Positron Annihilation Investigations Research Articles

Systematic investigation of positron annihilation in transition metals from first principles

Systematic investigation of positron annihilation in transition metals from first principles

Positron annihilation investigation of hardening behavior induced by Fe2+ irradiation in low-Cr FeCrAl alloys

Compared with 13Cr–FeCrAl alloy, micro-alloyed low-Cr FeCrAl alloy is expected to provide higher potential for application as accident-tolerant fuel (ATF) cladding material. In this study, low-Cr FeCrAl alloys with 7–10 wt.% Cr contents were irradiated to different doses of 1 and 5 dpa at 400 °C with 3 MeV Fe2+ ion. The 13Cr–FeCrAl alloy C35MN developed by Oak Ridge national laboratory was also studied for comparison. Samples were examined by Doppler Broadening of Annihilation Radiation (DBAR) and nanoindentation before and after irradiation. Nanoindentation hardness tests showed that irradiation hardening occurred in FeCrAl alloy samples with different Cr contents after irradiation. The hardening response of the 13Cr–FeCrAl alloy increased with the increase of irradiation dose, while the hardness of low-Cr micro-alloyed FeCrAl alloy changed little or decreased after higher dose irradiation. DBAR shows that the defect type of all samples before irradiation is the same and there are a large number of vacancy-type defects in all unirradiated samples. High temperature irradiation reduces the density of the internal vacancy defect of FeCrAl alloy, which is probably caused by the annealing-induced recovery effect on atomic displacement. The defect density of 13Cr-alloy increases with the increase of irradiation dose, while that of 10Cr–FeCrAl alloy decreases with the increase of irradiation dose. This is related to the much finer grain/precipitate size and the higher number density of precipitates, and thus the higher fraction of boundaries in the 10Cr–FeCrAl alloy. The results also showed that the vacancy recovery rate of the 10Cr–FeCrAl alloy is higher.

Microstructural and magnetic properties of Mn2FeSi and Mn2FeAl alloys prepared in bulk form

Microstructural and magnetic properties of the Mn2FeSi and Mn2FeAl alloys prepared in the bulk form have been investigated. Cylinder-shaped ingots produced by induction melting technique were analyzed in as-quenched state and additionally annealed at 773 K for 5 days in the protective argon atmosphere. The results show that Si and Al have different effects on the microstructural and magnetic properties of the alloys. The Mn2FeAl ingots are single-phase both before and after annealing, and their diffractograms surprisingly correspond not to the Heusler (L21 or XA) but to the primitive cubic β-Mn structure. Conversely, Mn2FeSi alloys show a two-phase behavior in the as-quenched state. From results of X-ray diffraction it was not possible to judge whether Mn2FeSi alloy has the inverse-Heusler (XA) or full Heusler (L21) structure. Annealing of Mn2FeSi leads to the formation of multiple phases. The EDX chemical area analyses resulted in only slight deviations of compositions compared to the nominal ones. The lattice parameters of 0.5672 nm and 0.6339 nm were estimated for the Mn2FeSi and Mn2FeAl samples from X-ray diffraction measurements. From the magnetic viewpoint, all samples are paramagnetic at room temperature and transform into antiferromagnetic state at the N é el temperatures about 50 K and 36 K for Mn2FeSi and Mn2FeAl, respectively. The negative Curie temperatures determined at all samples by Curie-Weiss law indicate an antiferromagnetic ordering of spins. Positron annihilation investigations revealed that Mn2FeSi contains a high concentration of vacancies. The local chemical environment of vacancies characterized by coincidence Doppler broadening is compatible with L21 rather than with XA structure. In contrast vacancy concentration in Mn2FeAl is very low and almost all positrons are annihilated in the free state.

