Cross Sections For Positron Collisions Research Articles

Higher order polarizabilities and the positron forward scattering problem: Convergence between calculated and measured cross sections in the very low energy regime

We have employed a well established correlation-polarization model potential to calculate the elastic cross sections for positron collisions with O2 , N2 and CO2 from 0.01 eV to Eps. The description of the outer component of the positron-molecule potential was improved introducing higher polarizability and the hyperpolarizabilities terms. The obtained cross sections were applied in the forward correction of the measured total cross sections and much improved convergence between the calculated elastic cross sections and the corrected measurements was found.

Elastic positron-uracil scattering cross sections

We report elastic integral and differential cross sections for positron collisions with uracil (${\mathrm{C}}_{4}{\mathrm{H}}_{4}{\mathrm{N}}_{2}{\mathrm{O}}_{2}$) for impact energies up to 10 eV. The cross sections were calculated employing the Schwinger multichannel (SMC) method in the static plus polarization approximation. The Born-closure procedure was applied to account for the long-range potential due to the permanent dipole moment of uracil. The present results are compared with experimental and theoretical data available in the literature, showing an overall good agreement with previously reported results. We also applied our differential cross sections to provide a correction to the total cross-section measurements by Anderson et al. [J. Chem. Phys. 141, 034306 (2014)] in order to account for the lack of angular resolution in the apparatus used by these authors. Besides, we applied a simple model employed by Franz and Gianturco [Phys. Rev. A 88, 042711 (2013)] to estimate the temperature effects on the uracil cross section. In particular, the level of agreement of the SMC differential cross sections as compared with the experimental findings is encouraging.

Elastic Scattering of Slow Positrons by Pyrazine.

We report elastic integral and differential cross sections for positron collisions with pyrazine (C4H4N2), for energies up to 10 eV. The cross sections were calculated with the Schwinger multichannel method in the static plus polarization approximation. Our computed elastic integral cross section indicates the presence of a Ramsauer-Townsend minimum and a bound state in the Ag scattering symmetry. In the absence of results in the literature on collisions of positrons with pyrazine, we compare the present results with results for pyrimidine and benzene molecules available in the literature.

Stopping Power, CSDA Range, Absorbed Dose and Cross Sections Calculations of F18 Simulated in Water using Geant4 Code

Objective: This study aims to calculate and estimate the stopping power, RCSDA, absorbed dose and cross sections for F18 simulated and propagated in water. Methods: In order to access the objective of this work, Monte Carlo Simulation – Geant4 code and Bethe-Bloch, Rudd, Seltzer, Berger, Wang and Rutherford equations are used. Finding: The simulation results are in a good agreement with many studies that deal with the same parameters and the same conditions. The change in the main dosimetric parameters (mass collision stopping power and absorbed dose) is clearly indicated at the energy range below 50 keV. The collisional stopping power holds a special importance in the radiation dosimetry field of this energy range. It can represent the total F18 stopping power and it is showing a close relation with the absorbed dose. Application: This study is devoted to the development of numerical analysis of the stopping power, absorbed dose and cross sections for positron collisions with water. Such quantitative studies are very relevant for medical, in particular, PET applications to predict the patient’s health status under exposure and for scientific knowledge on propagation of annihilation photons in media. This study analyzes a fundamental parameter of dosimetry in patients, the results will certainly be utilized to optimize patient exposure and to reduce effectively the radiation dose in the whole body without significantly sacrificing image quality. Keywords: Bethe-Bloch Equation, Image Quality, Monte Carlo Simulation, Numerical Analysis, Patient’s Dose

Scattering data for modelling positron tracks in gaseous and liquid water

We present in this study a self-consistent set of scattering cross sections for positron collisions with water molecules, in the energy range 0.1–10 000 eV, with the prime motivation being to provide data for modelling purposes. The structure of the database is based on a new model potential calculation, including interference terms, which provides differential and integral elastic as well as integral inelastic positron scattering cross sections for water molecules over the whole energy range considered here. Experimental and theoretical data available in the literature have been integrated into the database after a careful analysis of their uncertainties and their self-consistency. These data have been used as input parameters for a step-by-step Monte Carlo simulation procedure, providing valuable information on energy deposition, positron range, and the relative percentages of specific interactions (e.g. positronium formation, direct ionisation, electronic, vibrational and rotational excitations) in gaseous and liquid water.

Low-energy positron scattering by pyrimidine.

