High Incident Energies Research Articles

Isospin diffusion and equilibration forSn+Sncollisions atE/A=35MeV

Equilibration and equilibration rates have been measured by colliding Sn nuclei with different isospin asymmetries at beam energies of E/A=35 MeV. Using the yields of mirror nuclei of 7Li and 7Be, we have studied the diffusion of isospin asymmetry by combining data from asymmetric 112Sn+124Sn and 124Sn+112Sn collisions with that from symmetric 112Sn+112Sn and 124Sn+124Sn collisions. We use these measurements to probe isospin equilibration in central collisions where nucleon-nucleon collisions are strongly blocked by the Pauli exclusion principal. The results are consistent with transport theoretical calculations that predict a degree of transparency in these collisions, but inconsistent with the emission of intermediate mass fragments by a single chemically equilibrated source. Comparisons with ImQMD calculations are consistent with results obtained at higher incident energies that provide constraints on the density dependence of the symmetry energy.

Shape-controlled nanopores in single crystals

Nanometer length-scale holes (nanopores) are often formed in amorphous materials forfundamental studies of molecular mass transport. In the current study, electron beamirradiation in the transmission electron microscope was used to form nanopores in acrystalline material (Si). Analysis of the nanopores showed that they are formed byknock-on of atoms by the high energy incident electron beam, and surface diffusion ispartially responsible for the hour-glass shapes that are found for some nanopores.Energetically favorable three-dimensional shapes of nanopores were simulated, and thenanopores simulated in the model crystalline material were found to be more stable thanthe nanopores simulated in the amorphous material. The nanopore shape was also found todepend on the nanopore diameter-to-length ratio. Based on the above, we demonstrate theadvantage in using a crystalline material for nanopore formation and show that control ofthe three-dimensional shape of nanopores formed by electron beam irradiation is possible.

Calculation of energy deposition, photon and neutron production in proton therapy of thyroid gland using MCNPX

In this study, the MCNPX code has been used to simulate a proton therapy in thyroid gland, in order to calculate the proton energy deposition in the target region. As well as, we have calculated the photon and neutron production spectra due to proton interactions with the tissue. We have considered all the layers of tissue, from the skin to the thyroid gland, and an incident high energy pencil proton beam. The results of the simulation show that the best proton energy interval, to cover completely the thyroid tissue, is from 42 to 54 MeV, assuming that the thyroid gland has a 14 mm thickness and is located 11.2mm under the skin surface. The most percentage of deposited energy (78%) is related to the 54 MeV proton energy beam. Total photon and neutron production are linear and polynomial second order functions of the proton energy, respectively.

Multiphoton effects in laser-assisted ionization of a helium atom by electron impact

The dynamics of the electron impact multiphoton ionization of a He atom in the presence of an intense laser field (n $\gamma _e$ , 2e) is studied theoretically for laser polarization ( $\vert\vert^l$ ) and perpendicular ( $\bot^r$ ) to the incident momentum. The triple differential (TDCS) as well as the double differential (DDCS) cross sections are studied for the coplanar asymmetric geometry. The results are compared with the only available kinematically complete experiment at high incident energy (1000 eV). Significant laser modification (enhancement) is noted due to multiphoton effects in the present binary and recoil peak intensities of the TDCS for both the geometries, in qualitative agreement with the experiment. In the single photon case, the net effect of the laser field is to suppress the field free (FF) TDCS as well as the DDCS in the zeroth order approximation of the ejected electron wave function (CV), while in the first order (MCV), the cross sections are found to be enhanced. The CV multiphoton cross sections obey the famous Kroll Watson (KW) sum rule while the latter does not hold good in the corresponding MCV approximation.

Molecular Dynamical Simulations on a-C:H Film Growth from C and H Atomic Flux: Effect of Incident Energy

Molecular dynamical simulation is carried out to investigate the effects of the incident energy on a-C:H film growth from C and H atomic flux. Our simulations show that the film growth at low incident energy (1 eV) is dominated by the adsorption of H and C atoms. At moderate incident energy (10 and 20 eV), the abstraction reaction of incident H atoms with H atoms adsorbed at the surface becomes important. At high incident energy (30 and 40 eV), the a-C:H film growth is a two-step process: one is the adsorption and the shallow implantation of C atoms, and the other is the deep implantation of H atoms.

