Irradiation of carbon nanotubes with carbon projectiles: A molecular dynamics study

The irradiation of carbon based nanostructures with ions and electrons has been shown to be an appropriate tool to tailor their properties. The defects induced in the nanostructures during irradiation are able to modify their mechanical and electronic properties. Here we simulate the irradiation of carbon nanotubes with carbon ions using a molecular dynamics code. We use the Tersoff potential joined smoothly to the Universal Ziegler-Biersack-Littmark potential at short distances. We study the number of defects produced after irradiation with a single carbon ion finding a saturation with its energy at ∼ 3 keV. We observe, after continuum irradiation with low energy ions, the formation of bumps in the irradiated region. For larger energy ions we find that the diameter of the nanotube shrinks as shown in previous works.This work has been financially supported by Project FIS2010-17225 by the Spanish Ministerio de Ciencia e Innovación

Simulation of swift boron clusters traversing amorphous carbon foils

We use a simulation code to study the interaction of swift boron clusters (Bn+, n=2–6, 14) with amorphous carbon foils. We analyze different aspects of this interaction, such as the evolution of the cluster structure inside the target, the energy and angle distributions at the detector or the stopping power ratio. Our simulation code follows in detail the motion of the cluster fragments through the target and in the vacuum until reaching a detector, taking into account the following interactions: (i) wake force, (ii) Coulomb repulsion among cluster fragments, (iii) stopping force, and (iv) elastic scattering with the target nuclei. Electron capture and loss by each fragment is also included in the code, affecting the above-mentioned interactions. The clusters size grows inside the foil due mainly to the Coulomb explosion but this increase is less pronounced in the plane transversal to the beam direction because of the alignment effect of the wake forces. We obtain an enhancement of the stopping power ratio that increases with the projectile energy and with the number of molecular constituents. Our results agree very well with the available experimental data for the thicker foils (≳10 μg∕cm2) and are compatible (within the experimental error bars) for the thinner foils.This work has been financially supported by the Spanish Ministerio de Educación y Ciencia (Contract Nos. BFM2003-04457-C02-01 and BFM2003-04457-C02-02). S.H.A. thanks the Fundación CajaMurcia for financial support and C.D.D. thanks the Spanish Ministerio de Educación y Ciencia for support under the Ramón y Cajal Program

Collision cascade effects near an edge dislocation dipole in alpha-Fe: Induced dislocation mobility and enhanced defect clustering

Collision cascades near a 1/2⟨111⟩{110} edge dipole in alpha-iron have been studied using molecular dynamics simulations for a recoil energy of 20 keV and two temperatures, 20 K and 300 K. These simulations show that the collision cascade induces the migration of the dislocations through glide along its slip plane. The motion of the dislocations starts at the peak of the collision cascade and expands a time scale much longer than the cascade duration, until restoring the equilibrium distance of the dipole, regardless of the damage produced by the cascade. At the initial stages, kinks are formed at the dislocation that enhance glide. When defects reach the dislocations, jogs are produced. We show that the initial dislocation motion is triggered by the shock wave of the collision cascade. The cascade morphology is also strongly influenced by the presence of the dislocations, having an elongated form at the peak of the displacement, which demonstrates the strong interaction of the dislocations with the cascade even at the early stages. Finally, we show that larger vacancy clusters are formed in the presence of dislocations compared to isolated cascades and that these clusters are larger for 300 K compared to 20 K.This work was partly supported by the Generalitat Valenciana through PROMETEO2017/139. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 and 2019–2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. EM gratefully acknowledges support from the U.S. DOE, Office of Science, Office of Fusion Energy Sciences, and Office of Advanced Scientific Computing Research through the Scientific Discovery through Advanced Computing (SciDAC) project on Plasma-Surface Interactions (award no. DE-SC0008875)

Trabajo en grupo en la docencia universitaria de titulaciones científico-técnicas

Se propone una metodología para desarrollar y evaluar el trabajo en grupo en la docencia universitaria de titulaciones científico-técnicas mediante la realización de un informe de una práctica de laboratorio con un formato similar al de un artículo científico-técnico. Cada grupo entrega a mitad del curso las actas de las reuniones celebradas, una valoración del funcionamiento del grupo con propuestas de mejora, las valoraciones individuales de cada estudiante y un borrador del informe, que es corregido in situ por el docente. Los estudiantes deben trasladar estas correcciones al informe final, que se entrega al final del curso, junto con las actas de las nuevas reuniones celebradas, una valoración final del funcionamiento del grupo y las valoraciones finales individuales de cada estudiante. La evaluación del trabajo en grupo se efectúa en base a la documentación entregada. Esta actividad se ha llevado a cabo con éxito en la asignatura Acústica del Grado en Ingeniería en Sonido e Imagen en Telecomunicación y en la asignatura Fundamentos Físicos de la Ingeniería II del Grado en Ingeniería Química

