High Incident Energies Research Articles

Breakup of positronium in a collision with aLi+ion

Fragmentation of ground state ortho Positronium (Ps) in collision with Li ion (Li+) is studied in the framework of post collisional Coulomb distorted eikonal approximation (CDEA) for the target elastic case . The present model takes account of the two center effect on the ejected e which is crucial for a proper description of the projectile ionization involving an ionic target. Both the fully differential (TDCS) and the doubly differential (DDCS) cross sections (energy spectra) are investigated at intermediate and high incident energies. A broad distinct Electron loss peak (ELP) centered around v_e ~ v_p is noted in the e energy spectrum in contrast to the sharp ELP for a heavy projectile. Two salient features are noted in the present study: i) the shift of the e DDCS peak (summed over e+ angles) towards higher ejection energy with respect to half the residual energy of the system, ii) comparison of the e& e+ energy spectra reflect a strong e - e+ asymmetry with respect to the ratio v_e/v_p =1 >. Both these features could be attributed to the post collisional two center effect on the e due to its parent nucleus (e+) and the screened target ion . Two different wave functions of the Li ion are chosen in order to test the sensitivity of the present results with respect to the choice of the wave function.

Electron-lattice interactions in the perovskiteLaFe0.5Cr0.5O3characterized by optical spectroscopy andLDA+Ucalculations

We use resonance Raman scattering (incident photon energies between 1.8 and 4.13 eV), $\text{LDA}+\text{U}$ calculations, spectroscopic ellipsometry, and oblique IR reflectivity to characterize the strong electron-phonon interactions in the disordered perovskite ${\text{LaFe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3}$. When the photon energy coincides with a Cr to Fe Mott-Hubbard transfer gap around 2.4 eV the electron-phonon interaction is manifested by a Franck-Condon effect with exceptional first- and higher order scattering of a local oxygen breathing mode. At higher incident energies we observe a superposition of Franck-Condon scattering and Fr\"ohlich interaction induced infrared active longitudinal optical two-phonon scattering activated mainly by O to Fe charge transfer. Our results establish ${\text{LaFe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3}$ as a model compound for research on electron-phonon interactions in strongly correlated complex systems and show that Franck-Condon scattering in complex solids is not limited to Jahn-Teller active compounds.

DECAY OF 202Pb* FORMED IN 48Ca+154Sm REACTION USING THE DYNAMICAL CLUSTER-DECAY MODEL

The decay of compound nucleus 202 Pb *, formed in entrance channel reaction 48 Ca +154 Sm at different incident energies, is studied by using the dynamical cluster-decay model (DCM) where all decay products are calculated as emissions of preformed clusters through the interaction barriers. The calculated results show an excellent agreement with experimental data for the fusion-evaporation residue cross-section σ ER together with the fusion-fission cross-section σ FF (taken as a sum of the energetically favored symmetric [Formula: see text] and near symmetric A=65–75 plus complementary fragments), and the competing, non-compound-nucleus quasi-fission cross-section σ QF where the entrance channel is considered not to loose its identity (and hence with preformation factor P0=1). The interesting feature of this study is that the three decay processes (ER, FF and QF) are quite comparable at low energies, ER being the most dominant, whereas at higher energies FF becomes most probable followed by ER and QF. The prediction of two fission windows, the symmetric fission (SF) and the near symmetric fission (nSF) whose contribution is more at lower incident energies, suggests the presence of a fine structure effect in the fusion-fission of 202 Pb *. This result is attributed to the shell effects (magic shells) playing effective role in the fragment preformation yields for 48 Ca +154 Sm reaction at lower excitation energies, giving rise to "shoulders", to an otherwise Gaussian FF mass distribution, responsible for the QF process. As a further verification of this result, absence of "shoulders" (hence, the QF component) in the decay of 192 Pb * due to 48 Ca +144 Sm reaction is also shown to be given by the calculations, in agreement with experiments. The only parameter of the model is the neck-length ΔR which shows that the ER occurs first, having the largest values of ΔR, and the FF and QF processes occur almost simultaneously at lower incident energies but the FF takes over QF at higher incident energies. In other words, the three processes occur in different time scales, QF competing with FF at lower incident energies.

