ALBERT

Notes: The microstructure and composition of the SiO2/TiO2 analog (codeposited) and digital (thin layer pairs) gradient-index films were examined by transmission electron microscopy, Auger electron spectroscopy, Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, scatterometry, and in situ ellipsometry. Both analog and digital structures were amorphous as-grown. The SiO2 was incompletely oxidized at some of the interfaces in the layered structures and in the codeposited films, indicated competition from the TiO2 for available oxygen during growth. Digital structures with thin (65 A(ring)) layers remained well defined after annealing at 900 °C, but their order was completely destroyed by 1100 °C. Structures with thick (500 A(ring)) layers remained intact up to 1100 °C, with the TiO2 crystallizing throughout the layer width. Both anatase and rutile TiO2 crystallites were present in the layered and codeposited films after a high-temperature anneal (T(approximately-greater-than)650 °C), while only the anatase phase was observed for 300 °C〈T〈650 °C. The optical scatter of the digital films increased with increasing annealing temperature and layer thickness. TiO2 precipitated out of the codeposited films after a high-temperature anneal; however, the scatter of these films at 0.633 μm was several orders of magnitude lower than that of the annealed layered films. Codeposited films consisting of greater than ∼60% TiO2 displayed form birefringence in the as-grown state.

Notes: The construction of the wiggler W2 at DORIS is described together with the major components of the beamline. Details are given on the assembly and performance of the magnet structure. Three different monochromators are used in the beamline alternatively. One of them, a Laue–Bragg-type monochromator is described in more detail.

Notes: During the last years attempts have been underway at several synchrotron radiation laboratories1–4 to employ the abrupt change of absorption at the iodine K edge (33.17 keV) to the imaging of coronary arteries. The subtraction of two images taken with photon energies just above and below the edge strongly surpresses background contrast like bone and soft tissue and allows for amplification of iodinated structure contrast. At HASYLAB, for these studies, a beamline at a bending magnet was only available until 1987. At this beamline the system NIKOS I was tested to obtain information on its essential components like monochromator and detector by in-vivo investigations of dogs.The basic feature which differs from the approaches in other laboratoriesis simultaneous registration of two stationary beams in a two-line detector without any beam switching. Based on the experience with this system during the last year all its components have been improved and changed to dimensions capable of taking a 12-cm-wide image of the human heart. At present the new system NIKOS II is under testing at the new wiggler beamline HARWI. The new monochromator comprises two pairs of a thin Si Laue case crystal and a Ge Bragg crystal, each, with inherent focussing of the beam to a horizontal line focus of about 0.5 mm height and 12 cm width. The advantage of the Laue crystals which are optimized in thickness (30 μm) for maximum reflectivity, is the low heat absorption and the full use of the white beam in the production of two monochromatic beams. The crystals are cooled in a stream of helium. Compared to the NIKOS I version5 the detector was changed in the following points: The two detection lines are enlarged to 125 mm width and the phosphor at the entrance is exchangeable to be either made of powderlike Gd2O2S:Tb or of single crystals like CdWO4 or CaF2:Eu separated by thin walls for optimum spatial resolution. The numerical aperture of the glass fiber optics guiding the light via image intensifiers to the photodiode arrays is higher. The gain of the intensifiers is raised to 1000.Now two photodiode arrays are needed for the simultaneous readout of the two entrance lines. The readout time is halved by reading the odd and even number diodes by two ADCs per array. All these measures will increase the detective quantum efficiency from 0.2 to 0.6. The performance of the new system will be tested again by investigations of dogs before moving to investigations on humans, expected at the end of this year.

