Kengo Shibuya

Preparation of thermally stable TiO2-terminated SrTiO3(100) substrate surfaces

We have examined the thermal stability of TiO2-terminated SrTiO3(100) surfaces obtained by buffered HF etching and widely used as substrates for oxide film growth. In situ coaxial impact-collision ion scattering spectroscopy was used to measure the composition of the terminating atomic layer at temperatures up to 1000°C, simulating a broad range of thin-film growth conditions. The TiO2 termination of a nonannealed but HF-etched surface was found to start collapsing at temperatures as low as 300°C regardless of atmosphere, showing thermal instability of the chemically cleaved surface. Here, we introduce an alternative way to prepare a stabilized SrTiO3 surface, which maintains a perfect TiO2 termination up to 700°C, ideal for the growth of atomically sharp oxide heterointerfaces.

Scintillation properties of (C6H13NH3)2PbI4: Exciton luminescence of an organic/inorganic multiple quantum well structure compound induced by 2.0 MeV protons

Abstract We report a new type of scintillator especially suitable for pulse-radiation detection. Thin films of organic/inorganic perovskite compound (n-C6H13NH3)2PbI4, which is characterized by a multiple quantum well structure, were bombarded by 2.0 MeV protons, and their radiation-induced emission spectra were obtained. A single and sharp emission peak due to an exciton was observed at the wavelength of 524 nm. This emission was clearly detected even at room temperature, and its quantum efficiency was very high. The line shape of this emission did not change, retaining its sharpness, and no other emissions appeared throughout the irradiation. The optical response of (n-C6H13NH3)2PbI4 is very fast. (n-C6H13NH3)2PbI4 is a promising scintillator material, meeting requirements not satisfied by conventional scintillators.

Development of Ultra-Fast Semiconducting Scintillators Using Quantum Confinement Effect

The concept of a quantum scintillator, satisfying both a large light output and a quick response, is proposed. The temporal behavior of scintillation from (n-C6H13NH3)2PbI4, a natural multiple quantum well structure provided by the lead-halide-based perovskite-type organic-inorganic hybrid compound, was investigated using a short-pulsed electron beam and a streak camera. A decay component of 390 ps was efficiently observed even at room temperature. This response is notably quicker than conventional Ce3+-activated scintillators because of a quantum confinement effect that increases the overlapping region of electron and hole wavefunctions in the low-dimensional system. This achievement would be the next breakthrough in the development of ultra-fast inorganic scintillators.

Electrostatic modulation of the electronic properties of Nb-doped SrTiO3 superconducting films

We have performed ferroelectric field effect experiments using an epitaxial heterostructure composed of ferroelectric Pb(Zr0.2Ti0.8)O3 and superconducting Nb-doped SrTiO3. The films were prepared on (001) SrTiO3 substrates by off-axis radio-frequency magnetron sputtering and pulsed-laser deposition. By switching the polarization field of the 500-A-thick Pb(Zr0.2Ti0.8)O3 layer, a large change of about 30% in resistivity and a 20% shift of Tc (ΔTc∼0.05u2009K) were induced in the 400-A-thick epitaxial Nb-doped SrTiO3 layer. The relationship between Tc and the electrostatically modulated average carrier concentration can be mapped onto the phase diagram of chemically doped SrTiO3.

Quantum confinement for large light output from pure semiconducting scintillators

A method for creating a fast scintillator is proposed. Recently, much attention has been paid to pure semiconductors during development of subnanosecond fast solid scintillators. However, the bulky samples rarely exhibit high light yields at room temperature because of thermal instability at the excitonic levels. The authors employed the optimum three- and two-dimensional semiconducting systems provided by lead-halide-based compounds to demonstrate the advantage of low dimensionality in the scintillating efficiency. Their dimensional and temperature dependencies were investigated using a high-energy proton beam. Consequently, the quantum confinement system clearly prevented thermal quenching from excitonic level even at room temperature, and the result proposes the next breakthrough to create ultrafast solid scintillators.

Development of the X'tal Cube: A 3D Position-Sensitive Radiation Detector With All-Surface MPPC Readout

We have developed a new three-dimensional (3D) position sensitive radiation detector, called the Xtal cube. The Xtal cube is composed of a scintillation crystal block and a number of multi-pixel photon counters (MPPCs) which are coupled on all six sides of the block. The block is segmented into cubes and no reflector is used between the segments. Scintillation light originating in a crystal segment accordingly propagates three-dimensionally along the alignment of the crystal segments and is efficiently detected by the MPPCs. The Xtal cube could be used as the detector element of a PET system, for instance. We constructed two prototypes of the Xtal cube and evaluated their performance using gamma-ray sources. The crystal block of each prototype is composed of a 3D array of Lu2xGd2(1-x)SiO5: Ce (LGSO, x = 0.9) crystal segments. Each crystal volume is 3.0 mm × 3.0 mm × 3.0 mm. MPPCs of a 3.0 mm × 3.0 mm active area are coupled to each surface of the crystal block. In this work, we show that all crystal segments are identified by a simple Anger-type calculation performed on the MPPC signals for both prototypes. The Xtal cube provides high spatial resolution in three dimensions regardless of the incident angle of the radiation.

