Coincident Doppler Broadening Spectroscopy Research Articles

Characterization of the microstructure in friction stir welds of EN AW-2219 using coincident Doppler-broadening spectroscopy

Friction stir welding is an innovative welding technology that is often employed to weld aluminum alloys of the 2xxx-series. These alloys are primarily used in the aerospace industry to join structural parts or tanks. For these applications, high quality requirements have to be met. In a previous study, it was shown that controlling the welding temperature is one key to ensuring the quality of the welds. It was observed that the tensile strength varies depending on the welding temperature and the welding speed. In this paper, Positron Annihilation Spectroscopy (PAS) with a scanning positron beam was applied to detect crystallographic defects and changes of the microstructure of friction stir welds. The results were compared to optical microscopy and to hardness maps. Hereby, vacancy formation in the cross section of friction stir welds could be observed. Furthermore, it could be demonstrated that the state of the precipitates is the primary determinant for the mechanical properties of the welds. Finally, the observations made through PAS were employed to explain the trends in the mechanical properties, which were observed in the previous study. In particular, the varying ultimate tensile strength (UTS) could be clearly attributed to dissolution and re-precipitation phenomena, which occur during and after welding.

Positron Annihilation Spectroscopic Studies of Cu<sub>2</sub>Te Thermoelectric Material

Positron annihilation lifetime spectroscopy (PALS) and coincident Doppler-broadening spectroscopy (CDBS) have been used for investigating the evolution of vacancy-type defects in the thermoelectric material Cu2Te which annealed at different temperatures. The results of PALS show that a fraction of positrons has got annihilated at the surfaces and the sample which annealed at 450 °C has the highest concentration of surface defects. The average positron lifetime and the S parameter have the same trends which gradually increase with the increase of the annealing temperature. This change implies that the total concentration of the defects has been changed with the change of the annealed temperatures. The results of the CDBS ratio spectrum and S-W plot indicate that the defect species have no change after annealing at different temperatures.

Investigation of microstructure and mechanical properties of proton irradiated Zircaloy 2

Samples of Zircaloy 2 have been irradiated with 4 MeV protons to two different doses. Microstructures of the unirradiated and irradiated samples have been characterized by Electron Back Scatter Diffraction (EBSD), X-ray diffraction line profile analysis (XRDLPA), Positron Annihilation Lifetime Spectroscopy (PALS) and Coincident Doppler Broadening (CDB) Spectroscopy. Tensile tests and micro hardness measurements have been carried out at room temperature to assess the changes in mechanical properties of Zircaloy 2 due to proton irradiation. The correlation of dislocation density, grain size and yield stress of the irradiated samples indicated that an increase in dislocation density due to irradiation is responsible for the change in mechanical behavior of irradiated Zircaloy.

Ab-initio calculation of CDB spectra - a case study on transition metals

Experimental results of Coincident Doppler Broadening Spectroscopy (CDBS) in Si, Cr, Ni, Cu, Pt and Au references were compared with ab-initio calculations. These calculations were performed by use of the MIKA Doppler software package, which is provided for the community by Aalto University. This software calculates electron wavefunctions by an atomic superposition method. It is shown that despite this simple approach, the element specific information obtained by CDBS can be calculated with high reliability for the examined elements.

Positronbeam for μm Resolved Coincident Doppler Broadening Spectroscopy at NEPOMUC

A new high resolution coincidence Doppler spectrometer is under development at the positron beam facility NEPOMUC at the research neutron source FRM II. A Ni(100) re-moderation foil will be installed at the new spectrometer in transmission geometry for producing a positron beam with high brightness. Therefore, a magnetic lens for focusing the positrons onto the re-moderator and an electrostatic lens system for focusing the beam onto the sample have to be designed and simulated. The aim is to reach a beam diameter below 10 μm at the sample position for high resolution experiments. The current status of the project will be described and presented.

Novel data evaluation algorithm for Coincident Doppler Broadening Spectroscopy

In Coincident Doppler Broadening Spectroscopy (CDBS) the sum energy of the annihilation photons is checked to be 1022keV, to validate the measurement of an undisturbed two-gamma electron–positron decay event. The events are stored in a two-dimensional acquisition matrix. A new algorithm is presented, which optimizes the extraction of the one-dimensional CDBS spectrum from this matrix by enhanced background suppression by the use of variable size bins.

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.

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.

Dielectric properties and structural defects in BaTi1−xSnxO3 ceramics

As a function of Sn doping concentration x, dielectric properties and X-ray diffraction measurements were carried out on BaTi1-xSnxO3 (BTSx) ceramics fabricated by the solid-state reaction route. Positron annihilation lifetime spectroscopy and coincident Doppler-broadening spectroscopy were also measured for the evaluation of defects in the BTSx ceramics. Dielectric properties measurement reveals that the permittivity of BTSx ceramic gradually increases with increasing Sn dopant content for x≤3%, and then decreases. This change of permittivity is found to agree well with the relative defect concentration estimated using two positron annihilation techniques. The S-W plot indicates that the defect species do not change with Sn doping. The variation correlations between defects and dielectric properties further proves that BTSx ceramics with the higher relative defect concentration present a lower permittivity.