Coverage dependent adsorption dynamics in hyperthermal organic thin film growth.

We have examined the dynamics of adsorption of diindenoperylene (DIP) on SiO(2) and SiO(2) modified with an interfacial organic layer using in situ real time synchrotron x-ray scattering, focusing on the effects of coverage. On both surfaces we observe a substantial increase in the probability of adsorption with increasing coverage, which is most dramatic at the highest incident kinetic energies. On the initially uncovered surfaces, we observe a smooth decrease in the probability of adsorption with increasing incident kinetic energy, indicative of trapping-mediated adsorption. Once both surfaces are covered by DIP, the effects of incident kinetic energy are greatly reduced, and trapping is very efficient over the range of kinetic energies examined. Possible reasons for efficient trapping at high coverage and at high incident kinetic energy include more efficient momentum transfer due to mass matching, and possibly direct molecular insertion. Comparison to results on another small-molecule, pentacene, suggests that this behavior should be common to hyperthermal growth of a variety of other small-molecule thin films.

Decay ofBa118,122*compound nuclei formed inKr78,82+Ca40reactions using the dynamical cluster-decay model of preformed clusters

Application of the preformed clusters based dynamical cluster-decay model (DCM) is made to the recent data on decay of the compound systems $^{118,122}\mathrm{Ba}{}^{*}$ at a relatively low bombarding energy of 5.5 MeV/$A$. The same model has been applied earlier to the intermediate mass fragment (IMF) data of $^{116}\mathrm{Ba}{}^{*}$, observed at medium and higher incident energies. For the heavier $^{118,122}\mathrm{Ba}{}^{*}$ systems, however, a complete mass fragmentation spectrum is observed experimentally. Except for a small narrow region of heavier mass fragments ($8\ensuremath{\leqslant}{Z}_{L}\ensuremath{\leqslant}15$), the DCM gives an overall reasonable description of the observed data on both the intermediate mass fragments and the fusion-fission cross-sections, whereas the statistical model calculations based on BUSCO and GEMINI codes describe the intermediate mass fragment data and the heavier mass fragment and fusion-fission data, respectively. Within the DCM (with preformation factor ${P}_{0}=1$), the possibility of non-compound-nucleus decay contributing to the region $8\ensuremath{\leqslant}{Z}_{L}\ensuremath{\leqslant}15$ of heavier mass fragments is also explored. All three models use the maximum angular momentum ${\ensuremath{\ell}}_{\mathrm{max}}$ as a fitting parameter, which in the DCM is fixed via a neck-length parameter for the penetrability $P\ensuremath{\rightarrow}1$.

Fragmentation of positronium in collision with Li ion including electron loss to the continuum

Fragmentation of ground state ortho-positronium (Ps) in collision with Li ion (Li+) is studied in the framework of post-collisional Coulomb distorted eikonal approximation (CDEA), giving special emphasis on the dynamics of the electron loss to the continuum (ELC) that occurs when the electron (e) and the positron (e+) are very close to each other in the velocity space (υe ≈ υp). The present model takes account of the two-centre effect on the ejected e which is crucial for a proper description of the ELC phenomena. Both the fully differential cross-section (TDCS) and the doubly differential cross-section (DDCS) (energy spectra) are investigated at intermediate and high incident energies. A broad distinct ELC peak centred around υe ≈ υp is noted in the e energy spectrum in contrast to the sharp ELC peak for a heavy projectile, corroborating the experimental findings. Two salient features are noted in the present study: (i) the shift of the e DDCS peak (summed over e+ angles) towards higher ejection energy with respect to half the residual energy of the system, (ii) comparison of the e and e+ energy spectra reflect a strong e−e+ asymmetry with respect to the ratio υe/υp = 1. Both these features are in qualitative agreement with the experimental observations due to Armitage et al for Ps-He atom system and could be attributed to the post-collisional two-centre effect on e due to its parent nucleus (e+) and the screened target ion. Two different wave functions of the Li ion are chosen in order to test the sensitivity of the present results with respect to the choice of the wave function.