Imaging simulations of an “OpenPET” geometry with shifting detector rings

We have proposed a new “OpenPET” geometry consisting of two detector rings of axial length W each separated by a gap G. For obtaining an axially continuous field of view (FOV) of 2Wxa0+xa0G, the maximum limit for G must be W. However, two valleys of sensitivity appear on both sides of the gap. Setting a more limited range for the gap as Gxa0<xa0W, which is desirable for filling in the sensitivity valleys, results in not only a shortened gap, but also a shortened axial FOV. In this paper, we propose an alternative method for improving the uniformity of sensitivity by shifting two detector rings axially closer or further apart at the same velocity to each other. In addition, image reconstruction of the OpenPET is an incomplete problem, and low-frequency components are missing in the gap. Therefore, the proposed method is also expected to improve the conditions for the inverse problem. We simulated an OpenPET scanner which measures events simultaneously by shifting the detector rings. The results showed that the right and left peaks of the sensitivity approach each other upon shifting of the detector rings, and these valleys of sensitivity are effectively recovered. The results also showed that distortion, which is observed for objects containing low-frequency components, is reduced. Larger detector shifts allow a more uniform axial distribution of sensitivity and a higher image quality, but at the cost of a smaller minimum gap. Therefore, an appropriate detector-shifting pattern should be determined based on the desired scanner application.

Simulation studies of a new 'OpenPET' geometry based on a quad unit of detector rings.

We have proposed an OpenPET geometry which consists of two detector rings of axial length W each axially separated by a gap G. In order to obtain an axially continuous field-of-view (FOV) of 2W+G, the maximum limit for G must be W. However, two valleys of sensitivity appear, one on each side of the gap. In practice, the gap should be G<W in order to compensate for the sensitivity valleys. In this paper, we proposed an alternative method to improve uniformity of the sensitivity while maintaining the gap. The proposed geometry consisted of four units of detector rings obtained by dividing each right and left unit of detector rings into two units. The inner two units formed the main gap, and the outer two units were appropriately placed to improve the uniformity of sensitivity. The geometry was optimized to minimize the standard deviation of the sensitivity distribution. Numerical simulation results supported the effectiveness of the proposed method. The outer units compensated for the sensitivity valleys on both sides of the main gap. A more appropriate geometry should be designed for the desired application, such as a long axial FOV PET and in-beam PET.

The "X'tal cube" PET detector: 3D scintillation photon detection of a 3D crystal array using MPPCs

We have proposed a depth of interaction (DOI) PET detector named Xtal cube, in which a number of Multi-Pixel Photon Counters (MPPCs) are coupled on various positions of six surfaces of a segmented scintillation crystal block. There are no reflectors within the block, and the areas among MPPCs on the surface are covered with reflector. The MPPC is thin and light solid-state photo-detector so that it is possible to set the MPPCs on the subject side in the detector ring of a PET scanner and the crystal blocks can be closely placed each other in the PET detector ring. To study the characteristics of the Xtal cube, we constructed a segmented crystal block consisting of six layers of a 6 × 6 crystal array with Lu 2x Gd 2(1−x) SiO 5 : Ce (LGSO, x=0.9) crystals. Each crystal is 3.0 × 3.0 × 3.0 mm3 in dimensions. We examined crystal identification performance for different arrangements of photo-detector elements on the block surfaces. The preliminary experiments showed the possibility of realizing a 3D detector having isotropic resolutions.

Poly[bis(phenethyl­ammonium) [di­bromido­plumbate(II)]-di-μ-bromido]]

Crystals of the title compound, {(C6H5C2H4NH3)2[PbBr4]}n, were grown at room temperature from a solution in N,N-dimethylformamide (DMF) using nitromethane as the poor solvent. This perovskite-type organic–inorganic hybrid compound consists of well ordered sheets of corner-sharing disordered PbBr6 octahedra separated by bilayers of phenethylammonium cations. The octahedra are rotated and tilted due to N—H⋯Br hydrogen bonds with the ammonium groups, generating a superstructure in the unit cell similar to that of the tetrachloridoplumbate (C6H5C2H4NH3)2[PbCl4].