Oxide single crystal scintillators such as BGO have higher radiation sensitivity than those of conventional NaI:Tl, and are chemically stable without hygroscopicity. Therefore, they are installed as radiation detectors of X-ray CT, PET, underground resource exploration, astrophysics, nuclear and elementary particle experiment, etc. In this paper, the history of oxide single crystal scintillators is described, focusing on BGO, CWO, GSO, and LGSO, which are the materials that the author has developed over the past 40 years, and adding the history of equipment development in which these materials were used.

  Scintillators are essential components of gamma cameras and PET scanners, both key imaging systems in nuclear medicine. Following radiopharmaceutical administration, gamma rays emitted from the body first interact with the scintillator, generating photoelectrons that are subsequently processed to produce clinical images. Enhancing scintillator performance can drive advancements in nuclear medicine. While gamma camera imaging, including SPECT, utilizes various radiopharmaceuticals labeled with isotopes such as ¹²³I and ^99mTc, its performance remains insufficient for clinical applications among recent medical advances. In this regard, PET imaging surpasses gamma cameras in resolution and sensitivity, expected to be utilized more extensively. Furthermore, fast-decay scintillators enable time-of-flight gamma-ray measurement, significantly improving spatial resolution and reducing noise. PET holds the greatest promise particularly in neuroscience, given the brain’s complexity and limitations in invasive measurement. Notably, PET remains the only modality capable of quantitative imaging of deep brain regions.

  Real dose monitors under the high dose-rate condition are required to remove the debris in the Fukushima Daiichi Nuclear Power Plant as the decommissioning step, and I have proposed a dose-rate monitor consisting of a scintillator, optical fiber and photo detector. To realize such monitor, scintillation materials are required to have the long-emission-wavelength (550 - 1000 nm) and high light output. From this reason, several novel materials such as Cs₂HfI₆ and Rb₂HfI₆ for gamma-ray detection have been developed, and Li₂HfBr₆ had sensitive to both gamma ray and neutrons. These scintillators had an emission wavelengths of 600 to 750 nm, and I review the detail of such red and infrared emitting scintillator.

  The high purity of inorganic crystals is one of the most important properties for radiation measurement in nuclear and particle physics. The dark matter search project needs a low background rate of less than 1 keV⁻¹kg⁻¹day⁻¹ and energy of less than 10 keV. The neutrinoless double beta decay experiment needs less than one event per one ton of detector in one year. The radioactive impurities in an inorganic crystal suffer its sensitivity to dark matter and neutrinoless double beta decays. We aim to reduce the concentrations of uranium and thorium series impurities in the crystals to less than one ppt. The purification method should be optimized for both water-soluble and water-insoluble materials. This review report describes the purification methods. The measurement technique for tiny amounts of impurity is described. ICP-MS and radiation measurements should be applied to get precise contamination information. The prospects for the dark matter search and double beta decay experiments by inorganic crystals are discussed.

  Scintillators have long been used as radiation detectors and are essential tools for nuclear and particle physics research. Our goal is to understand the properties of neutrinos, a fundamental particle, through the study of double beta decay. In 2020, we launched the PIKACHU experiment (Pure Inorganic scintillator experiment in KAmioka for CHallenging Underground sciences), which aims to study the double beta decay of 160Gd using large, high-purity Ce:Gd₃Al₂Ga₃O₁₂ scintillator crystals.

  This paper introduces the physics of double beta decay, presents the current status and future prospects of the PIKACHU experiment, and discusses the challenges involved. Finally, we provide insights into the types of scintillator crystals that would be desirable for future experiments, including unique double beta decay searches and dark matter detection. These advancements in scintillator technology will be crucial for improving sensitivity and enabling groundbreaking discoveries in particle physics.

  Transparent semiconductors are useful for constructing gamma-ray detectors using charge induction and Cherenkov light signals for good energy and position resolution and fast time resolution. Thallium bromide (TlBr) is a compound semiconductor with the band gap energy of 2.68 eV. This makes TlBr crystals transparent to visible light. TlBr is an attractive material for gamma-ray detectors because of its high atomic number and density. This article reviews the development trends of gamma-ray detectors using transparent semiconductors, with emphasis on TlBr detectors.