219 research outputs found

    Strong mass effect on ion beam mixing in metal bilayers

    Full text link
    Molecular dynamics simulations have been used to study the mechanism of ion beam mixing in metal bilayers. We are able to explain the ion induced low-temperature phase stability and melting behavior of bilayers using only a simple ballistic picture up to 10 keV ion energies. The atomic mass ratio of the overlayer and the substrate constituents seems to be a key quantity in understanding atomic mixing. The critical bilayer mass ratio of δ<0.33\delta < 0.33 is required for the occurrence of a thermal spike (local melting) with a lifetime of τ>0.3\tau > 0.3 ps at low-energy ion irradiation (1 keV) due to a ballistic mechanism. The existing experimental data follow the same trend as the simulated values.Comment: 4 pages, 4 figures, preprin

    Transformation of five grape rootstocks with plant virus genes and a virE2 gene from Agrobacterium tumefaciens

    Get PDF
    Summary: To facilitate the development of transgenic grapevines that are resistant to grapevine fanleaf virus (GFLV), grapevine leafroll-associated closterovirus (GLRaV-3) and crown gall diseases, we developed a rapid system for regenerating root-stocks: Couderc 3309, Vitis riparia ‘Gloire de Montpellier', Teleki 5C, Millardet et De Grasset 101-14, and 110 Richter via somatic embryogenesis. Embryo culture and grape regeneration were accomplished with four media. Embryogenic calluses from anthers were induced in the initiation medium [MS basic medium containing 20 g sucrose per L, 1.1 mg 2,4-dichlorophenoxyacetic acid (2,4-D) per L, 0.2 mg N6-benzyladenine (BA) per L, and 0.8% Noble agar). The percentage of anthers that developed into embryogenic calli ranged from 2 to 16.3% depending on the rootstock. Calluses with early globular stage embryos were cocultivated with Agrobacterium tumefaciens strain C58Z707 containing the gene constructs of interest. The genes were sense-oriented translatable and antisense coat protein genes from GFLV and GLRaV-3, a truncated HSP90-related gene of GLRaV-3 (43K), and a virE2 del B gene from A. tumefaciens strain C58. Twenty independent transformation experiments were performed on five rootstocks. After 3-4 mo. under kanamycin selection, secondary embryos were recovered on differentiation medium (1/2 MS salts with 10 g sucrose per L, 4.6 g glycerol per L, and 0.8% Noble agar). Embryos that were transformed were regenerated on a medium containing MS salts with 20 g sucrose per L, 4.6 g glycerol per L, 1 g casein hydrolysate per L, and 0.8% Noble agar. Elongated embryos were then transferred to a rooting medium supplemented with 0.1 mg BA per L, 3 g activated charcoal per L, 1.5% sucrose, and 0.65% Bacto agar. A total of 928 independent putative transgenic plants were propagated in the greenhouse. All plants were tested for neomycin phosphotransferase II expression by enzyme-linked immunosorbent assay (ELISA). The presence of transgenes was assessed by polymerase chain reaction and Southern analysis. ELISA revealed various levels of expression of GFLV coat protein in transgenic plants of Couderc 3309. The transgenic rootstocks that have been generated are being screened to determine whether transgenes have conferred resistance to the virus and crown gall disease

    Measurement of polarisation observables in Ks0Σ+K^0_s\Sigma^+ photoproduction off the proton

    Get PDF
    The reaction γ p→KS0 Σ+\gamma \, p \rightarrow K^0_S\,\Sigma^+ is studied in the photon energy range from threshold. Linearly polarised photon beams from coherent bremsstrahlung enabled the first measurement of photon beam asymmetries in this reaction up to Eγ=2250E_\gamma = 2250 MeV. In addition, the recoil hyperon polarisation was determined through the asymmetry in the weak decay Σ+→pπ0\Sigma^+ \rightarrow p \pi^0 up to Eγ=1650E_\gamma = 1650 MeV. The data are compared to partial wave analyses, and the possible impact on the interpretation of a recently observed cusp-like structure near the K∗K^* thresholds is discussed.Comment: 6 pages, 5 figures. References [8,9,10,11] which were not on the original submission are now include

