**2.1.3 DIS dosimeters**

124 Environmental Monitoring

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.

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

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 40 [μSv].

Fig. 5. Schematic diagram of the DIS dosimeter

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

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


Table 1. Basic characteristics of each solid state dosimeter

Environmental Background Radiation Monitoring Utilizing Passive Solid Sate Dosimeters 127

The DIS as shown in Fig.7 (right) was supplied from RADOS Technology, Finland. The basic principle of the DIS 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 transistor in air or gas space surrounded by a conductive wall. The DIS dosimeter is based on Analog-EEPROM (Analog Electrically Erasable Programmable Read Only Memory). The DIS responds to X, γ, β-rays and neutron. This dosimeter has an excellent energy characteristic and can be read repeatable without quenching of the data. The DIS dosimeter can widely detect a radiation

The personal dosimeters GD-450, Luxel badge (Type S) and DIS-1 were set on 7 points in

Each of 7 points are represented as alphabet of from A to G. (A: Tsurugi-machi, B: Tatsunokuchi, C: Inside of house of Mt. Shishiku, D: Outside of Mt. shishiku, E: Inside of Ogoya Mines, F: Outside of Ogoya Mines, G: Public Health and Environmental Science). Each data was obtained monthly. Photographs of local points in which the dosimeters were set up are shown in Fig.9. Data were obrained monthly. The each accumulated monthly data was divided into daily data and multiplied 30 days. The each data was compensated appropriately with the each formula for the dosimeters (Sarai, 2004). The same point data were averaged and the standard deviations were calculated. The data of GD-450 were

Fig. 8. Map of seven points such as Tsurugi-machi (◆), Tatsunokuchi (●), outside of

Ogoya Mines (○) and rooftop of Ishikawa Prefecture Institute of Public health and Environmental Science (□). in Ishikawa prefecture, in which the environmental radiation

Mt.Shishiku (■), inside of house in Mt.Shishiku, (▲), outside of Ogoya Mines (◇), Inside of

dose within the range from 1 μSv to 40 Sv

compared with the data of the other dosimeters.

dose using the glass dosimeter were measured.

Ishikawa prefecture as shown in Fig.8.
