Positron Implantation Research Articles

A Positron Implantation Profile Estimation Approach for the PALS Study of Battery Materials

Positron annihilation spectroscopy is a powerful probe to investigate the interfaces in materials relevant for energy storage such as Li-ion batteries. The key to the interpretation of the results is the positron implantation profile, which is a spatial function related to the characteristics of the materials forming the battery. We provide models for the positron implantation profile in a cathode of a Li-ion battery coin cell. These models are the basis for a reliable visualization of multilayer geometries and their interfaces in thin cathodes of lithium-ion batteries.

Influence of the positron implantation profile on the study of the defect depth distribution by the positron annihilation technique

The formulas obtained for deconvolution of positron mean lifetime results in the sequential etching technique for detecting defect depth profiles are presented. In this experiment, only the conventional positron lifetime measurement with radioisotope-based positrons is used. The important role of the positron implantation profile is discussed. These formulas were successfully used to describe the depth profile of defects formed in sliding contact in pure vanadium. Two different layers were detected below the surface.

Defects Characterization of HgCdTe and CdZnTe Compounds by Positron Annihilation Spectroscopy

Infrared cooled photodetectors must operate at higher temperatures to reduce their size, weight and power consumption (SWaP context). Their stability and image quality are then challenged by extra electrical activity of crystal defects. Knowledge of defect populations is mandatory to improve the material quality of the epitaxial Hg1−xCdxTe (MCT) active layer and the Cd1−xZnxTe (CZT) substrate. Positron annihilation spectroscopy with a slow positron beam was used to study near-surface open-volume defects profiles. Low- and high-momentum fractions (\(S\),\( W\)) were used to characterize the Doppler broadening of the 511 keV electron-positron pair annihilation-line as a function of the positron implantation energy \(E\). The results show that three regions can be identified beneath the surface of the as-grown non-optimized MCT layer. The quasi-linear relationship between the annihilation characteristics in the regions suggests that the defect populations mainly correspond to the same open-volume defect in different concentrations. The probed defect is thought to be related to the mercury vacancy. This hypothesis is discussed in an original way with near-surface elemental profiles using scanning transmission electron spectroscopy combined to energy dispersive x-ray spectroscopy (STEM-EDX). Afterwards, this approach is extended to CZT substrates showing that surface and bulk properties of those fabricated by LYNRED tend to match those that are state-of-the-art. A common open-volume defect is probed, in concentration estimated by Hall effect around 1015 cm−3 and thought to be related to the cadmium vacancy.

Defect depth profiling of sputter-deposited Cu/Nb bilayers using a positron accelerator

Ultrathin bilayers of Cu/Nb viz. Nb/Cu/Si and Cu/Nb/Si have been deposited on Si substrate using DC and RF magnetron sputtering. The structural characterization of bilayers has been carried out using X-ray reflectivity (XRR) to determine the thickness and density of individual layers. The thickness of Nb and Cu layer in the bilayers ranges from ∼ 44–71 nm and 6–11 nm, respectively. Defect depth profiling of bilayers has been carried out using a positron accelerator. The measured depth profiles, i.e., S-parameter profiles have been fitted using VEPFIT, a program that factors the positron implantation profile and its annihilation to estimate S-parameter and positron diffusion length in individual layers. We present a detailed and systematic analysis of the measured S-profiles using VEPFIT to evaluate S-parameter and positron diffusion length in individual layers of the bilayer structures. These parameters are representative of defect characteristics of the layers. In this work we also highlight the nuances of VEPFIT analysis useful to extract defect characteristics in bilayers. The results indicate that defect microstructure of the layer depends on the thickness, deposition method and irradiation. Defect density is seen to increase with the reduction of thickness of the layer. RF sputtered layer has more defects as compared to DC sputtered layers. Positron depth profiling does not reveal distinct interface between the two metallic layers as well as layer and the substrate. A systematic characterization of defect in ultrathin structures is crucial to establish a correlation between defect microstructure and properties of nanolayer composites for advanced applications.

Time-of-flight apparatus for the measurement of slow positronium emitted by nanochannel converters at cryogenic temperatures

Silicon nanochannel plates (NCPs) are efficient positron to positronium converters and are employed as sources of abundant cold positronium. Characterization of the emitted positronium energy spectrum is crucial to assess the NCP performance and support their further optimization. We present here a Time-of-Flight (ToF) system optimized to measure the energy spectrum of positronium produced by NCP converters held at cryogenic temperatures. The ToF-apparatus was tested by implanting 7keV positrons into NCPs at room temperature before performing ToF experiments using positron implantation energies of 7 and 11keV into NCPs held at 20K. At these conditions we succeeded in determining the energy spectrum of the positronium escaping from the nanochannels into the vacuum down to an energy of 10meV.

