Positronium Formation Research Articles

Positron scattering from hydrogen atom embedded in dense quantum plasma

Scattering of positrons from the ground state of hydrogen atoms embedded in dense quantum plasma has been investigated by applying a formulation of the three-body collision problem in the form of coupled multi-channel two-body Lippmann-Schwinger equations. The interactions among the charged particles in dense quantum plasma have been represented by exponential cosine-screened Coulomb potentials. Variationally determined hydrogenic wave function has been employed to calculate the partial-wave scattering amplitude. Plasma screening effects on various possible mode of fragmentation of the system e++H(1s) during the collision, such as 1s→1s and 2s→2s elastic collisions, 1s→2s excitation, positronium formation, elastic proton-positronium collisions, have been reported in the energy range 13.6-350 eV. Furthermore, a comparison has been made on the plasma screening effect of a dense quantum plasma with that of a weakly coupled plasma for which the plasma screening effect has been represented by the Debye model. Our results for the unscreened case are in fair agreement with some of the most accurate results available in the literature.

Positron range estimations with PeneloPET

Technical advances towards high resolution PET imaging try to overcome the inherent physical limitations to spatial resolution. Positrons travel in tissue until they annihilate into the two gamma photons detected. This range is the main detector-independent contribution to PET imaging blurring. To a large extent, it can be remedied during image reconstruction if accurate estimates of positron range are available. However, the existing estimates differ, and the comparison with the scarce experimental data available is not conclusive. In this work we present positron annihilation distributions obtained from Monte Carlo simulations with the PeneloPET simulation toolkit, for several common PET isotopes (18F, 11C, 13N, 15O, 68Ga and 82Rb) in different biological media (cortical bone, soft bone, skin, muscle striated, brain, water, adipose tissue and lung). We compare PeneloPET simulations against experimental data and other simulation results available in the literature. To this end the different positron range representations employed in the literature are related to each other by means of a new parameterization for positron range profiles. Our results are generally consistent with experiments and with most simulations previously reported with differences of less than 20% in the mean and maximum range values. From these results, we conclude that better experimental measurements are needed, especially to disentangle the effect of positronium formation in positron range. Finally, with the aid of PeneloPET, we confirm that scaling approaches can be used to obtain universal, material and isotope independent, positron range profiles, which would considerably simplify range correction.

Study of Positronium formation in nano-channelled silicon as a function of sample temperature

Oxidized nanochannel in silicon have been demonstrated to be suitable for positronium (Ps) formation and cooling also at low sample temperature. To investigate the Ps yield and to clarify the Ps formation mechanism we studied, by Positron Annihilation Spectroscopy (PAS), nanochanneled Si p-type samples in the 150–430 K temperature range. Ps yield was found to be constant in the 150–300 K temperature range, then it increases up to ~50% of its value from 350–400 K. This effect is associated to a decrease of the fraction of positrons annihilating in Si and in the SiO2 layer on the nanochannels surface. This finding is compatible with the thermal decrease of the positive charge distribution at the Si/SiO2 interface limiting e+ reaching the SiO2 layer and to a charge rearrangement at the SiO2 surfaces.

Positronium formation in molecular oxygen

New measurements of the positronium (Ps) formation cross section in molecular oxygen have been made using the 'positron loss' technique used in earlier measurements in the noble gases – the reduction in beam intensity being attributed to Ps formation. The results are generally in agreement, in terms of dependence on incident positron energy, with earlier measurements, but are lower in magnitude than those deduced from total ionization measurements below the first threshold for direct ionization, which employed ion detection. QPs for molecular oxygen exhibits an interesting peak-like dependence on incident positron energy just above threshold, and possible reasons for this structure are discussed. These include the possibility of a dissociative attachment process; a unique peak at similar projectile energies has long been observed in electron-oxygen scattering. The most likely explanation, however, lies in coupling between Ps formation and excitation to the continuum, the latter having previously been shown also to have a peaked energy dependence above threshold.