Forward-backward correlation and its incident energy dependence in secondary-electron emission from a thin carbon foil upon proton penetration

The statistical distributions of the number of simultaneously emitted secondary electrons (SEs) from a carbon foil have been measured with proton beams of $0.5\ensuremath{-}3.5$ MeV. In this experiment, the forward- and backward-emitted SEs have been measured simultaneously with foil-transmitted protons using a digitizer. As a method to examine how the forward and backward SE emissions correlate to each other, the forward (backward) SE yields ${\ensuremath{\gamma}}_{\mathrm{F}}$ (${\ensuremath{\gamma}}_{\mathrm{B}}$), that is, the mean number of the forward-emitted (backward-emitted) electrons per projectile, have been evaluated as a function of the number of the backward-emitted (forward-emitted) SEs, ${n}_{\mathrm{B}}$ (${n}_{\mathrm{F}}$). At higher incident energies, ${\ensuremath{\gamma}}_{\mathrm{F}}$ (${\ensuremath{\gamma}}_{\mathrm{B}}$) increases with increasing ${n}_{\mathrm{B}}$ (${n}_{\mathrm{F}}$). With decreasing incident energy, this so-called positive correlation becomes weaker and then changes to negative at the lowest incident energy. Although measurements using a slightly thicker foil exhibit just the same trend, the correlation changes from positive to negative at the higher incident energy. For a given foil thickness, the range of the produced binary electron and hence the incident proton energy seems to determine the sign of the correlation. A simple Monte Carlo simulation for the forward and backward SE emission in the present experimental condition can qualitatively reproduce the observed incident-energy dependence of the positive correlation but cannot reproduce the negative one observed at the lower incident energies.

Influence of the rotational degrees of freedom on the initial sticking probability of water on Pt{110}-(1×2): A molecular dynamics study

This work focuses on a molecular dynamics (MD) study of the initial sticking probability of water on the Pt{110}-(1 x 2) surface. Previous studies of the system [T. Panczyk et al., J. Chem. Phys. 131, 064703 (2009)] led to the following conclusions: (i) adsorption of water is controlled by the efficiency of the dissipation of the initial kinetic energy during collision with the surface and (ii) the process is probably dominated by the electron-hole pair excitation mechanism. In the current work, we extend this study to understand the influence of the orientation of the water molecule and its rotational energy on the probability of the energy exchange during collision. The simulated MD trajectories correspond to various orientations of water molecule at different rotational energies. We found that assuming the angular dependence on the probability of the energy exchange can explain the experimental results obtained using supersonic molecular beams, especially for high incident molecular beam energies. For low beam energies, dispersion of the incident kinetic energy must be incorporated into the model. These are the key factors that enable to model the experimental results on a good qualitative level.

The (e, 2e) triple differential cross sections of Cu+ (3p) in coplanar asymmetric geometry

The three-body distorted-wave Born approximation has been used to calculate the (e, 2e) triple differential cross sections (TDCSs) of Cu+ (3p) in different kinematical variables in coplanar asymmetric geometry. The angles 4°, 10° and 20° were selected as the scattering electron angles. Under high incident energy (≥ 500 eV) and high asymmetric detection energy, the binary peaks showed abnormal splits. Such abnormal splits have not been observed in atomic target and outer valence orbitals of ionic target, which indicates that an (e, 2e) process for inner valence orbitals of ionic target would be more complicated than outer valence orbitals. Furthermore, some pronounced peaks appeared at certain ejected angles. We considered that these pronounced peaks are probably related to one kind of double-binary collision.

Comparison of secondary ion emission yields for poly-tyrosine between cluster and heavy ion impacts

Emission yields of secondary ions necessary for the identification of poly-tyrosine were compared for incident ion impacts of energetic cluster ions (0.8 MeV C 8 + , 2.4 MeV C 8 + , and 4.0 MeV C 8 + ) and swift heavy monoatomic molybdenum ions (4.0 MeV Mo + and 14 MeV Mo 4+) with similar mass to that of the cluster by time-of-flight secondary ion mass analysis combined with secondary ion electric current measurements. The comparison revealed that (1) secondary ion emission yields per C 8 + impact increase with increasing incident energy within the energy range examined, (2) the 4.0 MeV C 8 + impact provides higher emission yields than the impact of the monoatomic Mo ion with the same incident energy (4.0 MeV Mo +), and (3) the 2.4 MeV C 8 + impact exhibits comparable emission yields to that for the Mo ion impact with higher incident energy (14 MeV Mo 4+). Energetic cluster ion impacts effectively produce the characteristic secondary ions for poly-tyrosine, which is advantageous for highly sensitive amino acid detection in proteins using time-of-flight secondary ion mass analysis.

