Accurate Total Cross Sections Research Articles

Neutron transmission measurements for silica glass at the KURNS-LINAC

Silica glass has been used as a base and host material in vitrified radioactive waste and lithium glass scintillators for neutron detection because of its superb transparency, high heat resistance, and excellent chemical inertness. Therefore, an accurate total cross section of the silica glass is crucial to evaluate the criticality safety of vitrified wastes and understand the neutron response for lithium glass scintillators. This study performed neutron transmission measurements for silica glass using a pulsed neutron beam with the time-of-flight method at the Kyoto University Institute for Integrated Radiation and Nuclear Science − Linear Accelerator to provide an accurate total cross section in the thermal and epithermal energy range. We obtained the neutron total cross section of the silica glass in the energy region from 0.002–25 eV. The results were compared and discussed with previous results and evaluated data.

A semiempirical model for low energy electron–atom transport cross sections: The case of noble gases

A semiempirical approach to describe low energy electron–atom transport cross sections of easy implementation and reproduction is presented. The heart of the model is an energy independent two-parameter potential that was adjusted to reproduce the accurate total cross sections for He, Ne, Ar and Kr, measured with a threshold photoelectron source technique from meV up to 20 eV. Once the potential was conceived, the model was validated by comparing the values obtained for the calculated scattering lengths and phase shifts with the respective quantities previously reported in the literature. We close the article by presenting the momentum transfer and viscosity cross sections. Good agreement is found when compared to the similar data obtained from swarm experiments, from phase shifts according to differential cross section measurements and to the cross sections reported by sophisticated ab initio relativistic many-body calculations. Tables for the phase shifts and cross sections are provided for direct use and applications.

Integral Electron Scattering Cross Sections from N2O for Impact Energies Ranging from 1 to 1000 eV.

Accurate total cross sections (TCS), within 5%, for electron scattering by N2O molecules have been measured with a magnetically confined electron transmission apparatus for impact energies ranging from 1 to 200 eV. For higher energies, these measurements have been complemented with our independent atom-based screening corrected additivity rule, including interference (IAM-SCAR + I) method to determine a complete reference TCS data set in the energy range (1-1000 eV). After a critical discussion that includes our calculated integral elastic and ionization cross sections and the theoretical and experimental data available in the literature, a complete set of integral elastic and inelastic (rotational, vibrational, and electronic excitation, ionization and electron attachment) cross sections, consistent with the reference TCS data, have been derived. This update on the N2O collisional database may help to improve the accuracy of radiation-induced transport models.

Observation of Transient Anions That Do Not Decay through Dissociative Electron Attachment: New Pathways for Radiosensitization.

Low-energy electrons (LEEs) can very efficiently induce bond breaking via dissociative electron attachment (DEA). While DEA is ubiquitous, the importance of other reactions initiated by LEEs remains much more elusive. Here, we looked into this question by measuring highly accurate total cross sections for electron scattering from 1-methyl-5-nitroimidazole (1M5NI), a model radiosensitizer. The small uncertainty and high energy resolution allow us to identify many resonant features related to the formation of transient anions. In addition to novel insights about DEA reactions through the lower-lying resonances, our key finding is that the higher-lying resonances do not undergo DEA, implying alternative decay channels with significant cross sections. In particular, dissociation into two neutral fragments is probably involved in the case of 1M5NI. This finding has direct implications for the understanding of LEE-induced chemistry, particularly in the fundamental processes underlying the radiosensitization activity.

Effective one-electron approach to proton collisions with molecular hydrogen

The two-centre wave-packet convergent close-coupling approach to ion–atom collisions is extended to study proton collisions with molecular hydrogen including electron-capture channels. We use a model potential to represent the molecular target as an effective one-electron spherically symmetric system. This greatly simplifies the target structure, allowing us to use already existing code developed for ion collisions with single-electron targets. Calculated total cross sections for electron capture, single ionisation, and excitation processes generally agree well with experimental data and other theoretical calculations where available. However, the total electron capture cross section is found to overestimate the experimental data at low energies, while the total ionisation cross section is slightly underestimated. Additionally, we present state-resolved cross sections for capture into the 1s, 2ell , and 3ell states of the projectile where deviation between various previous calculations is substantial. Our results lead to overall improvement over previous theoretical studies although discrepancies with experiment are observed for 3p and 3d capture. We conclude that treating molecular hydrogen as an effective one-electron system within the two-centre coupled-channel approach to one-electron targets can give reasonably accurate total cross sections at intermediate and high energies, without the need for a complex and computationally demanding two-electron target representation.

