環境中の放射性核種による線量評価

(1)

細則様式第１－２号

学位請求論文の内容の要旨

領域医療生命科学分野放射線生命科学

氏名

Chanis Pornnumpa

（論文題目）

Dose assessment due to radionuclides in the environment

環境中の放射性核種による線量評価

主査柏倉幾郎

副査中村敏也

副査井瀧千恵子

副査床次眞司

【

Introduction

】

Accumulation of radiations in the environment is a cause of risk leading to cancer from breathing the air that contains radioactive aerosol particles or radiation exposure from high background areas. In general, radiation particles in air such as radon (

²²²

Rn), thoron (

²²⁰

Rn) and their progeny can enter into the body by inhalation. Those radionuclides are produced from soils, rocks and earth’s crust by uranium-238 and thorium-232.

²²²

Rn and

²²⁰

Rn with their gas status can be inserted through the air gap of the ground into atmosphere or accumulated in the dwelling. Some types of the building material that make from natural materials can release

²²²

Rn and

²²⁰

Rn as well. In addition, radioactive absorption from the

artificial

radionuclides such as

Iodine-131 (¹³¹I),

Cesium-134 (

¹³⁴Cs), and

Cesium-137 (

¹³⁷Cs)

is an important cause of risk from radiation. The

artificial

radionuclides are found in medical industries and nuclear power plants. Some of those radionuclides can accumulated and affect the target’s organ in the body.

Especially, long – lived radionuclides when are collected in the body.

¹³⁴

Cs and

137

Cs are one of the radionuclide that can be collected in muscles. The important radionuclide such as strontium-90 (

⁹⁰

Sr) can accumulate in the bone as well.

（注）論文題目が外国語の場合は，和訳を付すこと。

(2)

【細則様式第１－２号続き】

The aim of the present study is protection against environmental radiation exposures for human’s health. In this study, the dose assessment was divided into three parts in order to focus particularly from the viewpoint of radiological estimation in air. In the first chapter, indoor

²²²

Rn and its progeny were investigated in a Japanese dwelling by observing their characteristics during using air appliances to reduce

²²²

Rn progeny exposure. In the second chapter, the absorbed dose rate in air was estimated in Namie Town after the Fukushima Daiichi Nuclear Power Plant accident for dose assessment from long-lived radionuclides. In the third chapter, an exposure chamber was developed for calibration of radioactive gas monitors from the viewpoint of quality assurance and quality control (QA/QC) of radiation measurements.

【

First chapter

】

Characteristics of radon and its progeny in a Japanese dwelling while using air appliances.

Characteristics of

²²²

Rn and its progeny were investigated in different air conditions by turning four types of indoor air appliances on and off in a two-story concrete Japanese dwelling. The four appliances were: air-conditioner, air-cleaner, gas heater and cooker hood. The measurements were done using two devices: (1) a Si-based semiconductor detector for continuous measurement of indoor

²²²

Rn concentration and (2) a ZnS(Ag) scintillation counting system for equilibrium equivalent

²²²

Rn concentration (EERC). Throughout the entire experiment, the cooker hood was the most effective in decreasing indoor

²²²

Rn concentration over a long period of time and the second effective was the air-conditioner, while the air-cleaner and gas-heater did not affect the concentration of

²²²

Rn. However, the results measured in each air condition will differ according to the lifestyles and activities of the inhabitants. In this study, indoor

²²²

Rn and its progeny in a Japanese dwelling were characterized by the different air conditions.

【

Second chapter

】

Investigation of absorbed dose rate in air by car-borne survey in Namie Town, Fukushima Prefecture.

Namie Town, Fukushima Prefecture was instituted as an evacuation zone

(3)

and a controlled area following the Fukushima Daiichi Nuclear Power Plant accident in April, 2011. A significant amount of radionuclides were released in the atmosphere and environment immediately following the accident. Radionuclides with long half-lives, such as

¹³⁷

Cs ( T

^1/2

: 30.05 y),

¹³⁴

Cs ( T

^1/2

: 2.04 y), and

⁹⁰

Sr ( T

^1/2

: 28.8 y) may be a cause of a cancer risk from consuming contaminated food or water.