Positron annihilation studies of zirconia doped with metal cations of different valence

New results obtained by applying positron annihilation spectroscopy to the investigation of zirconia-based nanomaterials doped with metal cations of different valence are reported. The slow-positron implantation spectroscopy combined with Doppler broadening measurements was employed to study the sintering of pressure-compacted nanopowders of tetragonal yttria-stabilised zirconia (t-YSZ) and t-YSZ with chromia additive. Positronium (Ps) formation in t-YSZ was proven by detecting 3γ-annihilations of ortho-Ps and was found to gradually decrease with increasing sintering temperature. A subsurface layer with enhanced 3γ-annihilations, compared to the deeper regions, could be identified. Addition of chromia was found to inhibit Ps formation. In addition, first results of positron lifetime measurements on nanopowders of zirconia phase-stabilised with MgO and CeO2 are presented.

Influence of polar groups in binary polymer blends on positronium formation

The present work studied the role of the polar group unconjugated oxygen on the inhibition of positronium (Ps) formation in two binary blends made from a set of chosen constituent polymers with polar and weakly polar groups (nonpolar). The polymer blend samples of PVC-EVA and PVC-SAN were investigated by coincidence Doppler broadening and positron lifetime techniques. The strong polar acetate group in the EVA contributed to positron annihilation with electrons of unconjugated oxygen (-C(+)=O(-)) as revealed by the momentum distribution curves peaking around 17 P(L) (10(-3) m(0)c). The ortho-Ps intensity indicated the unconjugated oxygen shows about a 28% Ps reduction even in the presence of a strong Ps inhibiting halogen (Cl(-)). In contrast, this effect was not seen in the PVC-SAN blends since SAN contains a weakly polar (nonpolar) acrylonitrile group (C≡N). Our results indicate the chlorine of PVC in the blends is a major contributor to Ps inhibition through the formation of a (Cl(-)-e(+)) bound state but the unconjugated oxygen in EVA of the PVC-EVA blend also plays a similar, but lesser, role.

Model-potential approach to positron elastic scattering from noble gases

In a recent publication [D. Assafr\~ao et al., Phys. Rev. A 84, 022713 (2011)] a model-potential approach was proposed to positron-atom scattering based on an adiabatic method that treats the positron as a light nucleus. In this paper the formalism is further improved by the introduction of semiempirical energy terms which appropriately account for the positron-target interaction at long and short ranges. Our method allows us to describe the main observed features of the elastic cross sections for positron scattering from He, Ne, Ar, Kr, and Xe. Valuable insights into the cross-section behavior below the positronium formation threshold, the scattering lengths, and other aspects of the low-energy positron-noble gases scattering are presented.

Depth-dependent positron annihilation in different polymers

Depth-dependent positron annihilation Doppler broadening measurements were conducted for polymers with different chemical compositions. Variations of the S parameter with respect to incident positron energy were observed. For pure hydrocarbons PP, HDPE and oxygen-containing polymer PC, S parameter rises with increasing positron implantation depth. While for PI and fluoropolymers like PTFE, ETFE and PVF, S parameter decreases with higher positron energy. For chlorine-containing polymer PVDC, S parameter remains nearly constant at all incident positron energies. It is suggested that these three variation trends are resulted from a competitive effect between the depth-dependent positronium formation and the influence of highly electronegative atoms on positron annihilation characteristics.

Threshold behavior of positronium formation in positron–alkali-metal scattering

We consider positron scattering on the alkali-metal atoms of Li, Na, and K at very low energies, where only the elastic scattering and positronium formation in the ground state are the two open channels. Utilizing the recently developed two-center convergent close-coupling method [Lugovskoy, Kadyrov, Bray, and Stelbovics, Phys. Rev. A 82, 062708 (2010)] we investigate the behavior of the cross sections as the impact energy goes to zero and demonstrate their convergence. The study sets quantitative benchmarks for any rigorous theoretical treatment of the collision problems.

The influence of crystal morphology on the process of positronium formation in docosane. The accessibility of electrons for trapping

Various morphologies of docosane crystals have been obtained by crystallization of samples of the same origin with different methods. The positron annihilation lifetime spectra indicate some differences between them, which are related to the differences in the electron trapping capability. To find out the origin of these differences, some crystallographic and optical spectroscopic studies were performed. We argue that the differences in each structure are related to the ease of electron release from traps during excitation and not only to the density of the traps, as has been assumed so far.