Nonperturbative scaling behavior for net ionization of biologically relevant molecules by multiply charged heavy-ion impact

A recently developed model to describe proton collisions from molecules involving basic atoms such as hydrogen, carbon, nitrogen, oxygen and phosphorus (H, C, N, O, P) is extended to treat collisions with multiply charged ions. The ion-atom collisions are computed using the two-center basis generator method (TC-BGM), which has a proven track record of yielding accurate total cross sections for electron capture and ionization. The atomic net ionization cross sections are then used to assemble two models for ion-molecule collisions: an independent atom model (IAM) that follows the Bragg additivity rule (labeled IAM-AR), and also the so-called pixel-counting method (IAM-PCM). The latter yields reduced cross sections relative to IAM-AR near the maximum, since it takes into account the overlapping nature of effective cross sectional areas. The IAM-PCM for higher-charge projectiles leads to strong reductions of net ionization cross sections relative to the IAM-AR method, and is computed directly for projectile charges $Q=1, 2, 3$. The scaling behavior of the IAM-PCM is investigated over a wide range of energies $E$, and at high $E$ it converges towards the IAM-AR. An empirical scaling rule is established which allows to reproduce these results based on proton impact calculations. Detailed comparisons are provided for the uracil target ($\rm C_4 H_4 N_2 O_2$), for which other theoretical as well as experimental results are available. Based on the scaling model derived from the IAM-PCM data it is shown how the experimental data for uracil and water bombarded by multiply charged ions can be reduced to effective $Q=1$ cross sections respectively, and these are compared to proton impact data.

Pulse-duration dependence of the double-to-single ionization ratio of Ne by intense 780-nm and 800-nm laser fields: Comparison of simulations with experiments

Accurate ab initio calculations of the ratio of double-to-single ionization of Ne atoms in strong laser fields are difficult due to the many-electron nature of the target. Here, with accurate total cross sections carefully evaluated by using the state-of-the-art many-electron R-matrix theory for both electron-impact ionization and electron-impact excitation of Ne+, we simulate the total double-ionization yields of Ne2+ in strong laser fields at 780 and 800 nm for pulse durations in the range from 7.5 to 200 fs based on the improved quantitative rescattering model. The corresponding single-ionization yields of Ne+ are calculated within the nonadiabatic tunneling model of Perelomov, Popov, and Terent'ev. The ratio of double-to-single ionization of Ne is then obtained from the calculated double- and single-ionization yields. By normalizing the ratio to the one calculated from solving the time-dependent Schrodinger equation for a short few-cycle pulse, we make quantitative comparisons of our results with experimental data to show that our model predicts the experimental findings very well. Finally, we analyze the pulse-duration dependence of the double-to-single ionization ratio.

RBED cross sections for the ionization of atomic inner shells by electron-impact

The relativistic binary-encounter-dipole (RBED) model for electron-impact ionization of atoms combines classical binary-encounter theory and the asymptotic dipole interaction, which is based on the plane-wave Born approximation, with the only non-trivial ingredient being the optical oscillator strength (OOS). Due to the difficulty of obtaining accurate OOSs, the performance of the RBED model has so far not been fully assessed. In the present work we compare RBED inner-shell ionization cross sections (total and differential) of neutral atoms evaluated using three types of OOSs, namely an empirical power-law OOS, analytical hydrogenic OOSs and ab initio OOSs calculated numerically from self-consistent atomic potentials. We find that, compared to the distorted-wave Born approximation (DWBA), the RBED with either hydrogenic or numerical OOSs generally yields more accurate total cross sections (TCSs) than the RBED with the power-law OOS, especially for the most tightly bound shells. In the highly relativistic limit the RBED model does not recover the Bethe asymptotic behavior because of its different energy-dependent prefactor, hence we investigate an alternative prefactor which restores the correct Bethe asymptote. Finally, we suggest multiplying the RBED differential cross sections (DCSs) by the ratio of DWBA to RBED TCSs and verify that this renormalization improves the agreement with the DWBA DCSs.