Fast 3D kernel computation method for positron range correction in PET

Objective. The positron range is a fundamental, detector-independent physical limitation to spatial resolution in positron emission tomography (PET) as it causes a significant blurring of underlying activity distribution in the reconstructed images. A major challenge for positron range correction methods is to provide accurate range kernels that inherently incorporate the generally inhomogeneous stopping power, especially at tissue boundaries. In this work, we propose a novel approach to generate accurate three-dimensional (3D) blurring kernels both in homogenous and heterogeneous media to improve PET spatial resolution. Approach. In the proposed approach, positron energy deposition was approximately tracked along straight paths, depending on the positron stopping power of the underlying material. The positron stopping power was derived from the attenuation coefficient of 511 keV gamma photons according to the available PET attenuation maps. Thus, the history of energy deposition is taken into account within the range of kernels. Special emphasis was placed on facilitating the very fast computation of the positron annihilation probability in each voxel. Results. Positron path distributions of 18F in low-density polyurethane were in high agreement with Geant4 simulation at an annihilation probability larger than 10−2 ∼ 10−3 of the maximum annihilation probability. The Geant4 simulation was further validated with measured 18F depth profiles in these polyurethane phantoms. The tissue boundary of water with cortical bone and lung was correctly modeled. Residual artifacts from the numerical computations were in the range of 1%. The calculated annihilation probability in voxels shows an overall difference of less than 20% compared to the Geant4 simulation. Significance. The proposed method is expected to significantly improve spatial resolution for non-standard isotopes by providing sufficiently accurate range kernels, even in the case of significant tissue inhomogeneities.

Study of large axial space rebinning algorithm in γ-photon industrial tomography image reconstruction

The accuracy of the existing single slice and Fourier rebinning algorithms depends on the projection angle of the line of response. The increase of such projection angle with the detector size, typical in the large axial space of γ-photon industrial detection, and the loss of some projection data after rebinning, result in the degradation of the image quality. In addition, those algorithms consider the probability of positron annihilation equally distributed along the line of response, which prevents to estimate accurately the positions of the annihilation point, and can originate artifacts and noise in the reconstructed image. In this work, we propose an alternative large axial space rebinning algorithm. In that algorithm, initially the line of response is divided into transverse and axial components. Then, each line of response is uniformly rebinned into all the 2D sinogram data intersecting with it. To improve the accuracy of the estimate of the annihilation point location and suppress the noise effectively, we assign a Gaussian weight coefficient to the projection data, and optimise the rebinning algorithm with it. Finally, we reconstruct the image on the basis of the 2D sinograms with the optimised weights. On the computational side, the algorithm is also accelerated by making use of parallel computing. Both simulation and experimental results show that the proposed method improves the contrast and spatial resolution of 2D reconstructed images. Furthermore, the reconstruction time is not affected by the new method, which is therefore expected to meet the demand of γ-photon industrial inspection imaging.

Positron annihilation investigation of thermal cycling induced martensitic transformation in NiTi shape memory alloy

Thermal cycling of a Ni-excess NiTi alloy was conducted between 50 °C and liquid nitrogen temperature to induce martensitic transformations and to reverse them after. The starting point was an annealed and slowly cooled state, the end point a sample thermally cycled 1500 times. Positron annihilation lifetime spectra and Coincidence Doppler Broadening profiles were obtained in various states and at various temperatures. It was found that the initial state was low in defects with positron lifetimes close to that of bulk NiTi. Cycling lead to a continuous build-up of a defect structure up to 200−500 cycles after which saturation was reached. Two types of defects created during cycling were identified, namely pure dislocations and vacancies attached to dislocations.

Positron diffusion enhances the accumulation effect in small particles embedded in a low-density matrix

This report considers the mathematical model of positron’s behavior in a system consisting of small particles embedded in a matrix. It takes into account their initial distribution after the slowing down process, random walk, and finally annihilation. The theoretical results are compared with the results obtained from an experiment in which positron lifetime measurements were carried out for samples consisting of nickel particles of various diameters embedded in an epoxy resin. The model well describes the measurements extracted from the probability of positron annihilation in particles, as a function of their volume fraction. Based on these results, it was possible to determine the parameter characterizing the accumulation effect in this system and to estimate the positron diffusion length in the matrix. The model can be useful in studying, using positron annihilation techniques, polymer composites with particles and nanoparticles, as well as shedding light on the behavior of swift positrons in composites.