This work reports elastic integral and differential cross sections for positron collisions with pyrimidine, for energies up to 20 eV. The cross sections were computed with the Schwinger multichannel method in the static plus polarization approximation. We also employed the Born closure procedure to account for the long range potential due to the permanent dipole moment of the molecule. Our results are compared with the experimental total cross section of Zecca et al. [J. Phys. B 43, 215204 (2010)], the experimental grand-total, quasi-elastic integral and differential cross section of Palihawadana et al. [Phys. Rev. A 88, 12717 (2013)]. We also compare our results with theoretical integral and differential cross sections obtained by Sanz et al. [Phys. Rev. A 88, 62704 (2013)] with the R-matrix and the independent atom model with screening-corrected additivity rule methods, and with the results computed by Franz and Gianturco [Phys. Rev. A 88, 042711 (2013)] using model correlation-polarization potentials. The agreement between the theory and the experiment is encouraging.

Positron collisions with C3H6 isomers

We report calculated elastic integral (ICS) and differential (DCS) cross sections for positron collisions with C3H6 isomers, propene and cyclopropane. The ICSs computations were performed with the Schwinger multichannel method, at the static plus polarization level of approximation from 0.001 up to 8 eV. These ICSs are compared with the only experimental total cross sections (TCSs) available in literature for low energy positron scattering by these isomers [Makochekanwa C et al 2008 Phys. Rev. A 77 042717]. Even though there are significant differences between the two molecules (for instance propene has a dipole moment while cyclopropane has not), the ICSs for both systems are remarkably similar. This is mainly due to the fact that the dipole moment of propene is very small (0.39 D), and the increase of cyclopropane’s ICS at low impact energy is possibly due to the presence of a virtual state. This same resemblance occurs for the DCSs at lower energies, with the differences between the isomers becoming clearer as the energy increases.

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.

Positron collisions with ethene

We present experimental and theoretical cross sections for positron collisions with ethene molecules. The experimental total cross sections (TCSs) were obtained with a linear transmission technique, for energies from 0.1 eV up to 70 eV. The calculations employed the Schwinger multichannel method and were performed in the static plus polarization approximation for energies up to 10 eV. Our calculated elastic cross sections indicate a Ramsauer-Townsend minimum around 2.8 eV and a virtual state, in agreement with previous calculations by da Silva et al. [Phys. Rev. Lett. 77, 1028 (1996)]. We found reasonable agreement between the calculated elastic integral cross section and the measured total cross section below the positronium formation threshold. The present results are also in quite good agreement with available theoretical and experimental data, although for the experiments this is only true for TCSs above about 7 eV.

Positron and electron collisions with anti-protons in strong magnetic fields

Recent simulations of anti-hydrogen production rely on the rate that anti-protons are slowed in positron plasmas. These plasmas are typically cold and in strong magnetic fields. This paper describes Classical Trajectory Monte Carlo calculations of the momentum transfer cross sections for positron collisions with anti-protons. We also give results for electron collisions with anti-protons. For the typical temperatures and magnetic fields in the anti-hydrogen experiments, the dominant slowing mechanism is the close, non-perturbative collisions; all previous calculations of the slowing rate are for situations where the perturbative collisions dominate. These cross sections are converted into a slowing rate as a function of magnetic field and temperature. For the parameter range of current interest, we find this to be an extraordinarily simple function.

The effect of target representation in positron-impact ionization of molecular hydrogen

Ionization cross sections for positron collisions with hydrogen molecules have been calculated using a two-center molecular representation. The results are compared with our previous work which used sphericaly averaged H 2 wavefunctions as well as with existing experimental results. The use of a two-centre wave function for the molecular target, either of the Heitler–London type or a Gaussian representation, produces results which lie between the two most recent sets of experimental data. We find that our CPE4 model produces results in better agreement with experiment over the whole energy range than our CPE model.

Application of least-squares methods to atomic rearrangement collisions

The least-squares method used by Merts and Collins to solve the differential equations describing electron-atom collisions in the absence of exchange has been extended to treat the integro-differential equations representing atomic rearrangement collisions. To illustrate the method we consider positronium formation cross sections for positron collisions with hydrogen atoms and with helium ions. Variational corrections to the method are discussed.

