**Environmental Background Radiation Monitoring Utilizing Passive Solid Sate Dosimeters**

Hidehito Nanto1,2, Yoshinori Takei1,2 and Yuka Miyamoto2,3 *1Advanced Materials Science R&D Center, 2Research Laboratory for Integrated Technological Systems, Kanazawa Institute of Technology, Hakusan, Ishikawa, 3Oarai Research Center, Chiyoda Technol Corporation, Oarai-machi, Higashi Ibaragi, Japan* 

#### **1. Introduction**

120 Environmental Monitoring

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Natural environmental background radiation is radiation that is constantly present in the environment and is emitted from a variety of natural and artificial sources. Primary contribution comes from sources in the earth, from space and in the atmosphere. Naturally occurring sources are responsible for the vast majority of radiation exposure. However, not including direct exposure from radiological imaging or therapy, about 3% of background radiation comes from man-made sources such as self-luminous dials and signs, global radioactive contamination due to historical nuclear weapons testing, nuclear power station or nuclear fuel reprocessing accidents, normal operation of facilities used for nuclear power and scientific research, emission from burning fossil fuels and emission from nuclear medicine facilities and patients.

We are all exposed to ionizing radiation every day. In fact, the environmental background radiation contributes about two-thirds of our radiation exposure. Therefore, it is important to determine the exact environmental background radiation dose. Active dosimeters have been formally appropriate for monitoring dose equivalent rates of environmental background radiation. On 2001 in Japan, not only dose equivalent rate but also dose equivalent can be applied to environmental background radiation monitoring, which is based on the Japanese law modification concerned with radiation protection. Thus, there is the possibility that passive solid state dosimeters are also appropriate for environmental background radiation monitoring.

So far, some types of solid state dosimeter have been developed not only for personal monitoring but also for environmental background radiation monitoring. For instance, a thermoluminescence (TL) dosimeter has been studied to monitor the environmental background radiation (Nanto, 2011). Recently newly passive solid state dosimeters utilizing optically stimulated luminescence (OSL), direct ion storage (DIS) and radiophotoluminescence (RPL) phenomena have been developed to monitor the personal and environmental radiation (Ranogajec-Komor, 2008; Koyama, 2010).

In the following, the basic principle of the passive solid state dosimeters utilizing TL, OSL, DIS and RPL phenomenon are reviewed and the results on environmental background

Environmental Background Radiation Monitoring Utilizing Passive Solid Sate Dosimeters 123

then Ag+ ions change to Ag0 ions. On the other hand, the holes are captured initially by PO4 tetrahedra and then migrate to produce Ag2+ ions. It has been reported (Miyamoto, 2011) that both Ag0 and Ag2+ ions can be the centers of luminescence in the phosphate glass as shown in Fig.1. Morever, once trapped, luminescence centers are stable unless the glasses are annealed at high temperature at about 400℃. Figure 2 shows photograph of orange RPL from the glass dosimeter which was exposed to x-ray. As the RPL intensity is proportional to the amount of irradiation, the Ag+- doped phosphate glass can be used in individual

Fig. 2. Photographs of emitted RPL from the glass dosimeter which was exposed to x-ray

The OSL process as well as TL process is based on the presence of electron and/or hole traps and luminescence centers in storage phosphor materials (Nanto, 1998). Figure 3 shows the energy band diagram of Eu doped BaFBrI (BaFBrI:Eu) photostimulable storage phosphor which is used as the storage phosphor material of the imaging plate (IP) (Nanto, H. 2006) for the computed radiography. Upon irradiation with ionizing radiation such as x-ray to storage phosphor materials, free electrons in conduction band (C.B.) and holes in valenced band (V.B.) are promoted via band-to-band excitation. The free electrons are, then, trapped at anion vacancies such as F, I and Br vacancies to produce the F centers as the electron trap centers. While the free holes are trapped at the Eu2+ impurity centers to produce the Eu3+

In OSL process, the energy is provided by stimulating the phosphor materials with visible or near infrared light after irradiation. During a detrapping transition, free electrons stimulated from the F centers into the conduction band recombine with the luminescence centers of the Eu3+ ions, whereby visible photons (OSL) are emitted as

(upper photograph) and without x-ray irradiation (down photograph).

impurity centers. Detrapping of these carriers requires energy.

monitoring of ionizing radiation.

**2.1.2 OSL dosimeters** 

shown in Fig.3.

monitoring using these passive dosimeters, especially personal dosimeter utilizing RPL penopmenon, are shown and discussed.