Positron Annihilation Investigation of Embrittlement Behavior in Chinese RPV Steels after Fe-Ion Irradiation

Positron Annihilation Investigation of Embrittlement Behavior in Chinese RPV Steels after Fe-Ion Irradiation

Positron annihilation spectroscopy investigation of vacancy defects in neutron-irradiated3C-SiC

Positron annihilation spectroscopy characterization results for neutron-irradiated $3C$-SiC are described here, with a specific focus on explaining the size and character of vacancy clusters as a complement to the current understanding of the neutron irradiation response of $3C$-SiC. Positron annihilation lifetime spectroscopy was used to capture the irradiation temperature and dose dependence of vacancy defects in $3C$-SiC following neutron irradiation from 0.01 to 31 dpa in the temperature range from $380{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ to $790{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. The neutral and negatively charged vacancy clusters were identified and quantified. The results suggest that the vacancy defects that were measured by positron annihilation spectroscopy technique contribute very little to the transient swelling of SiC. In addition, coincidence Doppler broadening measurement was used to investigate the chemical identity surrounding the positron trapping sites. It was found that silicon vacancy-related defects dominate in the studied materials and the production of the antisite defect ${\mathrm{C}}_{\mathrm{Si}}$ may result in an increase in the probability of positron annihilation with silicon core electrons.

Evolution of embedded lithium nanoclusters in lithium implanted alumina

High dose of ion implantation followed by annealing is considered a feasible way to generate thermally stable nanoclusters inside a transparent host matrix. Low energy (50 keV) Li ions have been implanted into single crystals of alumina with different fluence (1 × 1015–1 × 1017 ions/cm2). The samples have been annealed at temperatures ranging from 500 to 1100 °C in air in step of 100 °C. Depth dependent Doppler broadening measurements have been carried out using high purity germanium detector coupled to a variable energy slow positron beam. Fractional area in the central and wing regions of Doppler broadened annihilation radiation spectrum, namely, S- and W- parameters, were evaluated from each spectrum. Any variation in positron annihilation probability with valence and core electrons which occurs on trapping of positrons at a defect site is reflected in these parameters. The effect of ion fluence and annealing temperature on evolution of defects and formation of embedded Li nanoclusters have been studied by indexing the variation in line shape S- (W-) parameter as a function of positron implantation depth. These studies supplemented by theoretical calculations confirm that with annealing up to 700 °C, vacancy clusters are created due to the aggregation of vacancies wherein Li nanoclusters are formed. On annealing at higher temperature, there is evidence for the breakdown of these Li clusters leaving behind vacancy clusters in the samples.

Positron annihilation investigation of BaSrFBr:Eu by X-ray irradiation

The mechanical property of the BaSrFBr:Eu phosphor layer of X-ray image plates was investigated by using resolution (LP/mm) and coincidence Doppler broadening (CDB) positron annihilation as well as positron annihilation lifetime (PAL). The image plate samples of BaSrFBr:Eu phosphors in this experiment were irradiated by using hospital X-rays. The LP/mm values of the irradiated BaSrFBr:Eu image plates varied from 3.35 to 1.25 for up to 20,000 exposures. CDB positron annihilation and lifetime spectroscopy were used to analyze defect structures in the phosphor layer. Even when the LP/mm values were greatly decreased due to exposures, the S parameter and the lifetime (τ1, τ2) values were almost constant with increasing number of exposures. A positive relationship existed between the SEM images and positron annihilation spectroscopy (PAS). According to the SEM images and the positron annihilation spectroscopy (PAS) results, measurements of the defects with PAS indicate that the image-plate phosphor can be safely used for hospital X-rays in the course of diagnostic radiography at an average rate of 20,000 times for one year.

Defects and hyperfine interactions in binary Fe-Al alloys studied by positron annihilation and Mössbauer spectroscopies

The defects, the behavior of 3d electrons and the hyperfine interactions in binary Fe-Al alloys with different Al contents have been studied by measurements of positron lifetime spectra, coincidence Doppler broadening spectra of positron annihilation radiation and Mössbauer spectra. The results show that on increasing the Al content in Fe-Al alloys, the mean positron lifetime of the alloys increase, while the mean electron density of the alloys decrease. The increase of Al content in binary Fe-Al alloys will decrease the amount of unpaired 3d electrons; as a consequence the probability of positron annihilation with 3d electrons and the hyperfine field decrease rapidly. Mössbauer spectra of binary Fe-Al alloys with Al content less than 25 at.% show discrete sextets and these alloys make a ferromagnetic contribution at room temperature. The Mössbauer spectrum of Fe70Al30 shows a broad singlet. As Al content higher than 40 at.%, the Mössbauer spectra of these alloys are singlet, that is, the alloys are paramagnetic. The behavior of a 3d electron and its effect on the hyperfine field of the binary Fe-Al alloy has been discussed.

