**2. Passive solid state dosimeters**

Active dosimeters have been formally appropriate for monitoring dose equivalent rates of environmental natural radiation. In 2001, not only dose equivalent rate but also dose equivalent can become applied to environmental natural radiation monitoring; the dose equivalents at the boundary of the controlled area and the area is limited to be less than or equals to 1.3 mSv/3 months. Thus, there is the possibility that passive dosimeters are also appropriate for the environmental natural radiation monitoring. An application of various kinds of passive dosimeters, especially the dosimeter utilizing TL phenomenon (TL dosimeter), has been studied to monitoring the environmental natural radiation (Saez-Vergara, 1999). Recently, new passive dosimeters such as the dosimeter utilizing OSL phenomenon (OSL dosimeter), the dosimeter utilizing DIS phenomenon (DIS dosimeter) and the glass dosimeter utilizing RPL phenomenon (RPL glass dosimeter) have been developed as the personal dosimeter. In this study, the RPL glass dosimeters as well as the OSL dosimeter and the DIS dosimeter has been applied to monitor the environmental natural background radiation .

#### **2.1 Operation principle of the solid state dosimeters**

In this section, the operation principle of the solid state dosimeters, such as RPL glass dosimeter, the OSL dosimeter and the DIS dosimeter are discussed.

#### **2.1.1 Glass dosimeters**

Ag+-doped phosphate glass after exposure to ionizing radiation has an intense luminescence by the excitation with ultraviolet light. This phenomenon is called radiophotoluminescence (RPL). When Ag+-doped phosphate glass is exposed to ionizing radiation, electron and hole pairs are produced. The electrons are captured by the Ag+ ions in the glass structure, and

Fig. 1. Energy band diagram for RPL centers in Ag+-doped phosphate glass.

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 monitoring of ionizing radiation.

Fig. 2. Photographs of emitted RPL from the glass dosimeter which was exposed to x-ray (upper photograph) and without x-ray irradiation (down photograph).

#### **2.1.2 OSL dosimeters**

122 Environmental Monitoring

monitoring using these passive dosimeters, especially personal dosimeter utilizing RPL

Active dosimeters have been formally appropriate for monitoring dose equivalent rates of environmental natural radiation. In 2001, not only dose equivalent rate but also dose equivalent can become applied to environmental natural radiation monitoring; the dose equivalents at the boundary of the controlled area and the area is limited to be less than or equals to 1.3 mSv/3 months. Thus, there is the possibility that passive dosimeters are also appropriate for the environmental natural radiation monitoring. An application of various kinds of passive dosimeters, especially the dosimeter utilizing TL phenomenon (TL dosimeter), has been studied to monitoring the environmental natural radiation (Saez-Vergara, 1999). Recently, new passive dosimeters such as the dosimeter utilizing OSL phenomenon (OSL dosimeter), the dosimeter utilizing DIS phenomenon (DIS dosimeter) and the glass dosimeter utilizing RPL phenomenon (RPL glass dosimeter) have been developed as the personal dosimeter. In this study, the RPL glass dosimeters as well as the OSL dosimeter and the DIS dosimeter has been applied to monitor the environmental

In this section, the operation principle of the solid state dosimeters, such as RPL glass

Ag+-doped phosphate glass after exposure to ionizing radiation has an intense luminescence by the excitation with ultraviolet light. This phenomenon is called radiophotoluminescence (RPL). When Ag+-doped phosphate glass is exposed to ionizing radiation, electron and hole pairs are produced. The electrons are captured by the Ag+ ions in the glass structure, and

penopmenon, are shown and discussed.

**2. Passive solid state dosimeters** 

natural background radiation .

**2.1.1 Glass dosimeters** 

**2.1 Operation principle of the solid state dosimeters** 

dosimeter, the OSL dosimeter and the DIS dosimeter are discussed.

Fig. 1. Energy band diagram for RPL centers in Ag+-doped phosphate glass.

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+ impurity centers. Detrapping of these carriers requires energy.

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 shown in Fig.3.

Environmental Background Radiation Monitoring Utilizing Passive Solid Sate Dosimeters 125

The DIS dosimeter is composed of metal-oxide-semiconductor field effect transistor (MOSFET) with ionizing chamber (Wernli, 1998) as shown in Fig.5. The basic principle of the DIS dosimeter is as follows; a nonvolatile solid state memory cell is stored in the form of electric charge being trapped on the floating gate of a MOSFET in air or gas space surrounded by a conductive wall. The DIS dosimeter (Type DIS-1) which responds to X, γ and β-rays (Kobayashi, 2004) can widely detect a radiation dose within the range from 1to

The comparison of various basic characteristics, such as the readout process, the sensitivity for x-ray, β-ray and neutron, the energy dependence, of each solid state dosimeter is shown in Table 1. We would like to emphasize here is that the RPL glass dosimeter has good fading characteristics which means the luminescence centers are stable at room temperature unless


**source drain**


**secondary particles**



**(Materials) Readout Sensitivity Energy Dependence Fading** 

signal 1~40 [μSv] 6keV - 9 MeV (Χ・γ-ray)

10keV~10MeV(x-ray, γray), 300keV~3MeV(βray) 0025eV-15MeV(Neutron)

+

**ionizing radiation**

<sup>+</sup> <sup>+</sup> <sup>+</sup>

5keV~10MeV(Χ・γ-ray) 150keV~10MeV(β-ray) 0.025eV~0.5eV(Neutron)

0.06 – 0.8 MeV (β-ray) Good

Excllent

OK

**2.1.3 DIS dosimeters** 

Fig. 5. Schematic diagram of the DIS dosimeter

**channel**

Excitation with UV light

Excitation with visible light

Electrical

**2.2 Comparison of features of each solid state dosimeter** 

the glasses are annealed at high temperature at about 400℃.

Table 1. Basic characteristics of each solid state dosimeter

0.1 - 10,000 [mSv]

0.01 [mSv] – 10 [Sv] (Χ・γ-ray) 0.01 [mSv] – 10 [Sv] (β-ray) 0.1 [mSv] – 6 [mSv] (Neutron)

40 [μSv].

**Floating gate**

**Phenomenon** 

**RPL (Phosphate glass)** 

**OSL (Al2O3:C, BaFIBr:Eu, KCl:Eu)** 

**Ionization of air (Si –MOSFET)** 

Fig. 3. Energy band diagram for OSL process in BaFIBr:Eu phosphor.

In TL process the energy is provided by heating the phosphor materials. Since the OSL intensity as well as TL intensity is proportional to x-ray dose, the phosphor materials which exhibit the OSL or TL phenomenon offer an alternative to conventional x-ray film (Nanto, 1999). Figue 4 shows typical OSL spectra and their stimulation spectra of various IPs. In all IPs, the OSL peaked at about 400 – 450 nm is observed by stimulating with about 550-650 nm light. The OSL phenomena can, therefore, be applied to the computed radiography using IP with BaFBrI:Eu phosphor materials as well as to individual radiation monitoring and environmental monitoring using LiF:Mg (Saez-Vergara, 1999) TL dosimeters or Al2O3:C OSL dosimeter (Sarai, 2004). The OSL of the Luxel badge using Al2O3:C photostimulabule phosphor can be observed at about 420 nm by stimulating with about 520 nm light.

Fig. 4. OSL spectra and its stimulation (excitation) spectra of various imagig plates. Here, the IP, type-BAS-MS using BaFBrI:Eu photostimulable phosphor is commercially available from Fuji Film Corp.
