Magnitude Of Cross Section Research Articles

Research on proton-impact excitation process of hydrogen atom in Debye plasma

The high-energy H++H impact excitation process in Debye plasma is investigated using impact parameter Born approximation method. In the cases of several different values of Debye length D, the inter-nuclear distance R of matrix elements, the weighted probability at a collision energy of 160 keV/u, and impact excitation cross sections in an energy range of 100–1000 keV/u for direct 1s → 2p transition in H for both the unscreened and screened Coulomb interactions are calculated. It is demonstrated that the magnitude of impact excitation cross section gradually reduces as screening parameter increases. According to the excitation cross section formula and the calculated results, a detailed analysis of the reason for the reduction caused by excitation cross section is given in this paper. The effects of screened Coulomb interaction on the potential of between incident particles and excited electronic and on the hydrogenatomic structure (wave function and energy level) have very important influence on the excitation cross section.

Computational screening of one- and two-photon spectrally tuned channelrhodopsin mutants

Optogenetics is by now a well-established field within neuroscience where neuro-response is controlled at the molecular level using the photochemical properties of channelrhodopsin (ChR). In this study the recently published X-ray structure of retinal inside the ChR binding pocket serves as the basis for conducting high-level polarizable embedding quantum mechanical/molecular mechanical (QM/MM) mutation studies with the aim of providing insight into the tuning mechanisms of this remarkable protein. The levels of theory applied are the recently developed PERI-CC2 and PE-DFT approaches. Their computational efficiency makes it possible to rapidly carry out a large number of spectral calculations. This is exploited to construct in silico mutated ChR variants which are characterized in terms of the location of the relevant excitation energy and the magnitude of the two-photon absorption cross section. In turn, this allows us to pinpoint the amino acids that have the largest electrostatic effect on the studied excited state properties. We show that a single/double site mutation strategy in ChR does not perturb the electronic properties of retinal to a degree that satisfies the experimental demand for a significant red-shift. With respect to non-linear absorption we conjecture that the recently synthesized ChETA variant possesses an even larger two-photon cross section than the C1C2 variant and it is thus an ideal candidate for further studies on the two-photon activation of ChR.

Intermediate Structures in Fission and Consequences on Average Partial Cross Sections

This paper is willing to illustrate and quantify the impact of a double fission barrier in terms of average fission cross section and in particular, to status on both the additional class-II width correction factor and the degree of freedom of the overall double fission barrier which is a key parameter in the calculation of the standard Wn, f factor of Hauser-Feshbach theory. 1 Problematic related to average fission cross section simulation The purpose of this communication is to both recall and emphasize the existence of the class-II state width fluctuation factor, WII , whose treatment is usually disregarded in standard average cross section evaluation codes. The impact of fission channel Intermediate Structures (IS) is well pictured by Fig. 1 which displayed the observed and current evaluated fission cross sections within the neutron-incident Unresolved Resonance energy Range (URR) for a fissile and a fertile isotope borrowed from the plutonium family. Below the fission threshold energy ( En < 500keV), we observe large manifestations of the IS (Fig. 1- right side), relatively to the fission cross section magnitude, although IS e ffects show up as well for fissile isotopes (Fig. 1- left side). We guess that more complexity in the classically (1) used URR fission cross section model will be required for a fertile isotope but are we allowed to make some approximations for a fissile isotope without collateral damage? This is the question raised by this paper. In the next sections, the impact of both the barrier tunneling and WII factor for fissile and fertile isotopes is clarified using a new way of calculating the inextricably admixed IS and WII effects based on accurate formal R-matrix Monte-Carlo-fashion calculations which have been recently applied to the whole plutonium family in a consistent and macro-microscopic approach (2).

Relative magnitude of cross-sections for elastic and inelastic scattering of a fast particle on a many-particle target

In this work, we examine the available experimental and theoretical data of scattering cross-sections for fast incoming particles on a many-body target. We found that use of the Born approximation and the Hartree model together with the Thomas–Fermi model gives a general rule that describes the increasing dominance of elastic scattering processes for targets with an increasing amount of particles. Here, significant correlation is found for targets with similar electron configurations, while a deviation is observed with increasing molecular complexity. Furthermore, we discuss the limitations of this rule concerning its divergence for large molecules and condensed matter.

Nuclear fragmentation study at ITEP heavy ion facility

In an experiment performed at ITEP TWA heavy ion accelerator, the yields of hydrogen (p,d,t) and helium (from 3He to 8He) isotopes at 3.5° from fragmentation of 12C at T0 = 0.2 − 3.2 GeV/nucleon on a Be target have been measured. Momentum spectra of the fragments in the projectile rest frame have been obtained in larger momentum intervals than in the previous experiments with heavy ion beams. The main attention was given to the region of high momentum where fragment velocity exceeds the velocity of the projectile nucleus. The obtained data cover about 6 orders of the differential cross section magnitude. It made possible the observation of a transition from the Gaussian shape of the longitudinal momentum spectra in projectile rest frame, expected for the evaporation mechanism, to the exponential shape, typical for the cumulative (pre-equilibrium) processes. The Feynman x distributions for protons are analyzed in the framework of quark-gluon string model. The probabilities of existence of six-and nine-quark clusters are estimated and compared with the results on two- (three-) nucleon short range correlations in nuclei measured at Jefferson Laboratory.