Functionalizing Multi-Walled Carbon Nanotubes Using ECR Plasma and a Mild Nitric Acid Treatment

In the present work, a two-step process combining electron cyclotron resonance (ECR) plasma and 0.25 M nitric acid treatments is utilized to functionalize multi-walled carbon nanotubes (MWCNTs). In the process, high density and high incident energy hydrogen and oxygen cations generated by using ECR plasma system are used to bombard the MWCNTs for 5 min to break C-C bonds, creating free radical defects on the nanotube surface; the treated nanotubes are subsequently sonicated in the mild acid for 2 h, forming functional groups at the defects. Several characterization techniques, including X-ray photoelectron spectroscopy, scanning electron microscopy, Raman spectroscopy, and thermogravimetric analysis, are utilized to systematically evaluate the effect of the method. The results show that the MWCNTs treated by the presented method can contain a high concentration of oxygen-containing functional groups on the nanotube surface with the added advantage of high thermal stability and minimal destructive effects to the material structure.

RUBIDIUM DISTRIBUTION IN KIDNEYS OF IMMATURE RATS

Synchrotron radiation X-ray fluorescence (SR-XRF) using high energy incident X-rays was combined with micro-PIXE to analyze the renal distribution of Rb and essential elements in immature rats. The Rb concentrations in the kidneys of newborn (1 week old) and young (3 weeks old) rats were 4.19 ± 0.71 and 4.13 ± 0.30 µg/g, respectively, similar to those of adult rats. Rubidium as well as Zn was detected more in the cortex than the medulla, while Fe was concentrated in the outermost areas of the medulla of newborn rats. The renal structures of young rats achieved adult zonation into the cortex, the outer medulla and the inner medulla. Rubidium was high in the outer stripe of the outer medulla. Further analysis at higher resolution revealed Rb in the proximal tubules in the innermost part of the cortex. These results indicated that Rb was distributed downstream of the proximal tubules. This profile was similar to that of potassium, an Rb congener.

Effects of gas composition on highly efficient surface modification of multi-walled carbon nanotubes by cation treatment.

High incident energy hydrogen and/or oxygen cations are generated by electron cyclotron resonance system, and then used to highly efficiently modify multi-walled carbon nanotubes (MWCNTs). The effects of various H2/O2 gas compositions on the modification process are studied. A systematic characterization method utilizing a combination of X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, Raman spectroscopy, and thermogravimetric analysis (TGA) is used to evaluate the effects of various H2/O2gas compositions on MWCNT functionalization. The Raman results show that the ID/IG ratio is directly affected by H2 concentration in gas mixture, and the treatment applying a H2/O2 gas mixture with ratio of 40/10 (sccm/sccm) can yield the nanotubes with the highest ID/IG ratio (1.27). The XPS results suggest that the gas mixture with ratio of 25/25 (sccm/sccm) is most effective in introducing oxygen-containing functional groups and reducing amorphous carbon. The TGA suggests that the structural change of the treated nanotubes is marginal by this method with any gas condition.

(e,3e)processes on two-electron atoms: Cusp conditions and scaling law

We study the double ionization by electron impact of the ground state of heliumlike atoms and propose a scaling law for fully differential $(e,3e)$ cross sections. Within the first Born approximation, cross sections are calculated with a three-body Coulomb (3C) double-continuum wave function and initial states represented by highly accurate wave functions, which satisfy all two-body Kato cusp conditions. We first consider the helium atom in the kinematical and geometrical conditions of the only absolute, high incident energy, experimental data available: our calculations confirm unambiguously that satisfying or not Kato cusp conditions is not a relevant feature of the ground state. Other heliumlike atoms are then considered. Under similar conditions, cross sections for ${\mathrm{H}}^{\ensuremath{-}}$ are much larger than for helium while the reverse is true for positive ions; a comparison with the rare other theoretical calculations is performed. Finally, within our theoretical framework, we propose an approximate scaling law for $(e,3e)$ cross sections for heliumlike positive ions, and confirm it by calculations.