Positron Annihilation Lifetime of Irradiated Polyimide

Ortho-positronium pick off annihilation lifetime gives the information of the microscopic free volume of polymers. Polyimide polymers such as Kapton are applied in many fields, but it was impossible to apply the positron lifetime method for free volume investigation because of no positronium formation. Here, we apply the idea of the free positron annihilation probability that is sum of the probability of annihilation by the positron and electrons on the molecular chain where the positron localized and that for the annihilation with the electrons on the neighboring molecular chains. The second term is probably affected by the free volume change. We have successfully shown the temperature dependence and the electron beam irradiation effect on the free volume change by observing the free positron annihilation lifetime for Kapton.

Investigation of positron annihilation in Fe52Co34Hf7B6Cu1 amorphous alloy treated by intermediate frequency magnetic pulse

The effect of intermediate frequency magnetic pulse treatments on Fe52Co34Hf7B6Cu1 amorphous alloy prepared by melt-spun technique is investigated. The microstructure and structural defects of the treated specimens are investigated by TEM, Mössbauer spectroscopy and position annihilation lifetime spectra. The results show that the treated specimens by intermediate frequency magnetic pulse are partially crystallized, the content of the crystallization phase increases with the increase of the magnetic pulse frequency, and the crystalline volume fraction is 33.1% at a frequency of 2000 Hz. For the as-quenched amorphous alloy, the annihilation lifetime τ1 is 150.5 ps in the monovacancy-like free volume, and the relative intensity I1 is 77.7%. The annihilation lifetime τ2 is 349.7 ps in the microvoid, and the relative intensity I2 is 22.3%. With the increase of the magnetic pulse frequency, the values of τ1 and τ2 of the treated specimens decrease, I1 increases, I2 and τ decrease compared with the as-quenched amorphous alloy.

Spectroscopic ellipsometry and positron annihilation investigation of sputtered HfO 2 films

Hafnium oxide (HfO 2) films were prepared using a pulsed sputtering method and different O 2/(O 2 + Ar) ratios, deposition pressures, and sputtering powers. Spectroscopic ellipsometry (SE) and positron annihilation spectroscopy (PAS) were used to investigate the influence of the deposition parameters on the number of open volume defects (OVDs) in the HfO 2 films. The results reveal that a low O 2/(O 2 + Ar) ratio is critical for obtaining films with a dense structure and low OVDs. The film density increased and OVDs decreased when the deposition pressure was increased. The film deposited at high sputtering power showed a denser structure and lower OVDs. Our results suggest that SE and PAS are effective techniques for studying the optical properties of and defects in HfO 2 and provide an insight into the fabrication of high-quality HfO 2 thin films for optical applications.

Theoretical aspects of studies of oxide and semiconductor surfaces using low energy positrons

This paper presents the results of a theoretical study of positron surface and bulk states and annihilation characteristics of surface trapped positrons at the oxidized Cu(100) single crystal and at both As- and Ga-rich reconstructed GaAs(100) surfaces. The variations in atomic structure and chemical composition of the topmost layers of the surfaces associated with oxidation and reconstructions and the charge redistribution at the surfaces are found to affect localization and spatial extent of the positron surface-state wave functions. The computed positron binding energy, work function, and annihilation characteristics reveal their sensitivity to charge transfer effects, atomic structure and chemical composition of the topmost layers of the surfaces. Theoretical positron annihilation probabilities with relevant core electrons computed for the oxidized Cu(100) surface and the As- and Ga-rich reconstructed GaAs(100) surfaces are compared with experimental ones estimated from the positron annihilation induced Auger peak intensities measured from these surfaces.

Positron probing of gamma-irradiated Ge doped with P, As, Sb, and Bi: Changes in atomic structures of defects due to n→p conversion

