Cross Sections For Positron Research Articles

Comment on ‘Total and positronium formation cross sections for positron scattering from H2O and HCOOH’

We would like to clarify some misunderstandings we noticed in a recent work by Casten Makochekanwa et al (2009 New J. Phys. 11 103036) with regard to our theoretical results on positronium (Ps) formation cross sections in collisions of positrons impacting H2O molecules in the gas phase. Thus, here we clearly discuss again the threshold for Ps formation as well as the domain of validity of our theoretical predictions.

Calculation of transport cross sections for positrons in solid targets via improved expressions

Transport cross sections (TCS) for positron impinging on a number of selected atomic targets have been calculated in the energy range 1–4 keV. The calculations were performed by using a simplified correction of the approximate analytical expression of transport cross sections reported by Jablonski [A. Jablonski, Phys. Rev. B 58 (1998) 16470]. An argued parameter–obtained from both an exponential and polynomial fits–was then introduced. The agreement generally observed between our results and those previously reported in the literature is quite good, especially for positron energies greater than 2 keV. Furthermore, an attempt has been made for scaling the TCS with the atomic number of solid targets of interest. Such a scaling was found to be possible for both exponential and third order polynomial fits. The information gathered by the present study may be useful for the evaluation of parameters needed for quantitative low-energy positron annihilation spectroscopy.

Elastic and inelastic positron–helium scattering

Elastic and inelastic positron–helium scatterings have been investigated with the coupled-channel optical method (CCO). Ionization continuum and positronium formation channels are included via a complex equivalent-local optical potential. Calculations are reported of cross sections of elastic scattering, total excitation and n = 2, 3, and 4 excitations of ground-state helium for incident energies from 30 eV to 400 eV. The present calculation shows that the ionization and Ps-formation channels significantly affect the cross sections of elastic and inelastic positron–helium scatterings.

Simultaneous Ionization and Excitation of Molecules by Positron Impact

Cross sections for ionization excitation of molecules by positron impact have been measured for the first time by scattering a positron beam from CO2 and N2. The cross sections have been observed to exceed those for electron impact by up to a factor of approximately 3 for CO2 and approximately 5 for N2. The enhancement arises primarily via positronium formation. The cross sections account for up to approximately 12% and 20% of the total cross sections for positron scattering from N2 and CO2, respectively.

Improving the wavefunction of the Schwinger multichannel method for positron scattering—II: Application for elastic and inelastic e+-H2 scattering

In the present paper, we propose a development to the Schwinger multichannel (SMC) method (Germano J S E and Lima M A P 1993 Phys. Rev. 47 3976) for the elastic and inelastic scattering of positrons by targets of arbitrary geometry. The essence of this work is based on the fact that the main limitation of the SMC method for positron scattering lies in what makes it a general method, i.e. the expansion of the scattering function is done in an L2 basis (Cartesian Gaussian functions) and this is very effective for short-range potentials. Our effort is to improve the SMC method through the use of plane waves as trial functions (SMC-PW) (Lino J L S 2007 Phys. Scr. 76 521). In order to do this, we have to evaluate reliable codes involving matrix elements of the type , which is done here by direct numerical quadrature. Calculated elastic differential and inelastic integral cross sections for positron–H2 collisions are reported. For the elastic scattering, we discuss a mechanism for the Buckingham polarization model using an adjustable energy-dependent parameter at intermediate energies in the range 50–200 eV. We also present a preliminary study on the excitation of the X 1Σ+g→B 1Σ+u transitions in H2. Our calculations for inelastic scattering cover the impact range of 13.5–30 eV investigated by Arretche and Lima (2006 Phys. Rev. A 74 042713) using the traditional SMC method. Our inelastic and elastic cross sections are found to be in reasonable agreement with other theoretical studies and experimental data.

Total and positronium formation cross sections for positron scattering from H2O and HCOOH

Total and positronium formation cross sections have been measured for positron scattering from H2O and HCOOH using a positron beam with an energy resolution of 60 meV (full-width at half-maximum (FWHM)). The energy range covered is 0.5–60 eV, including an investigation of the behavior of the onset of the positronium formation channel using measurements with a 50 meV energy step, the result of which shows no evidence of any channel coupling effects or scattering resonances for either molecule.

High-resolution positron scattering from helium: Grand total and positronium-formation cross sections

Measurements of the grand total and positronium-formation cross sections for positrons scattered by helium within the impact energy range from 10 to 60 eV are presented. All measurements presented here use a high-resolution $(\ensuremath{\sim}70\text{ }\text{meV})$ trap-based pulsed positron beam. Scattering is studied using a high-magnetic field, and absolute measurements of the scattering cross sections are obtained without the need for normalization to other cross sections. We also present single center, convergent close coupling calculations of the total cross section. A detailed study of the cross section to investigate the possibility of scattering resonances and channel coupling has been made. Comparisons with previous cross-section measurements and theoretical calculations are also included.

