Abstract: In Japan, Crookes tube has been used in the teaching of science for junior-high school students where the first purpose is to learn characters of electron and current. During experiments, the X-rays radiated from a Crookes tube caused a high dose (Hp(0.07) up to 250 mSv/h at 5 cm) that may expose to the participants. However, a low energy X-ray is hard to measure using conventional survey meters. From March 2017, a project named “Establishment of radiation safety management system for low energy X-rays at education sites” was started in Japan. Its scope aims to establish a guideline on radiation safety management at the educational sites. In this study, as the partials of the project, we investigated characteristics of X-rays emitted by Crookes tubes, the relevance of operational factors and the effects of electrical setting on leakage dose from Crookes tube.
A variable applied voltage (HV) from a power supply (an induction coil) to a Crookes tube created inhomogeneous energy of X-rays radiated from a Crookes tube. In a conventional Crookes tube, the maximum applied voltage was approximately 25 – 40 kV, and the tube emits X-rays at the low energy of approximately 16 – 20 keV. By increasing the applied voltage, the X-ray energy peak was shifted to a higher region in the X-ray energy spectrum. As a result, the X-ray energy spectrum and applied voltage were changed by HV equipment setting that gives a large change in leakage dose. The results also indicated that the dose depended on a change of current in the Crookes tube, and changed with a distance as a square of the distance. Recently, we initially applied cloud chamber to simulate the X-ray energy spectrum from the Crookes tube. It showed a correlation between X-ray track length and X-ray energy. With a low energy X-ray of 19.5 keV, the length was observed at 3~4 mm by the cloud chamber.
Keywords: Crookes tube, radiological education, radiation protection, X-ray energy spectrum