Positron annihilation in Cr, Cu, and Au layers embedded in Al and quantum confinement of positrons in Au clusters

Defect-sensitive and element-selective measurements on ultrathin chrome, copper, and gold layers embedded in aluminium are presented using coincident Doppler broadening spectroscopy (CDBS) with a monoenergetic positron beam. The amounts of positrons implanted in the layers of different thicknesses are calculated and compared with the experimentally gained fractions of positrons annihilating in the buried layers. A high sensitivity was already reached at an Au layer of only 2 nm thickness embedded below 200 nm Al, which was attributed to the highly efficient positron trapping in the Au layer and Au clusters. An implantation and diffusion model describes this high sensitivity for positron trapping layers. A quantum-well model of the positron wave function limits the trapping to gold clusters of a radius larger than 0.23 nm. This result was confirmed experimentally and validated with complementary TEM measurements.

Spatially resolved positron annihilation spectroscopy on friction stir weld induced defects

A friction stir welded (FSW) Al alloy sample was investigated by Doppler broadening spectroscopy (DBS) of the positron annihilation line. The spatially resolved defect distribution showed that the material in the joint zone becomes completely annealed during the welding process at the shoulder of the FSW tool, whereas at the tip, annealing is prevailed by the deterioration of the material due to the tool movement. This might be responsible for the increased probability of cracking in the heat affected zone of friction stir welds. Examination of a material pairing of steel S235 and the Al alloy Silafont36 by coincident Doppler broadening spectroscopy (CDBS) indicates the formation of annealed steel clusters in the Al alloy component of the sample. The clear visibility of Fe in the CDB spectra is explained by the very efficient trapping at the interface between steel cluster and bulk.

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)

High elemental selectivity to Sn submonolayers embedded in Al using positron annihilation spectroscopy

In the present work, we demonstrate that metal layers in the submonolayer range embedded in a matrix are revealed with unprecedented sensitivity by coincident Doppler-broadening spectroscopy of the positron annihilation using a monoenergetic positron beam. The measured electron momentum distribution specific for Sn is clearly observable in $\mathrm{Al}∕\mathrm{Sn}∕\mathrm{Al}$-layered samples even at a Sn area density of as low as $7.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{g}∕{\mathrm{cm}}^{2}$ below $200\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ Al. An explanation for the high elemental selectivity for the thin Sn layers is set forward in terms of efficient positron trapping due to the changing positron affinity at the $\mathrm{Al}∕\mathrm{Sn}$-interface and quantum-dot-like positron states in Sn nanoparticles.

Investigation of the chemical vicinity of crystal defects in ion-irradiated Mg and a Mg-Al-Zn alloy with coincident Doppler broadening spectroscopy

Crystal defects in magnesium and magnesium based alloys like AZ31 are of major importance for the understanding of their macroscopic properties. We have investigated defects and their chemical surrounding in Mg and AZ31 on an atomic scale with Doppler broadening spectroscopy of the positron annihilation radiation. In these Doppler spectra the chemical information and the defect contribution have to be thoroughly separated. For this reason samples of annealed Mg were irradiated with Mg-ions in order to create exclusively defects. In addition Al- and Zn-ion irradiation on Mg-samples was performed in order to create samples with defects and impurity atoms. The ion irradiated area on the samples was investigated with laterally and depth resolved positron Doppler broadening spectroscopy (DBS) and compared with preceding SRIM-simulations of the vacancy distribution, which are in excellent agreement. The investigation of the chemical vicinity of crystal defects in AZ31 was performed with coincident Doppler broadening spectroscopy (CDBS) by comparing Mg-ion irradiated AZ31 with Mg-ion irradiated Mg. No formation of solute-vacancy complexes was found due to the ion irradiation, despite the high defect mobility.

Deconvolution of 2D coincident Doppler broadening spectroscopy using the Richardson–Lucy algorithm

Coincident Doppler Broadening Spectroscopy (CDBS) measurements are popular in positron solid-state studies of materials. By utilizing the instrumental resolution function obtained from a gamma line close in energy to the 511 keV annihilation line, it is possible to significantly enhance the quality of the CDBS spectra using deconvolution algorithms. In this paper, we compare two algorithms, namely the Non-Negativity Least Squares (NNLS) regularized method and the Richardson–Lucy (RL) algorithm. The latter, which is based on the method of maximum likelihood, is found to give superior results to the regularized least-squares algorithm and with significantly less computer processing time.