Phosphors for Radiation Detectors. Группа авторов

that readers understand solid state physics, quantum mechanics, and inorganic chemistry. Some key concepts are explained in Chapters 1 and 2. We hope that this book will be useful to all its readers.

      Takayuki Yanagida & Masanori Koshimizu May 2021

      Series Preface

      Wiley Series in Materials for Electronic and Optoelectronic Applications

      This book series is devoted to the rapidly developing class of materials used for electronic and optoelectronic applications. It is designed to provide much‐needed information on the fundamental scientific principles of these materials, together with how these are employed in technological applications. The books are aimed at (postgraduate) students, researchers, and technologists, engaged in research, development, and the study of materials in electronics and photonics, and industrial scientists developing new materials, devices, and circuits for the electronic, optoelectronic and, communications industries.

      The development of new electronic and optoelectronic materials depends not only on materials engineering at a practical level, but also on a clear understanding of the properties of materials, and the fundamental science behind these properties. It is the properties of a material that eventually determine its usefulness in an application. The series therefore also includes such titles as electrical conduction in solids, optical properties, thermal properties, and so on, all with applications and examples of materials in electronics and optoelectronics. The characterization of materials is also covered within the series in as much as it is impossible to develop new materials without the proper characterization of their structure and properties. Structure–property relationships have always been fundamentally and intrinsically important to materials science and engineering.

      Materials science is well known for being one of the most interdisciplinary sciences. It is the interdisciplinary aspect of materials science that has led to many exciting discoveries, new materials, and new applications. It is not unusual to find scientists with a chemical engineering background working on materials projects with applications in electronics. In selecting titles for the series, we have tried to maintain the interdisciplinary aspect of the field, and hence its excitement to researchers in this field.

      Arthur Willoughby

      Peter Capper

      Safa Kasap

1 Ionizing Radiation Induced Luminescence

       Takayuki Yanagida

       Nara Institute of Science and Technology, Ikoma, Japan

1.1 Introduction

      Ionizing radiation was discovered more than one hundred years ago [1]. Ionizing radiation is defined as high energy quanta that ionize materials. For some years, the physical properties, and the uses and harmful effects of ionizing radiation, have been widely recognized. A typical property is its high penetration of materials, especially with high energy photons such as X‐ and γ‐rays. Such properties makes it possible for us to investigate the inside of materials, including the human body, without damaging any of the internal structures. On the other hand, if the human body absorbs too much ionizing radiation, the radiation can cause harm such as a cancer. In order to merit the use of ionizing radiation, control of the amount and energy generated is necessary, for which accurate detection techniques are required. Ionizing radiation is invisible and odorless, and in order to detect it, we must first convert it into something which we can easily access. In most cases, we can use various tools to convert ionizing radiation into a current. From this current, we can easily gain the information desired by using common electronics. Here, the tools used to convert ionizing radiation into recognizable information are known as radiation detectors.

Figure 1.1 illustrates the classification of typical ionizing radiation detectors. Mainly, there are two types of solid materials that are most commonly used as radiation detectors. One is the semiconductor, and the other is in the form of luminescent materials known as scintillators and storage phosphors [2]. The former functions to absorb the energy of the ionizing radiation and converts it into a large number of carriers. The latter converts the ionizing radiation into a large number of photons, which can be detected by photodetectors such as the photomultiplier tube (PMT) and the photodiode (PD). The number of these carriers or photons is proportional to the quantity or energy of the incident ionizing radiation, and in this way, we can measure any type of ionizing radiation. Radiation detector types comprise two kinds of detection methodologies; photon counting‐type and integration‐type. In the photon counting‐type detectors, because each radiation signal is processed event‐by‐event, a fast time‐response typically in the order of ns to μs is important. In contrast, the integration‐type detectors detect multiple events over a few ms, so a fast time‐response is not required. If the counting rate of the target radiation is limited, we generally use the photon counting‐type detectors, while we select the integration‐type when the rate is high.

Figure 1.1 Classification of solid state ionizing radiation detectors.

      Semiconductors and scintillators can be applied to both types of detectors, while storage phosphors can only be applied to the integration‐type detectors with a very long integration time (e.g., several weeks to months). These storage phosphors are mainly used for personnel protection dosimetry and imaging plates in the nondestructive study of humans (e.g., dental application) or objects. The storage phosphors used for such applications are mainly classified into three types by emission mechanisms, namely optically stimulated luminescence (OSL) [3], thermally stimulated luminescence (TSL) [4], and radiophotoluminescence (RPL) [5] materials. Hereafter, the two kinds of luminescent materials (scintillators and storage phosphors) are focused. Although the emission mechanisms of storage phosphors are different from those of scintillators, the readout system of dosimeters is similar to that of scintillation detectors. Their emissions are typically read out by PMT under photo‐ or thermal‐stimulation (excitation). Details are described in later chapters.

      Typically, luminescent materials for ionizing radiation measurements consist of the host material and the dopant for luminescence centers. For example, Tl‐doped NaI is the most common scintillator [6], NaI is the host which is generally an insulator or semiconductor, and Tl is the dopant with a typically very low concentration. In these kind of materials, the host has a role to absorb the target ionizing radiation efficiently, and the dopant has a role to emit photons whose number is proportional to the incident radiation energy or amount. The combination of the host and the dopant is one of the more recent trends of R&D in this field. The same trend is true, not only for scintillators, but also for storage phosphors for dosimeters.

In addition to host‐dopant type materials, undoped materials have also been investigated for the measurement of radiation. Although we can control the detection properties of luminescent materials by doping, some difficulties in the synthesis, such as a uniformity of dopant distribution, are always problematic. To avoid such problems, continuous effort has been paid to the study of undoped materials. The most common undoped materials for radiation measurement are semiconductor scintillators, and in addition to these, self‐activated scintillators such as Bi₄Ge₃O₁₂ (BGO) [7] and Auger‐free luminescence (AFL) scintillators (e.g., BaF₂ [8]) are also common. Undoped materials have been also investigated for dosimeter applications, and a common material used is BeO [9].