In this study, a car-borne survey which used a Nal(Tl) scintillation spectrometer was carried out in Namie on August 22

^nd

–24

^th

, 2014 and September 14

^th

–16

^th

, 2015 to make a dose rate distribution map for monitoring the time variation of absorbed dose rate in air. The maximum, minimum and geometric mean with their standard deviation of absorbed dose rates in air for 2014 were estimated to be 5.8 ± 0.1 µGy h

^-1

, 0.08 ± 0.001 µGy h

^-1

and 1.0 µGy h

^-1

, respectively, while in 2015 the maximum, minimum and geometric mean were measured at 5.6 ± 0.1 µGy h

^-1

, 0.05 ± 0.001 µGy h

^-1

and 0.9

µGy h^-1

respectively. However, since 2011, the dose rate level has decreased around 88% when compared to the maximum value in 2014 and 88% in 2015.

【

Third section

】

Development of the exposure chamber for QA/QC.

A 150-L stainless steel exposure chamber was developed for radioactive gas monitor calibration. The system was setup with three main sections. In the first section,

²²⁰

Rn concentration controlling system was setup using lantern mantles for generating

²²⁰

Rn gas, which could achieve the concentration of 10,000 – 26,000 Bq m

^-3

. In the second section, two gas monitors were connected with the exposure chamber for measuring the

²²⁰

Rn concentration inside. A Si-based semiconductor detector was used to continuously measure

²²⁰

Rn concentrations, while a portable radiation monitor with a scintillation cell was used for grab sampling of

²²⁰

Rn concentration as using a scintillation cell was regarded as a standard technique. In the third section, a humidifier was used for controlling relative humidity inside the exposure chamber which could be set to around 30 – 60 %.

As an example of the calibration exercise for the radioactive gas monitor,

alpha track density was measured using a solid state nuclear track detector

(4)

(SSNTD) with high and low-air exchange chambers (RADUET). The exposure level was assigned to four different levels for the

²²⁰

Rn concentration: 500 kBq h m

^-3

, 1,000 kBq h m

^-3

, 2,000 kBq h m

^-3

, and 3,000 kBq h m

^-3

. The radioactive gas monitor could be calibrated by this system with high performance.

The results in this study will be important information for the reduction of

risks from environmental radiation exposure for protection of human’s health.

(5)

学位論文のもととなる研究成果としての筆頭著者原著

論文題目

Characteristics of indoor radon and its progeny in a Japanese dwelling using air appliances

著者名

C. Pornnumpa, S. Tokonami, A. Sorimachi and C. Kranrod

掲載学術誌名

Radiation Protection Dosimetry

巻，号，項

5

項

掲載年月日

環境中の放射性核種による線量評価

Chanis Pornnumpa

Dose assessment due to radionuclides in the environment

Introduction

Accumulation of radiations in the environment is a cause of risk leading to cancer from breathing the air that contains radioactive aerosol particles or radiation exposure from high background areas. In general, radiation particles in air such as radon (

Rn), thoron (

Rn) and their progeny can enter into the body by inhalation. Those radionuclides are produced from soils, rocks and earth’s crust by uranium-238 and thorium-232.

Rn and

Rn with their gas status can be inserted through the air gap of the ground into atmosphere or accumulated in the dwelling. Some types of the building material that make from natural materials can release

Rn and

Rn as well. In addition, radioactive absorption from the

radionuclides such as

Cesium-134 (

Cesium-137 (

is an important cause of risk from radiation. The

radionuclides are found in medical industries and nuclear power plants. Some of those radionuclides can accumulated and affect the target’s organ in the body.

Especially, long – lived radionuclides when are collected in the body.

