**2. Instrumental**

The apparatus collected to the world-wide gamma ray spectra in the deep sea environment, through hundreds dives of manned submersibles Shinkai6500/2000 and Remotely Operated

Environmental Gamma-Ray Observation in Deep Sea 57

spherical NaI(Tl) scintillator has been adopted and stored into Al-pressure container with high voltage supply and with multi-channel pulse height analyzer (PHA). The power supply for the underwater part is +12 V or +24 V according to the specifications of platforms (submersibles, towed-fish, etc.). When the γ-ray was detected by the system, its energy level was determined by PHA and transferred via serial communication of 9600 baud of RS-232C. Under this constraint, the channels of PHA are limited to 256. Firstly, the system was equipped on a towed-fish (Deep-tow system) and then applied on submersibles and other platforms, including a real time observatory (Hattori et al., 2000; Iwase et al., 2001). The approximate sensitivity of the apparatus is 0.25 nGy/h/cps, which is significantly reduced

Once NaI(Tl) scintillation system rolled out, Ge-semiconductor detector had also tried to use. Instead of its high resolution to resolve the energy distribution of γ-ray radiations in environment, the sensitivity of Ge-semiconductor system is one-third or one order lower than that of NaI(Tl) scintillation system. Thus, the Ge-semiconductor system has limited to a trial production in JAMSTEC although another institutions, e.g. Japan Atomic Energy Research Institute (JAERI), tried further development for environmental monitoring (Ito et

In analysis, the peel-off (stripping) method is applied to resolve the contributions from different nuclides to the total gamma-ray intensity. The method identifies the total energy peaks from approx. 3MeV to lower. In this energy region, the scintillation spectra are composed of photopeaks and their Compton continuums. Here, each of Compton continuums is assumed to have flat distribution. Thus, to resolve the photon energy distribution, the Compton continuums were sequentially peeled off from higher energy level to lower (Okano et al., 1982). These protocols are partly available on commercial applications, e.g. Seiko EG&G Co. Energy calibrations are performed for all time-series data typically for individual dives by using frequently identified three photopeaks: 609 keV of

Due to the effective shield by seawater, total count rate in water is rather lower than that of sub aerial measurement (Yoshida and Tsukaraha, 1987). According to the geological environment, total count rate of sub aerial environment is 20-100 nGy/h in Japan, which is equivalent to approx. 80-400 counts per second (cps) in our system: 1 nGy/h is equivalent to 4cps. Figure 2 is an example of the time-series record of total count rate in submersible dive:

by the shielding by their pressure vessels.

al., 2005).

**2.1 Protocols of analysis** 

214Bi, 1460 keV of 40K and 2614 keV of 208Tl.

Fig. 1. Schematic block diagram of the Deep-sea gamma-ray sensor.

**3. Intensities of gamma-radiation in seawater and on seafloor** 

Vehicles (ROVs). In addition, an on-line real-time environmental gamma ray observatory has operated at deep seafloor of 1174m water depth for more than ten-years at Hatsushima Observatory, Sagami-Bay, southern coast of mainland Japan.

All the deep-sea systems developed are based on an on-land system originally developed at RIKEN (the Institute of Physical and Chemical Research; Okano et al., 1980; Kumagai and Okano, 1982). Since 1984, Deep-Sea Research department of JAMSTEC had developed deepsea gamma ray sensor available down to 6500m of water depth (Hattori et al., 1997; 2000; 2002). To achieve required sensitivity for the system, NaI(Tl) scintillation counter has applied to the measurement that stored into Al-pressure vessel (Plate 1). Figure 1 is a schematic block diagram of the system. During the development, various assemblages of high voltage supply, data transfer, and sizes or shapes of scintillator were tried; three or two inches spherical, or three inches cylindrical scintillators. As the current model, three inches

Plate 1. Deep sea gamma-ray sensor for a manned submersible *Shinkai6500*. NaI(Tl) scintillator are in the Al-pressure container (dark green colour, wrapped by black plastic tape, left). Under an operation, the sensor unit is connected thorough an underwater connector on the pressure container (silver coloured cover) to a feed-thorough on the pressure hull of submersible by underwater cable. The data signals are transferred to PC via power supply box (silver coloured) where +12V DC power is converted from AC power supplied from submersible to feed to the sensor unit.

