Suchergebnisse

We present a manageable approach to include, within the context of optical-data models of the dielectric response function, exchange and correlation (XC) effects in inelastic electron scattering, thus, going beyond the standard random-phase approximation (RPA). The many-body local-field correction in its static limit, G(q) , is employed to incorporate XC effects to all orders in q at both the level of "screening" and the level of "scattering" by computing the so-called test-charge–test-charge (t–t), electron–test-charge (e–t), and electron–electron (e–e) dielectric functions. Some of the most used analytic approximations for G(q) are examined, ranging from the early Hubbard-like expressions to more recent parameterized formulations that satisfy some of the known asymptotic limits. The effect of the different G(q) models upon the inelastic scattering of low-medium energy electrons in condensed matter is examined using solid (amorphous) carbon as an example. It is shown that when XC corrections at all levels are considered, a net reduction of the inelastic scattering cross section by up to 20%–30% from the corresponding RPA value is obtained. Interestingly, a screened Hubbard approximation to G(q) reproduces (to a few %) the results of more accurate representations. Based on the present results, the controversial high-q asymptotic behaviour of G(q) is inconsequential to inelastic electron scattering in the examined energy range. ; The work of D.E. and I.K. has been financially supported by the European Union FP7 (Marie Curie Actions) program "RADDEL" (REA Grant Agreement No. 290023). The work of R.G.M. and I.A. has been supported by the European Regional Development Fund and the Spanish Ministerio de Economía y Competitividad (Project No. FIS2010-17225).

The energy dissipation pattern of low-energy electron beams (0.3–30 keV) in multi-walled carbon nanotube (MWCNT) materials is studied by Monte Carlo simulation taking into account secondary-electron cascade generation. A quasi first-principles discrete-energy-loss model deduced from a dielectric response function description of electronic excitations in MWCNTs is employed whereby both single-particle and plasmon excitations are included in a unified and self-consistent manner. Our simulations provide practical analytical functions for computing depth-dose curves and charged-carrier generation volumes in MWCNT materials under low-energy electron beam irradiation. ; Financial support for D.E., I.K., and K.K. by the European Union FP7 ANTICARB (HEALTH-F2-2008-201587) and for R.G.M. and I.A. by the Spanish Ministerio de Ciencia e Innovación (project FIS2010-17225).

The effect of bulk and surface excitations to inelastic scattering in low-energy electron beam irradiation of multi-walled carbon nanotubes (MWNTs) is studied using the dielectric formalism. Calculations are based on a semiempirical dielectric response function for MWCNTs determined by means of a many-pole plasmon model with parameters adjusted to available experimental spectroscopic data under theoretical sum-rule constrains. Finite-size effects are considered in the context of electron gas theory via a boundary correction term in the plasmon dispersion relations, thus, allowing a more realistic extrapolation of the electronic excitation spectrum over the whole energy-momentum plane. Energy-loss differential and total inelastic scattering cross sections as a function of electron energy and distance from the surface, valid over the energy range ∼50–30,000 eV, are calculated with the individual contribution of bulk and surface excitations separated and analyzed for the case of normally incident and escaping electrons. The sensitivity of the results to the various approximations for the spatial dispersion of the electronic excitations is quantified. Surface excitations are shown to have a strong influence upon the shape and intensity of the energy-loss differential cross section in the near surface region whereas the general notion of a spatially invariant inelastic mean free path inside the material is found to be of good approximation. ; Financial support of D.E., I.K., and K.K. by the European Union FP7 ANTICARB (Grant No. HEALTH-F2-2008-201587) and of I.A. and RGM by the Spanish Ministerio de Ciencia e Innovación (Project FIS2010-17225).

We use a simulation code, based on Molecular Dynamics and Monte Carlo, to investigate the depth-dose profile and lateral radial spreading of swift proton beams in liquid water. The stochastic nature of the projectile-target interaction is accounted for in a detailed manner by including in a consistent way fluctuations in both the energy loss due to inelastic collisions and the angular deflection from multiple elastic scattering. Depth-variation of the projectile charge-state as it slows down into the target, due to electron capture and loss processes, is also considered. By selectively switching on/off these stochastic processes in the simulation, we evaluate the contribution of each one of them to the Bragg curve. Our simulations show that the inclusion of the energy-loss straggling sizeably affects the width of the Bragg peak, whose position is mainly determined by the stopping power. The lateral spread of the beam as a function of the depth in the target is also examined. ; This work has been financially supported by the the Spanish Ministerio de Ciencia e Innovación (Project FIS2010-17225) and the European Union FP7 ANTICARB (HEALTH-F2-2008-201587). PdV thanks the Conselleria d'Educació, Formació i Ocupació de la Generalitat Valenciana for its support under the VALi+d program. We acknowledge support provided by the COST Action MP 1002, Nanoscale Insights into Ion Beam Cancer Therapy.

We have calculated the electronic energy loss of proton and α-particle beams in dry DNA using the dielectric formalism. The electronic response of DNA is described by the MELF-GOS model, in which the outer electron excitations of the target are accounted for by a linear combination of Mermin-type energy-loss functions that accurately matches the available experimental data for DNA obtained from optical measurements, whereas the inner-shell electron excitations are modeled by the generalized oscillator strengths of the constituent atoms. Using this procedure we have calculated the stopping power and the energy-loss straggling of DNA for hydrogen- and helium-ion beams at incident energies ranging from 10 keV/nucleon to 10 MeV/nucleon. The mean excitation energy of dry DNA is found to be I = 81.5 eV. Our present results are compared with available calculations for liquid water showing noticeable differences between these important biological materials. We have also evaluated the electron excitation probability of DNA as a function of the transferred energy by the swift projectile as well as the average energy of the target electronic excitations as a function of the projectile energy. Our results show that projectiles with energy ≤ 100 keV/nucleon (i.e., around the stopping-power maximum) are more suitable for producing low-energy secondary electrons in DNA, which could be very effective for the biological damage of malignant cells. ; This work has been supported financially by the Spanish Ministerio de Ciencia e Innovación (projects FIS2006-13309-C02-01 and FIS2006-13309-C02-02). CDD thanks the Spanish Ministerio de Ciencia e Innovación and Generalitat Valenciana for support under the Ramón y Cajal Program. IK and DE acknowledge financial support from the Research Committee of the University of Ioannina (Grant no. 80037) and the European Union FP7 ANTICARB (HEALTH-F2-2008-201587).