J. García López

Boron in Very Metal-poor Stars*

We have observed the B I λ2497 line to derive the boron abundances of two very metal-poor stars selected to help in tracing the origin and evolution of this element in the early Galaxy: BD +23°3130 and HD 84937. The observations were conducted using the Goddard High Resolution Spectrograph on board the Hubble Space Telescope. A very detailed abundance analysis via spectral synthesis has been carried out for these two stars, as well as for two other metal-poor objects with published spectra, using both Kurucz and OSMARCS model photospheres and taking into account consistently the non-LTE (NLTE) effects on the line formation. We have also reassessed all published boron abundances of old disk and halo unevolved stars. Our analysis shows that the combination of high effective temperature (Teff 6000 K, for which boron is mainly ionized) and low metallicity ([Fe/H] -1) makes it difficult to obtain accurate estimates of boron abundances from the B I λ2497 line. This is the case of HD 84937 and three other published objects (including two stars with [Fe/H] ~ -3), for which only upper limits can be established. BD +23°3130, with [Fe/H] ~ -2.9 and log N(B)NLTE = 0.05 ± 0.30, appears then as the most metal-poor star for which a firm measurement of the boron abundance presently exists. The evolution of the boron abundance with metallicity that emerges from the seven remaining stars with Teff < 6000 K and [Fe/H] < -1, for which beryllium abundances were derived using the same stellar parameters, shows a linear increase with a slope of ~1. Furthermore, the B/Be ratio found is constant at a value of ~20 for stars in the range -3 < [Fe/H] < -1. These results point to spallation reactions of ambient protons and α-particles with energetic particles enriched in CNO as the origin of boron and beryllium in halo stars.

CNA: The first accelerator-based IBA facility in Spain

Abstract The recently created Centro Nacional de Aceleradores (National Center for Accelerators, CNA) in Seville emerges as the first ion beam analysis facility in Spain. The laboratory is based on a 3 MV tandem accelerator model 9SDH-2 of NEC and it is primarily focused on material research and modification by means of IBA techniques: PIXE, RBS, NRA, PIGME and ERDA. The ions are delivered by two ion sources: Alphatross radio-frequency source and SNICS-II sputtering source. The ion beam handling system includes equipment for beam focusing, steering and diagnosis, a 90° analyzing magnet and a seven-port switcher magnet. A system based on magnetic steerers has been installed for high-precision beam energy scanning. In this paper the main elements of the laboratory will be described, focusing on the electronic equipment, detectors and the four beamlines planned for the moment: channeling line, microbeam line, multipurpose vacuum chamber and external microbeam. Moreover, the characteristics of the system as observed until now by means of performance tests, beam energy spread measurements and energy calibration experiments will be summarized.

External microbeam set-up at the CNA (Sevilla) and its application to the study of Tartesic jewellery

Abstract A new external microbeam system has been installed recently at the +45° beam line of the 3 MV Pelletron accelerator at the Centro Nacional de Aceleradores (Sevilla, Spain). The facility, based on the system from Oxford Microbeams (OM), includes two sets of slits, a doublet of magnetic quadrupole focusing lenses, a Faraday cup and an accurate positioning device. In addition, a beam profile monitor (BPM) and a quartz viewer have been installed in the line to facilitate the microbeam production. The first PIXE application of the new facility was made in the field of Archaeometry, on the study of Tartesic gold artefacts (700–500 BC, SW of Spain) from Ebora and other archaeological sites. The aim of the analysis was to perform an exhaustive and systematic study about the soldering procedures that were employed by the goldsmiths along the Valley of Guadalquivir River after the Phoenician colonisation.

Structural properties of LiNbO3 thin films grown by the pulsed laser deposition technique

Abstract LiNbO 3 thin films have been deposited onto R -cut and C -cut single crystalline sapphire substrates ( α -Al 2 O 3 , (1102) and (0001)) by the pulsed laser deposition technique at different oxygen pressures and substrate temperatures. Thin film composition and structure have been determined using Rutherford backscattering spectroscopy (RBS) and X-ray diffraction experiments. The atomic composition is dependent on the oxygen pressure in the range 0.5–1.3 mbar. At pressures lower than 1.3 mbar, we have observed a deviation from the stoichiometry. Nearly stoichiometric thin films have been obtained for a pressure equal to 1.3 mbar. For that pressure, the atomic composition does not depend on the substrate temperature in the range 650–800°C. Under optimised conditions the 〈1102〉 and 〈0001〉 preferential orientations of growth have been obtained on (1102) and (0001) sapphire substrates, respectively.

