Phosphors for Radiation Detectors. Группа авторов

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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.