β-Ga₂O₃ is a well-known semiconductor material for power devices and other applications. Recently, β-Ga₂O₃ has also been reported as a scintillator material with a light yield of approximately 8400 ph./MeV, scintillation decay time of <1 μs, and density of 6.44 g/cm³. In this study, 45 cm diameter β-Ga₂O₃ single crystals were prepared via oxide crystal growth using the cold crucible (OCCC) method under various oxygen partial pressures. In the OCCC method, as in the cold crucible method, a high frequency is applied directly to the oxide materials, which are heated and melted, and the melt is held by the outermost solid material itself that is cooled by water using a copper hearth. In the OCCC method, crystal growth is performed while rotating the seed crystal, as in the Czochralski method, to increase the crystal diameter. The optical properties and radiation responses of the crystals grown under various oxygen partial pressures were evaluated.

To improve positron emission tomography (PET) image quality, depth-of-interaction (DOI) information and timeof-flight (TOF) information are key technologies. In this work, we developed the DOI-TOF detector based on our original singleended readout scheme with the continuous layered gadolinium fine aluminum garnet (GFAG) array. The size of each GFAG crystal is 1.45×1.45×20 mm3. The multi-pixel photon counter (MPPC) used as a photo-sensor has a surface area of 3.0×3.0 mm2. One pair of crystals coupled with a partial optical window is arranged across two MPPCs. Boundaries without the partial optical window are covered with optical reflectors. Each pair is coupled with paired MPPCs of different patterns. Crystal identification is obtained from paired MPPCs of different patterns and output rates. By limiting the spread of scintillation light, the proposed DOI detector based on the local centroid calculation was expected to improve crystal response including the edge. For performance comparison, we prepared three discrete layers of the GFAG array with the same total thickness. For the continuous layer, the DOI resolution, the energy resolution and the timing resolution of the pair of detectors are 4.7 mm, 14% and 402 ps, respectively. Their respective values are 29%, 20% and 33% better than those with the three discrete layers.

A novel digital PET scanner based on Time over Threshold method is developed. The positron emission tomography (PET) is composed of 144channel Ce:Gd₃Al₂Ga₃O₁₂ (GAGG)-Avaranche photodiode (APD) detector arrays individually coupled with custom designed Time over Threshold (ToT) application-specific integrated circuit (ASIC) to realize the high count rate and good spatial resolution. Such an imaging system provides a simple front-end circuit and flexible digital signal processing like multiplexing such as a pulse train method. The measured energy resolution of the detector system was 6.7% for the 511 keV peak, and 4.25 ns time resolution was measured with a single detector module. The measured spatial resolution for a point source was 1.37 mm FWHM for our initial data with a columnar ²²Na source.