function of F127 loading or porosity. Strong correlations between positronium 3γ annihilation fraction, S parameter and porosity of the mesoporous silica films with isolated pores are obtained, which may provide a complementary method to determine closed porosities of mesoporous silica films by DBAR.

Interstitial Solution Carbon Concentration and Defects of Ti + Nb ULC-BH Steel by Internal Friction and Positron Annihilation Methods

The interstitial solution carbon concentration and defects in continuously annealed Ti+Nb bearing ULC-BH (ultra-low carbon bake hardening) steel samples are investigated by multi-functional internal friction apparatus, positron annihilation lifetime spectroscopy (PALS) and coincidence Doppler broadening spectroscopy (CDBS). The relationship of internal friction peaks, interstitial solution carbon concentrations and movable dislocations in the samples under different conditions is analyzed. A correlation of lifetime component τ1 values with interstitial solution carbon concentrations in the samples for different continuous annealing processes is established, while a correlation of lifetime component τ2 values with multi-vacancies, vacancy clusters, microvoids and other types of defects for various continuous annealing processes is also demonstrated. Furthermore, the average lifetime results illustrate the overall defect densities for various continuous annealing processes. The CDBS analysis reflects the chemical surroundings of the defects at the annihilation sites and reveals that the peak heights of the ratio curves relate to the total number of defects such as interstitial carbon atoms, dislocations, vacancies and other types of defects. The results show that internal friction, PALS and CDBS are effective techniques to identify and characterize the interstitial solution carbon concentration, multi-vacancies, vacancy clusters, microvoids and other types of microscopic defects in annealed Ti+Nb bearing ULC-BH steel.

Positron annihilation sites in nano lead sulfide powders

The coincident Doppler broadening of the positron-electron annihilation radiation has been applied for the identification of vacancies in nano lead sulfide (nano-PbS) powders. The results of the positron annihilation studies on nano-PbS with different particle size are presented. Nano-PbS specimens have been synthesized by the chemical bath deposition in aqueous medium. Independently of the size of nanoparticles (from 12 to 16 nm) the positron lifetime (PLT) for all specimens has been found to be the same and equal to 391±1 ps. Independently of the size of nanoparticles the chemical surrounding of positron annihilation sites identified by coincident Doppler broadening measurements was the same, namely lead (Pb).

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.

Single crystal growth of [formula omitted] semiconductors and defect studies via positron annihilation spectroscopy

Small single crystals of Ga2(SexTe1−x)3 semiconductors, for x=0.1, 0.2, 0.3, were obtained via modified Bridgman growth techniques. High resolution powder x-ray diffractometry confirms a zincblende cubic structure, with additional satellite peaks observed near the (111) Bragg line. This suggests the presence of ordered vacancy planes along the [111] direction that have been previously observed in Ga2Te3. Defect studies via positron annihilation spectroscopy show an average positron lifetime of ≈400ps in bulk as-grown specimens. Such a large lifetime suggests that the positron annihilation sites in these materials are dominated by defects. Moreover, analyzing the electron momenta via coincidence Doppler broadening measurements suggests a strong presence of large open-volume defects, likely to be vacancy clusters or voids.

Effect of interfacial interaction on free volumes in phenol-formaldehyde resin–carbon nanotube composites: positron annihilation lifetime and age momentum correlation studies

The phenol-formaldehyde-carbon nanotube composites were characterized for their free volume properties and interfacial interactions between nanotubes and the polymer matrix. The base polymeric material was a novolac type phenol-formaldehyde (PF) condensation resin cross-linked with para-toluene sulfonic acid. Multi-wall carbon nanotubes (MWCNTs) were synthesized using a catalytical chemical vapor deposition method and characterized using high-resolution transmission electron microscopy. The PF resin-carbon nanotubes composites having 2, 5, 10 and 20% (w/w%) MWCNTs were prepared. The crystallinity and morphology of the samples were characterized using X-ray diffraction and scanning electron microscopy. The free volume size in the polymer nanocomposites was observed to increase with the increase in nanotube content. Positron age momentum correlation (AMOC) studies revealed the electronic environment around different positron annihilation sites. The studies showed that ortho-positronium principally annihilates from interfacial regions of polymer and nanotubes in the nanocomposite. The positron lifetime studies together with AMOC measurements indicate an increase in the free volumes at the interface of polymer and MWCNTs in the composite. The free positron intensities showed that the polymer and nanotubes are weakly interacting in this system.