Notes: High photon fluxes are crucial in dichromatic digital subtraction angiography with line scan systems to allow for adequate image quality. To obtain images wide enough and with sufficient vertical resolution, each of the two beams used has to be at least 100 mm wide but not more than 0.5 mm high at the patient's heart with the beam energies bracketing the iodine-K edge at 33.17 keV. A new monochromator in Laue transmission geometry has been developed, successfully tested at beamline ID11 of the European Synchrotron Radiation Facility (ESRF) and is now in use at the angiography wiggler beamline W2 at HASYLAB. The monochromator consists of two almost identical goniometer heads which are independently controlled. Within each head a bent silicon single crystal extracts a quasimonochromatic beam and focuses it vertically to the patient thus also increasing intensity compared to an unbent crystal. With a storage ring current of 50 mA the new monochromator provides 0.8×1011 photons/mm2/s in front of the patient which is an increase by a factor of 6 compared to the formerly used Bragg design with Ge crystals. Also, it is remarkably more stable against variations of the input power load. © 1995 American Institute of Physics.

Notes: The performance of large area directly cooled monochromator crystals at high-power wiggler beamlines is degraded by the heat load from insertion devices and by the pressure of the cooling agent. Based on the experience gained with directly water-cooled crystals at HASYLAB, a new adaptive crystal with remotely controlled crystal bender was built. A characterization of the first new Si(111) crystal under different heat load conditions up to 850 W is presented. © 1995 American Institute of Physics.

Notes: The effect of S segregation to grain boundaries on the intergranular embrittlement of Ni has been studied at room temperature using Auger electron spectroscopy and slow strain rate tensile tests. The grain-boundary S concentration was varied by time-controlled annealing of dilute Ni–S alloy specimens at 625 °C. The ductile-to-brittle transition in Ni, as determined from percent integranular fracture and reduction-in-area measurements, occurred over a narrow range of S concentrations centered on 15.5±3.4 at. % S. This critical S concentration for 50% intergranular fracture of polycrystalline Ni is similar to the 14.2±3.3 at. % S required to induce 50% amorphization of single-crystal Ni by S+-ion implantation. This suggests that segregation-induced intergranular fracture, like implantation-induced amorphization, may be a disorder-induced polymorphous melting process. In agreement with experimental observations, the polymorphous melting curve for the Ni–S solid solution on the phase diagram drops rapidly to zero as the alloy composition approaches ∼18 at. % S. The critical grain-boundary concentration for intergranular fracture, while slightly less, is within experimental error of the concentration predicted for polymorphous melting as well as that measured for ion-implantation-induced amorphization. © 2000 American Institute of Physics.

Notes: In situ fracture studies have been carried out on thin films of the NiTi intermetallic compound under plane stress, tensile loading conditions in the high-voltage electron microscope. Local stress-induced amorphization of regions directly in front of moving crack tips has been observed. The upper cutoff temperature, TC–Amax, for the stress-induced crystalline-to-amorphous transformation was found to be 600 K, identical to that for heavy ion-induced amorphization of NiTi and for ion-beam mixing-induced amorphization of Ni and Ti multilayer specimens. 600 K is also both the lower cutoff temperature, TA–Cmin, for radiation-induced crystallization of initially-unrelaxed amorphous NiTi and the lowest isothermal annealing temperature, TXmin, at which stress-induced amorphous NiTi crystallizes. Since TXmin should be TK, the ideal glass transition temperature, the discovery that TC–Amax=TA–Cmin=TXmin=TK implies that disorder-driven crystalline-to-amorphous transformations result in the formation of the ideal glass, i.e., the glassy state that has the same entropy as that of the defect-free crystal. As the glassy state with the lowest free energy, its formation can be understood as the most energetically-favored, kinetically-constrained response of crystalline alloys driven far from equilibrium. © 1998 American Institute of Physics.

Notes: The high-heat-load monochromator, based on the HASYLAB (Hamburger Synchrotronstrahlungslabor) `Torii' design, has been used successfully for 2 years at the BW2 wiggler beamline. The bowing of the reflecting surface induced by the heat load and water pressure is compensated mechanically. This is achieved by mounting the specially shaped crystal in a bending mechanism and by heat transfer from the directly water-cooled crystal. We present the latest step in the development of this design. The piezoceramic-driven actuators are replaced by actuators based on the thermal expansion of copper rods. The direct water cooling of the crystal is replaced by a safer indirect water-cooling scheme. Characterization results of the crystal are presented. The Si(111) rocking-curve width at 9.5 keV was measured to be 8.3 arcsec under 500 W heat load.