Incident Positron Energy Research Articles

Positron re-emission, reflection, and diffraction from W(100) surface at very low energies

The energy distributions of scattered and re-emitted low-energy positrons from a W(100) surface were measured as a function of incident positron energy from 0 to 25 eV. Given that tungsten has a negative work function of about −3 eV for positrons, one can envisage three scenarios of very low-energy positron scattering from such a surface. First, a positron approaching the sample surface with energy say 1 eV above the vacuum level will see a potential barrier of about 2 eV height and will be reflected back to the vacuum. Second, when the energy of incident positrons increases up to the top of the surface potential barrier (positron work function), they start entering the solid and, therefore, the reflectivity of positrons from the surface reduces. Positrons entering the solid are thermalised within few picoseconds and have a chance to escape back to the vacuum with kinetic energy about 3 eV above the vacuum level undergoing so-called re-emission. Third, coherent scattering of low-energy positrons may occur on the crystal surface, i.e. positron diffraction. All the three scenarios of low-energy positrons scattering are studied here experimentally. Measured spectra are very sensitive to the surface conditions of the sample: they change dramatically after surface oxidation or thin film deposition.

Positron Scattering from the Group IIB Metals Zinc and Cadmium: Recommended Cross Sections and Transport Simulations

Results from the application of our optical potential and relativistic optical potential models to positron scattering from gas-phase zinc (Zn) and cadmium (Cd) are presented. In particular, integral cross sections (ICSs) for elastic scattering, positronium formation, summed discrete electronic-state excitation, and ionization scattering processes are reported for both species and over an extended incident positron energy range. From those ICSs, the total cross section is subsequently constructed by taking their sum. We note that there are currently no experimental data available for any of these scattering processes for either species, with earlier computational results being limited to the elastic channel and restricted to relatively narrow incident positron energy regimes. Nonetheless, we construct recommended positron cross section datasets for both zinc and cadmium over the incident positron energy range of 0–10 000 eV. The recommended positron cross section data are subsequently employed in a multi-term Boltzmann equation analysis to simulate the transport of positrons, under the influence of an applied (external) electric field, through the background Zn and Cd gases. Qualitatively similar behavior in the calculated transport coefficients was observed between both species. Finally, for the case of zinc, the present positron transport coefficients are compared against corresponding results from electron transport with some significant differences now being observed.

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.

Dynamics of positron scattering from lithium, sodium and potassium atoms in hot and dense plasmas

The dynamics of positron scattering from the ground state of lithium, sodium and potassium atoms in hot and dense plasmas has been investigated by applying a two-state approximation that includes elastic scattering and rearrangement scattering. The wave functions and energies of the target alkali atoms have been determined quite accurately within the framework of the ’method of model potential’. An inclusive study was made on the effects of plasma screening on the differential and total cross sections for elastic scattering and positronium formation in the ground state for the incident positron energy lying within 10 eV to 500 eV. For the unscreened case, our present results agree reasonably with the results of other calculations. It was found that the cross sections suffer considerable change due to the effects of the background plasma.

An approach to study electron and positron scattering from NH3 and PH3 using the analytic static potential

A detailed study of the elastic scattering of electrons and positrons from two C3v symmetrical molecules viz ammonia and phosphine molecules is presented. The partial wave phase shift analysis method with complex optical potential consisting of static, exchange, polarization and an imaginary absorption potentials has been used. In contrast to the previously used similar approaches, the present method is quite different in the sense that we have obtained the required static potential in an analytical form using correctly represented Gaussian molecular wave functions for NH3 and PH3 in our calculations. The differential, integrated, momentum transfer, absorption and total cross sections are calculated and reported in the incident electron and positron energy range of 10–500 eV. In order to test the applicability of our approach, the present results are compared with the available previous measurements and theoretical calculations and a good agreement is found.