Systematic behaviour of the average parameters required for the Los Alamos model of prompt neutron emission

A large number of fissioning systems have been studied in the frame of the Los Alamos model which showed interesting regular behaviours of the average model parameters (the energy release in fission 〈 E r 〉, the total kinetic energy of the fission fragments 〈 TKE〉, the average neutron separation energy 〈 S n 〉 from the fission fragments, the prompt gamma-ray energy 〈 E γ 〉 and the level density parameter 〈 a〉 parameterized as 〈 C〉 = A/〈 a〉, where A is the mass number of the fissioning nucleus)) as well as of other quantities in connection with the prompt fission neutron emission (such as the total average prompt neutron multiplicity at thermal incident energy, the total average fission fragment excitation energy leading to prompt neutron emission, the total average prompt fission energy deposition, the average excitation energy given to the fragments, and the average center-of-mass energy of prompt neutrons and so on) and to elaborate systematics of model parameters. These systematic behaviours include two parts, the first one refers to the systematic behaviour of the average model parameters at thermal incident neutron energy and the second part contains the systematics concerning the average model parameter variation with the incident energy (or the same with the excitation energy of the fissioning nucleus). The relations expressing the systematic behaviour of the average model parameters can be a useful tool for prompt fission neutron data evaluation, their utility being relevant especially for fissioning nuclei that are far from the known and studied systems; these nuclei act as secondary fission chances of the main compound nucleus chain as well as of the secondary compound nucleus chains formed by charged particle emission at high incident energies. In cases with no experimental data and integral benchmarks, the regular behaviour of the nuclear quantities used to calculate prompt fission neutron emission can make possible an indirect validation of evaluation results.

A New Glauber Theory Based on Multiple Scattering Theory

Glauber theory for nucleus-nucleus scattering at high incident energies is reformulated so as to become applicable also for the scattering at intermediate energies. We test validity of the eikonal and adiabatic approximations used in the formulation, and discuss the relation between the present theory and the conventional Glauber calculations with either the empirical nucleon-nucleon profile function or the modified one including the in-medium effect.

Molecular dynamics simulation of Pd clusters colliding onto MgO(001) surface

Abstract The atomistic simulation of metal clusters colliding onto oxide surface is a challenge in surface science and technology. It requires a reasonable way to get the interatomic potentials for the metal–oxide system. In this work, we use a Chen–Mobius inversion method to extract the atom–atom, atom–ion and ion–ion potentials from ab initio cohesive and adhesive energies. Based on these potentials, the dynamic process of Pd cluster colliding onto MgO(0 0 1) surface is studied by molecular dynamics. The simulation result shows that there are three kinds of collision processes say “adsorption”, “reflection” and “penetration” correspond to low, middle and high incident energies, respectively. In addition, in the case of “adsorption”, the resultant cluster structure is strongly dependent on the incident energy. In summary, our work gives a good qualitative understanding of Pd cluster colliding onto MgO(0 0 1) surface with some interesting predictions. These results might be helpful to the techniques of thin film growth and surface cleaning as well as the related physics.

Backscattered Electron Imaging in the Scanning Electron Microscope: the Use of Either: (a) High Incident Energy or (b) an Array Detector

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Collision dynamics of He@C60+He@C60 at low energies

A semi-empirical molecular dynamics model is developed. The central collisions of C60+C60 and He@C60+He@C60 at different incident energies are investigated based on this model. It is found that the dimer structures have been produced at proper incident energies and these fullerene dimers could be formed by a self-assembly of C60 fullerene and He@C60. The He atom has a significant effect at higher incident energy and this embedded He atom can enhance the stability of the dimer structure.

Formation of negative hydrogen ion in positronium-hydrogen collisions

The importance of the excited states of Positronium (Ps) in the formation cross sections (both differential and total) of the negative hydrogen ion (H-) are investigated theoretically for the charge transfer reaction, Ps (n = 1, 2) + H ↦ e+ + H- for a wide range of incident energies (e.g., threshold – 500 eV). The calculations are performed in the frame work of a qualitative model, the post collisional Coulomb modified eikonal approximation (CMEA). A comparative study is also made between the capture from ground and excited states of the Ps. The present CMEA model takes account of higher order effects which is essential for a rearrangement process where the first Born type approximation (Coulomb Born for the ionic case) is not supposed to be adequate. At low incident energies, the excited states of Ps (2s, 2p) are found to play a dominant role in the H- formation cross sections. Significant deviations are noted between the present CMEA and the Coulomb Born (CBA) results even at very high incident energies (e.g., Ei = 500 eV), indicating the importance of higher order effects. At high incident energies the present CMEA differential cross section (DCS) exhibits a double peak structure which is totally absent in the CBA and could again be attributed to higher order effects.

