Establishment of Radiation Safety Management Guidelines for Crookes Tubes Used in the Teaching of Science

概要

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 used with an induction coil as a power supply. By applying a voltage of several tens of kV between the cathode and the anode in the tube, the cations in the evacuated tube are accelerated to hit the cathode, which knock out secondary electrons. These electrons 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 for observation of electricity and current in the second year of junior high school, and all five science textbooks contain information about Crookes tubes. In addition, due to the revision of the curriculum guidelines in March 2017, a content of understanding 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 lack of radiation education in school for a long time. Practically, teachers and students may be exposed to X-ray radiation during a demonstration. Some studies showed that 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 5 cm). 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 since May 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 operating conditions.
• 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 teaching of science. Practically, actual learning has a better learning effect than via video or theory. 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 of radiation 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 the following parts: (a) the X-ray energy emitted from the Crookes tube; (b) electrical parameters and the correlation of operation factors to the characteristics of output X-ray spectrum; (c) leakage dose; (d) transmission of X-rays. To estimate the effectiveness of operational conditions to the output X-ray spectrum, the Crookes tube operated with various applied voltages from the induction coil. Since the Crookes tube emitted very low energy X-rays, all X-ray spectra were acquired with a 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. The induction coil generated unstable voltage with a pulse-shaped width of 20 µs. The inhomogeneous high-voltage pulses were measured by a digital oscilloscope, and a 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 matched well with the distribution of the most frequent voltage. In addition, the attenuation measurement estimated the transmission of X-rays through Al layers. The effective energy was interpolated from the attenuation coefficient showed a good agreement with 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 the discharge 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 might cause exposure to participants, but the characteristics of dose distribution was an ambiguous 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 the present 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 the range 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 the body) 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 to collect 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 the electron beam inside the Crookes tube. The present study concentrated on investigating the transmission properties of transparent such as acrylic, lead acrylic, glass materials, and pure aluminum used as the compared material. The lead acrylic had the same transmission factor with aluminum, while that of acrylic was greatly higher. The glass aquarium covered the whole Crookes tube with the transmission factor of 30% that showed the good effect of radiation shielding. In addition, the transmitted dose for each demonstration (for 10 min) was below the limited 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 this side was also the highest dose distribution. From the present results, the participants should keep a distance of 1m from the tube. It strictly avoids the position 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.