Cross Sections For Positron Research Articles

Electron and positron scattering from the benzene derivative: Toluene

Electron and positron cross sections for scattering from toluene molecules have been investigated both experimentally and theoretically over the energy range $0.4--1000\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Peaks have been observed in the electron total cross sections (TCSs) at 1.4, 4.5, and 8.0 and a shoulder at about $40\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Vibrational and elastic differential cross section experiments were carried out to probe the origin and nature of these peaks. The continuum multiple scattering method was used to calculate elastic integral cross sections for electron impact. Although the $1.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ peak is dominated by the elastic channel, strong contributions from the ${\mathrm{CH}}_{3}$ asymmetric bending and stretching vibrational modes are also observed. The $4.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ feature is also observed to be strongly due to vibrational excitation. The broad $7--13\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ peak includes contributions from the ${\mathrm{CH}}_{3}$ asymmetric bending (peaked at $7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$) and the ${\mathrm{CH}}_{3}$ stretching (peaked at $9.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$) vibrational modes. Positron TCSs are studied and compared to positron-benzene TCSs. The effect of the ${\mathrm{CH}}_{3}$ substitution is observed to make a significant contribution to both electron and positron TCSs below $20\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$.

Positron scattering on benzene and cyclohexane: Experiment and modified effective range theory

Total cross sections for positron scattering on benzene and cyclohexane measured with high angular resolution in the 0.5–20 eV range are compared with previous experimental data. Present data showing a rise in the zero energy range agree much better with the theory than previous experiments. The rise can be reproduced by the modified effective range theory, using three partial waves.

Dispersive contributions to e+p/e−p cross section ratio in forward regime

Abstract Two-photon exchange (TPE) contributions to elastic electron–proton scattering in the forward regime are considered. The imaginary part of TPE amplitude in these kinematics is related to the DIS nucleon structure functions. The real part of the TPE amplitude is obtained from the imaginary part by means of dispersion relations. We demonstrate that the dispersion integrals for the relevant elastic ep -scattering amplitude converge and do not need subtraction. This allows us to make clean prediction for the real part of the TPE amplitude at forward angles. We furthermore compare e + p and e − p cross sections which depends on the real part of TPE amplitude, and predict the positron cross section to exceed the electron one by a few per cent, with the difference ranging from 1.4% to 2.8% for electron lab energies in the range from 3 to 45 GeV. We furthermore predict that the absolute value of this asymmetry grows with energy, which makes it promising for experimental tests.

Positron probing of point V-group impurity-vacancy complexes in γ-irradiated germanium

For the first time the positron probing of the vacancy—V-group-atom impurity complexes, which were created by γ-irradiating 60Co ( T irr.≈280 K) in the oxygen-lean n-Ge〈V〉 (V=As, Sb, or Bi), has been conducted systematically both before and after n-p-conversion of material. The one-dimensional angular correlation of the annihilation radiation (ACAR) has been registered for crystallographic directions [1 1 1], [1 1 0], and [1 0 0]. The concentration of defects in the same samples was determined from the temperature dependencies of the carrier density and mobility obtained over the range of 4.2–300 K. It was found that (i) the trapping cross section of positrons by the radiation complexes is over the range of (0.7–4.3)×10 −15 cm 2, and (ii) the size of V-group-atom situated in nearest environment of the positron plays crucial role in generating elementally specific emission of the annihilation γ-quanta. The positron-sensitive vacancy—V-group-atom impurity complexes, or E centres, are formed under γ-irradiation at room temperature in oxygen-lean n-Ge〈V〉 (V=As, Sb, or Bi); this process is accompanied by removing free electrons and at higher doses of irradiation the n-p-conversion occurs. Deeply converted material of p-type contains acceptor centres whose energy levels depending on V-group-atom size are over the range of E v+(0.1–0.16) eV. The positron annihilation data are interpreted in terms of successive trapping of vacancies by the E centres to be formed, in particular, before n-p-conversion of γ-irradiated material.

Total cross sections for positron scattering in argon, nitrogen and hydrogen below 20 eV

Total cross sections for positron scattering on argon, nitrogen and hydrogen have been measured at 0.4–20 eV by an absolute, transmission method. The apparatus uses a weak magnetic field but is characterised by small entrance and exit apertures of the scattering cell, assuring a good angular resolution down to the sub-eV energy range. Present measurements in all three gases show a fall of the cross section from the zero energy limit, then constant cross section values, within the error bar, up to the positronium formation threshold. Agreement with other experiments and theories is good but present data in the low energy limit are generally higher.

Positronium- and positron–H2O total cross sections

The total cross sections for positron and positronium scattering from H2O molecules have been measured for incident energies between 7 and 417 eV, and 10 and 100 eV, respectively. The experimental system has been characterized with respect to its angular acceptance of both scattered positrons and positronium in order to correct the data for forward-scattering errors once differential cross sections become available. The present data are compared with previous results for electron and positron total cross sections.

