Do Duy Khiem

称号及び氏名博士（工学）

学位授与の日付

令和 2 年 9 月 25 日論

「Establishment of radiation safety management guidelines

文

名for Crookes tubes used in the teaching of science（学校教育現場で用いられるクルックス管の放射線安全管理ガイドラインの確立）」

論文審査委員

主査

松浦 寛人

副査

古田 雅一

副査

宮丸 広幸

副査

秋吉 優史

論文要旨

Crookes tube is the oldest fundamental X-ray device that was used by
Roentgen to start the study of radiation. It is a type of discharge tube, usually usedwith an induction coil as a power supply. By applying a voltage of several tens ofkV between the cathode and the anode in the tube, the cations in the evacuatedtube are accelerated to hit the cathode, which knock out secondary electrons. Theseelectrons emitted by the cold cathode are accelerated to collide with the glass tube
to create a bremsstrahlung X-ray.

In Japan, the curriculum guidelines demand to use Crookes tubes forobservation of electricity and current in the second year of junior high school, andall five science textbooks contain information about Crookes tubes. In addition,
due to the revision of the curriculum guidelines in March 2017, a content ofunderstanding the nature of the radiation associated with the vacuum discharge
was newly added, and further utilization of the Crookes tube has been required.
Currently, many teachers are demonstrating on Crookes tube in a science class.
But they do not know that the X-rays emitted from the Crookes tube owing to lackof radiation education in school for a long time. Practically, teachers and students
may be exposed to X-ray radiation during a demonstration. Some studies showedthat Crookes tubes emitted X-rays with very low energy (approximately 20 keV)but the 1 cm dose equivalent, Hp(10), was remarkably high (up to 143 mSv/h at 5cm). It caused a maximum dose (Hp(10)) of 0.15 mSv per experiment for students,
which is higher than the recommended value in ICRP publication 36.
The X-rays emitted from the Crookes tube can be used as highly educational
contents if it is used properly. Even if the Crookes tube is in such use, radiation
safety must be ensured. However, radiation protection and safety guidelines have
not yet been evaluated sufficiently to date. Basing on that urgency, the “Crookes
tube project” has been launched nationwide in Japan by volunteer scientists sinceMay 2017, and it is achieving the goal at a fast speed. The project aims to establish
and promulgate the radiation safety management guidelines on Crookes tube at
educational sites. This thesis involved a variety of tasks in the “Crookes tube
project” and performed the following findings:

Measurement: X-ray energy spectrum, electric operational parameters, dose
spatial distribution,
Evaluation: equivalent dose, the relevance between the dose and operatingconditions.
Protection: ALARA principle in radiation protection, improving operating
conditions of equipment, improving in usage,
Recommendation: provisional guidelines to manage radiation safety.
In Vietnam, although radiological education added to high school curricula
tens of years ago, there was not a practical lesson of Crookes tube in the teachingof science. Practically, actual learning has a better learning effect than via video ortheory. The research in the thesis is the premise for the application of the Crookes
tube in teaching science in Vietnam future.
Chapter 1 introduced general information about the history of the Crookes tube,
its applications in teaching science, and the “Crookes tube project” in Japan. The
Crookes tube is a device that plays a prominent role in talking about the history ofradiation in science class. This chapter also presented the essentials of the Crookes
tube project, progressing works, and involving tasks.
Chapter 2 presented characteristics of low-energy X-ray radiated from a
Crookes tube used in the teaching of science. This chapter investigated thefollowing parts: (a) the X-ray energy emitted from the Crookes tube; (b) electricalparameters and the correlation of operation factors to the characteristics of output
X-ray spectrum; (c) leakage dose; (d) transmission of X-rays. To estimate theeffectiveness of operational conditions to the output X-ray spectrum, the Crookestube operated with various applied voltages from the induction coil. Since theCrookes tube emitted very low energy X-rays, all X-ray spectra were acquired witha high-performance X-ray and gamma-ray CZT detector. A handcrafted collimator
kit was used to reduce the pile-up effect due to the high intensity of X-rays. Theinduction coil generated unstable voltage with a pulse-shaped width of 20 µs. The
inhomogeneous high-voltage pulses were measured by a digital oscilloscope, anda voltage divider circuit was used to avoid damaging the oscilloscope’s probes.
The effective energy of X-rays was approximately 20 keV, and the applied voltage
was in the range of 16 kV – 40 kV. The distribution of the peak energy matchedwell with the distribution of the most frequent voltage. In addition, the attenuation
measurement estimated the transmission of X-rays through Al layers. The effectiveenergy was interpolated from the attenuation coefficient showed a good agreementwith the results obtained by the CZT detector. The present results indicated that
effective energy and exposure increased with the increase of the applied voltage.
However, the energy and dose were limited when the in-air spark occurred between
the electrodes on the induction coil. It was found that the distance between thedischarge electrodes controlled the effective energy and also dose.
Chapter 3 investigated the dose distribution from the Crookes tube using
thermoluminescent dosimeters. During the demonstration, the leakage dose mightcause exposure to participants, but the characteristics of dose distribution was anambiguous factor. In this chapter, the dose spatial distribution of X-rays
surrounding the Crookes tube was estimated using thermoluminescent dosimeters(TLDs) and obtained the 70 µm dose equivalent, Hp(0.07). The results in thepresent study characterize and map the spatial dose distribution of the Crookestube. It indicates where is the minimum exposure position to prevent radiation
hazards. To evaluate sufficiently the inhomogeneous radiation field, 2-D
measurement of the dose distribution was performed. TLDs were attached to 2-D
human body shape, in which the size was fitted to a junior-high-school student.
The 2-D human body shape was placed at four aspects of the tube (front, back,right, and left) for the dose assessment. Hp(0.07) doses on the body were in therange of 0.23 ± 0.01 – 0.46 ± 0.02 mSv/h at 1 m. The highest dose distributed on
the aligned direction to the head of the Crookes tube (central-upper portions of thebody) and declined on the lower parts. On the other hand, the doses were severely
low for the back side and the left side, and they distributed in the range of 0.07 ±0.01 – 0.16 ± 0.01 mSv/h, and 0.10 ± 0.01 – 0.23 ± 0.01 mSv/h, respectively. At
the highest output, the maximum Hp(0.07) dose was 0.08 ± 0.01 mSv for 10
minutes at the distance of 1 m from the front of the tube.
Chapter 4 estimated the transmission of X-rays through the popular materials tocollect workable materials for radiation shielding. According to the ALARA
principle, exposure is possible to minimize by distance, time, and shielding. For
teaching science, radiation shielding material must attenuate radiation intensity
effectively, and its light-transmittance ensures to observe the behavior of theelectron beam inside the Crookes tube. The present study concentrated oninvestigating the transmission properties of transparent such as acrylic, lead acrylic,glass materials, and pure aluminum used as the compared material. The leadacrylic had the same transmission factor with aluminum, while that of acrylic wasgreatly higher. The glass aquarium covered the whole Crookes tube with thetransmission factor of 30% that showed the good effect of radiation shielding. Inaddition, the transmitted dose for each demonstration (for 10 min) was below thelimited dose of the ICRP 36. These results revealed that lead acrylic and the glass
aquarium were suitable to protect participants against exposure to X-ray radiation
irradiated from Crookes tubes in the science class.
Chapter 5 included the conclusion of the thesis, the summary of the previous
chapters, and guidelines of radiation safety management for the Crookes tubes.
The major output of the Crookes tube was the front surface of the tube, and thisside was also the highest dose distribution. From the present results, theparticipants should keep a distance of 1m from the tube. It strictly avoids theposition facing the front of the tube. On the induction coil, the distance of discharge
electrodes should be set shorter than 20 mm and never remove them. A
demonstration should conduct within 10 minutes with low output power to keep
the minimum radiation exposure. Besides, it is effective in reducing radiation
exposure using transparent shields such as lead acrylic or glass aquarium during
demonstrations.

審査結果の要旨
本論文は、主に中学校の教育現場で用いられるクルックス管を安全に取り扱うためのガイドラインの確立の基礎となる低エネルギーX 線のエネルギー分布、空間線量分布、遮蔽特性の実験的研究を行ったものである。クルックス管は陰極線管とも呼ばれ、中学校学習指導要領において、電流、放電、放射線に関連した単元での授業での実演が進められている。他方、古い製品の一部には、非常に高い強度の X 線が漏洩してくるものがあることが指摘されている。しかるに、生成される X 線のエネルギーは極めて低く、通常の放射線サーベイメーターなどでは正確な線量測定もできない。本研究では、クルック
ス管から漏洩してくる X 線の基礎特性を正確に測定し、その成果を中学校の理科教育のためのガイドライン制定に還元している。特に、注目すべき成果は以下のとおりである。
(1)クルックス管に印加する誘導コイルの出力電圧は一定ではなく、振幅の異なるパルス状である。パルスピークの高電圧で加速された電子は管壁を容易に透過する高エネルギーX 線を生じる。本研究では、クルックス管外部に漏洩する X 線のエネルギースペクト
ルおよび被ばく線量と運転条件の関係を初めて明らかにすることができた。
(2)クルックス管ではビーム状の電子がガラス壁に衝突して X 線を発生するため、X 線の放射には強い異方性があると予想される。本研究では被ばく線量の空間分布を詳細に測定し、被ばくのリスクを最小限にするための重要な知見を得ることができた。
(3)低エネルギーX 線は金属板で容易に遮蔽できるが、教育現場の利用を考えると身の回りにある透明な材料の使用が望ましい。本研究では市販のガラス水槽がかなりの遮蔽効果を示すことができ、ガイドラインにも反映させることができた。
以上の諸成果は、教育現場でのクルックス管使用にとどまらず、同じく低エネルギーX 線を日常的に使用している医療現場の看護師の被ばく低減に重要な知見を与える。本研究はこのように X 線の発生の物理機構のみならず、放射線案管理の上からも有益な情報を提供したものであり、関連分野の学術的・産業的な発展に貢献するところ大である。また、申請者が自立して研究活動を行うのに必要な能力と学識を有することを証したものである。
学位論文審査委員会は、本論文の審査および最終試験の結果から、博士（工学）の学位を授与することを適当と認める。