Tokuumi Fukazawa

Cerium‐activated Gd2SiO5 single crystal scintillator

Cerium‐activated phosphors are characterized by their fast luminescence decay. Gadolinium orthosilicate (Gd2SiO5) is a material possessing a high atomic number, and can also play host to the cerium activator. Cerium‐doped Gd2SiO5 single crystals were grown by the Czochralski technique, and their luminescence properties were examined. The light output was 1.3 times larger than that of the best Bi4Ge3O12, and the decay constant was 60 ns at room temperature.

Scintillation study of ZnWO4 single crystals

Scintillation characteristics of the Czochralski‐grown ZnWO4 single crystal were studied. This crystal, with light red‐brown color, exhibits a maximum luminescence wavelength of 480 nm under excitation at 254 nm. The total luminescence output under x‐ray excitation at 254 nm. The total luminescence output under x‐ray excitation was found to be 2.3 times larger than that of the best Bi4Ge3O12 at room temperature with small afterglow.

Inversion of the direction of the solid-liquid interface on the Czochralski growth of GGG crystals

In the process of the crystal growth of Gd3Ga5O12 by the Czochralski method, the shape of the solid-liquid interface drastically changes from convex to concave toward the melt at a certain crystal diameter under a constant seed rotation rate condition. It is due to the remelting of the old crystal and is related to the balance between a natural convection and a forced one caused by crystal rotation. The relationship between the conditions of the inversion and the growth parameters have been examined by growing the crystals under various conditions. The inversion diameter which gives the inversion of the interface was expressed as (Re η/ϱ)12 ωs-12 by introducing the Reynolds number Re.

Thin Film Growth of YBa2Cu3O7-x by ECR Oxygen Plasma Assisted Reactive Evaporation

A new apparatus equipped with an ECR oxygen plasma source, a co-evaporation system of Y, Ba and Cu and a differential pumping system was developed. YBa2Cu3O7-x superconducting films were obtained at a substrate temperature of 450–500°C. The critical temperatures of films deposited on SrTiO3, MgO and Si substrates were 87 K, 80 K and 63 K, respectively. These properties were closely related to the crystallinity of the film.

Superconducting characteristics of a planar‐type HoBa2Cu3O7−x −La1.5Ba1.5Cu3O7−y−HoBa2Cu3O7−x junction

A planar‐type junction of HoBa2Cu3O7−x−La1.5Ba1.5Cu3O7−y −HoBa2Cu3O7−x having an electrode spacing as small as 0.1 μm is fabricated and its electrical characteristics are measured. Supercurrent through the La1.5Ba1.5Cu3O7−y layer is detected up to the temperature of 69 K. A superconducting region extends from the YBa2Cu3O7−x electrodes into a normal La1.5Ba1.5Cu3O7−y layer with a submicrometer scale. The superconducting decay length is 65 nm, which is two orders of magnitude larger than the value calculated from the conventional proximity theory.

Improvement in the scintillation conversion efficiency of Bi4Ge3O12 single crystals

Abstract Single crystals of Bi4Ge3O12, which are greatly needed as scintillators for positron computed tomography, are grown by the Czochralski technique. Their sensitivity can be brought up to 12% of that of NaI by eliminating voids and purifying the crystals. Radiation damage is also prevented. These crystals provide useful scintillators with high sensitivity and energy resolution.

Growth of SrTiO3 thin films by pulsed-laser deposition

Abstract SrTiO 3 thin films were grown on MgO(001) substrates at a substrate temperature of 600 °C by pulsed-laser deposition. The preferential a axis orientation is enhanced and the lattice parameter is strongly expanded as the oxygen pressure during growth is lowered. The thin film grown at 10 −6 Torr has a lattice parameter as large as 0.395 nm. This is thought to result from oxygen vacancies introduced during growth in a non-thermal equilibrium state. The lattice of the film has a cube-on-cube orientation. Such an epitaxial-like thin film was thought to arise because thin film growth at a low oxygen pressure brings about a molecular-beam-epitaxy-like condition and thus a large migration energy is retained on the substrate surface.

Low temperature growth of superconducting YBa2Cu3O7−x thin films by organometallic chemical vapour deposition

Abstract A superconducting YBa 2 Cu 3 O 7− x thin film with a zero-resistivity temperature of 83 K was grown on an SrTiO 3 (100) substrate at 700°C by organometallic chemical vapour deposition. Yttrium, barium and copper tetramethylheptanedione complexes and oxygen were used as source materials and the deposition pressure was 1.5 mmHg. The onset temperature for the superconducting transition was 86 K. Above this temperature the resistivity of the film decreased with decreasing temperature. The thin film is polycrystalline and has a crystallographic orientation in which the a axis is perpendicular to the substrate plane.

Preparation of YBa2Cu3O7-x Superconducting Thin Films by RF-Magnetron Sputtering

Thin films of Y-Ba-Cu-O system have been prepared by rf-magnetron sputtering technique. The sputtered film was a multilayer composed of (Ba, Cu, O) and (Y, Cu, O). The films were crystallized by a high temperature chemical reaction between BaCuO2 and Y2Cu2O5 layers. Critical temperatures of YBa2Cu3O7-x thin fims in this study were 80-85 K.

Electrical Characteristics of HoBa2Cu3O7-x-La1.5Ba1.5Cu3O7-y-HoBa2Cu3O7-x Junctions with Planar-Type Structures

A HoBa2Cu3O7-x-La1.5Ba1.5Cu3O7-y-HoBa2Cu3O7-x junction having a planar structure is fabricated and its electrical characteristics are investigated. The planar junction is formed by sectioning a HoBa2Cu3O7-x layer into two superconducting electrodes keeping a submicrometer between them. Supercurrent is detected for the junction with an electrode spacing smaller than 0.4 µm. Critical current is exponentially dependent on electrode spacing. Superconducting decay length for the La1.5Ba1.5Cu3O7-y layer is 65 nm and is almost independent of temperature up to 70 K. This extraordinary long decay length and anomalous superconducting characteristics are not shown to be explained by the conventional proximity theory, and phenomenologically outlined.