Analysis of positron profiling data using e[formula omitted]DSc computer code

The Green’s function method was applied to solve the one-dimensional positron diffusion equation for a system consisting of up to four layers that contain defects with different trapping rates. These allow us to obtain the analytical relationships valid for the evaluation of data obtained from variable energy positron measurements. They have been implemented in user-friendly free computer code available to users. Fitting strategies are presented to extract the relevant physical parameters. The code was used to determine positron diffusion length in samples of polycrystalline pure, well-annealed iron, depleted uranium, and titanium. Program summaryProgram Title: e+DSC-1CPC Library link to program files: https://doi.org/10.17632/jxpj25kjvr.1Licensing provisions: MIT licenseProgramming language: Microsoft Visual Basic 2015External routines/libraries: Accord.NETNature of problem: The program enables the analysis of data obtained from a variable energy positron beam. The shape parameter of the annihilation line as a function of the incident positron energy is evaluated using a positron diffusion trapping model in which the positron trapping rate function is expressed as a four-step function.Solution method: The one-dimensional diffusion equation was solved by the Green’s function method. This allows the use of an exact form for the positron implantation profile, which is used in the program as the Makhov’s function. The parameters of this function are obtained from the MC simulation with GEANT4 and stored in the program.

Absolute fraction of emitted Ps determined by geant4-supported analysis of gamma spectra

The fraction of positronium (Ps) emitted from a surface of a germanium single crystal at high temperature is usually assumed to approach unity at zero positron implantation energy. In the experiment, however, the determination of the absolute Ps fraction is not straight forward since recording a reference spectrum with $100\,\%$ Ps formation remains demanding. We use GEANT4-simulated detector responses to $2\gamma$ and $3\gamma$ radiation sources mimicking positron and Ps annihilation inside the (coincidence) Doppler-broadening spectrometer at NEPOMUC, FRM II, in order to derive a reliable value for the Ps fraction reemitted from a Ge(100) target heated close to its melting point. Analysis of the measured spectra by fitting the simulated spectra shows an absolute value of $(72\,\pm{4})\,\%$ maximum Ps formation, contradicting the $100\,\%$ assumption.

Efficient machine learning approach for optimizing the timing resolution of a high purity germanium detector

We describe here an efficient machine-learning based approach for the optimization of parameters used for extracting the arrival time of waveforms, in particular those generated by the detection of 511 keV annihilation γ-rays by a 60 cm3 coaxial high purity germanium detector (HPGe). The method utilizes a type of artificial neural network (ANN) called a self-organizing map (SOM) to cluster the HPGe waveforms based on the shape of their rising edges. The optimal timing parameters for HPGe waveforms belonging to a particular cluster are found by minimizing the time difference between the HPGe signal and a signal produced by a BaF2 scintillation detector. Upon applying these variable timing parameters to the HPGe signals, we achieved a γ–γ coincidence timing resolution of ∼4.3 ns at the 511 keV photopeak (defined as 511 ± 50 keV) and a timing resolution of ∼6.5 ns for the entire γ-ray spectrum—without rejecting any valid pulses. This timing resolution approaches the best obtained by analog nuclear electronics, without the corresponding complexities of analog optimization procedures. We further demonstrate the universality and efficacy of the machine learning approach by applying the method to the generation of secondary electron time-of-flight spectra following the implantation of energetic positrons on a sample.

More accurate parameterization of positron implantation depth profiles for the sensitivity range of positron-based characterization techniques

Techniques that employ positron annihilation spectroscopy are powerful tools to investigate defect structures and concentrations in materials. A hindrance to experimental design and the interpretation of results lies in the lack of agreement in the literature concerning the proper form of the positron implantation profile, a function that determines the sensitivity range for all non-slow positron annihilation spectroscopy techniques. Employing the dominant 22Na isotopic source, a positron implantation profile database of 270 common materials is published. The parameters for a novel implantation profile functional form providing superior agreement with simulation are derived. Finally, and most critically, an algorithm is presented and validated, which permits utilization of the published elemental implantation profile parameters to produce the positron implantation profile for any material of interest. This tool provides rapid calculation of the sensitivity range for all positron annihilation techniques, enabling more informed experimental design and more accurate knowledge of the spatial distribution of defects in materials.