Running into trouble with the time-dependent propagation of a wavepacket

The propagation in time of a wavepacket is a conceptually rich problem suitable to be studied in any introductory quantum mechanics course. This subject is covered analytically in most of the standard textbooks. Computer simulations have become a widespread pedagogical tool, easily implemented in computer labs and in classroom demonstrations. However, we have detected issues raising difficulties in the practical effectuation of these codes which are especially evident when discrete grid methods are used. One issue—relatively well known—appears at high incident energies, producing a wavepacket slower than expected theoretically. The other issue, which appears at low wavepacket energies, does not affect the time evolution of the propagating wavepacket proper, but produces dramatic effects on its spectral decomposition. The origin of the troubles is investigated, and different ways to deal with these issues are proposed. Finally, we show how this problem is manifested and solved in the practical case of the electronic spectra of a metal surface ionized by an ultrashort laser pulse.

Depth-profiling of implanted 28Si by (α,α) and (α,p0) reactions

Silicon nanocrystals enclosed in thin films (Si quantum dots or Si QDs) are regarded to be the cornerstone of future developments in new memory, photovoltaic and optoelectronic products. One way to synthesize these Si QDs is ion implantation in SiO2 layers followed by thermal annealing post-treatment. Depth-profiling of these implanted Si ions can be performed by reactions induced by α-particles on 28Si. Indeed, for high incident energy, nuclear levels of 32S and 31P can be reached, and cross-sections for (α,α) and (α,p0) reactions are more intense. This can help to increase the signal for surface silicon, and therefore make distinguishing more easy between implanted Si and Si coming from the SiO2, even for low fluences. In this work, (α,α) and (α,p0) reactions are applied to study depth distributions of 70 keV 28Si+ ions implanted in 200 nm SiO2 layers with fluences of 1 × 1017 and 2 × 1017 cm-2. Analysis is performed above ER = 3864 keV to take advantage of resonances in both (α,α) and (α,p0) cross-sections. We show how (α,p0) reactions can complement results provided by resonant backscattering measurements in this complex case. © 2010 Elsevier B.V. All rights reserved.

광섬유 방사선량계를 이용한 선량보강 영역에서의 심부선량 백분율과 피부 선량률 측정

본 연구에서는 고 에너지 광자선을 조사 할 때, 선량보강 영영에서의 피부 선량률을 측정을 위해 유기 섬광체와 플라스틱 광섬유를 사용한 광섬유 방사선량계를 제작하였다. 광섬유 방사선량계의 센서부에서 발생된 섬광빛은 30 m 길이의 광섬유를 통해 전달되어 광증배관과 전류계로 측정된다. 광섬유 방사선량계로 측정한 선량보강 영역에서의 피부 선량률은 이온 전리함 및 GAFCHROMIC EBT 필름의 측정 결과와 비교 및 분석 하였다. In this study, we have fabricated a fiber-optic dosimeter using an organic scintillator and a plastic optical fiber. The dosimeter measure skin dose and percentage depth dose in a build-up region for an incident high energy photon beam. The scintillating light generated in the organic sensor probe embedded in a solid water phantom is guided by 30 m plastic optical fiber to a light-measuring device such as a PMT or an electrometer. In addition, using a fiber-optic dosimeter or a GAFCHROMIC EBT film, skin dose and percentage depth dose in the build-up region are measured and compared.

Precision calculation of low-energy electron-impact excitation cross sections of sodium

The collision cross sections of sodium from the ground state to the first four excited states at the incident energy ranging from 0 to 5.4 eV are calculated using the $R$-matrix method. The convergences of the cross sections are checked systematically by using four sets of high-quality target states, i.e., 5, 9, 14, and 19 physical target states. The influence of the Rydberg target states on the collision cross sections is also elucidated at higher incident energies; i.e., the amplitude of resonance structures will decrease with respect to the effective quantum number $\ensuremath{\upsilon}$ of the Rydberg target states. This result is very useful for the calculations of these cross sections at intermediate energy with finite target states by combining the partial-wave-expansion methods valid at low energy with the first Born approximation method valid at high energy, which would be of great importance in obtaining complete cross-section data for related scientific fields.

Double ionization of two-electron systems

We address various issues related to the double ionization by electron impact of two-electron systems. The emphasis will be put on the theoretical description of high incident energy (e,3e) processes, for which the first Born approximation should be suitable. In the case of helium, absolute experimental data for fivefold differential cross sections are available in coplanar geometry. We will review and discuss the divergencies existing between the results obtained with different theoretical models, and those appearing when compared to the experiments in particular with respect to the absolute scale. We will then discuss some results obtained in a recently proposed out of plane geometry.

