Publications

An option of increasing the luminosity of the Large Hadron Collider (LHC) at CERN to 1035 cm-2 s-1 has been envisaged to extend the physics reach of the machine. An efficient tracking down to a few centimetres from the interaction point will be required to exploit the physics potential of the upgraded LHC. As a consequence, the semiconductor detectors close to the interaction region will receive severe doses of fast hadron irradiation and the inner tracker detectors will need to survive fast hadron fluences of up to above 1016cm-2. The CERN-RD50 project "Development of Radiation Hard Semiconductor Devices for Very High Luminosity Colliders" has been established in 2002 to explore detector materials and technologies that will allow to operate devices up to, or beyond, this limit. The strategies followed by RD50 to enhance the radiation tolerance include the development of new or defect engineered detector materials (SiC, GaN, Czochralski and epitaxial silicon, oxygen enriched Float Zone silicon), the improvement of present detector designs and the understanding of the microscopic defects causing the degradation of the irradiated detectors. The latest advancements within the RD50 collaboration on radiation hard semiconductor detectors will be reviewed and discussed in this work. 2005 Published by Elsevier B.V.

Three compositions of metallic glasses, Fe58Co24Nb3Ta1Mo1B13, Fe61.5Co20.5Nb3Ta1Mo1B13 and Fe66Co18Si1B15 were prepared by rapid quenching from the melt and subsequently annealed for 1 hour at 450, 650 and 750 degrees C. The samples were analyzed by X-ray diffraction (XRD) and Mossbauer spectroscopy. All Mossbauer spectra were fitted with a six-line pattern corresponding to the crystalline alpha-(FeCo) phase and a hyperfine magnetic field distribution representing the amorphous component. The Moss-bauer spectra of annealed Fe66CO18B15 revealed the presence of a secondary crystalline phase, namely (FeCO)(3)(BSi). Moreover, the last Mossbauer spectrum in the set was fitted with an additional sextet, corresponding to the appearance of hematite in the system. It is inferred that the addition of Nb, Ta and Mo considerably delays the onset of bulk crystallization and iron oxidation in these systems. The XRD patterns are in qualitative agreement with the Mossbauer results and are consistent with a surface layer of hematite nanoparticles in the system. The activation energy of 4.34 eV for oxidation was estimated from the Mossbauer results. (c) 2005 Published by Elsevier B.V..

NaCl and KCI single crystals were irradiated with swift heavy ions (An, Ph, Bi and U) of energy between 560 and 2640 MeV. The damage induced by the ions was studied using optical absorption spectroscopy, chemical etching and small-angle X-ray scattering. The damage in the tracks is dominated by the creation of simple color centers (F and V-3 centers). Their concentration increases as a function of fluence, saturating when single tracks overlap. The F-center containingly track zone extends up to several tens of nanometers. In addition to this large track halo, there exists also a much smaller region (1-2 nm) close to the ion trajectory in which defect aggregates are formed. Indications for this track core are given by the preferential chemical etchability of ion tracks and also by the anisotropic pattern produced by small-angle X-ray scattering. This halo-core track structure is very similar as in LiF, however given by the higher mobility of the primary hole centers in NaCl and KCl, recombination with electrons is facilitated resulting in a lower efficiency for defect creation. (c) 2005 Elsevier B.V. All rights reserved.

We study the dynamics of a one-dimensional Bloch electron subjected to a constant electric field. The periodic potential is supposed to be less singular than the delta-like potential (Dirac comb). We give a rigorous proof of Ao's result that for a large class of initial conditions (high momentum regime) there is no localization in momentum space. The proof is based on the mathematical substantiation of the two simplifying assumptions made in physical literature: the transitions between far away bands can be neglected and the transitions at the quasicrossing can be described by Landau-Zener-type formulas. Using the connection between the above model and the driven quantum ring (DQR) shown by Avron and Nemirovski, our results imply the increase of energy for weakly singular such DQR and appropiate initial conditions. (C) 2005 American Institute of Physics.

Scanning Hall probe microscopy has been used to make a microscopic study of flux structures and dynamics in yttrium barium copper oxide thin-film disks containing a regular 10-mu m-period square array of 2.5-mu m-sized holes (antidots). Images obtained after field cooling the sample to 77 K in very low fields reveal that the holes can trap two flux quanta at this temperature. Scans obtained after zero-field cooling (ZFC) to 77 K and a subsequent applied field cycle clearly display preferential flux channeling along chains of antidots in the direction of maximum induction gradient. Remarkably, upon reversal of field sweep direction, we observe flux "streaming" out of the holes towards the sample edges with almost uniform density flux "stripes" bridging the holes in the exit direction. We estimate that the antidots can preferentially trap about 15 flux quanta in these ZFC experiments. Classical electrodynamics simulations of our samples appear to be in good qualitative agreement with our results, indicating that many of the observed phenomena may be geometrical effects that depend primarily on the shape and topology of the sample, and potential applications are discussed.

Single layer TiAlN and alternate TiN/TiAlN multilayered hard coatings were deposited on Si, plain carbon steel and high-speed steel substrates by the cathodic are technique. Chemical composition, phase composition, texture, Vickers microhardness, adhesion, bilayer period, sliding friction coefficient and wear resistance were investigated by XPS, EDX and XRD analyses, microhardness measurements, scratch and tribological tests.

Electronic spin-lattice relaxation measurements performed in X and Q bands on electrons irradiated polyethylene, polycarbonate and polymethylmethacrylate have shown that at liquid helium temperatures and higher, up to 12K, the spin-lattice relaxation time of radiation induced free radicals does not depend on the magnetic. field but varies inverse proportionally with the square of absolute temperature and free radicals concentration. These peculiarities have been well explained by admitting that below 77 K there is a considerable amount of stable free radicals coupled in pairs that mediate the spin-lattice relaxation process of single radicals. Consequently, we have proposed an analytical equation that, within this hypothesis, quantitatively describes all these particularities.

Experimental results concerning the pulsed laser deposition parameters of in-situ epitaxially grown high crystalline quality YBa2Cu3O7-x thin films are reported. The structural and morphological properties of the deposited films analyzed by transmission electron microscopy, selected area electron diffraction, X-ray diffraction, and Raman spectroscopy are also presented. The YBa2Cu3O7-x thin films, predominantly c-axis oriented had required the smallest oxygen pressure during the cool-down time reported to date.

30-mu m thick polycarbonate foils were irradiated with single swift heavy ions (,e.g. Au 11.4 MeV/nucleon). After etching with solutions containing NaOH and methanol, templates containing single conical pores were obtained. The size and the shape of the nanopores depend on etching conditions such as time of etching, concentration and temperature of the etching solution. By electrochemical deposition of nickel, nanowires were grown in the single pores. The nanowires were contacted by sputtering a gold layer on top of the membrane. The magnetic measurements indicated that the nanowires possess around 1% anisotropic magnetoresistance. The current-voltage characteristic of a Ni nanowire shows a linear behavior for current densities smaller than 10(8) A/cm(2). The maximum current density that a single Ni wire can withstand was found to be 3x 10(8) A/cm(2).

Microspheres and planoconvex lenses (50 divided by 400 mu m in diameter) based on glassy As2S3, an important infrared material, have been produced by a special flame melting technique. Glassy fibres of 45 mu m in diameter, with the length of 10-20 cm have been drawn from melt. The morphological properties of these optical elements have been investigated.