Positronium formation in the noble gases

We present results based upon our ab initio relativistic optical potential method for the simulation of positronium formation in the noble gases neon, argon, krypton and xenon. The method used in this simulation is based upon an approach, first suggested by Reid and Wadehra, whereby the ionization threshold of the atom is effectively reduced by 6.8 eV, the binding energy of positronium in its ground state. This procedure is then modified in order to account for the fact that positronium formation cross sections tend to zero much more rapidly than the corresponding direct ionization cross sections as the energy of the incident positron increases. The agreement of our positronium formation cross sections with experiment is quite good for argon, krypton and xenon over most of the energy region while for neon our cross sections are generally too small at all energies. Nonetheless, our method predicts positronium formation cross sections in better agreement with experiment than the much more sophisticated theoretical approaches used in the past. We also show that adding positronium formation in this way improves agreement with the measured elastic differential cross section at small scattering angles.

Cross sections for positron scattering from ethane

We report experimental and theoretical cross sections for positron scattering from the fundamental organic-chemistry molecule ethane (C${}_{2}$H${}_{6}$). The experimental total cross sections (TCSs) were obtained using a linear transmission technique, for energies in the range 0.1--70 eV and with an energy resolution of $\ensuremath{\sim}$0.25 eV (full width at half maximum). Agreement, over the common energy range, with the earlier TCS measurements of Floeder et al. [J. Phys. B 18, 3347 (1985)] is excellent, while both the present results and those of Floeder et al. are consistently higher in magnitude than the data of Sueoka and Mori [J. Phys. B 19, 4035 (1986)]. The present calculations employed the Schwinger multichannel method and were performed in the static plus polarization approximation for energies up to 10 eV. Our calculated elastic integral cross sections (ICSs) indicate a Ramsauer-Townsend minimum at around 1.4 eV in the ${A}_{g}$ scattering symmetry, and a virtual state. In addition we calculated from our scattering cross section a scattering length of $\ensuremath{-}$13.83${a}_{0}$. Agreement between our measured TCS and calculated elastic ICS is found to be only qualitative, although this is perhaps not so surprising given the TCS below 10 eV in principle includes contributions from rotational, vibrational, and electronic-state excitation and positronium formation whereas the calculation does not.

Comparative study of the three- and four-body boundary-corrected Born approximations for positronium formation

The three- and four-body boundary-corrected first Born approximations are formulated for the single-electron capture from the K shell of helium, carbon and neon atoms by impact of positrons. These approximations are applied to calculate the post and prior forms of the T-matrix elements. The differential and total cross sections obtained from these transition amplitudes are compared. In order to avoid the discrepancy between the post and prior formalisms, their average amplitude is calculated to obtain the total cross sections. For helium atoms, the total cross sections obtained from the post-, prior- and averaged-transition amplitudes calculated by the three-body approximation are compared with available experimental data as well as other theories, and also with the corresponding values obtained from a four-body formalism. In comparison with the three-body version of the specified method, the results obtained from the averaged four-body transition amplitude are in better agreement with the experiment, while the situation is the reverse for the prior calculations.

Experimental and theoretical cross sections for positron collisions with 3-hydroxy-tetrahydrofuran

Cross section results from a joint experimental and theoretical investigation into positron scattering from 3-hydroxy-tetrahydrofuran (3H-THF) are presented. Total and positronium (Ps) formation cross sections have been measured from 1 to 190 eV using the positron beamline at the Australian National University, which has an energy resolution between 60 and 100 meV. The total cross section (TCS) and the elastic and total inelastic integral cross sections in the energy range between 1 and 1000 eV have been computed within the Independent Atom Model using the Screening Corrected Additivity Rule approach. In addition, we have calculated elastic differential cross sections at selected incident energies. Our computations represent the first theoretical results reported for this target species, while our measured Ps formation cross sections are also novel. Comparison of the present TCS with the previous results from the University of Trento shows a good level of agreement at the lowest energies. We also provide a comparison between the present cross sections for 3H-THF and those from our earlier study on the parent molecule tetrahydrofuran.

Total, elastic, and inelastic cross sections for positron and electron collisions with tetrahydrofuran

We present total, elastic, and inelastic cross sections for positron and electron scattering from tetrahydrofuran (THF) in the energy range between 1 and 5000 eV. Total cross sections (TCS), positronium formation cross sections, the summed inelastic integral cross sections (ICS) for electronic excitations and direct ionization, as well as elastic differential cross sections (DCS) at selected incident energies, have been measured for positron collisions with THF. The positron beam used to carry out these experiments had an energy resolution in the range 40-100 meV (full-width at half-maximum). We also present TCS results for positron and electron scattering from THF computed within the independent atom model using the screening corrected additivity rule approach. In addition, we calculated positron-impact elastic DCS and the sum over all inelastic ICS (except rotations and vibrations). While our integral and differential positron cross sections are the first of their kind, we compare our TCS with previous literature values for this species. We also provide a comparison between positron and electron-impact cross sections, in order to uncover any differences or similarities in the scattering dynamics with these two different projectiles.