Determination of nanoobject sizes and concentrations in irradiated metals and alloys via positron annihilation spectroscopy

Positron annihilation spectroscopy is shown to be an efficient method for determining the size of nanoobjects (vacancies and their clusters), the free volumes of pores, cavities, voids, and their concentrations and chemical compositions at the annihilation sites of porous systems and certain defective materials, including metals. The results of experimental studies on nanodefects of metals and alloys have been reviewed comprehensively.

Positron annihilation in neutron irradiated iron-based materials

The hardening and embrittlement of reactor pressure vessel steels is of great concern in the actual nuclear power plant life assessment. This embrittlement is caused by irradiation-induced damage, like vacancies, interstitials, solutes and their clusters. But the reason for the embrittlement of the material is not yet totally known. The real nature of the irradiation damage should thus be examined as well as its evolution in time. Positron annihilation spectroscopy has been shown to be a powerful method for analyzing some of these defects. In fact, both vacancy type clusters and precipitates can be visualized by positrons. Recently, at SCK·CEN, a new setup has been constructed, calibrated and optimized to measure the coincidence Doppler broadening and lifetime of neutron irradiated materials. To be able to compare the results obtained by the positron studies, with those of other techniques (such as transmission electron microscopy, atom probe tomography and small angle neutron scattering), quantitative estimations of the size and density of the annihilation sites are needed. Using the approach proposed by Vehanen et al., an attempt is made to calculate the needed quantities in Fe and Fe-Cu binary alloys that were neutron irradiated to different doses. The results obtained are discussed highlighting the difficulties in defining the annihilation centres even in these simple model alloys, in spite of using both lifetime and Doppler broadening measurements in the same samples.

Defect investigations of micron sized precipitates in Al alloys

A lot of light aluminium alloys achieve their favourable mechanical properties, especially their high strength, due to precipitation of alloying elements. This class of age hardenable Al alloys includes technologically important systems such as e.g. Al-Mg-Si or Al-Cu. During ageing different precipitates are formed according to a specific precipitation sequence, which is always directed onto the corresponding intermetallic equilibrium phase. Probing the defect state of individual precipitates requires high spatial resolution as well as high chemical sensitivity. Both can be achieved using the finely focused positron beam provided by the Bonn Positron Microprobe (BPM) [1] in combination with the High Momentum Analysis (HMA) [2]. Employing the BPM, structures in the micron range can be probed by means of the spectroscopy of the Doppler broadening of annihilation radiation (DBAR). On the basis of these prerequisites single precipitates of intermetallic phases in Al-Mg-Si and Al-Cu, i.e. Mg2Si and Al2Cu, were probed. A detailed interpretation of these measurements necessarily relies on theoretical calculations of the DBAR of possible annihilation sites. These were performed employing the DOPPLER program. However, previous to the DBAR calculation the structures, which partly contain vacancies, were relaxed using the ab-initio code SIESTA, i.e. the atomic positions in presence of a vacancy were recalculated.

Black holes in our galactic halo: compatibility with FGST and PAMELA data and constraints on the first stars

10−105m☉ black holes with dark matter spikes that formed in early minihalos and still exist in our Milky Way Galaxy today are examined in lightof recent data from the Fermi Gamma-Ray Space Telescope (FGST). The dark matter spikes surrounding black holes in our Galaxy are sites of significant dark matter annihilation. We examine the signatures of annihilations into gamma-rays, e+/e−,and neutrinos. We find that some significant fraction of the point sources detected by FGST might be due to dark matter annihilation near black holes in our Galaxy. We obtain limits on the properties of dark matter annihilations in the spikes using the information in the FGST First Source Catalog as well as the diffuse gamma-ray flux measured by FGST.We determine the maximum fraction of high redshift minihalos that could have hosted the formation of the first generation of starsand, subsequently, their black hole remnants. The strengthof the limits depends on the choice ofannihilation channel and black hole mass; limits are strongest for the heaviest black holes andannhilation to bb̄ and W+W− final states. The larger black holes considered in this paper may arise as the remnants of Dark Stars after the dark matter fuel is exhausted and thermonuclear burning runs its course; thus FGST observations may be used to constrain the properties of Dark Stars. Additionally, we comment on the excess positron flux found by PAMELA and its possible interpretation in terms of dark matter annihilation around these black hole spikes.

