Total Cross Sections For Positron Research Articles

2p-L-7 C_2H_2によるe^+及びe^-の全断面積の前方散乱の補正

2p-L-7 C_2H_2によるe^+及びe^-の全断面積の前方散乱の補正

Theoretical Formalism To Estimate the Positron Scattering Cross Section.

A theoretical formalism is introduced in this article to calculate the total cross sections for positron scattering. This method incorporates positron-target interaction in the spherical complex optical potential formalism. The study of positron collision has been quite subtle until now. However, recently, it has emerged as an interesting area due to its role in atomic and molecular structure physics, astrophysics, and medicine. With the present method, the total cross sections for simple atoms C, N, and O and their diatomic molecules C2, N2, and O2 are obtained and compared with existing data. The total cross section obtained in the present work gives a more consistent shape and magnitude than existing theories. The characteristic dip below 10 eV is identified due to the positronium formation. The deviation of the present cross section with measurements at energies below 10 eV is attributed to the neglect of forward angle-discrimination effects in experiments, the inefficiency of additivity rule for molecules, empirical treatment of positronium formation, and the neglect of annihilation reactions. In spite of these deficiencies, the present results show consistent behavior and reasonable agreement with previous data, wherever available. Besides, this is the first computational model to report positron scattering cross sections over the energy range from 1 to 5000 eV.

Experimental and theoretical cross sections for positron scattering from the pentane isomers.

Isomerism is ubiquitous in chemistry, physics, and biology. In atomic and molecular physics, in particular, isomer effects are well known in electron-impact phenomena; however, very little is known for positron collisions. Here we report on a set of experimental and theoretical cross sections for low-energy positron scattering from the three structural isomers of pentane: normal-pentane, isopentane, and neopentane. Total cross sections for positron scattering from normal-pentane and isopentane were measured at the University of Trento at incident energies between 0.1 and 50 eV. Calculations of the total cross sections, integral cross sections for elastic scattering, positronium formation, and electronic excitations plus direct ionization, as well as elastic differential cross sections were computed for all three isomers between 1 and 1000 eV using the independent atom model with screening corrected additivity rule. No definitive evidence of a significant isomer effect in positron scattering from the pentane isomers appears to be present.

Structures in positron–atom and molecule total cross sections

Resonances in positron scattering from several atomic and molecular species are predicted by theoretical models. However, only positron–molecule binding was experimentally confirmed so far. Sharp structures in positron total cross sections were also measured; however, their detection is still in a very preliminary stage owing to instrumental limitations. Here we briefly review the present knowledge about structures in low-energy positron scattering. Based on further measurements and re-analysis of the experimental data collected at the University of Trento, we also present evidence of sharp structures in the total cross sections for several atomic and molecular targets. We find that the behaviour of some of those features can be described using the equation of a Fano resonance line-shape, despite the fact that an actual resonant scattering process might not be involved. Possible mechanisms that can give rise to such structures still need to be identified. Further independent experimental work using high-resolution spectrometers is needed in order to confirm the existence of structures in positron scattering cross sections.

Cross Sections for Positron Impact with 2,2,4-Trimethylpentane

2,2,4-Trimethylpentane (C8H18), a hydrocarbon produced all over the world on a large scale in the processing of crude oil, has long been known and used in the energy sector. It has also recently attracted the attention of the radiation physics and chemistry community, owing to its applications in medical imaging techniques. Charged-particle interactions with this species unfortunately remain mostly unknown. In this study, we report on measured total cross sections for positron scattering from 2,2,4-trimethylpentane in the energy range from 0.12 to 50 eV. We also present calculations of the total cross sections, elastic integral and differential cross sections, positronium formation cross sections, and inelastic integral cross sections at energies from 1 to 1000 eV using the independent atom model with screening corrected additivity rule. A knowledge of those scattering cross sections might, through simulation models, help to improve the accuracy of current radiation detection devices and hence provide better estimates of the extent of any charged-particle-induced damage in biomolecular systems.

Positron scattering from the cyclic ethers oxirane, 1,4-dioxane, and tetrahydropyran

In this paper we report original measurements of total cross sections (TCSs) for positron scattering from the cyclic ethers oxirane (C(2)H(4)O), 1,4-dioxane (C(4)H(8)O(2)), and tetrahydropyran (C(5)H(10)O). The present experiments focus on the low energy range from ∼0.2 to 50 eV, with an energy resolution smaller than 300 meV. This study concludes our systematic investigation into TCSs for a class of organic compounds that can be thought of as sub-units or moieties to the nucleotides in living matter, and which as a consequence have become topical for scientists seeking to simulate particle tracks in matter. Note that as TCSs specify the mean free path between collisions in such simulations, they have enjoyed something of a recent renaissance in interest because of that application. For oxirane, we also report original Schwinger multichannel elastic integral cross section (ICS) calculations at the static and static plus polarisation levels, and with and without Born-closure that attempts to account for the permanent dipole moment of C(2)H(4)O. Those elastic ICSs are computed for the energy range 0.5-10 eV. To the best of our knowledge, there are no other experimental results or theoretical calculations against which we can compare the present positron TCSs. However, electron TCSs for oxirane (also known as ethylene oxide) and tetrahydropyran do currently exist in the literature and a comparison to them for each species will be presented.

