Vol.2 No.3 2009
Research paper : Development of battery-operated portable high-energy X-ray sources (R. Suzuki)−225−Synthesiology - English edition Vol.2 No.3 (2009) For this accelerator, new circuit using a high-voltage high-current semiconductor switch was developed for the generation circuit of the high-voltage pulses to be supplied to the microwave amplifying tube (klystron). High-power microwaves of about 2 MW were generated by running the C-band klystron with small semiconductor switches of about 18 cm × 7 cm × 7 cm, and the electron beam was successfully accelerated.The attainment, in the process of R&D of this C-band accelerator, of higher frequency of microwaves, the high-voltage semiconductor switch, and the high-voltage pulse generation was the technological base for realizing the new X-ray source.3.3 Battery-operated ultra-small electron acceleratorsIn the 2000s, there was an increase in accidents due to aging of pipes in plants as well as leakage of steam from the pipes in nuclear power plants, and the social demand for on-site evaluation increased. Pipes with covering materials such as heat insulations were extremely troublesome to inspect since the covering had to be removed, and an inspection method without removing the covering material was desired. At that time, the author et al. were conducting research on downsizing and energy savings in electron accelerators as described in sections 3.1 and 3.2. As mentioned in chapter 1, thinking that the findings from our research could fulfill the social demands, we did a conceptual design for an electron accelerator and an X-ray source system that pursued ultimate downsizing and energy savings. Here, we returned to the basics to redesign the conventional accelerator technologies such as efficient high-voltage pulse generation, microwave generation, electron generation, and control technology. We developed and fabricated prototypes based on the details obtained, and succeeded in generating high-energy X-rays by operating an ultra-small electron accelerator with AA batteries.This ultra-small electron accelerator is composed of the electron gun, the accelerating tube, the microwave source, the vacuum pump, the pulse generator, and the control system, just as in large electron accelerators. Since the conventional accelerator had several accelerating tubes, temperature controlled water was necessary to synchronize their resonance frequencies. Since the new accelerator has only one accelerating tube, no cooling is necessary as the heat load is low when operated on dry cell batteries. This accelerator employs a method of synchronizing the resonance frequency by varying the frequencies rather than keeping the resonance frequency constant by controlling the temperature of the accelerating tube. It is a system without a water cooling/heating system that required large amounts of power consumption like in the conventional accelerator. The vacuum pump is an ion pump that uses a little amount of power in high vacuum.In this accelerator, a 9.4 GHz X-band pulse magnetron tube that has higher frequency than the aforementioned C-band is used as a microwave source. To operate this magnetron and the electron gun of the accelerator, the battery power source is boosted to 12 kV or higher, the electric energy is stored in the storage circuit, and the high-voltage pulses of about 100 kW are generated at intervals of about 1 microsecond using the semiconductor switch. By supplying to the accelerating tube the 9.4 GHz microwaves generated in the magnetron tube with the high voltage pulses, the electron beam is accelerated and a high-energy electron beam of 100 keV or over is generated. X-rays are generated by injecting this electron beam onto the heavy metal target.Figure 7 shows a photograph of the prototype of the main body of the accelerating tube composed of the electron gun, the accelerating tube, the vacuum pump (ion pump), the X-ray target (gold film), and the X-ray emission window. It is about palm size (the accelerating tube is about 3 cm) and the weight is about 1.5 kg. The flange and valve in the photograph are necessary only in the prototype, and the weight will be reduced to half when they are removed. This main body and the components for microwave and power sources can be fit in a small camera case and carried easily with one hand.This ultra-small accelerator has a peak electric consumption of 100 kW order, but since the pulse width is 1 microsecond, the average power consumption can be 20 W or less by lowering the pulse rate, and X-rays can be generated with 10 to 12 AA batteries. X-ray transmission imaging is possible by combining this X-ray source with the X-ray imaging system. To complete this system, we created the technology to generate high-voltage high-power pulses from dry cell batteries by trial-and-error. This could be applied to the electron emission property tests for carbon nanostructures and X-ray generators mentioned below, and X-ray emission windowTargetAccelerating tubeVacuum pumpElectron gunFig. 7 Main body of the X-band ultra-small electron accelerator.