Convergent-close-coupling formalism for positron scattering from molecules

The $ab$ $initio$ convergent-close-coupling method has been extended to positron-molecule collisions within the adiabatic (fixed-nuclei) approximation. Application to molecular hydrogen at energies from 0.1 to 1000 eV has yielded convergent total ionization and grand total cross sections over most of the energy range. We find that very large calculations are required for convergence, even in the case of low-energy elastic scattering, due to the effects of positronium formation. In general, the comparison with experiment is good.

Positronium formation in methanol and ethanol

Cross sections for the formation of positronium (Ps) in collisions of positrons with methanol and ethanol molecules, QPs, have been measured for positrons of incident energies from 3 to 40 eV. The threshold energies for Ps formation were found to be 4.0 ± 0.2 eV and 3.5 ± 0.5 eV, respectively. QPs for the two molecules are very similar in size and energy dependence, suggesting that Ps formation is associated preferentially with the OH group, whereas (Qtot–QPs), where Qtot is the total scattering cross section, is related more to the physical size of the molecules.

Positron-Excited Lithium Atom Collisions

The inelastic scattering of positrons by excited lithium alkali atoms Li*(2p) have been investigated within the frame work of the coupled-static and frozen-core approximations with the assumption that the elastic and rearrangement channels are open. In the present work, a rather complicated computer code is developed based on the coupled-static, frozen-core and Green’s function partial wave expansion technique. The partial and total elastic and positronium (Ps) formation cross sections of e+-Li*(2p) are calculated through a wide range of incident energy of positrons ranging from 0.3 eV to 1000 eV. Also, we have calculated the partial and total elastic and rearrangement (reversal of the Ps formation) cross sections of Ps-Li+ collisions through the low, intermediate and high energy regions. The effect of polarization potential of the Ps atom is taken into our consideration. The total cross sections which corresponding to twelve partial cross sections (calculated at twelve values of the total angular momentum l = 0 to l = 11) are calculated for each channel. Our calculated total positronium formation cross sections are compared with experimental results and those calculated by other authors. The present calculations encourage the experimental physicists to carry out positron-lithium experiments by taking the excited lithium target into accounts in order to obtain more positronium especially in the low and intermediate energy regions.

Positron scattering from hydrogen atom embedded in weakly coupled plasma

The positron-hydrogen collision problem in weakly coupled plasma environment has been investigated by applying a formulation of the three-body collision problem in the form of coupled multi-channel two-body Lippmann-Schwinger equations. The interactions among the charged particles in the plasma have been represented by Debye-Huckel potentials. A simple variational hydrogenic wave function has been employed to calculate the partial-wave scattering amplitude. Plasma screening effects on various possible mode of fragmentation of the system e++H(1s) during the collision, such as 1s→1s and 2s→2s elastic collisions, 1s→2s excitation, positronium formation, elastic proton-positronium collisions, have been reported. Furthermore, a detailed study has been made on differential and total cross sections of the above processes in the energy range 13.6-350 eV of the incident positron.

Positron Annihilation Study of ZnO Nanoparticles Grown Under Folic Acid Template

Positron lifetime spectroscopy (PAL), Doppler broadening (DB) as well as coincidence Doppler broadening (CDB) spectroscopy of a new variety of Folic acid (FA) capped zinc oxide nano-particle samples has been performed at room temperature. The results show interesting patterns of observation, hither-to unobserved in ZnO wurtzite crystalline samples, such as predominance of positronium formation, as reflected in the (PAL) analysis, phase transition in the nano crystalline samples at ~0.8-1.0 % FA concentration as depicted from DB results. Also, the chemical environment of the samples has been analysed from the ratio curves of CDB studies. Beside these, other independent results from X-ray diffraction (XRD) data and Debye- Scherrer method, Transmission electron microscopic (TEM) observations and as well as Fourier transformed infrared spectroscopic (FT-IR) analysis are reported for comparison.