A comparative study on low‐energy elastic electron‐NHx (x = 1,2,3) collisions

AbstractIn this work, a theoretical study on elastic electron‐NHx (x = 1,2,3) collisions in the low‐energy range is presented. More specifically, calculated elastic differential, integral, and momentum transfer cross‐sections are reported in the (1–30)‐eV energy range. An optical potential composed of static, exchange, and correlation–polarization contributions is used to represent the electron‐target interaction whereas the iterative Schwinger variational method and the method of continued fractions are used to solve the scattering equations. Comparison of the calculated cross‐sections for electron scattering by these targets shows that the results are very similar to each other at higher incident energies. This similarity indicates that the interaction between the scattering electron and the central Nitrogen is dominant. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

The rotational excitation partial cross sections for the He-HI collisions

The close coupling approach is used to calculate the elastic and inelastic partial cross sections for collisions of helium atoms with hydrogen iodide molecules based on the intermolecular potential introduced in our previous research. The calculations are performed separately at the incident energies of 40, 75 and 100 meV. This study indicates that the excitation partial cross sections converge faster than the elastic partial cross section for each of the incident energies considered here. The excitation partial cross sections converge more rapidly for the higher excited states, and more slowly for the higher incident energies.

Calculation of the electron–electron bremsstrahlung cross-section in the field of atomic electrons

An accurate expression for the free electron–electron bremsstrahlung (ee-brems) cross-section was derived by Haug in 1975, but unfortunately it must be combined with a rather prosaic correction to account for electron binding in atoms. We report on a numerical calculation of the ee-brems cross-section which include the effect of binding explicitly from the outset. Electron–electron bremsstrahlung occurs when a photon is emitted during the collision of two electrons, and an accurate treatment of ee-brems is well-advised in light elements, where it is significant compared to the nuclear bremsstrahlung process dominating in heavier elements. Our calculation allows us to verify and refine the ee-brems cross-section calculation recommended as part of ICRU Report 37. The results are auspicious in that they confirm the validity of the recommended approximations for electrons with high incident energies and over most of the emitted photon energy range. For incident energies on the order of 1MeV or lower, however, we find a considerable discrepancy between the ee-brems radiative cross-section obtained with the ICRU calculation and with our more accurate approach. Accordingly, we correct existing cross-section data tables for the EGSnrc radiation transport simulation software.

Quantum State-Resolved CO2 Collisions at the Gas−Liquid Interface: Surface Temperature-Dependent Scattering Dynamics

Energy transfer dynamics at the gas-liquid interface are investigated as a function of surface temperature both by experimental studies of CO2 + perfluorinated polyether (PFPE) and by molecular dynamics simulations of CO2 + fluorinated self-assembled monolayers (F-SAMs). Using a normal incident molecular beam, the experimental studies probe scattered CO2 internal-state and translational distributions with high resolution infrared spectroscopy. At low incident energies [Einc = 1.6(1) kcal/mol], CO2 J-state populations and transverse Doppler velocity distributions are characteristic of the surface temperature (Trot approximately Ttrans approximately TS) over the range from 232 to 323 K. In contrast, the rotational and translational distributions at high incident energies [Einc = 10.6(8) kcal/mol] show evidence for both trapping-desorption (TD) and impulsive scattering (IS) events. Specifically, the populations are surprisingly well-characterized by a sum of Boltzmann distributions where the two components include one (TD) that equilibrates with the surface (TTD approximately TS) and a second (IS) that is much hotter than the surface temperature (TIS > TS). Support for the superthermal, yet Boltzmann, nature of the IS channel is provided by molecular dynamics (MD) simulations of CO2 + F-SAMs [Einc = 10.6 kcal/mol], which reveal two-temperature distributions, sticking probabilities, and angular distributions in near quantitative agreement with the experimental PFPE results. Finally, experiments as a function of surface temperature reveal an increase in both sticking probability and rotational/translational temperature of the IS component. Such a trend is consistent with increased surface roughness at higher surface temperature, which increases the overall probability of trapping, yet preferentially leads to impulsive scattering of more highly internally excited CO2 from the surface.