    High statistics study of the reaction γp→p  2π0\gamma p\to p\;2\pi^0

    Get PDF
    The photoproduction of 2π0\pi^0 mesons off protons was studied with the Crystal Barrel/TAPS experiment at the electron accelerator ELSA in Bonn. The energy of photons produced in a radiator was tagged in the energy range from 600\,MeV to 2.5\,GeV. Differential and total cross sections and pπ0π0p\pi^0\pi^0 Dalitz plots are presented. Part of the data was taken with a diamond radiator producing linearly polarized photons, and beam asymmetries were derived. Properties of nucleon and Δ\Delta resonances contributing to the pπ0π0p\pi^0\pi^0 final state were determined within the BnGa partial wave analysis. The data presented here allow us to determine branching ratios of nucleon and Δ\Delta resonances for their decays into pπ0π0p\pi^0\pi^0 via several intermediate states. Most prominent are decays proceeding via Δ(1232)π\Delta(1232)\pi, N(1440)1/2+πN(1440)1/2^+\pi, N(1520)3/2−πN(1520)3/2^-\pi, N(1680)5/2+πN(1680)5/2^+\pi, but also pf0(500)pf_0(500), pf0(980)pf_0(980), and pf2(1270)pf_2(1270) contribute to the reaction.Comment: 28 pages, 17 figures, 7 table

    In-medium ω\omega mass from the γ+Nb→π0γ+X\gamma + Nb \to \pi^{0}\gamma + X reaction

    Full text link
    Data on the photoproduction of ω\omega mesons on nuclei have been re-analyzed in a search for in-medium modifications. The data were taken with the Crystal Barrel(CB)/TAPS detector system at the ELSA accelerator facility in Bonn. First results from the analysis of the data set were published by D. Trnka et al. in Phys. Rev. Lett 94 (2005) 192303 \cite{david}, claiming a lowering of the ω\omega mass in the nuclear medium by 14% at normal nuclear matter density. The extracted ω\omega line shape was found to be sensitive to the background subtraction. For this reason a re-analysis of the same data set has been initiated and a new method has been developed to reduce the background and to determine the shape and absolute magnitude of the background directly from the data. Details of the re-analysis and of the background determination are described. The ω\omega signal on the NbNb target, extracted in the re-analysis, does not show a deviation from the corresponding line shape on a LH2LH_2 target, measured as reference. The earlier claim of an in-medium mass shift is thus not confirmed. The sensitivity of the ω\omega line shape to different in-medium modification scenarios is discussed.Comment: 13 pages and 11 figures, submitted for publicatio

    Modification of the ω\omega-Meson Lifetime in Nuclear Matter

    Full text link
    The photo production of ω\omega mesons on the nuclei C, Ca, Nb and Pb has been measured using the Crystal Barrel/TAPS detector at the ELSA tagged photon facility in Bonn. The dependence of the ω\omega meson cross section on the nuclear mass number has been compared with three different types of models, a Glauber analysis, a BUU analysis of the Giessen theory group and a calculation by the Valencia theory group. In all three cases, the inelastic ω\omega width is found to be 130−150MeV/c2130-150 \rm{MeV/c^2} at normal nuclear matter density for an average 3-momentum of 1.1 GeV/c. In the restframe of the ω\omega meson, this inelastic ω\omega width corresponds to a reduction of the ω\omega lifetime by a factor ≈30\approx 30. For the first time, the momentum dependent ω\omegaN cross section has been extracted from the experiment and is in the range of 70 mb.Comment: 5 pages, 4 figure

    Quasi-free photoproduction of eta-mesons of the neutron

    Full text link
    Quasi-free photoproduction of eta-mesons off nucleons bound in the deuteron has been measured with the CBELSA/TAPS detector for incident photon energies up to 2.5 GeV at the Bonn ELSA accelerator. The eta-mesons have been detected in coincidence with recoil protons and recoil neutrons, which allows a detailed comparison of the quasi-free n(gamma,eta)n and p(gamma,eta)p reactions. The excitation function for eta-production off the neutron shows a pronounced bump-like structure at W=1.68 GeV (E_g ~ 1 GeV), which is absent for the proton.Comment: accepted for publication in Phys. Rev. Let