A simple dislocation model of the influence of high-angle boundaries on the deformation behavior of ultrafine-grained materials

The deformation resistance of ultrafine-grained (UFG) materials is modelled on the basis of the evolution of the average dislocation density with strain in the course of glide and recovery of dislocations. In contrast to materials with conventional grain size (CG), dislocations are stored and annihilated solely at the high-angle boundaries, where screw dislocations glide and edge dislocations climb towards annihilation sites. The high-angle boundaries enhance both the rates at which dislocations are stored and recovered. Depending on the spacing of high-angle boundaries, temperature and strain rate, UFG materials are softer or harder than their CG counterparts.

Possibilities of studying nanoobjects in porous silicon and silicon substrates irradiated with protons by positron annihilation spectroscopy

The chemical composition of the material at the annihilation sites (silicon atoms of the pore “wall”), the size of nanodefects, and their concentration in porous silicon and single-crystal silicon wafers irradiated by protons have been determined using the angular distribution of annihilation photons.

Thermally activated precipitation at deformation-induced defects in Fe-Cu and Fe-Cu-B-N alloys studied by positron annihilation spectroscopy

We have investigated the influence of deformation-induced defects on the isothermal precipitation at $550\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ in as-quenched (solute-supersaturated) and annealed (solute-depleted) Fe-Cu and Fe-Cu-B-N alloys by positron annihilation spectroscopy and hardness tests. Using the coincidence Doppler broadening technique, the evolution of local environment at the positron annihilation sites (open-volume defects, Cu precipitates, and matrix) was monitored as a function of the aging time. For all samples, plastic deformation causes a pronounced change in $S$ and $W$ parameters signaling the formation of open-volume defects. For the as-quenched samples, aging results in a sharp decrease in the amount of open-volume defects combined with the rise of a strong copper signature, which can be attributed to preferential copper precipitation at the open-volume defects introduced by plastic deformation. In contrast, the open-volume defects of the annealed samples can only be reduced partially. Both the hardness tests and the positron annihilation spectroscopy indicate that the addition of B and N to the Fe-Cu alloy causes a significant acceleration of the precipitation in the as-quenched alloys.

Positronics and nanotechnologies: Possibilities of studying nanoobjects in critical engineering materials using positron annihilation spectrometry

It is shown that positron annihilation spectroscopy (PAS) is one of the efficient methods for determining the sizes of nanodefects (vacancies, vacancy clusters); free volumes of pores; cavities, and voids; their concentrations and the chemical composition at the annihilation site (location) in nanomaterials and other critical engineering materials. A brief review of experimental studies of nanodefects in porous silicon, silicon, and quartz monocrystal irradiated by protons as well as quartz powders are given.

Coincident Doppler‐broadening spectroscopy of Si, amorphous SiO2, and α‐quartz using mono‐energetic positrons

AbstractExperiments on oxides and on silicon by coincident Doppler broadening spectroscopy (CDBS) using the mono‐ener‐ getic positron beam at the neutron induced positron source NEPOMUC are presented. Doppler‐broadening spectroscopy (DBS) as a function of positron implantation energy is applied in order to compare the crystalline quartz and single crystalline Si with amorphous quartz. More detailed information about the chemical surrounding at the positron annihilation site is gained by CDBS‐experiments. For this reason, the CDBS‐measurements are performed to detect the signature of positrons annihilating with electrons from oxygen. The comparison of the CDB spectra obtained for the quartz samples and ice with that of the Si surface clearly shows that the surface of single‐crystalline Si is terminated with oxygen. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Application of positron lifetime spectroscopy to the study of material processing of Nd–Fe–B permanent magnets

A positron lifetime spectroscopy was applied to a series of samples of Nd–Fe–B based alloys prepared by hydrogenation-disproportionation-desorption-recombination (HDDR) process with various desorption-recombination (DR) times and the results were compared to the results of cross-sectional observation with scanning electron microscopy (SEM). From these studies, it is strongly suggested that microstructural changes, which causes the sudden onset of coercivity during DR stage progress abruptly in a relatively short period of time. Positron lifetime spectroscopy can detect the microstructural changes during DR treatment and furthermore, it is suggested that positron annihilation site is directly related to the region which determines the coercivity of the HDDR-processed Nd–Fe–B alloys.