Implantation profiles and depth distribution of slow positron beam simulated by Geant4 toolkit

The positron implantation profiles and depth distribution in metal, semiconductor, and polymer surfaces are simulated using the Monte-Carlo-method-based Geant4 toolkit. As per the slow positron beam technique, the monoenergetic positron beams (in the range from 1 to 35 keV) with 1.5 mm radius is injected into semi-infinite samples in the present work. The implantation profile of 3.1 keV positrons in Fe is in good agreement with the Makhovian profile. The mean depth and depth resolution exhibit a general negative correlation with the material densities and incident position energy, respectively, while the fixed peak energy of backscattered positrons exhibits a net correlation with atomic number Z. Furthermore, the implantation profiles present better z-axis resolution with increasing incident angle, and this effect is more prominent for low-density materials. The results can provide theoretical support for new measurement methods based on the slow positron beam technique, such as segment measurements and micro-beam scanning measurements.

Monte Carlo simulation of the implantation profile of e+ in nanochanneled silicon

The process of implantation and diffusion of positron in nanochanneled silicon crystals has been simulated in detail through the Monte Carlo technique. Our implantation simulations evidenced the fraction of empty volume inside the sample to be the decisive factor in the determination of the shape of the implantation profile, with the specific shape of the nanoscopic structure playing a marginal role for implantation processes with an energy above 3 keV. Moreover we observed that, due to the high density of surfaces inside of the silicon sample, the subsequent diffusion process is highly suppressed and that thermalized positrons reach the surface of a nanoscopic channel close to their implantation depth. Due to this suppression of the diffusion process, 60–80% of the positrons implanted at an energy comprised between 4 and 13 keV will reach, at thermal energy, the surface of a channel without escaping the sample or undergoing annihilation.

Improving depth resolutions in positron beam spectroscopy by concurrent ion-beam sputtering

Abstract The depth resolution of mono-energetic positron annihilation spectroscopy using a positron beam is shown to improve by concurrently removing the sample surface layer during positron beam spectroscopy. During ion-beam sputtering with argon ions, Doppler-broadening spectroscopy is performed with energies ranging from 3 keV to 5 keV allowing for high-resolution defect studies just below the sputtered surface. With this technique, significantly improved depth resolutions could be obtained even at larger depths when compared to standard positron beam experiments which suffer from extended positron implantation profiles at higher positron energies. Our results show that it is possible to investigate layered structures with a thickness of about 4 microns with significantly improved depth resolution. We demonstrated that a purposely generated ion-beam induced defect profile in a silicon sample could be resolved employing the new technique. A depth resolution of less than 100 nm could be reached.

Excitation of positronium: from the ground state to Rydberg levels

Following bombardment of a mesoporous silica sample by positrons ejected from a two-stage buffer gas trap, ortho-positronium (o-Ps) was emitted into vacuum with an efficiency of around 27%–28%. This ensemble, with a density close to 1011 m−3, was then irradiated by multiple solid state-based laser beams up to 50 ns after positron implantation. Tunable over a wide range (230–2200 nm), the lasers have enabled excitation of the o-Ps Lyman-α transition, followed by subsequent excitation and, if desired, ionisation to vacuum. Excitation from the 2P state to intermediate states in the principal quantum number, , range has been achieved with efficiencies , whilst the excitation efficiency of ground state o-Ps to the 2P state of is currently limited by a mismatch between the Doppler broadening of the 1S-2P transition and the 225 GHz laser bandwidth at 243 nm.

Evaluation of Positron Implantation Profiles in Various Materials for 22Na Source

Evaluation of Positron Implantation Profiles in Various Materials for ²²Na Source

Characterization of a transmission positron/positronium converter for antihydrogen production

In this work a characterization study of forward emission from a thin, meso-structured silica positron/positronium (Ps) converter following implantation of positrons in light of possible antihydrogen production is presented. The target consisted of a ∼1μm thick ultraporous silica film e-gun evaporated onto a 20nm carbon foil. The Ps formation and emission was studied via Single Shot Positron Annihilation Lifetime Spectroscopy measurements after implantation of pulses with 3-4·107 positrons and 10ns temporal width. The forward emission of implanted positrons and secondary electrons was investigated with a micro-channel plate – phosphor screen assembly, connected either to a CCD camera for imaging of the impinging particles, or to a fast photomultiplier tube to extract information about their time of flight. The maximum Ps formation fraction was estimated to be ∼10%. At least 10% of the positrons implanted with an energy of 3.3keV are forward-emitted with a scattering angle smaller than 50° and maximum kinetic energy of 1.2keV. At least 0.1–0.2 secondary electrons per implanted positron were also found to be forward-emitted with a kinetic energy of a few eV. The possible application of this kind of positron/positronium converter for antihydrogen production is discussed.