    Measurement of the Beam Asymmetry Σ\Sigma in the Forward Direction for pi0 Photoproduction

    Get PDF
    Photoproduction of neutral pions has been studied with the CBELSA/TAPS detector for photon energies between 0.92 and 1.68~GeV at the electron accelerator ELSA. The beam asymmetry~Σ\Sigma has been extracted for 115∘<θc.m.<155∘115^\circ < \theta_{\rm c.m.} < 155^\circ of the π0\pi^0~meson and for θc.m.<60∘\theta_{\rm c.m.} < 60^\circ. The new beam asymmetry data improve the world database for photon energies above 1.5~GeV and, by covering the very forward region, extend previously published data for the same reaction by our collaboration. The angular dependence of Σ\Sigma shows overall good agreement with the SAID parameterization.Comment: 11 pages, 10 figures, published in Phys. Rev. C, included LEPS data and MAID 2007 predictions for comparison with our dat

    K^0 pi^0 Sigma^+ and K^*0 Sigma^+ photoproduction off the proton

    Full text link
    The exclusive reactions γp→K∗0Σ+(1189)\gamma p \to K^{*0} \Sigma^+(1189) and γp→K0π0Σ+(1189)\gamma p \to K^{0} \pi^{0}\Sigma^+(1189), leading to the p 4π0\pi^{0} final state, have been measured with a tagged photon beam for incident energies from threshold up to 2.5 GeV. The experiment has been performed at the tagged photon facility of the ELSA accelerator (Bonn). The Crystal Barrel and TAPS detectors were combined to a photon detector system of almost 4π\pi geometrical acceptance. Differential and total cross sections are reported. At energies close to the threshold, a flat angular distribution has been observed for the reaction γp→K0π0Σ+\gamma p\to K^{0} \pi^{0}\Sigma^+ suggesting dominant s-channel production. Σ∗(1385)\Sigma^*(1385) and higher lying hyperon states have been observed. An enhancement in the forward direction in the angular distributions of the reaction γp→K∗0Σ+\gamma p \to K^{*0}\Sigma^+ indicates a tt-channel exchange contribution to the reaction mechanism. The experimental data are in reasonable agreement with recent theoretical predictions.Comment: 11 pages, 13 figures, submitted to EPJ

    Photoproduction of {\omega} Mesons off the Proton

    Get PDF
    The differential cross sections and unpolarized spin-density matrix elements for the reaction γp→pω\gamma p\to p\omega were measured using the CBELSA/TAPS experiment for initial photon energies ranging from the reaction threshold to 2.5 GeV. These observables were measured from the radiative decay of the ω\omega meson, ω→π0γ\omega\to\pi^0\gamma. The cross sections cover the full angular range and show the full extent of the tt-channel forward rise. The overall shape of the angular distributions in the differential cross sections and unpolarized spin-density matrix elements are in fair agreement with previous data. In addition, for the first time, a beam of linearly-polarized tagged photons in the energy range from 1150 MeV to 1650 MeV was used to extract polarized spin-density matrix elements. These data were included in the Bonn-Gatchina partial wave analysis (PWA). The dominant contribution to ω\omega photoproduction near threshold was found to be the 3/2+3/2^+ partial wave, which is primarily due to the sub-threshold N(1720) 3/2+N(1720)\,3/2^+ resonance. At higher energies, pomeron-exchange was found to dominate whereas π\pi-exchange remained small. These tt-channel contributions as well as further contributions from nucleon resonances were necessary to describe the entire dataset: the 1/2−1/2^-, 3/2−3/2^-, and 5/2+5/2^+ partial waves were also found to contribute significantly.Comment: 8 pages, 5 figures, accepted for publication in Physics Letters
    • …
    corecore