Vol.2 No.3 2009
Research paper : Development of battery-operated portable high-energy X-ray sources (R. Suzuki)−226−Synthesiology - English edition Vol.2 No.3 (2009) led to the advancement of the device development in a short time period.3.4 Application of the carbon nanostructure electron source to the X-ray sourceThe electron source of the aforementioned ultra-small electron accelerator must have the heater power source ON at all times including when X-rays are not being generated, since it employs the principle of thermionic electron emission, and can operate for only about four hours on 10 AA batteries. Therefore, dry cell batteries tend to be used as emergency power source, and this is not an X-ray source that could be truly used any time, anywhere. This problem of thermionic electron emission electron sources does not only apply to high-frequency electron acceleration but also applies to general portable X-ray sources.This issue can be solved if there is a high-performance electron source that does not require a heater or a filament, but carbon nanotubes (CNT) that produce electron emission at room temperature have a disadvantage in that the structure is destroyed and deteriorates readily under a strong electric field like X-ray tubes. Therefore we looked for a cold cathode electron source with high stability under high electric field, and focused on a carbon nanostructure (CNX) electron source developed by certain companies. This electron source had a coniferous form that became thicker on the substrate side, and the tip was a nanometer structure as in CNT and the electric field concentrated at the tip. Since it was considered to be more stable in high electric fields than CNT, we expected it to be a hopeful X-ray source.Therefore, we started the product development in July 2008, after checking its function in a preliminary experiment using the CNX electron source. Other carbon-based cold cathode electron sources were commercially available, but we selected the CNX electron source because the company brought the manufacturing machine for the electron source to AIST, so an environment that allowed free trial-and-error became available for the development of the X-ray source. Experiments under various conditions were possible, and the new X-ray generator described in chapter 2 was realized.4 DiscussionHere, the findings from the development of the battery-operated high-energy X-ray generator are discussed.The development of the battery-operated high-energy X-ray generator using carbon nanostructures was realized through the combination of the researches for energy saving and downsizing of the electron accelerator and the technology for a battery-operated ultra-small electron accelerator that were being conducted at AIST, and the technology for the carbon nanostructure electron source of private sector companies.The motivation for the downsizing research of electron accelerators was from the necessity that arose in the course of research, to solve the issues of the large electron accelerator that was owned by AIST. We became aware that the energy saving and downsizing of the accelerator might find wide industrial applications in nondestructive evaluation, medicine, and sterilization, and set them as new development topics. The development of the components for small C-band accelerators led to the development of the energy saving technology and of ultra-small accelerators. In the C-band small electron accelerators and the X-band ultra-small electron accelerators, the human resources and facilities including those for the conventional S-band electron accelerators, radiation detection technology, and radiation-controlled area were greatly useful. This result would not have been achieved without the large electron accelerator facility.For energy savings in accelerators, we were able to execute various energy saving measures, rather than leave them as mere desk plans, because we ran into an opportunity of renovating the aged air conditioning and water cooling/heating systems. The measures included experimental ones that would not have been employed in an ordinary electron accelerator system. We were able to accumulate the technologies and know-hows for energy savings in accelerators by observing the effectiveness of the various measures.The development of the X-band ultra-small electron accelerator was the result of the combination of the social demand for a portable high-energy X-ray source and the necessity for downsizing and energy savings in the electron accelerator at AIST. Although this accelerator had a heater for thermionic electron emissions, and could not be called a truly practical portable X-ray source, by presenting our technological level by publishing our result to the outside world, we were able to find a new technology called the carbon nanostructure electron source. Moreover, the companies worked on this development with passion, brought the manufacturing machines for the electron source to AIST, and through repeated trial-and-error and concentrated effort, we were able to develop a truly practical X-ray generator in just half a year.The results of this research is the result of the integration of various factors in addition to individual technologies, including facilities, people, change in the research environment, accumulation of technology, social demands, and the publication of the results. However, the factors do not lead to new results if they are simply collected. For example, if the linac was in operation without any problem, one would not think actively about energy savings or downsizing or employing an electron source with carbon nanostructure, and the developments that followed might not have occurred.