Total cross sections for scattering of positron from CO, HCl, NH3 and CH4 at intermediate and high energies

A complex optical model potential modified by incorporating the concept of bonded atom, which takes into consideration the overlapping effect of electron clouds between two atoms in a molecule, is firstly employed to calculate the total cross sections for positron scattering from CO, HCl, NH3, SiH4 in the incident energy range 30—3000eV by using additivity rule model at Hartree-Fock level. In the study, the complex optical model potential composed of static, correlation polarization plus absorption contributions firstly uses the bonded-atom concept. The results of quantitative molecular total cross sections are compared with those obtained by experiments and other theories wherever available, and good agreement is obtained. It is shown that the additivity rule model together with the complex optical potential model modified by incorporating the concept of bonded atom can give much better calculation results than the unmodified one.

Have positron-helium scattering resonances been observed?

We comment on the recent claim by Karwasz [Eur. Phys. J. D 35, 267 (2005)] that he observed resonant structures in the total cross-section for positron scattering from helium. We frame our observations in terms of both the general capabilities of the Trento spectrometer, and new checks we have made on the measurement procedure employed by Karwasz. We conclude that the observed structures are most likely an experimental artefact, rather than being due to the positron-helium interaction.

Positronium formation cross section for positron–lithium collisions

The hyperspherical hidden crossing method (HHCM) has been used to calculate the positronium formation cross section for positron–lithium collisions in the energy range 0–1.8eV. Results for the s-, p- and d-wave positronium formation cross sections were previously reported; here we present new results for the f-wave. In energy range studied, we found that the Stückelberg phase ΔL varied only slightly with incident positron momentum ki and decreased in a systematic way with increasing partial wave L. The HHCM cross section for positronium formation (summed over the four partial waves) lies between experimental measurements of the lower and upper limits of this cross section; the f-wave contribution to the cross section dominates near the lithium excitation threshold.

Optical Potential Calculations of Positron–H2 ElasticCollision

We report the calculations of the elastic scatteringdifferential cross section for positron–H2 collisions with theimpact energy below the positronium formation threshold.Our calculation is based on the static-exchange-optical model which hasobtained great success in the case of electron scattering. Theeffective potential used here includes the static and opticalpotentials. The optical method can completely include the second-ordereffect arising from real and virtual excitation of target states, whichis important for the scattering. A comparison is made with theavailable theoretical calculations.

Positron scattering in helium: Virtual-positronium resonances

Total cross sections for positron scattering in helium and argon were measured at 0.5–25eV with a new spectrometer from Trento University, with a good angular resolution (3.1×10−4sr). Data in argon fall in-between earlier measurements; those for He agree well with the previous above the positronium formation thresholds but are up to 50% higher at low energies. Data for He show four resonant-like structures, one at the positronium threshold formation (17.8eV) predicted by Van Reeth and Humberston [J. Phys. B 32 (1999) L103], another at about 6.8eV and two very prominent centred at 1.6eV and 2.2eV. Following Gribakin and King [J. Phys. B 27 (1994) 2639] the two latter could be signs of virtual-positronium formation in helium.

Rotational cooling of molecular gases by positron impact at vanishing collision energies

The behaviour of the rotationally inelastic cross sections for ultracold positrons injected in molecular gases is examined computationally by considering the ambient molecules to be rotationally ‘hot’ (j = 10) and by letting the relative translational energies tend to zero. The size of the corresponding state-to-state transitions are obtained from an exact quantum calculation carried out in the space-frame representation, using non-empirical model interactions between e+ and the target test molecules (H2, N2, O2, Li2). The structural features of the molecules are related, via the dynamical coupling effects, to the size and efficiency of the cooling process along the above series of target gases. It is shown that positron projectiles can provide a possible route to the cooling of molecular rotations for simple homonuclear diatomics within the framework of ultracold dynamical conditions.

The lives and deaths of positrons in the interstellar medium

We reexamine in detail the various processes undergone by positrons in the ISM from their birth to their annihilation using the most recent results of positron interaction cross sections with H, H2 and He. The positrons' lives are divided into two phases: the 'in-flight' phase and the thermal phase. The first phase is treated with a Monte Carlo simulation that allows us to determine the fraction of positrons that form positronium and annihilate as well as the characteristics of the annihilation emission as a function of the medium conditions. The second phase is treated with a binary reaction rate approach, with cross sections adopted from experimental measurement or theoretical calculations. An extensive search and update of the knowledge of positron processes was thus undertaken. New reaction rates and line widths have been obtained. We investigate the treatment of the complicated interactions between positrons and interstellar dust grains. New reaction rates and widths of the line resulting from the annihilation inside and outside of the grain have been obtained. The final results of our calculations showed that dust is only important in the hot phase of the ISM, where it dominates all other processes. Combining the new calculations, we have constructed annihilation spectra for each phase of the ISM, considering various grain contents, as well as an overall combined spectrum for the ISM as a whole.