Origin of the Larger Values of Total Cross Section for Helium Scattering from Isolated Atomic Steps

The effect of surface relaxation and the electronic re-arrangement in the vicinity of a step on the total step cross section for helium scattering is investigated. A realistic helium interaction potential at a Cu(001) step is modeled by summing non-spherical pair potentials which allows for the possibility of varying the smoothing across the surface due to the itinerant aspects of the surface electronic structure. Numerical calculations reveal a significant increase in the magnitude of the total step cross section with large charge re-arrangement in the vicinity of the step. Also, the relaxation of surface layers has no effect whatsoever. The present study clearly shows that the origin of the experimentally observed large step cross section is the hard wall scattering from charge re-arrangement in the proximity of the step. Further, the charge re-arrangements probed by thermal helium atoms must be greater than predicted by pairwise models.DOI: http://dx.doi.org/10.3329/dujs.v60i2.11532 Dhaka Univ. J. Sci. 60(2): 271-275, 2012 (July)

Electron impact excitation of the ã 3B1u electronic state in C2H4: An experimentally benchmarked system?

We report on differential and integral cross section measurements for the electron impact excitation of the lowest-lying triplet electronic state (ã (3)B(1u)) in ethylene (C(2)H(4)). The energy range of the present experiments was 9 eV-50 eV, with the angular range of the differential cross section measurements being 15°-90°. As the ground electronic state of C(2)H(4) is a (1)A(g) state, this singlet → triplet excitation process is expected to be dominated by exchange scattering. The present angular distributions are found to support that assertion. Comparison, where possible, with previous experimental results from the University of Fribourg group shows very good agreement, to within the uncertainties on the measured cross sections. Agreement with the available theories, however, is generally marginal with the theories typically overestimating the magnitude of the differential cross sections. Notwithstanding that, the shapes of the theoretical angular distributions were in fact found to be in good accord with the corresponding experimental results.

L-shell photoionization of Ar+to Ar3+ions

Absolute photoionization cross sections of Ar${}^{+}$, Ar${}^{2+}$, and Ar${}^{3+}$ ions have been measured in the 250--280 eV photon energy range. These results are compared to theoretical cross sections extracted from two new calculations we performed. They reproduce well the general behavior of the experimental spectra and the magnitude of the direct photoionization cross sections. However, the oscillator strength of the resonant structure in the spectra, dominated by $2p\ensuremath{\rightarrow}3d$ transitions, is overestimated up to 80$%$ by our calculations.

Argon 420.1–419.8 nm emission line ratio for measuring plasma effective electron temperatures

We explore the feasibility of using the ratio of two argon emission line intensities at 420.1 and 419.8 nm to measure the effective electron temperature in argon-containing plasmas. Experimental measurements in numerous plasma sources reveal the ratio varies from a value of approximately 1 at high electron temperatures, to a value near 4 at low electron temperatures. This variation is understood in terms of the magnitudes of the electron excitation cross sections into the upper energy levels of the two transitions. At high electron temperatures, the upper levels of the two emission lines, the J = 3 3p9 level for the 420.1 nm line and the J = 0 3p5 level for the 419.8 nm line, are both primarily populated by excitation from the ground state and have similar optical emission cross sections. At low electron temperatures, excitation is dominated by excitation from the metastable levels which have very different cross sections into the two levels. Temperatures obtained with this line pair ratio in an inductively coupled plasma are found to be consistent with values obtained from a Langmuir probe as well as an analysis of the entire set of 2px → 1sy emission lines (665–1150 nm) under a wide variety of plasma conditions.

Constraints on WIMP and Sommerfeld-enhanced dark matter annihilation from HESS observations of the galactic center

We examine the constraints on models of weakly interacting massive particle (WIMP) dark matter from the recent observations of the Galactic Center by the High Energy Spectroscopic System (HESS) telescope. We analyze canonical WIMP annihilation into Standard Model particle final states, including bb̄, tt̄ and W+W−. The constraints on annihilation into bb̄ is within an order of magnitude of the thermal cross section at ∼ 3 TeV, while the τ+τ− channel is within a factor of ∼ 2 of thermal. We also study constraints on Sommerfeld-enhanced dark matter annihilation models, and find that the gamma-ray observational constraints here rule out all of the parameter space consistent with dark matter annihilation interpretations of PAMELA and the Fermi-LAT e+e− spectrum, in specific classes of models, and strongly constrains these interpretations in other classes. The gamma-ray constraints we find are more constraining on these models, in many cases, than current relic density, cosmic microwave background, halo shape and naturalness constraints.