Vehicles (ROVs). In addition, an on-line real-time environmental gamma ray observatory has operated at deep seafloor of 1174m water depth for more than ten-years at Hatsushima

All the deep-sea systems developed are based on an on-land system originally developed at RIKEN (the Institute of Physical and Chemical Research; Okano et al., 1980; Kumagai and Okano, 1982). Since 1984, Deep-Sea Research department of JAMSTEC had developed deepsea gamma ray sensor available down to 6500m of water depth (Hattori et al., 1997; 2000; 2002). To achieve required sensitivity for the system, NaI(Tl) scintillation counter has applied to the measurement that stored into Al-pressure vessel (Plate 1). Figure 1 is a schematic block diagram of the system. During the development, various assemblages of high voltage supply, data transfer, and sizes or shapes of scintillator were tried; three or two inches spherical, or three inches cylindrical scintillators. As the current model, three inches

Plate 1. Deep sea gamma-ray sensor for a manned submersible *Shinkai6500*. NaI(Tl) scintillator are in the Al-pressure container (dark green colour, wrapped by black plastic tape, left). Under an operation, the sensor unit is connected thorough an underwater connector on the pressure container (silver coloured cover) to a feed-thorough on the pressure hull of submersible by underwater cable. The data signals are transferred to PC via power supply box (silver coloured) where +12V DC power is converted from AC power

supplied from submersible to feed to the sensor unit.

Observatory, Sagami-Bay, southern coast of mainland Japan.

spherical NaI(Tl) scintillator has been adopted and stored into Al-pressure container with high voltage supply and with multi-channel pulse height analyzer (PHA). The power supply for the underwater part is +12 V or +24 V according to the specifications of platforms (submersibles, towed-fish, etc.). When the γ-ray was detected by the system, its energy level was determined by PHA and transferred via serial communication of 9600 baud of RS-232C. Under this constraint, the channels of PHA are limited to 256. Firstly, the system was equipped on a towed-fish (Deep-tow system) and then applied on submersibles and other platforms, including a real time observatory (Hattori et al., 2000; Iwase et al., 2001). The approximate sensitivity of the apparatus is 0.25 nGy/h/cps, which is significantly reduced by the shielding by their pressure vessels.

Fig. 1. Schematic block diagram of the Deep-sea gamma-ray sensor.

Once NaI(Tl) scintillation system rolled out, Ge-semiconductor detector had also tried to use. Instead of its high resolution to resolve the energy distribution of γ-ray radiations in environment, the sensitivity of Ge-semiconductor system is one-third or one order lower than that of NaI(Tl) scintillation system. Thus, the Ge-semiconductor system has limited to a trial production in JAMSTEC although another institutions, e.g. Japan Atomic Energy Research Institute (JAERI), tried further development for environmental monitoring (Ito et al., 2005).

#### **2.1 Protocols of analysis**

In analysis, the peel-off (stripping) method is applied to resolve the contributions from different nuclides to the total gamma-ray intensity. The method identifies the total energy peaks from approx. 3MeV to lower. In this energy region, the scintillation spectra are composed of photopeaks and their Compton continuums. Here, each of Compton continuums is assumed to have flat distribution. Thus, to resolve the photon energy distribution, the Compton continuums were sequentially peeled off from higher energy level to lower (Okano et al., 1982). These protocols are partly available on commercial applications, e.g. Seiko EG&G Co. Energy calibrations are performed for all time-series data typically for individual dives by using frequently identified three photopeaks: 609 keV of 214Bi, 1460 keV of 40K and 2614 keV of 208Tl.