Nano- and micro-scale free volume in ultrafine grained Cu–1 wt.%Pb alloy deformed by equal channel angular pressing

Nano- and micro-scale free volumes in ultrafine grained Cu–1 wt.%Pb alloy deformed by equal channel angular pressing (ECAP) were examined by a combination of radiotracer diffusion measurements, transmission electron microscopy, depth milling by focused ion beam, and positron annihilation spectroscopy. The positron lifetime spectroscopy revealed the existence of vacancy-type defects, presumably related to grain boundary states, and vacancy clusters. The positron annihilation sites probed by coincident Doppler broadening spectroscopy are surrounded by both Cu and Pb atoms, implying preferred nucleation of nanovoids near small Pb inclusions. Larger free volume defects, such as nano- and submicron-sized pores and submicrometer cracks, were directly observed by the focused ion beam technique. Ultra-fast penetration of liquid tracer solution in the sample bulk was established in the radiotracer diffusion experiments. The results provide an unambiguous proof of the existence of open porosity in ultrafine grained Cu–Pb alloy deformed by ECAP. Under specific deformation conditions, a connected network of open channels, which promotes long-range penetration of a liquid radiotracer solution, was shown to form during the severe deformation. The relative volume fraction of the percolating porosity was estimated at about 2 × 10 −6.

THE GALACTIC POSITRON ANNIHILATION RADIATION AND THE PROPAGATION OF POSITRONS IN THE INTERSTELLAR MEDIUM

The ratio of the luminosity of diffuse 511 keV annihilation radiation, measured by INTEGRAL in its four years, from a Galactic positron ( 3.5 kpc), the cascade of the magnetic turbulence, which scatters the positrons, is damped by ion-neutral friction, allowing positrons to stream along magnetic flux tubes. We find that nearly 1/2 of the positrons produced in the disk escape from it into the halo. On the other hand, we show that within the extended, or interstellar, bulge (<3.5 kpc), essentially all of the positrons are born in the hot plasmas which fill that volume. We find that the diffusion mean free path is long enough that only a negligible fraction annihilate there and ~80% of them escape down into the H II and H I envelopes of molecular clouds that lie within 1.5 kpc before they slow down and annihilate, while the remaining ~20% escape out into the halo and the disk beyond. This propagation accounts for the low observed annihilation radiation luminosity of the disk compared to the bulge. In addition, we show that the primary annihilation sites of the propagating positrons in both the bulge and the disk are in the warm ionized phases of the interstellar medium. Such annihilation can also account for those two basic properties of the emission, the fraction (~93% ± 7%) of annihilation via positronium and the ratio (~0.5) of broad (~5.4 keV) to narrow (~1.3 keV) components of the bulge 511 keV line emission. Moreover, we expect that the bulk of this broad line emission comes from the tilted disk region (0.5 < R < 1.5 kpc) with a very large broad/narrow flux ratio of ~6, while much of the narrow line emission comes from the inner bulge (R < 0.5 kpc) with a negligible broad/narrow flux ratio. Separate spectral analyses of the 511 keV line emission from these two regions should be able to test this prediction, and further probe the structure of the interstellar medium. Lastly, we show that the asymmetry in the inner disk annihilation line flux, which has been suggested as added evidence for new sources, can also be fully understood from propagation and the asymmetry in the inner spiral arms as viewed from our solar perspective without any additional sources.

Structural analysis of rapidly solidified Mg–Cu–Y glasses during room-temperature embrittlement

The structural changes of rapidly-quenched amorphous Mg–Cu–Y alloys during time-dependent room-temperature embrittlement were investigated. A continuous reduction in enthalpic content with time, corresponding to a loss in free volume, and radial atomic distances obtained from synchrotron X-ray diffraction experiments imply an overall reduction in volume. Two distinct lifetimes were found by positron annihilation lifetime spectra, with the longer-lifetime component (attributed to areas of larger free volume) vanishing after an ageing time corresponding roughly to the time of embrittlement. Coincidence Doppler broadening suggests that the immediate environment of the annihilation sites is not altered during ageing. The mechanical behaviour correlates with the structural changes in the alloy. The densification observed is assumed to influence the sharp ductile-to-brittle transition in these Mg-based glasses, and the abrupt change in bending ductility is found to result from disappearing large free volume.