High-energy neutron scattering from hydrogen using a direct geometry spectrometer

Deep inelastic neutron scattering experiments using indirect time-of-flight spectrometers have reported a smaller cross section for the hydrogen atom than expected from conventional scattering theory. Typically, at large momentum transfers, a deficit of 20-40% in the neutron scattering intensity has been measured and several theories have been developed to explain these results. We present a different approach to this problem by investigating the hydrogen cross section in polyethylene using the direct geometry time-of-flight spectrometer MARI with the incident energy fixed at a series of values ranging from Ei=0.5 eV to 100 eV. These measurements span a much broader range in momentum than previous studies and with varying energy resolutions. We observe no momentum dependence to the cross section with an error of 4% and through a comparison with the scattering from metal foil standards measure the absolute bound cross section of the hydrogen atom to be sigma(H)= 80 +/- 4 barns. These results are in agreement with conventional scattering theory but contrast with theories invoking quantum entanglement and neutron experiments supporting them. Our results also illustrate a unique use of direct geometry chopper instruments at high incident energies and demonstrate their capability for conducting high-energy spectroscopy.

Surface Temperature Dependence of Methane Activation on Ni(111)

Vibrational state resolved measurements of methane’s dissociation on Ni(111) show a strong surface temperature dependence near the translational energy threshold for reaction. The reactivity of molecules excited to v = 1 of the ν3 C−H stretching vibration and incident on the surface with a translational energy of 40 kJ mol−1 increased 8-fold as the surface temperature increased from 90 to 475 K. This enhancement is much larger than that reported for earlier studies at higher incident energies. These results support recent calculations that predict an important role for lattice deformation in transition state access. At higher surface temperatures, surface phonon excitation allows substrate atoms to sample lattice geometries with more favorable transition state energetics. We have also measured the coverage-dependent reactivity of these molecules at both surface temperatures and report a simple model that quantitatively predicts the observed coverage-dependent reactivity.

Scaling laws in (e,3e) processes

We study the double ionization of helium-like ions by impact of electrons with high incident energy. Within the isoelectronic sequence, an approximate scaling law for (e,3e) differential cross sections is proposed and confirmed by calculations. The latter are performed using 14-parameters Hylleraas-like wave functions to represent the bound electrons in the initial channel, plane waves for the fast incoming and scattered electrons, and a continuum distorted wave approach for the two ejected electrons in the final channel.

Photoabsorption of hydrocarbons in Titan's atmosphere

AbstractTitan, the largest satellite of the planet Saturn, has a thick atmosphere which consists of nitrogen (N2) and methane (CH4). In 2004, the Cassini-Huygens mission observed the occultation of two stars through the atmosphere of Titan and measured ultraviolet (UV) absorption spectra. Through these spectra it was possible to identify the molecular species contained in this environment. In the present work, we have simulated a spectrum of this atmosphere using some molecules such as CH4, C2H2, C2H4, C2H6, C4H2, and C6H6. Our cross sections data were experimentally obtained using the electron energy-loss technique, where the electron energy-loss spectra, measured high incident energies and in small scattering angles, are similar to photoabsorption spectra. The comparison of our synthetic spectrum with that measured by Cassini shows that this method is very efficient for identifying molecules as well as estimating abundances.

Double ionization ofH2by fast bare-ion collisions

A time-dependent close-coupling method is developed to treat the double ionization of ${\text{H}}_{2}$ by fast bare-ion collisions. At high incident energies, for which charge transfer to the projectile may be ignored, multipole expansions are made for the electron-electron and electron-projectile interactions in a fixed target nuclei coordinate system. The time-dependent Schr\"odinger equation for the six dimensional target electron wave function is reduced to a set of close-coupled equations on a four dimensional numerical lattice in $({r}_{1},{\ensuremath{\theta}}_{1},{r}_{2},{\ensuremath{\theta}}_{2})$ center-of-mass spherical polar coordinates. Time-dependent close-coupling calculations are carried out for $p+{\text{H}}_{2}$ collisions at an incident energy of 1.0 MeV. The ratio of double to single ionization is found to be 0.3%, which is in reasonable agreement with experimental measurements.

Laser assisted (e, 2e) process of the helium atom

The dynamics of the electron impact multiphoton ionization of a Helium atom in the presence of an intense laser field (n γe, 2e) is studied theoretically. Significant modifications are noted in the laser assisted cross sections with respect to the field free ones. The results are compared with the recent kinematically complete experiments for high incident energies (1 KeV).