THE CATALYTIC ROLE OF CORONENE FOR MOLECULAR HYDROGEN FORMATION

We present the results of an experimental study on the interaction of atomic deuterium with coronene films. The effects of D atom irradiation have been analyzed with infrared spectroscopy. The spectral changes provide evidence for deuteration of the outer edge coronene C sites via a D addition reaction. A cross section of 1.1 A2 is estimated for the deuteration process of coronene. HD and D2 molecules form, through abstraction reactions, on deuterated coronene sites with a cross section of 0.06 A2. The magnitude of both cross sections is in line with an Eley-Rideal type process. The results show that hydrogenated neutral polycyclic aromatic hydrocarbon molecules act as catalysts for the formation of molecular hydrogen.

Ionization of a lithium ion by electron impact in a strong laser field

The modification in the dynamics of the electron-impact ionization process of a ${\mathrm{Li}}^{+}$ ion due to an intense linearly polarized monochromatic laser field ($n\ensuremath{\gamma}e,2e$) is studied theoretically using coplanar geometry. Significant laser modifications are noted due to multiphoton effects both in the shape and magnitude of the triple-differential cross sections (TDCSs) with respect to the field-free (FF) situation. The net effect of the laser field is to suppress the FF cross sections in the zeroth-order approximation [Coulomb-Volkov (CV)] of the ejected electron wave function, while in the first order [modified Coulomb-Volkov (MCV)], the TDCSs are found to be enhanced or suppressed depending on the kinematics of the process. The strong FF recoil dominance for the ($e$,2$e$) process of an ionic target at low incident energy is destroyed in the presence of the laser field. The FF binary-to-recoil ratio changes remarkably in the presence of the laser field, particularly at low incident energies. The difference between the multiphoton CV and the FF results indicates that for the ionic target, the Kroll-Watson sum rule does not hold well at the present energy range in contrast to the neutral atom (He) case. The TDCSs are found to be quite sensitive with respect to the initial phase of the laser field, particularly at higher incident energies. A significant qualitative difference is noted in the multiphoton ejected energy distribution (double-differential cross sections) between the CV and the MCV models. Variation of the TDCSs with respect to the laser phase is also studied.

Measurement of the inclusive isolated prompt photon cross-section in pp collisions at [formula omitted] using [formula omitted] of ATLAS data

A measurement of the differential cross-section for the inclusive production of isolated prompt photons in pp collisions at a center-of-mass energy s=7 TeV is presented. The measurement covers the pseudorapidity ranges |η|<1.37 and 1.52⩽|η|<2.37 in the transverse energy range 45⩽ET<400 GeV. The results are based on an integrated luminosity of 35 pb−1, collected with the ATLAS detector at the LHC. The yields of the signal photons are measured using a data-driven technique, based on the observed distribution of the hadronic energy in a narrow cone around the photon candidate and the photon selection criteria. The results are compared with next-to-leading order perturbative QCD calculations and found to be in good agreement over four orders of magnitude in cross-section.

Formula omitted] bremsstrahlung as the dominant annihilation channel for dark matter, revisited

We revisit the calculation of electroweak bremsstrahlung contributions to dark matter annihilation. Dark matter annihilation to leptons is necessarily accompanied by electroweak radiative corrections, in which a W or Z boson is also radiated. Significantly, while many dark matter models feature a helicity suppressed annihilation rate to fermions, bremsstrahlung process can remove this helicity suppression such that the branching ratios Br(ℓνW), Br(ℓ+ℓ−Z), and Br(ν¯νZ) dominate over Br(ℓ+ℓ−) and Br(ν¯ν). We find this is most significant in the limit where the dark matter mass is nearly degenerate with the mass of the boson which mediates the annihilation process. Electroweak bremsstrahlung has important phenomenological consequences both for the magnitude of the total dark matter annihilation cross section and for the character of the astrophysical signals for indirect detection. Given that the W and Z gauge bosons decay dominantly via hadronic channels, it is impossible to produce final state leptons without accompanying protons, antiprotons, and gamma rays.