Positron Impact Excitation of the nS States of Atomic Hydrogen

The excitation cross sections of the nS states, n = 2 to 6, of atomic hydrogen at various incident positron energies (10.23 to 300 eV) were calculated using the variational polarized-orbital method. Nine partial waves were used to obtain converged cross sections. The present results should be useful for comparison with results obtained from other theories and approximations. The positron-impact cross section was found to be higher than the electron-impact cross sections. Experimental and other theoretical results are discussed. The threshold law of excitation is discussed and the cross sections in this region were seen to obey the threshold law proportional to ( ln k f ) − 2 . Cross sections were calculated in the Born approximation also and compared to those obtained using the variational polarized orbital method.

Positron-impact single ionization of helium using the Faddeev–Merkuriev equations

We present the study of the positron-impact single ionization process of helium in the case where the residual ion remains in its ground state. We calculated the total cross-section for incident-positron energies ranging from the ionization threshold to 150 eV, and the triple differential cross-sections (TDCSs), using a single continuum wave function built through the Faddeev–Merkuriev differential equations. A comparison of the results of the total cross-section obtained with our approach and other theoretical models as well as with existing experimental data, shows a good agreement. Due to the lack of experimental and theoretical results for comparison, and to be more general and predictable in our investigation, the two- and three-dimensional representation of the TDCSs covering all possible values of the angle of the ejected electron for different values of its energy are presented and discussed.

Calculations of positron scattering on the hydrogen molecular ion

Positron scattering from the vibrational ground state of the hydrogen molecular ion has been investigated using the single-centre convergent close-coupling (CCC) method. Cross sections for dissociative excitation, dissociative ionisation, total inelastic scattering, and proton production have been calculated at incident positron energies from 10 to 500 eV. For this energy range the mass stopping power and mean excitation energy have also been calculated for both the positron and electron cases. Since there are no experiments or other calculations for positron scattering on the hydrogen molecular ion, the obtained results have been compared with corresponding CCC results for positron collisions with He+ and electron collisions with . Comparative analysis allows to estimate the quality of the approximations used in the present calculations.

Positron Scattering from Gas-Phase Beryllium and Magnesium: Theory, Recommended Cross Sections, and Transport Simulations

Results from the application of our optical potential and relativistic optical potential models to positron scattering from gas-phase beryllium (Be) and magnesium (Mg) are presented. Specifically, total cross sections and integral cross sections for the elastic, positronium formation, summed discrete electronic-state excitation, and ionization scattering processes are reported for both species and over an extended incident positron energy range. Where possible, these results are compared against the existing theoretical and experimental data, although it must be noted here that no current measurements are yet available for Be and those that are available for Mg are largely restricted to the total cross section. Nonetheless, on the basis of that comparison, recommended cross section datasets, for all the aforementioned cross sections, are formed. Those recommended cross section data are subsequently employed in a Boltzmann equation analysis to simulate the transport of positrons, under the influence of an applied (external) electric field, through the background Be and Mg gases. Note that relativistic optical potential results for the elastic momentum transfer cross section are also reported, to allow us to account for anisotropy effects in our transport simulations. Finally, our positron simulation results for quantities such as the ionization rate coefficients and flux and bulk drift velocities are compared with the corresponding electron transport results with significant differences being observed.

Positron Impact Excitation of the 2S State of Atomic Hydrogen

The excitation cross-sections of the 2S state of atomic hydrogen at low (near threshold energy) to high incident positron energies (10.30 to 300 eV) have been calculated using the variational polarized-orbital method. Nine partial waves have been used to obtain converged cross-sections in the above energy range. The cross sections compared to the electron-impact excitation of the S state of atomic hydrogen are larger in the present case. The maximum cross section is 3.63(−1) π a 0 2 at 16.5 eV compared to 1.37(−1) π a 0 2 at 11.14 eV for the electron-impact excitation. The present results are compared with other calculations. Cross-sections have also been calculated in the Born approximation in which the polarization of the target has been included. Differential cross sections were calculated at k = 1.0 (13.6eV), 2.5 (85 eV), 3.483 (200 3V), and 4.696 (300 eV).

