Kazumasa Takagi

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.

SrRuO3 Thin Films Grown under Reduced Oxygen Pressure

SrRuO3 thin films are grown under reduced oxygen pressures between 10-6 Torr and 100 mTorr by pulsed laser deposition. The thin films grown at temperatures above 640° C and at a pressure of 10-6 Torr are characterized in terms of the lattice shrinkage due to oxygen deficiency. The resistivity increases and the temperature dependence changes to semiconductive from metallic as the oxygen pressure during growth is decreased. The Hall coefficient of the film grown at 740° C and at a pressure of 10-6 Torr is thirty times higher at low temperatures than at room temperature. The transport properties of the film are thought to result from the high carrier concentration but the low mobility due to the oxygen deficiency.

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.

Work function of LaB6

By means of Richardson plots, low work functions of 2.3–2.4 eV have been measured for three cyrstal planes including (100) of single‐crystal LaB6. Modifying Smith’s model of a metallic surface, the work function of LaB6 has been theoretically estimated for a jellium model at 2.31±0.05 eV which is in good agreement with experimental values.

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.

Investigation of ramp-type Josephson junctions with surface-modified barriers

We have investigated the properties of YBa/sub 2/Cu/sub 3/O/sub 7-x/ ramp-edge Josephson junctions with surface-modified barriers produced by Ar-ion irradiation followed by oxygen annealing. The fabricated junctions displayed RSJ-like I-V characteristics and excellent uniformity. The stray capacitance of the junctions was estimated from the ramp-edge structure. The junction capacitance was obtained by subtracting the stray capacitance from the shunting capacitance. We estimated the barrier thickness from the junction capacitance, and found that the critical current density of the junction increased exponentially with decreasing barrier thickness. The relative dielectric constant of the barriers ranged from 13 to 18.