Abstract The emission of the high-momentum annihilation radiation from the subvalent ion core shells and electron density around a positron localized at a vacancy-group-V-impurity atom complexes produced in oxygen-lean Ge doped with P, As, Sb, and Bi by irradiation with 60Co gamma-rays at room temperature have been investigated with the help of the angular correlation of annihilation radiation (ACAR) before and after n→p conversion. The probability of positron annihilation in the subvalent shells of atoms incorporated in dominant radiation centers was found to be dependent on the ratio of the ion core radii ri(P5+, As5+)/ri(Ge4+) 1, respectively. In passing from P to As impurity atoms the activation energy ΔEe of electron emission to be detected by DLTS measurements is increased by ~(+0.017 eV) vs. the increase of the electron density parameter to be reconstructed by ACAR data, Δr′s=r′s(As)−r′s(P)≈0.029 a.u. On the contrary, in passing from Sb to Bi impurity atoms, ΔEe value is decreased by ~(−0.028 eV) whereas the electron density parameter rises by Δr′s=r′s(Bi)−r′s(Sb)≈0.04 a.u. After n→p conversion a marked decrease in both the electron density and the number of ion cores around the positron points to the fact that the radiation-produced complexes with group-V-impurity atoms (P, As, Sb, Bi) are of a multi-vacancy character. The deep acceptor states in the forbidden gap (Ev+0.1), (Ev+0.12), (Ev+0.16) eV to be attributed to the P-, As-, Sb-, and Bi-containing multi-vacancy centers, respectively, were found to contribute to lessening the electron density around the trapped positron. It is argued that a close similarity of the As5+ and Ge4+ ion cores results in a small (but marked) augmentation in the electron density around the positron in As-containing multi-vacancy centers after n→p conversion. A trend for inward relaxation of the ion cores is observed in all radiation-produced centers studied.

Study of the behavior of 3d-shell electrons in binary Nd–Fe alloys

The behavior of 3d electrons in binary Nd–Fe alloys with different Nd content from 7 to 13at.% has been studied by using positron coincidence Doppler broadening techniques. It has been found that the 3d electron signal in Nd2Fe17 alloy is relatively high as compared with other alloys. In Fe-rich Nd2Fe17 alloys, as the content of Fe increasing, the phase boundaries between α-Fe and Nd2Fe17 phases will increase, which gives rise to the decrease in the probability of positron annihilation with 3d electrons. In Nd-rich Nd2Fe17 alloys, with the decrease of Fe content, the d–d interactions are weakened, and the probability of positron annihilation with 3d electrons will decrease. The coercivity JHc and remanence Jr of the Nd–Fe alloy increase with the d–d interaction.

Microdefects and 3d electrons in ordered B2-FeAl alloys investigated by positron annihilation techniques

Microdefects and 3d electrons in B2-FeAl alloys with different chemical composition, single crystal of Fe and cold-rolled Fe has been studied by positron lifetime and coincidence Doppler broadening spectroscopy. The coincidence Doppler broadening spectrum of the single crystal of Fe shows the highest 3d electron signal in the spectra of all tested samples. The 3d electron signal in the spectrum of Fe50Al50 alloy is much lower than that of the cold-rolled Fe. This indicates that some of the 3d electrons of Fe atoms and 3p electrons of Al atoms in B2-FeAl alloy are localized to form strong covalent bonds, thus decreasing the probability of positron annihilation with 3d electrons of Fe atoms. With the increase of Al content in B2-FeAl alloys, the 3d electron signal in the spectrum of the alloy decreases, while the open volume of defect increases.

Monte Carlo investigation of positron annihilation in medical positron emission tomography

A number of Monte Carlo codes are available for simulating positron emission tomography (PET), however, physics approximations differ. A number of radiation processes are deemed negligible, some without rigorous investigation. Some PET literature quantify approximations to be valid, without citing the data source. The radiation source is the first step in Monte Carlo simulations, for some codes this is 511 keV photons 180° apart, not polyenergetic positrons with radiation histories of their own. Without prior assumptions, we investigated electron–positron annihilation under clinical PET conditions. Just before annihilation, we tallied the positron energy and position. Right after annihilation, we tallied the energy and separation angle of photon pairs. When comparing PET textbooks with theory, PENELOPE and EGSnrc, only the latter three agreed. From 10 6 radiation histories, a positron source of 15O in a chest phantom annihilated at as high as 1.58 MeV, producing photons with energies 0.30–2.20 MeV, 79–180° apart. From 10 6 radiation histories, an 18F positron source in a head phantom annihilated at energies as high as 0.56 MeV, producing 0.33–1.18 MeV photons 109–180° apart. 2.5% and 0.8% annihilation events occurred inflight in the chest and the head phantoms, respectively. PET textbooks typically either do not mention any deviation from 180°, or state a deviation of 0.25° or 0.5°. Our findings are founded on the well-established Heitler cross-sections and relativistic kinematics, both adopted unanimously by PENELOPE, EGSnrc and GEANT4. Our results highlight the effects of annihilation in-flight, a process sometimes forgotten within the PET community.