Electron and positron scattering from the benzene derivative benzotrifluoride: Total and vibrational excitation cross sections

Electron and positron cross sections have been investigated experimentally for scattering from benzotrifluoride $({\text{C}}_{6}{\text{H}}_{5}{\text{CF}}_{3})$ molecules over the energy range 0.2--1000 eV. The current results have been compared with our previous results for ${\text{C}}_{6}{\text{H}}_{6}$ molecules. Similar to ${\text{C}}_{6}{\text{H}}_{6}$, a peak has been observed in electron total cross sections (TCS) centered at 8.0 and a shoulder at about 40 eV. Vibrational excitation cross section experiments were carried out to probe the origin and nature of the 8 eV peak. For positron TCS, a broad peak spanning the region 0.8--15 eV and a change in slope at about 40 eV are observed with the only difference between the current results and those of ${\text{C}}_{6}{\text{H}}_{6}$ observed in the region 1--100 eV.

Positron scattering from formic acid

We report on measurements of total cross sections for positron scattering from the fundamental molecule formic acid (HCOOH). In this case, the energy range of our experimental work is $0.3--50.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Our interpretation of these data was somewhat complicated by the fact that at room temperature, formic acid vapor consists of about 95% monomer and 5% dimer forms, so that the present cross sections represent an average for that ensemble. To assist us in interpreting the data, rigorous Schwinger multichannel level calculations for positron elastic scattering from the formic acid monomer were also undertaken. These calculations, incorporating an accurate model for the target polarization, are found to be in good qualitative agreement with our measured data, particularly when allowance is made for the target beam mixture (monomer versus dimer) in the experiment.

Low-energy positron scattering from methanol and ethanol: Total cross sections

We report total cross sections for positron scattering from two primary alcohols, methanol $(\mathrm{C}{\mathrm{H}}_{3}\mathrm{O}\mathrm{H})$ and ethanol $({\mathrm{C}}_{2}{\mathrm{H}}_{5}\mathrm{O}\mathrm{H})$. The energy range of the present study is $0.1--40\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The ethanol measurement appears to be original while for methanol we compare our data to the only previous result from Kimura and colleagues [Adv. Chem. Phys. 111, 537 (2000)], with a significant discrepancy between them being found at the lower energies. Positronium formation threshold energies for both species, deduced from the present respective total cross section data sets, are found to be consistent with those expected on the basis of their known ionization energies. There are currently no theoretical results against which we can compare our total cross sections.

Low-energy positron scattering byCO2

In this work we present results of integral (ICS) and differential (DCS) cross sections for positron-${\text{CO}}_{2}$ scattering at low incident energies. Our ICS shows a significant improvement toward the experimental data, especially below 2 eV, and all the way up to the positronium formation threshold (7.8 eV), in comparison to our previous calculations [S. d'A. Sanchez, F. Arretche, M. T. do N. Varella, and M. A. P. Lima, Phys. Scr. T110, 276 (2004)]. Our calculated DCSs show a better resemblance in shape with the quasielastic experimental points of the Detroit group [D. A. Przybyla, W. Addo-Asah, W. E. Kaupilla, C. K. Kwan, and T. S. Stein, Phys. Rev. A 60, 359 (1999)], but the agreement is still not fully satisfactory, indicating a need for further theoretical and experimental investigation.

Measurements of total cross sections for positron scattering by uracil molecules

We present measurements of total cross sections for 1 to 30 eV positrons scattered by uracil molecules. The measurements are done using a beam transmission technique on a setup adjusted to accommodate the nature of uracil and we discuss possibilities of future experiments studying positron interactions with more complicated targets. These and future investigations of positron interactions with biomolecules have potential relevance to various biomedical applications.

Monte Carlo simulation of non-conservative positron transport in pure argon

The main aim of this paper is to apply modern phenomenology and accurate Monte Carlo simulation techniques to obtain the same level of understanding of positron transport as has been achieved for electrons. To this end, a reasonably complete set of cross sections for low energy positron scattering in argon has been used to calculate transport coefficients of low energy positrons in pure argon gas subject to an electrostatic field. We have analyzed the main features of these coefficients and have compared the calculated values with those for electrons in the same gas. The particular focus is on the influence of the non-conservative nature of positronium formation. This effect is substantial, generally speaking much larger than any comparable effects in electron transport due to attachment and/or ionization. As a result several new phenomena have been observed, such as negative differential conductivity (NDC) in the bulk drift velocity, but with no indication of any NDC for the flux drift velocity. In addition, there is a drastic effect on the bulk longitudinal diffusion coefficient for positrons, which is reduced to almost zero, in contrast to the other components of the diffusion tensor, which have normal values. It is found that the best way of explaining these kinetic phenomena is by sampling real space distributions which reveal drastic modification of the usual Gaussian profile due to pronounced spatial differentiation of the positrons by energy.