Cs and

Cs are one of the radionuclide that can be collected in muscles. The important radionuclide such as strontium-90 (

Sr) can accumulate in the bone as well.

The aim of the present study is protection against environmental radiation exposures for human’s health. In this study, the dose assessment was divided into three parts in order to focus particularly from the viewpoint of radiological estimation in air. In the first chapter, indoor

Rn and its progeny were investigated in a Japanese dwelling by observing their characteristics during using air appliances to reduce

First chapter

Characteristics of radon and its progeny in a Japanese dwelling while using air appliances.

Characteristics of

Rn concentration and (2) a ZnS(Ag) scintillation counting system for equilibrium equivalent

Rn concentration (EERC). Throughout the entire experiment, the cooker hood was the most effective in decreasing indoor

Rn concentration over a long period of time and the second effective was the air-conditioner, while the air-cleaner and gas-heater did not affect the concentration of

Rn. However, the results measured in each air condition will differ according to the lifestyles and activities of the inhabitants. In this study, indoor

Rn and its progeny in a Japanese dwelling were characterized by the different air conditions.

Second chapter

Investigation of absorbed dose rate in air by car-borne survey in Namie Town, Fukushima Prefecture.

Namie Town, Fukushima Prefecture was instituted as an evacuation zone

and a controlled area following the Fukushima Daiichi Nuclear Power Plant accident in April, 2011. A significant amount of radionuclides were released in the atmosphere and environment immediately following the accident. Radionuclides with long half-lives, such as

Cs ( T

: 30.05 y),

Cs ( T

: 2.04 y), and

Sr ( T

: 28.8 y) may be a cause of a cancer risk from consuming contaminated food or water.

In this study, a car-borne survey which used a Nal(Tl) scintillation spectrometer was carried out in Namie on August 22

–24

, 2014 and September 14

–16

, 2015 to make a dose rate distribution map for monitoring the time variation of absorbed dose rate in air. The maximum, minimum and geometric mean with their standard deviation of absorbed dose rates in air for 2014 were estimated to be 5.8 ± 0.1 µGy h

, 0.08 ± 0.001 µGy h

and 1.0 µGy h

, respectively, while in 2015 the maximum, minimum and geometric mean were measured at 5.6 ± 0.1 µGy h

, 0.05 ± 0.001 µGy h

and 0.9

respectively. However, since 2011, the dose rate level has decreased around 88% when compared to the maximum value in 2014 and 88% in 2015.

Third section

Development of the exposure chamber for QA/QC.

A 150-L stainless steel exposure chamber was developed for radioactive gas monitor calibration. The system was setup with three main sections. In the first section,

Rn concentration controlling system was setup using lantern mantles for generating

Rn gas, which could achieve the concentration of 10,000 – 26,000 Bq m

. In the second section, two gas monitors were connected with the exposure chamber for measuring the

Rn concentration inside. A Si-based semiconductor detector was used to continuously measure

Rn concentrations, while a portable radiation monitor with a scintillation cell was used for grab sampling of

Rn concentration as using a scintillation cell was regarded as a standard technique. In the third section, a humidifier was used for controlling relative humidity inside the exposure chamber which could be set to around 30 – 60 %.

As an example of the calibration exercise for the radioactive gas monitor,

alpha track density was measured using a solid state nuclear track detector

(SSNTD) with high and low-air exchange chambers (RADUET). The exposure level was assigned to four different levels for the

Rn concentration: 500 kBq h m

, 1,000 kBq h m

, 2,000 kBq h m

, and 3,000 kBq h m

. The radioactive gas monitor could be calibrated by this system with high performance.

The results in this study will be important information for the reduction of

risks from environmental radiation exposure for protection of human’s health.

Characteristics of indoor radon and its progeny in a Japanese dwelling using air appliances

C. Pornnumpa, S. Tokonami, A. Sorimachi and C. Kranrod

Radiation Protection Dosimetry

5

April 27, 2015