Load/strain distribution between ulna and radius in the mouse forearm compression loading model

Finite element analysis (FEA) of the mouse forearm compression loading model is used to relate strain distributions with downstream changes in bone formation and responses of bone cells. The objective of this study was to develop two FEA models – the first one with the traditional ulna only and the second one in which both the ulna and radius are included, in order to examine the effect of the inclusion of the radius on the strain distributions in the ulna. The entire mouse forearm was scanned using microCT and images were converted into FEA tetrahedral meshes using a suite of software programs. The performance of both linear and quadratic tetrahedral elements and coarse and fine meshes were studied. A load of 2N was applied to the ulna/radius model and a 1.3N load (based on previous investigations of load sharing between the ulna and radius in rats) was applied to the ulna only model for subsequent simulations. The results showed differences in the cross sectional strain distributions and magnitude within the ulna for the combined ulna/radius model versus the ulna only model. The maximal strain in the combined model occurred about 4mm toward the distal end from the ulna mid-shaft in both models. Results from the FEA model simulations were also compared to experimentally determined strain values. We conclude that inclusion of the radius in FE models to predict strains during in vivo forearm loading increases the magnitude of the estimated ulna strains compared to those predicted from a model of the ulna alone but the distribution was similar. This has important ramifications for future studies to understand strain thresholds needed to activate bone cell responses to mechanical loading.

Magnetoelectric point scattering theory for metamaterial scatterers

We present a new, fully analytical point scattering model which can be applied to arbitrary anisotropic magneto-electric dipole scatterers, including split ring resonators (SRRs), chiral and anisotropic plasmonic scatterers. We have taken proper account of reciprocity and radiation damping for electric and magnetic scatterers with any general polarizability tensor. Specifically, we show how reciprocity and energy balance puts constraints on the electrodynamic responses arbitrary scatterers can have to light. Our theory sheds new light on the magnitude of cross sections for scattering and extinction, and for instance on the emergence of structural chirality in the optical response of geometrically non-chiral scatterers like SRRs. We apply the model to SRRs and discuss how to extract individual components of the polarizability matrix and extinction cross sections. Finally, we show that our model describes well the extinction of stereo-dimers of split rings, while providing new insights in the underlying coupling mechanisms.

Dielectric mismatch effect on the photoionization cross section and intersublevel transitions in GaAs nanodots

A theoretical study of the electronic states in a spherical quantum dot with and without a hydrogenic impurity is performed within the effective mass approximation taking into account the dielectric mismatch effect. By considering the joint action of the quantum confinement and polarization charges, the photoionization cross section for an on-center donor and intersublevel optical absorption are investigated. We found that: i) the subband energies increase while the 1 s and 2 p impurity levels decrease when the dielectric mismatch between the dot and its environment enhances; ii) the dielectric mismatch has a significant effect on the peak position and magnitude of the photoionization cross section so that the behavior of this quantity can indicate the material in contact with the nanostructure; iii) the absorption spectrum is less sensitive to the environment dielectric properties but it significantly depends on the dot radius as well as on the impurity presence. The possibility of tuning the resonant energies by using the combined effect of the quantum confinement and dielectric mismatch between the dot and the surrounding medium can be useful in designing new optoelectronic devices.

Photoionization study of Kr+ and Xe+ ions with the combined use of a merged-beam set-up and an ion trap

Photoionization cross sections of halogen-like Kr+ and Xe+ ions have been measured in the photon energy range extending up to 15 eV above the threshold. Two different devices were used, a merged-beam set-up and an ion trap. Combination of the two techniques allows for the extraction of the pure ground state ionization cross section on an absolute scale. Multiconfiguration Dirac–Fock calculations reproduce the magnitude of the direct photoionization cross sections well.

Dynamics ofO6+ + H electron capture in Debye plasmas and properties of resultingO5+(nl) emission spectra

The electron capture in ${\mathrm{O}}^{6+}$-H collisions is studied by the two-center atomic orbital close-coupling (TC-AOCC) method when the interactions of charged particles are screened and have a Yukawa form. Atomic orbitals and eigenenergies of $n$ $\ensuremath{\leqslant}$ $6$ states on ${\mathrm{O}}^{5+}$ and $n$ $\ensuremath{\leqslant}$ $2$ states on H are calculated as a function of the interaction screening parameter and used in the AOCC dynamics scheme to calculate the electron capture cross sections in the energy range $1$--$200$ keV/u. It is shown that the degree of interaction screening reduces the electron binding energies, the number of open electron capture channels, and the strength of exchange couplings, thus affecting the entire collision dynamics and the magnitude and energy behavior of electron capture cross sections. The state-selective electron capture cross sections in this collision system, as well as the intensities of a number of charge exchange spectral lines, for a number of representative interaction screening strengths are presented and discussed.

Estimating the Raman Cross Sections of Single Carbon Nanotubes

The order of magnitude of Raman differential cross sections of radial breathing modes (RBMs) of individual carbon nanotubes is measured for 633 and 785 nm laser excitations. This is shown by both a calibration applied to previously published data from other authors at 785 nm and our own measurements of individual nanotubes at 633 nm excitation. We find typical values of differential cross sections of RBMs to be on the order of approximately 10(-22) cm(2)/sr for resonant nanotubes on a silicon substrate. This study therefore provides a rigorous quantification of the accepted view that Raman cross sections of carbon nanotubes are "huge".