Coverage induced regulation of Au nanoparticles during pulsed laser deposition

The effects induced during the covering/embedding of metal nanoparticles (NPs) produced by pulsed laser deposition (PLD) and their impact on the structural and optical properties have been studied by producing pairs of samples containing Au NPs that are either uncovered (i.e., at the surface) or covered (i.e., embedded in an amorphous a-Al2O3 host). The main result is that covering species can sputter up to 100% of the Au atoms, the smaller the NPs the higher the sputtered fraction. This fraction has been simulated using standard models for ion bombardment and taking into account the kinetic energy distribution of arriving species and the cohesive energy dependence on NPs dimensions. Although all models well predict the order of magnitude of the sputtering yield, the calculated values are generally smaller than the experimental ones and do not account for the experimental dependence on NPs dimensions. This disagreement is discussed in terms of the limitations of standard models that do not take into accou...

Model Photospheres for Late-Type Stars from the Inversion of High-Resolution Spectroscopic Observations: The Sun

An inversion technique has been developed to recover LTE, one-dimensional, model photospheres for late-type stars from very high resolution, high signal-to-noise ratio stellar line profiles. It is successfully applied to the Sun by using a set of clean Ti I, Ca I, Cr I, and Fe I normalized line profiles with accurate transition probabilities, taking advantage of the well-understood collisional enhancement of the wings of the Ca I line at 6162 A. Line and continuum center-to-limb variations, continuum flux, and wings of strong metal lines are synthesized by means of the model obtained and are compared with solar observations, as well as with predictions from other well-known theoretical and empirical solar models, showing the reliability of the inversion procedure. The prospects for and limitations of the application of this method to other late-type stars are discussed.

Tuning the Er3+ sensitization by Si nanoparticles in nanostructured as-grown Al2O3 films

Nanostructured films consisting of single Si nanoparticles (NPs) and Er3+ ions layers separated by nanometer-scale Al2O3 layers of controlled thickness have been prepared in order to tune the energy transfer between Si NPs and Er3+ ions. The amorphous Si NPs with an effective diameter of ∼4.5 nm are formed during growth and are able to sensitize the Er3+ ions efficiently with no postannealing treatments. The characteristic distance for energy transfer from Si NPs to Er3+ ions in Al2O3 is found to be in the 1 nm range. It is shown that in the nanostructured films, it is possible to achieve an optimized configuration in which almost all the Er3+ ions have the potential to be excited by the Si NPs. This result stresses the importance of controlling the dopant distribution at the nanoscale to achieve improved device performance.

The response of a chromium doped alumina screen to keV and MeV ions

We have characterised the response of a chromium-doped alumina screen ‘Chromox’ to light ions (H+ and He+) accelerated to keV and MeV energies. In particular, we have determined the absolute luminosity in terms of the number of photons emitted per incident ion from the front and back faces of such a screen. This work has been motivated by the application of this material to a diagnostic for measuring fast ion losses, close to the plasma edge, from the hot plasmas in fusion devices, where its radiation hardness, compared to that of standard phosphors, makes it very attractive. We also discuss the persistent afterglow observed after removal of the ion beam in terms of its cause and possible repercussions for this diagnostic.

The direct growth of SrTiO3 (100) layers on silicon (100) substrates; application as a buffer layer for the growth of DyBa2Cu3O7−δ thin films

Highly (100)-oriented SrTiO3 thin films were sputtered on Si (100) substrates. After the optimization of the deposition conditions, the fraction of (110)-oriented material in the SrTiO3 films was about 3%, but the rocking curve of the SrTiO3 (200) peak was quite large (full width at half maximum (FWHM) 2.3°). Indeed grain boundaries crossing the whole SrTiO3 film and an amorphous layer between Si and SrTiO3 could be observed by transmission electron microscopy. DyBa2Cu3O7−δ (DBCO) films were successively sputtered on the polycrystalline SrTiO3 layers on Si. An Auger analysis revealed the presence of barium at the Si/SrTiO3 interface that presumably diffused along the grain boundaries through SrTiO3 to form barium silicates. By reducing both the DBCO deposition temperature and the SrTiO3 thickness, we could prevent this diffusion and improve the electrical properties (R300/R1001 and Tc (R=0) in the 40–60 K range). The specific problems for the growth of DBCO thin film on SrTiO3-buffered silicon substrates by sputter deposition are finally pointed out.

Effect of the substrate heterostructure on the texture of lanthanum modified lead titanate thin films

Abstract Modified lead titanate thin films with a nominal composition of Pb 0.88 La 0.08 TiO 3 (PTL) have been prepared by chemical solution deposition (CSD) onto annealed Ti/Pt/Ti/(100)Si and Pt/TiO 2 /(100)Si. Composition and heterostructure of the PTL films on the two substrates have been studied by Rutherford backscattering spectroscopy (RBS). Composition of the PTL films on the two substrates is similar and close to the nominal one. However, the Ti/Pt/Ti/(100)Si and Pt/TiO 2 /(100)Si substrates induces different textures in the PTL films. The films on Ti/Pt/Ti/(100)Si developed a preferred orientation, whereas the films on Pt/TiO 2 /(100)Si have a mixed preferred orientation.