Papers by Keyword: Positron Annihilation Spectroscopy

Abstract: Positron annihilation spectroscopy (PAS) offers a transformative approach to medical imaging, providing detailed insights into molecular structures. Although PAS has been extensively applied in studying defects in semiconductors and synthetic materials—yielding quantitative data on their microscopic properties—its potential in medical imaging could significantly enhance diagnostic methodologies. The application of positrons and other forms of radiation in analyzing living tissues necessitates careful consideration of potential damage. In this work, a model method designed to determine the optimal dose for experimental measurements is introduced. While Positron Emission Tomography (PET) has been instrumental in clinical diagnostics using radiopharmaceuticals to visualize metabolic processes, PAS presents a cutting-edge tool for improving the specificity and accuracy of biological imaging. Its capability to non-destructively explore structural transformations and micro-environmental changes in biological samples positions it as a promising innovation in diagnostics, paving the way for enhanced healthcare outcomes globally.

Abstract: Photoconductive amorphous selenium (a-Se) layers are utilized in flat panel X-ray imaging detectors as a direct conversion medium, converting X-ray photons directly into electric charge. Commercial a-Se direct conversion Active Matrix Flat Panel Imagers (AMFPIs) have demonstrated superior image quality in mammography, showcasing the potential of this X-ray imaging technology [1-2]. The use of a-Se is limited, however, by its low Z, resulting in low stopping of high energy X-rays [2]. This limitation is not shared by PbO thin films. Earlier PbO films consisted of small poly-crystalline platelets with low film density and suffered from the presence of both oxygen vacancies and impurity phases (PbO₂). Recent advances [3-4] have yielded dense amorphous PbO (a-PbO) films with apparently uniform stoichiometry, as confirmed by X-ray photoelectron spectroscopy (XPS). More careful analysis [5] using X-ray absorption spectroscopy (XAS) indicated some tailing of the conduction band, which was attributed to suspected O-vacancies. An annealing study on a-PbO [3] indicated a transition to β-PbO around 500 C. X-ray diffraction (XRD) data of the β-PbO (annealed a-PbO) film matched that of a β-PbO reference, while XAS data did not. This was attributed to the different depths of the sample volumes probed by the two techniques. Doppler-broadened positron annihilation spectroscopy (DBPAS) was conducted on several a-PbO samples synthesized under different conditions using the McMaster Variable-Energy Positron Beam (MVEPB) and the results were modelled using VEPFIT [6]. All samples were found to have a three-layer structure, with the bulk S-parameters between 0.4725 and 0.4753. The two other layers were contained within the first 300nm of the film and varied in thickness, diffusion length and S-parameter value. This confirms the suitability of DBPAS, as a sensitive probe of vacancy-type defects and the layer structure of thin films, to guide the optimization of a-PbO synthesis for photoconductive detectors. Work is underway to produce a series of samples which vary systematically in their synthesis conditions to establish synthesis-structure relationships.

Abstract: Doppler Broadening Spectroscopy (DBS) of the positron electron annihilation line allows the detection and analysis of defects in materials. DBS uses the fact that during the annihilation momentum has to be conserved and is therefore transferred from the annihilating positron-electron pair to the annihilation gamma quanta. As a sufficient approximation the positron is assumed to be thermalized in the solid when it annihilates with an electron. In that case, the dominant fraction of the transferred momentum originates from the electrons. However, when implanting high-energy positrons a small fraction of positrons (typicylly less than 1%) will annihilate with electrons before thermalizing. This process of in-flight annihilation can be differentiated from annihilation of thermalized positrons by Coincidence DBS (CDBS). We used the CDB Spectrometer at the NEutron induced POsitron source MUniCh (NEPOMUC) in order get a deeper understanding of in-flight annihilation in several materials of different densities and core annihilation probabilities. We utilized a ²²Na source as positron emitter which not only provides insight into the thermalization process of positrons, but also extends the capabilities of the CDB spectrometer.

Abstract: Glasses with the compositions (100-x)(0.16Na₂O/0.10MnO/0.74SiO₂)/xFe₂O₃ (x = 0-15 mol%) were prepared and characterized using positron annihilation lifetime spectroscopy (PALS) and coincidence Doppler broadening (CDB) spectroscopy. The PALS method applied seems to be sensitive to verify the Verwey phase transition, discovered for bulk magnetite (Fe₃O₄), from a high-temperature ‘bad metal’ conducting phase to a low-temperature insulating phase occurring at about 120 K in the glasses examined. It means that at relatively low concentration of Fe₂O₃ up to 15 mol%, the magnetite crystals could be synthesized in the amorphous matrix of Na₂O/MnO/SiO₂/Fe₂O₃ glass that is important for numerous practical applications. The CDB measurements showed that the majority of positrons in the glass samples studied are annihilated in the vicinity of oxygen anions and iron-oxide containing glasses have lower concentration of oxygen-vacancy defects compared to the non-iron containing base glass.