Calculation of Total Cross Sections for Positron Scattering byLithium at Intermediate Energies

The total cross sections for positron scattering bylithium at intermediate energies (10–200 eV) are calculated by usingthe coupled-channel optical method with a complex equivalent-localpolarization potential which incorporates ionization continuum andpositronium (Ps) formation channels contributions into the coupledchannels framework. The effects of the two-body Ps rearrangement andthree-body ionization process on the total cross section are found tobe significant at lower energies and this effect is not negligible upto 30 eV. Compared to the available theoretical data, the predictedtotal cross sections agree quite well with the calculations ofMcAlinden et al. [J. Phys. B: At. Mol. Opt. Phys. 30(1997) 1543] and Campbell et al. [Nucl. Instrum.Methods Phys. B 143 (1998) 41]

Evaluation of elastic-scattering cross sections for electrons and positrons over a wide energy range

Quantification of surface- and bulk-analytical methods, e.g. Auger-electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), electron-probe microanalysis (EPMA), and analytical electron microscopy (AEM), requires knowledge of reliable elastic-scattering cross sections for describing electron transport in solids. Cross sections for elastic scattering of electrons and positrons by atoms, ions, and molecules can be calculated with the recently developed code ELSEPA (Elastic Scattering of Electrons and Positrons by Atoms) for kinetic energies of the projectile from 10 eV to 50 eV. These calculations can be made after appropriate selection of the basic input parameters: electron-density distribution, a model for the nuclear-charge distribution, and a model for the electron-exchange potential (the latter option applies only to scattering of electrons). Additionally, the correlation-polarization potential and an imaginary absorption potential can be considered in the calculations. We report comparisons of calculated differential elastic-scattering cross sections (DCSs) for silicon and gold at selected energies (500 eV, 5 keV, 30 keV) relevant to AES, XPS, EPMA, and AEM, and at 100 MeV as a limiting case. The DCSs for electrons and positrons differ considerably, particularly for medium- and high-atomic-number elements and for kinetic energies below about 5 keV. The DCSs for positrons are always monotonically decreasing functions of the scattering angle, while the DCSs for electrons have a diffraction-like structure with several minima and maxima. A significant influence of the electron-exchange correction is observed at 500 eV. The correlation-polarization correction is significant for small scattering angles at 500 eV, while the absorption correction is important at energies below about 10 keV. Copyright © 2005 John Wiley & Sons, Ltd.

Scattering of low- to intermediate-energy positrons from molecular hydrogen

Using a complex model potential, we have calculated the total, integrated elastic, momentum transfer, absorption, and differential cross sections for positrons scattered from molecular hydrogen. The widely available software package GAUSSIAN is used to generate the radial electronic charge density of the molecule which is used to produce the interaction potentials. The quasifree absorption potential, previously developed and used for positron-atom scattering, is extended to positron scattering from molecular targets. It is shown that this model potential approach produces accurate results even into the low-energy regime.

Positronium Formation in e+–K Collision at Low Energies

Positronium (Ps) formation cross section for positron scattering onpotassium is calculated at low impact energies (0.2–15.0 eV). Thepresent calculation uses an optical-model method with an equivalentlocal polarization potential. The results for the Ps (n = 1) and Ps(n = 2) formation cross sections are calculated and compared with theexperimental measurements and other theoretical results.

Elsepa—Dirac partial-wave calculation of elastic scattering of electrons and positrons by atoms, positive ions and molecules

elsepa—Dirac partial-wave calculation of elastic scattering of electrons and positrons by atoms, positive ions and molecules

Experimental study of total cross sections for positron and electron scattering bySF6molecules

The total cross sections (TCSs) for $0.2--1000\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ positrons and for $0.8--1000\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ electrons colliding with ${\mathrm{SF}}_{6}$ molecules were measured by a relative method. In the present TCS investigation for electrons, peaks attributed to the ${a}_{1g}$, ${t}_{1g}$, ${t}_{2g}$, and ${e}_{g}$ shape resonances identified earlier were observed clearly at 2.2, 7.0, 12.5, and $30\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. In positron TCSs, a hump centered at about $2.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ was observed, followed by a larger and broader peak at around $50\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. These TCSs are compared with those for ${\mathrm{C}}_{2}{\mathrm{F}}_{6}$ molecules, a hexafluoro molecule showing roughly comparative structures at higher energies than $100\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, a feature that is attributable to the similarity in molecular size and polarizability. Especially for positron scattering, the TCSs are quite similar to each other over all the energy range investigated. Possible rationales for this similarity are provided.

Total cross sections for electron and positron scattering from 4-fluorobenzaldehyde (C6H4(CHO)F) molecules

Total cross sections for electron and positron scattering from 4-fluorobenzaldehyde (C6H4(CHO)F) molecules