Effective and Definite Resonances in the Photoproduction of η′ Mesons on Protons

A qualitative description of the most accurate experimental differential cross sections of the reaction γp→η′p from the CLAS 2009 Collaboration and unprecedentedly accurate total cross sections of this reaction, measured by the A1 Mainz 2015 Collaboration in the isobaric model of photoproduction has been obtained. Masses and widths of four resonances, extracted from such fits, lay claim to a real existence and lie in the usual mass and width intervals of nucleon resonances. But in order for our model not to contradict the results of measurements of the Σ-asymmetry of this reaction made by the GRAAL 2013 Collaboration, the need arose for an additional resonance with mass equal to the threshold of the reaction under discussion, and width Γ = 110 MeV.

Ionization of helium by slow antiproton impact: Total and differential cross sections

We theoretically investigate the single and double ionization of the He atom by antiproton impact for projectile energies ranging from $3$~keV up to $1000$~keV. We obtain accurate total cross sections by directly solving the fully correlated two-electron time-dependent Schr\"odinger equation and by performing classical trajectory Monte-Carlo calculations. The obtained quantum-mechanical results are in excellent agreement with the available experimental data. Along with the total cross sections, we also present the first fully \textit{ab initio} doubly differential data for single ionization at 10 and 100~keV impact energies. In these differential cross sections we identify the binary-encounter peak along with the anticusp minimum. Furthermore, we also point out the importance of the post-collisional electron-projectile interaction at low antiproton energies which significantly suppresses electron emission in the forward direction.

Total cross sections for positron scattering fromH2at low energies

This paper revisits positron scattering from molecular hydrogen, in an attempt to provide accurate total cross-section data against which theoretical calculations might be benchmarked. The present data were measured over the energy range 0.1--50 eV and, where possible, are compared to results from previous experiments and calculations. Agreement with the earlier data was typically very good at energies above 10 eV but becomes progressively more marginal as we go to lower energies. None of the current theories quantitatively reproduce our measurements over the entire energy range, although at a qualitative level the main features driving the scattering dynamics are apparent.

Beryllium and Graphite High-Accuracy Total Cross-Section Measurements in the Energy Range from 24 to 900 keV

This paper presents new measurements of the carbon and beryllium neutron total cross section in the energy range of 24 to 950 keV. The measurements were done using a pulsed neutron source driven by an electron LINAC. The neutron beam passed through a 30-cm-thick iron filter, which results in neutron transmission only in energies where resonance scattering and potential interference exist. The neutron filter removes most of the neutrons at other energies and significantly attenuates the gamma background resulting in 20 energy windows and a high signal-to-background ratio. The filtered beam was used for transmission measurements through graphite that results in ~1% accurate total cross sections that are in excellent agreement with current evaluations. The carbon measurement provides a verification of the accuracy of the filtered beam method. Measurements of three samples of different thicknesses of beryllium resulted in accurate total cross-section values that agree with one previous measurement and show discrepancies from current evaluations. The high accuracy of the new measurements can be used for improvement of future total cross-section evaluations of beryllium.

Logarithmically accurate total cross sections of fast electron scattering by atoms

A simple analytical formula for the total integrated cross section of fast electron scattering by atoms is obtained. This formula, being logarithmically accurate, is derived from the approximate rule of sum that does not take into account exchange effects in the atomic wave function (Hartree approximation). It is shown that exchange effects can be neglected in that scattered electron energy range where the first Born approximation is valid. The results of calculation for He, Ne, and Ar atoms agree well with experimental data.

Cross-sections for electron scattering by O 2 at intermediate and high energies, 0.1–10 keV

Cross-section data, both for elastic and inelastic electron collisions with molecular oxygen, in the energy range 0.1–10 keV, have been obtained in this study by combining theoretical and experimental methods. Accurate total cross-sections (within 3%) have been measured in a transmission beam system for electron energies ranging from 400 to 5500 eV. Differential and integral elastic cross-sections have been calculated by means of a scattering potential method in the framework of the independent atom model at electron energies ranging from 0.1 to 10 keV. The consistency of our theoretical and experimental results has been confirmed by comparison with previous elastic and ionisation experimental cross-sections.