Efficient and surface site-selective ion desorption by positron annihilation

We compared positron- and electron-stimulated desorption (e+SD and ESD) of positive ions from a TiO2(110) surface. Although desorption of O+ ions was observed in both experiments, the desorption efficiency caused by positron bombardment was larger by one order of magnitude than that caused by electron bombardment at an incident energy of 500 eV. e+SD of O+ ions remained highly efficient with incident positron energies between 10 eV and 600 eV. The results indicate that e+SD of O+ ions is predominantly caused by pair annihilation of surface-trapped positrons with inner-shell electrons. We also tested e+SD from water chemisorbed on the TiO2 surface and found that the desorption of specific ions was enhanced by positron annihilation, above the ion yield with electron bombardment. This finding corroborates our conclusion that annihilation-site selectivity of positrons results in site-selective ion desorption from a bombarded surface.

Positron scattering from pyridine.

We present a range of cross section measurements for the low-energy scattering of positrons from pyridine, for incident positron energies of less than 20 eV, as well as the independent atom model with the screening corrected additivity rule including interference effects calculation, of positron scattering from pyridine, with dipole rotational excitations accounted for using the Born approximation. Comparisons are made between the experimental measurements and theoretical calculations. For the positronium formation cross section, we also compare with results from a recent empirical model. In general, quite good agreement is seen between the calculations and measurements although some discrepancies remain which may require further investigation. It is hoped that the present study will stimulate development of ab initio level theoretical methods to be applied to this important scattering system.

Mode coupling and multiquantum vibrational excitations in Feshbach-resonant positron annihilation in molecules

The dominant mechanism of low-energy positron annihilation in polyatomic molecules is through positron capture in vibrational Feshbach resonances (VFR). In this paper we investigate theoretically the effect of anharmonic terms in the vibrational Hamiltonian on the positron annihilation rates. Such interactions enable positron capture in VFRs associated with multiquantum vibrational excitations, leading to enhanced annihilation. Mode coupling can also lead to faster depopulation of VFRs, thereby reducing their contribution to the annihlation rates. To analyze this complex picture, we use coupled-cluster methods to calculate the anharmonic vibrational spectra and dipole transition amplitudes for chloroform, chloroform-$d_1$, 1,1-dichloroethylene, and methanol, and use these data to compute positron resonant annihilation rates for these molecules. Theoretical predictions are compared with the annihilation rates measured as a function of incident positron energy. The results demonstrate the importance of mode coupling in both enhancement and suppression of the VFR. There is also experimental evidence for the direct excitation of multimode VFR. Their contribution is analyzed using a statistical approach, with an outlook towards more accurate treatment of this phenomenon.

Experimental and computational studies of positron-stimulated ion desorption from TiO2(1 1 0) surface

Experimental and computational studies of the positron-stimulated O ion desorption process from a TiO2(1 1 0) surface are reported. The measured data indicate that the O ion yields depend on the positron incident energy in the energy range between 0.5 keV and 15 keV. This dependence is closely related to the fraction of positrons which diffuse back to the surface after thermalization in the bulk. Based on the experimental and computational results, we conclude that the ion desorption via positron-stimulation occurs dominantly by the annihilation of surface-trapped positrons with core electrons of the topmost surface atoms.

Nuclear dynamics in a positron-CO collision using close-coupling methods

Apart from the electronic motion of the target in the molecular collision process, collision probabilities strongly depend on the dynamics of nuclear motions (rotation and vibration). Here we have studied this nuclear dynamical dependency of collision cross sections for a positron--carbon monoxide collision using rovibrational close-coupling, rotational laboratory frame close-coupling (rotational LFCC), and LFCC adiabatic nuclear vibration (LFCC-ANV) methods. Here we have computed the angle-integrated elastic and state-to-state rotational excitation, the elastic and state-to-state vibrational (summed over rotational) excitation, and the total (summed over rotational and vibrational) cross sections for the incident positron energy between 0.0 and 7.0 eV using the rovibrational close-coupling method. The rotational LFCC method is also employed to calculate elastic, (state-to-state) rotational excitation, and total cross sections. To estimate the effects of the nuclear dynamics we have calculated vibrational elastic and state-to-state vibrational excitation cross sections using the LFCC-ANV method. In these calculations the model correlation-polarization potential is used to include the correlation-polarization potential. The effect of nuclear dynamics on the collision probabilities is discussed comparing calculated total cross sections using the rovibrational close-coupling and the rotational LFCC method with other theoretical calculations and experimental results. The discussion includes comparison of the present vibrational $0\ensuremath{\rightarrow}1$ excitation process with the theoretical and measured values. The other vibrational and rotational elastic and excitation cross sections are also compared with the existing theoretical results and we have discussed the relevance of the effect of nuclear rotational and vibrational dynamics. We have also presented a comparison among the energy transfer parameters using different coupling schemes and have discussed the implications of the results.