Calculated electronic energy loss of swift proton and helium ion beams in liquid water

Paper submitted to the 9th International Conference on Applications of Nuclear Techniques, Crete, Greece, 8–14 June 2008.The electronic energy loss of swift proton and helium beams in liquid water is theoretically evaluated. Our model is based in the dielectric formalism, taking into account the charge exchange of the projectile during its travel through the target. The electronic properties of liquid water are described by the MELF-GOS model, where the outer electron excitations are represented by a sum of Mermin functions fitted to the experimental data in the optical limit, whereas the inner-shell electron excitations are modelled by the corresponding atomic generalized oscillator strength. The inverse mean free path, the stopping power and the energy loss straggling are calculated, showing a reasonably good agreement with the available experimental data.This work has been financially supported by the Spanish Ministerio de Educación y Ciencia (Contract Nos. FIS2006-13309-C02-01 and FIS2006-13309-C02-02). C.D.D. thanks the Spanish Ministerio de Educación y Ciencia for support under the Ramón y Cajal Program

Energy loss of hydrogen- and helium-ion beams in DNA: calculations based on a realistic energy-loss function of the target

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)

Energy loss of swift H2+ and H3+ molecules in gold: vicinage effects

We present experimental and theoretical results of vicinage effects due to the interaction of H2+ and H3+ molecules with thin gold foils. High-energy-resolution backscattering experiments were carried out at energies ranging from 80 up to 200 keV per nucleon for the H2+ molecules and up to 140 keV per nucleon for the H3+ molecules. The results show small enhancements (about 5% and 15% for H2+ and H3+ molecules, respectively) of the stopping ratios. The values obtained by the simulation code seics indicate small vicinage effects as well and are in satisfactory agreement with the experimental data. Moreover, the same simulations carried out for carbon foils yield relatively higher stopping ratios. The differences between the vicinage effects obtained for C and Au are interpreted in terms of different excitation spectra of each material. Finally, our results obtained for Au are in clear disagreement with those reported in the seminal work of Brandt, Ratkowski, and Ritchie [Phys. Rev. Lett. 33 1325 (1974)].The authors are indebted to the Brazilian agency Conselho Nacional de Desenvolvimento Científico e Tecnológico, Projetos de Núcleos de Excelência, and to the Spanish Ministerio de Ciencia e Innovación (project no. FIS2010-17225) for the support of this work

Aggregation effects in the energy loss of swift H and He ions in SiC and TiC

Paper submitted to the 9th International Conference on Applications of Nuclear Techniques, Crete, Greece, 8–14 June 2008.We use the dielectric formalism to evaluate the electronic energy loss of swift H and He ions in SiC and TiC targets, as a function of the projectile energy. The electronic properties of these materials are described by the MELF-GOS model, where the excitation of outer electrons is characterized by a linear combination of Mermin-type energy-loss functions (ELF), whereas the contribution due to the target inner-shell ionizations is included through hydrogenic generalized oscillator strengths (GOS). In this scheme, the differences between the energy loss of the projectile in the compound clearly differ from that calculated from its elemental component when applying Bragg’s additivity rule, which assumes that the energy loss in each species is unaffected by the state of aggregation. Our calculations show that the results provided by Bragg’s additivity rule fails near and below the stopping power maximum due to aggregation effects, which affect mainly the outer electrons excitation spectrum. We obtain that aggregation effects in the energy loss are more important for H projectiles in TiC targets. A good agreement with available experimental data is obtained.This work has been financially supported by the Spanish Ministerio de Educación y Ciencia (Projects Nos. FIS2006-13309-C02-01 and FIS2006-13309-C02-02). CDD thanks the Spanish Ministerio de Educación y Ciencia for support under the Ramón y Cajal program

Simulated carbon irradiation of carbon nanotubes – A comparative study of interatomic potentials

We simulate the irradiation of carbon nanotubes (CNT) with carbon ions using a molecular dynamics code. In order to describe the interaction between carbon ions we use the Tersoff or Brenner potential, both joined smoothly to the Universal ZBL potential at short distances. We have analyzed the defects produced after irradiation, the subsequent modification of the CNT structure, and their dependence on the used interatomic potential, the projectile energy (from 10 eV to 5 keV) and the dose. For single projectile irradiation, we have obtained that the coordination defect number increases with the projectile energy, although a saturation value is achieved at high projectile energies (∼3 keV). For continuous projectile irradiation, we have observed that for low energies (∼10 eV) the accumulation of adatoms produces a bump in the irradiated region. However, at intermediate energies (∼100 eV) the irradiation produces vacancies which are healed through non-hexagonal rings. This gives rise to a shrinking of the CNT diameter in the irradiated region. Finally, if the projectile energy is high enough (∼1 keV) the continuous irradiation produces the breaking of the CNT.This work has been financially supported by the Spanish Ministerio de Economía y Competitividad and the European Regional Development Fund (Project FIS2010-17225)