Abstract: Initially, this work briefly outlines how ultrasound can modify and characterize the defect system in semiconductors. Then, the study experimentally examines the effect of different types of acoustic waves on the association of FeB pairs in monocrystalline silicon. The results reveal that as the frequency of longitudinal waves increases, the ultrasound's effectiveness in accelerating the association rate decreases. Conversely, exciting transverse waves show the opposite trend. The study also assesses the potential to obtain a positron-annihilation response from the FeB complex in silicon, highlighting the advantages of conducting such measurements under ultrasound loading of the crystal.

Abstract: Irradiation of materials in space or nuclear applications is unavoidable and it is well known that it modifies their properties (electronic, optical, thermal, mechanical, ..) due to the formation of point and complex defects (vacancies Vs, self-interstitial atoms SIAs, cavities, bubbles, dislocation loops, dislocation lines, precipitates). As this review shows, irradiation can also be very useful for intentionally optimizing material properties and, when performed under very well controlled conditions, for understanding defects properties and their impact on large-scale material properties. Knowledge of how damage is created and accumulated in materials is needed to better understand the behavior of materials under irradiation, in particular their radiation resistance for nuclear applications or to know the best irradiation conditions for optimizing their properties in electronic or optical applications. Experimental characterization of damage is an essential element in achieving this objective, and is very often coupled with simulation. This paper presents general information on the introduction of damage during irradiation of materials and various examples illustrating the typical advantages of the Positron Annihilation Spectroscopy (PAS) technique for the study of radiation damage.

Abstract: This paper introduces a Coincidence Doppler Broadening (CDB) analysis method, as presented during the Alfredo Dupasquier [1] Summer School held in Brunate preceding the 2024 International Workshop on Positron Studies of Defects (PSD-24) in Como. CDB spectroscopy provides quantitative information on the chemical environment surrounding defects in various materials, including metal alloys, oxides, and polymers. Reference materials were analyzed to estimate the average chemical environment around defects, voids and porosities. The methodology is applied not only to homogeneous materials, such as metallic alloys, but also to investigate depth profiles in thin films of various materials. The statistical accuracy of element separation is also examined, along with the necessity to refine the method’s calibration through theoretical calculations and the integration of direct observation techniques.

Abstract: Utilizing positron annihilation spectroscopy for studying the energetics, kinetics or charge states of open volume point defects in semiconductors is seldom straight forward. Although obtaining usable experimental results with the technique is usually fairly easy, designing a suitable experiment for a specific case and/or interpreting the results in an unambiguous manner can be challenging. The goal of this lecture is to give advice and suggestions on what to consider when planning experiments with Positron Annihilation Spectroscopy (PAS) in semiconductors, through various example cases. This contribution is not meant to be scientific, rather educational.

Abstract: Undoped and Cu-doped ZnO grown on sapphire using pulsed laser deposition (PLD) were studied by positron annihilation spectroscopy (PAS), photoluminescence (PL), high-resolution transmission electron microscopy (HRTEM) and secondary ion mass spectroscopy (SIMS). In the undoped samples, two kinds of V_Zn-related defects, namely VZn1 and VZn2 are identified. VZn1 was identified in as-grown samples grown at relatively low substrate (~300 °C). After annealing at 900 °C, VZn-2, the green luminescence (GL) peaking at 2.47 eV and the near band edge (NBE) emission at 3.23 eV in the low temperature photoluminescence (LT-PL) were simultaneously introduced. Another kind of V_Zn-related defect is identified in the Cu-doped ZnO sample, and is tentatively assigned to the V_Zn decorated with the Cu.

Abstract: In the present work, defects created by implantation of hydrothermally grown ZnO single crystals of high quality with H⁺ ions were investigated by positron annihilation lifetime (LT) spectroscopy combined with measurements of optical transmittance (OT) and photoluminescence (PL). First, zinc vacancies attached with one hydrogen impurity (V_Zn – 1H) atom were identified in the virgin ZnO single crystal. The ZnO single crystals were then bombarded by H⁺ ions with the energy of 2.5 MeV to the fluence of 10¹⁶ cm^-2. It was found that V_Zn – V_O divacancies were introduced into ZnO by H⁺-implantation. Effects of H⁺-implantation on the optical activity of defects in ZnO lattice are characterised in the light of the present OT and PL data.