Positron scattering from hydrogen atom in dense quantum plasmas: Positronium formation in Rydberg states

Effects of dense quantum plasmas on positronium (Ps) formation in an arbitrary nlm–state in the scattering of positrons from the ground state of hydrogen atoms have been investigated within the framework of a distorted wave theory that incorporates the effect of screened dipole polarization potential. The interaction of charged particles in plasmas has been modeled by a modified Debye-Huckel potential. Effects of plasma screening on the structures of differential and total cross sections have been explored for various incident positron energies in the range 20–300 eV. For the free atomic case, our results are in conformity with the existing results available in the literature. It has been found that for small screening effects, the cross section presents the oscillatory behaviour. To the best of our knowledge, this is the first attempt to estimate the screening effects on the differential and total cross sections for Ps formation in Rydberg states in dense quantum plasmas.

Antifouling enhancement of polysulfone/TiO2 nanocomposite separation membrane by plasma etching

A polysulfone/TiO2 nanocomposite membrane was prepared via casting method, followed by the plasma etching of the membrane surface. Doppler broadened energy spectra vs. positron incident energy were employed to elucidate depth profiles of the nanostructure for the as-prepared and treated membranes. The results confirmed that the near-surface of the membrane was modified by the plasma treatment. The antifouling characteristics for the membranes, evaluated using the degradation of Rhodamin B, indicated that the plasma treatment enhances the photo catalytic ability of the membrane, suggesting that more TiO2 nanoparticles are exposed at the membrane surface after the plasma treatment as supported by the positron result.

Helium-Implanted ODS-Fe Alloy Investigated by Positron-Annihilation Spectroscopy

At room temperature, oxide dispersion strengthened (ODS)-Fe (Fe-2%Al2O3) and pure Fe were implanted with 140-keV helium ions by doses up to 1e+15, 5e+15, and 1e+16/cm2. Vacancy-type defects induced by the implantation were investigated with positron-annihilation spectroscopy (PAS). Defect profiles of the S-parameter derived from PAS as a function of positron incident energy up to 20 KeV were analyzed. The S-parameter of the damaged layers in the ODS-Fe tend to be smaller than those in the pure Fe when the irradiation doses reach 1e+16/cm2. This finding indicates that the ODS-Fe sample exhibits a higher irradiation resistance than the pure Fe. The reason might be that helium irradiation-induced point defects in ODS-Fe are trapped and annihilated at interfaces between the Fe matrix and the Al2O3 nanoparticles. Vacancy clusters and helium-vacancy complexes were identified as the main defects. Through the $S$ – $W$ curves, however, we observed only one type of defect in the irradiated ODS-Fe. While for the pure Fe case, things seem to be more complicated, where the surface and the bulk have different types of defects.

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.

Angular distribution of reemitted positrons from W(100) with over layers of oxygen and LiF

The angular distribution of reemitted positrons has been measured from clean and oxygen exposed W(100), as well as from W(100) with LiF deposited on the surface, at an incident positron energy of 600 eV. Both the deposition of LiF and oxygen adsorption on W(100) caused a drastic reduction in the reemitted positron intensity and broadening of the angular distribution of the remitted positrons from W(100). The angular distribution of reemitted positrons in each case could be best fitted to the sum of two Gaussians, a narrow Gaussian whose width is in the range expected from a one-dimensional step model of positron reemission and an additional broad Gaussian. The area under the narrow Gaussian is more sensitive to surface over layers than the broad Gaussian.