Multilevel resonance analysis of 59Co neutron transmission measurements

Large discrepancies exist between the recent high-resolution ORELA neutron transmission data of 59Co and transmissions computed from the resolved resonance parameters of ENDF/B-VI. In order to provide new resonance parameters consistent with these data, the transmission measurements have been analyzed with the computer code sammy in the energy range 200 eV to 100 keV. The resonance parameters reported in this paper provide an accurate total cross section from 10 −5 eV to 100 keV and correctly reproduce the thermal capture cross section. Thermal cross-section values and related quantities are also reviewed here.

Application of a coupled-surface time-dependent Hartree grid method to excited state optical spectroscopy

The dynamics of multidimensional wave packet motion on nonadiabatically coupled electronic potential surfaces is explored by numerically exact time-dependent quantum mechanics and by the time-dependent Hartree grid (TDHG) approximation. Excellent agreement is found between the TDHG and the exact evolution of the wave packet; in particular, the approximation yields accurate total cross sections for electronic absorption, excitation profiles for resonance Raman scattering, and partial cross sections for photofragmentation in a two degree of freedom model of direct dissociation (which is qualitatively related to experimentally observed processes in methyl halides and ICN).

Deconvolution of experimental differential cross sections

A method to deconvolute experimental differential cross sections is presented. The procedure removes the effect of the angular divergence of the primary beam, the finite size of the detector, and multiple scattering. The resulting cross section is the optimal experimental result. The method also determines ε=nlQ0, the average number of times each beam particle is scattered. Here n is the number density of scattering gas, l is the length of the scattering region, and Q0 is the true absolute cross section. If the product nl is known, Q0 can be determined. Since the effect of the finite angular resolution of the apparatus has been removed, this method should yield accurate total cross sections. This, in turn, allows the differential cross section to be put on an absolute scale.

Utilization of Fe-filtered neutron beam for total cross section measurements up to 1 MeV

An iron filtered neutron beam has been made to be available at the time-of-flight facility of the Japan Atomic Energy Research Institute electron linac in order to measure more accurate total cross sections in the keV region. Twenty five neutron peaks from 24.3 keV to 1 MeV, which have extremely large signal-to-background ratio of 10 4, were observed with a 6Li-glass and a plastic scintillator at a 100 m station. The usefulness of the present filtered neutron beam facility was made sure by measuring the total cross section of polyethylene. These values are in excellent agreement with values computed from evaluated data in JENDI-I and ENDF/B-V.

K, L, and M shell generalized oscillator strengths and ionization cross sections for fast electron collisions

Generalized oscillator strengths and ionization cross sections by fast electron impact are calculated for K, L, and M shells. A nonrelativistic Hartree–Slater program is used to generate the initial states as well as the continuum wave functions. Core edge shapes and their dependence on momentum transfer are computed within this atomic model up to some tens or hundreds of eV above threshold. Some comparisons are made with experimental measurements and though details near threshold are not predicted, the gross shape of the spectrum is in quite good agreement with these data. While we confirm that the hydrogenic model is a reasonable approximation for K edges, we expect our computations to be useful in obtaining more accurate total cross sections for L and M shell excitation.

Measurements of Total Cross Sections for the Scattering of Potassium Atoms by Rare Gases

Abstract Total cross sections for the scattering of potassium by He, Ne, Ar, Kr, and Xe have been measured in the thermal energy range. The velocity of potassium beam is selected by a velocity selector of a slotted disk type. To obtain accurate total cross sections, the pumping effect of a McLeod gauge for the pressure measurements, the fringe effect of the scattering chamber, corrections for angular resolution and for thermal motion of the target gases are taken into consideration. A new type of scattering chamber with a variable length is used to eliminate the error of the effective scattering path length. Velocity dependence of total cross sections has been obtained and glory undulations are observed for potassium-xenon systems. For argon, krypton, and xenon the van der Waals constants are calculated from the absolute total cross sections. The values of CK–Ar, CK–Kr, and CK–Xe are 237, 354, and 535×1060 erg·cm6, respectively, and are in good agreement with the theoretical ones of Dalgarno and Davison. For neon the velocity dependence of the total cross section can not be explained by the van der Waals potential, −C⁄rs, where s=6. For helium the correction factors for thermal motion of the scattering gas are large and the value of s for attractive potential can not be determined unambiguously.