Embedded Si nanoclusters in α-alumina synthesized by ion implantation: An investigation using depth dependent Doppler broadening spectroscopy

Embedded Si nanoclusters in optically transparent host materials have shown potential applications in opto-electronic devices. Ion implantation followed by annealing at higher temperatures is considered a promising way to produce embedded nanoclusters in variety of host materials. In the present work, α-alumina crystals were irradiated with Si ion beam (50 keV) with varying total dose (1 × 1016, 5 × 1016 and 1 × 1017 ions/cm2). The ion implantation and vacancy defect profiles in alumina have been calculated using computer code SRIM. The irradiated samples were annealed at 500 and 1000 °C in a reducing atmosphere for 30 min. Depth dependent Doppler broadening measurements using high purity Germanium (HPGe) detector coupled to a slow positron beam were carried out on as-irradiated as well as annealed samples. The line shape (S- and W-) parameters showing contribution from valence (low momentum) and core (high momentum) electrons were evaluated from the positron implantation energy dependent Doppler broadened spectra. The S-E profiles of as irradiated samples indicated the formation of open volume defects in the damaged region. On annealing, S-E profiles are modified in the damaged region that is attributed to the formation of vacancy clusters and Si nanoclusters as confirmed through S-W correlation plots. The S-E profiles have been fitted using variable energy positron fit (VEPFIT) to evaluate the characteristic S-parameter corresponding to the damaged region. It is observed that ∼90% of implanted positrons in the damaged region are confined to the Si nanoclusters due to their high affinity toward Si as compared to host material, α-alumina. The present study confirms that positrons are confined in embedded nanoclusters and acts as a self seeking probe for their characterization.

Four-dimensional positron age-momentum correlation

We have performed first four-dimensional age-momentum correlation (4D-AMOC) measurements at a pulsed high intensity positron micro beam and determined the absolute value of the three-dimensional momentum of the electrons annihilating with the positrons in coincidence with the positron age in the sample material. We operated two position sensitive detectors in coincidence to measure the annihilation radiation: a pixelated HPGe-detector and a microchannel plate image intensifier with a CeBr3 scintillator pixel array. The transversal momentum resolution of the 4D-AMOC setup was measured to be about 17 × 10−3 (FWHM) and was circa 3.5 times larger than the longitudinal momentum resolution. The total time resolution was 540 ps (FWHM). We measured two samples: a gold foil and a carbon tape at a positron implantation energy of 2 keV. For each sample discrete electron momentum states and their respective positron lifetimes were extracted.

Insights into the Mechanism of Zeolite Detemplation by Positron Annihilation Lifetime Spectroscopy

Templating approaches play a key role in expanding the structural diversity of many important classes of porous materials. Understanding the interaction of the templating agent with the resulting porous framework and the mechanism of detemplation is critical to ensure their effective application. This study explores the scope of positron annihilation lifetime spectroscopy (PALS) to gain insight into the location of tetrapropylammonium (TPA+) industrially employed as a structure directing agent (SDA) in the synthesis of MFI-type zeolites. ortho-Positronium (o-Ps) formed upon positron implantation are initially confined within the zeolite crystals. Simulations using a modified Tao potential confirm that o-Ps atoms localize between the propyl chains of adjacent TPA+ molecules within the sinusoidal channels of the micropore network. Controlled removal of the SDA leads to a linear increase in the micropore volume determined by nitrogen sorption. In contrast, the out-diffusion of o-Ps is found to be crystal siz...

COMPUTATION OF POSITRON IMPLANTATION PROFILE IN SOLIDS

Positron implantation profile in solids is very vital in understanding the process of positron annihilation in solids. In this work, a model for positron implantation profile in solids is presented; a computer program for computing and analyzing positron implantation profile in solids based on the model was developed and implemented. The program was used to compute and analyze positron implantation profile in aluminium, copper, steel and silicon. The results obtained revealed that the height of the positron implantation profile depends inversely on the incident positron energy while the implantation depth depends directly on the incident positron energy. Also, positron implantation profile depends directly on the nature of the solid. The results obtained in this work are in one to one agreement with the experimental positron implantation profiles in solids showing the predictability power of the model.

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.