Hyperspherical close-coupling calculation of D-wave positronium formation and excitation cross sections in positron–hydrogen scattering

The hyperspherical close-coupling method is used to calculate the elastic, positronium-formation, and excitation cross sections for positron collisions with atomic hydrogen at energies below the H (n = 4) threshold for the J = 2 partial wave. The resonances below each inelastic threshold were also analyzed. The adiabatic hyperspherical potential curves are used to identify the nature of these resonances.

Hyperspherical close-coupling calculation of positronium formation and excitation cross sections in positron-hydrogen scattering at energies below the H(n=4) threshold

A hyperspherical close-coupling method is used to calculate the elastic, positronium formation and excitation cross sections for positron collisions with atomic hydrogen at energies below the H(n=4) threshold for L=0 and 1 partial waves. A new two-dimensional coordinate transformation is used such that the Schrodinger equation at large hyper-radii can be solved accurately. The coupled hyper-radial equations are integrated to a large hyper-radius at which the solution is matched to the dipole states in the outer region. From the extracted K-matrix, the elastic, positronium formation and atomic hydrogen excitation cross sections are computed. Resonance positions, total widths, and partial cross sections are also examined and compared with those from other calculations.

Coupled-channel calculations of positron-atom scattering at intermediate energies

We present the cross sections for positron collisions with the alkali atoms Li, Na and K in the close-coupling approximation within the framework of a single-active-electron model. Our target basis sets are represented by Li(2s, 3s, 2p, 3p), Na(3s, 4s, 3p, 4p), and K(4s, 5s, 4p, 5p) respectively. The effects of coupling to the lowest lying Ps channels have been investigated by augmenting the target basis sets with the Ps(1s, 2s, 2p) eigenstates. The calculations have been made for impact energies below 50 eV. The interaction between the positron and the alkali target atom is represented by a model potential obtained using a single-active-electron approximation. The effect on elastic scattering and excitation due to the coupling between the direct and the Ps formation channels is found to be of great significance at low impact energies. The case of K is of particular interest. The inclusion of the Ps channels quantitatively reproduces the maximum in the total cross section at about 6 eV as reported in a recent experiment by Parikh et al. We find that the calculated total cross sections for Na and K are in good agreement with the experimental data of Parikh et al. and Kwan et al. respectively in the energy range of 2–50 eV provided allowance is made for the incomplete elastic discrimination in the experiment. The Ps formation reduces the K(4s→4p) cross section at all energies below 30 eV and a reduction is also seen in the elastic cross section for energies below about 7 eV. Similar effects are observed in the case of Li. In the case of Na for impact energies between 2 and 20 eV Ps formation significantly reduces the Na(3s→3p) excitation cross section, while the elastic scattering cross section increases. The calculated total Ps formation cross sections for Na and K are in good agreement with the preliminary experimental data of Kwan, Stein and co-workers.

Hyperspherical close-coupling calculation of positronium formation cross sections in positron-hydrogen scattering at low energies

The hyperspherical close-coupling method is applied to calculate both the elastic and positronium formation cross sections for positron collisions with atomic hydrogen at low energies. By treating the hyperradius as a slow variable, the Schrodinger equation in the body frame at each fixed hyperradius is solved using the higher-order finite-element method and the resulting hyperradial equations are solved using the diabatic-by-sector method. The coupled hyperradial equations are integrated to a large hyperradius where the wavefunctions are matched to the known asymptotic solutions to extract the scattering matrix. Both the elastic and the positronium formation cross sections are calculated at energies below the H(n=2) excitation threshold for J=0, 1, 2 and 3. It is shown that the present hyperspherical close coupling method gives results comparable to those obtained by elaborate variational methods.

Total cross sections for electron and positron collisions with rubidium in a model-potential approach

The total cross sections for electron and positron collisions with rubidium are calculated in the energy range 15-1000 eV, employing the modified Glauber approximations within the model-potential approach. The total cross sections for positron collision are in better agreement with experimental data, in comparison to those previously calculated in the frozen-core approach. The effect of the core polarization is significant. As in the case of scattering from Na and K, this effect causes the electron and positron cross sections obtained in the model-potential approach to no longer merge with each other at low energies.

Total cross sections for positron collisions with rubidium

The total cross sections for positron and electron collisions with rubidium are calculated in the energy range 15-1000 eV, employing the modified Glauber approximation. The MG total cross sections are found to be in reasonably good agreement with experimental data at energies below 100 eV, and consistent with the total cross sections calculated at energies below 30 eV in the close-coupling approximations.