Magnetic-field-free measurements of the total cross sections for positron scattering from neon

Magnetic-field-free measurements of the absolute total cross sections (TCS) for positron-neon scattering have been performed. In the energy range of 9–15 eV, the present results are larger than those of the other groups except for the Toronto group.

Positron scattering from methane

We report on measurements of total cross sections for positron scattering from the fundamental organic molecule methane (CH${}_{4}$). The energy range of these measurements was 0.1--50 eV, whereas the energy resolution was $\ensuremath{\sim}$100 meV when our Ni moderator was used and $\ensuremath{\sim}$260 meV when the W moderator was employed. To assist us in interpreting these data, Schwinger multichannel calculations were performed at both static and static plus polarization levels of approximation for elastic positron scattering from 0.001 to 10 eV. These calculations are found to be in quite good qualitative agreement with our measured data, and they clearly educe the crucial role played by the target polarization in the low energy positron--CH${}_{4}$ scattering dynamics.

Measurements of Total Cross Sections for Positron Scattering from He under Magnetic-Field-Free Conditions Using an Electrostatic High-Brightness Slow Positron Beam System

Magnetic-field-free measurements of absolute total cross sections for positron scattering from helium have been performed using an electrostatic high-brightness slow positron beam apparatus. In the energy region above 20 eV, the present results are in good agreement with those of other studies. On the other hand, in the energy range of 4–20 eV, our values are higher than most of the other measurements except one study.

Total cross-sections for positron and electron scattering from α-tetrahydrofurfuryl alcohol

In this paper, we report original measurements of total cross-sections (TCSs) for positron scattering from an important biomolecule, α-tetrahydrofurfuryl alcohol (THFA). The energy range of these measurements was 0.15–50.15 eV, whereas the energy resolution was ∼260 meV. In addition, we report theoretical results, calculated within the independent-screened additivity rule (IAM-SCAR) formalism, on the corresponding electron impact total cross-sections. In this case, the energy range is 1–10 000 eV. With the advent of new particle track simulation codes, which incorporate accurate atomic and molecular data in order to provide interaction details at the nanoscale, interest in positron and electron TCSs has enjoyed something of a recent renaissance as they specify the mean free path between collisions in such codes. Because the present data are, to the best of our knowledge, the first TCSs to be reported for positron scattering from THFA, they fill an important void in the knowledge available to us from the literature.

Forward angle scattering effects in the measurement of total cross sections for positron scattering

Measurements of total scattering by positron impact have typically excluded a significant portion of the forward scattering angles of the differential cross section. This paper demonstrates the effect that this can have on measurements of the total cross section. We show that much of the apparent disagreement between experimental measurements of positron scattering from atoms and molecules may be explained by this excluded angular range. It is shown that this same effect may also lead to an anomalous energy dependence of some cross sections.

Measurements of the total cross sections for positron scattering from He using an electrostatic high-brightness slow positron beam apparatus

We have measured the total cross sections for positron scattering from He using an electrostatic high-brightness slow positron beam apparatus. Our results are in reasonable agreement with the data by Jaduszliwer et al in the energy region below 15 eV, which have been obtained under the influence of the lower axial magnetic field than other groups.

Total cross sections for positron and electron scattering from pyrimidine

In this paper we report original measurements of total cross sections for positron scattering from the important biomolecule pyrimidine. The energy range of these measurements was 0.3–45 eV, while the energy resolution was ∼260 meV. In addition, we report theoretical results, calculated within the independent atom–screened additivity rule (IAM-SCAR) formalism, for the corresponding electron impact total cross sections. In that case the energy range is 1–10 000 eV. Total cross sections are very important input data for codes that seek to simulate charged-particle tracks in matter, as they define the mean-free path between collisions. As the present data and computations are to the best of our knowledge the first total cross sections to be reported for either positron or electron scattering from pyrimidine, they fill an important void in our available knowledge in the literature.

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.

Energy scale determination and energy resolution in positron total cross section measurements

We compare the recent values of positron total cross sections in benzene, cyclohexane and aniline by Zecca et al. with our previous measurements [Karwasz et al. Acta Phys. Pol. A, 107 (2005) 666] on the same apparatus. An apparent discrepancy disappears if Zecca's data are shifted by +0.2 eV; then their data can be approximated by the modified effective range theory, as well. This observation induced us to compare recent Trento data with other experiments and theories for polar targets, H2O and HCOOH. The evidence points out towards the need of a careful energy calibration in positron experiments.

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.

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.

Electron and positron scattering cross sections for propene and cyclopropane

In this paper we investigate electron and positron total and electron vibrational excitation scattering cross sections for propene and cyclopropane molecules. The electron and positron total cross sections were measured over the energy range 0.2--1000 eV using a retarding-potential time-of-flight method while the electron impact vibrational excitation cross sections were measured using a crossed-beam method. For both molecules, bending and stretching vibrational modes are studied at loss energies 0.12 and 0.37 eV, respectively, for propene, and 0.13 and 0.37 eV, respectively, for cyclopropane, at the scattering angle of 90\ifmmode^\circ\else\textdegree\fi{} and impact energy range 1--16 eV.