Fragmentation of positronium with excitation and ionization of the target

Calculations of positron and electron ejection cross sections are made for collisions of Ps $(1s)$ with the noble gases He, Ne, Ar, Kr, and Xe, allowing for target excitation (i.e., discrete excitation and ionization). Cross sections doubly differential and singly differential with respect to ejected energy and angle are reported, as well as total ionization cross sections. Our results show clearly that excitation of the target becomes very important at impact energies greater than about 2 times the ionization threshold of the atom. Comparison is made with absolute experimental measurements on He and Xe targets and, with one exception, good agreement is found at both total and single differential levels. Unfortunately, most of the measurements are at impact energies where target excitation is small or negligible. There is a need for experiments at higher impact energies to assess the predictions for target excitation.

Positron scattering from 3-hydroxy-tetrahydrofuran

We report on the first measurements of total cross sections for positron scattering from the important bio-molecule 3-hydroxy-tetrahydrofuran. In this case the energy range of our work is 0.4–18.4 eV. While there are currently no theoretical data against which we can compare these results, a comparison with corresponding measurements for the geometrically similar species tetrahydrofuran is made. This comparison enables us to infer information on the role that the conformers of 3-hydroxy-tetrahydrofuran play in the scattering process.

Low energy elastic scattering of positrons by CO: An application of continued fractions and Schwinger variational iterative methods

Iterative Schwinger variational methods and the method of continued fractions, widely used for electron–molecule scattering, are applied for the first time to investigate positron–molecule interactions. Specifically, integral and differential cross sections for elastic positron scattering by CO in the (0.5–20)eV energy range are calculated and reported. In our calculation, a static plus correlation–polarization potential is used to represent the collisional dynamics. Our calculated results are in general agreement with the theoretical and experimental data available in the literature.

Total cross sections for positron scattering on benzene – angular resolution corrections

We compare recent measurements of total cross sections for positron scattering in benzene from Trento laboratory [G.P. Karwasz, R.S. Brusa, Z. Idziaszek, A. Karbowski, Eur. J. Phys. D 144 (2007) 197] with early [O. Sueoka, J. Phys. B 21 (1988) L631], later [O. Sueoka, M.K. Kawada, M. Kimura, Nucl. Instr. Method. Phys. B 171 (2000) 96] and the most recent [C. Makochekanwa, O. Sueoka, M. Kimura, Phys. Rev. A 68 (2003) 32707-1] data from Tokyo laboratory. The latter data are significantly lower than the two other sets. A simple calculation shows that an angular resolution correction, rising strongly in the limit of zero energy, should be applied to measured values if wide apertures in the scattering cell and or strong guiding magnetic fields are used. We show, with the help of the modified effective range theory, that the data from Trento and those of Sueoka (1988) would agree well with those of Sueoka et al. (2000) if the data from Sueoka et al. (2000) were artificially low, as a result of the big angular resolution error due to the experimental conditions present in Sueoka et al. (2000).

Near-threshold vibrational excitation ofH2by positron impact: A projection-operator approach

We report vibrational excitation $({\ensuremath{\nu}}_{i}=0\ensuremath{\rightarrow}{\ensuremath{\nu}}_{f}=1)$ and total cross sections for positron scattering by ${\mathrm{H}}_{2}$. The Feshbach projection operator formalism was employed to vibrationally resolve the fixed-nuclei phase shifts obtained with the Schwinger multichannel method. The near-threshold behavior of the vibrational excitation cross section is in good agreement with available experimental data [Sullivan et al., Phys. Rev. Lett. 86, 1494 (2001)]. Though our fixed-nuclei calculations do not indicate the existence of a ${e}^{+}$-${\mathrm{H}}_{2}$ virtual state, a proper description of the $T$ matrix threshold behavior is essential (the adiabatic approximation is inadequate). The projection operator approach has long been a powerful tool for studies of nuclear dynamics in electron-molecule collisions, and its application to positron scattering is timely since couplings to nuclear degrees of freedom are known to be very important at low impact energies.

Calculation of Total Cross Sections for Positron–Mg Collision

Total cross sections in positron–Mg collision arecalculated in the range 2.0–60.0 eV using the momentum space coupledchannels optical potential approach. The target ionizationcontinuum and the positronium formation are included in theoptical potential. The present total cross sections are comparedwith the experiment data and the other theoretical results.

Total cross sections for positron scattering from benzene, cyclohexane, and aniline

We use a linear transmission technique to measure total cross sections for positron scattering from benzene, cyclohexane, and aniline. In the case of cyclohexane, the energy range of the present study is $0.1--20\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, while for benzene and aniline it is $0.2--20\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. With respect to benzene and cyclohexane, comparison is made to the only other existing results we know of [Makochekanwa and co-workers, Phys. Rev. A 68, 032707 (2003); 72, 042705 (2005)]. Agreement with those data is only marginal, being particularly poor at the overlap lower energies. Unlike Kimura et al. [J. Phys. B 37, 1461 (2004)], we find the low-energy dependence of the positron-benzene total cross sections to be qualitatively similar to those found in the electron channel [Gulley et al., J. Phys. B 31, 2735 (1998)]. We believe that the present positron-aniline total cross sections represent the first time such data have been measured. These cross sections are almost identical to those we found for benzene, suggesting that substitution of hydrogen by the amine group on the aromatic ring is largely irrelevant to the scattering process in the energy regimes considered.