Vol.2 No.3 2009
Research paper : Development of battery-operated portable high-energy X-ray sources (R. Suzuki)−222−Synthesiology - English edition Vol.2 No.3 (2009) and energy consumption can be solved.The carbon nanostructure used in the newly created X-ray tube was developed by the companies engaging in joint research. The graphene sheet composed of carbon has a coniferous form, and the tip has a nanometer size tubular structure that becomes thicker on the substrate side. It is therefore mechanically stable and the electric field readily concentrates at the tip, and this allows electron emissions with high current density of 100 mA/cm2 or higher at room temperature.Initially, an X-ray tube was created as a prototype with this carbon nanostructure as an electron source, molybdenum mesh as an extraction electrode, and a metal plate as a target. However, in this structure, the mesh electrode became hot and emitted gas, and abnormal electrical discharge occurred. The carbon nanostructure had a critical problem where the nanostructure was damaged by discharge and the emission property deteriorated, and this caused frequent abnormal discharges. To overcome this flaw, we designed and fabricated an X-ray tube, using the simulation code used in accelerator development, with a structure in which the electron beam focused on the target efficiently without using mesh electrodes. This X-ray tube is a bipolar X-ray tube where the electron source of the cathode is charged with a negative high voltage and the target of the anode is charged with a positive high voltage (Fig. 1).The fabrication process of the X-ray tube using carbon nanostructure involved the stabilization treatment of the emission current called aging. Since the abnormal discharge in this phase damaged the electron source, we searched by trial-and-error the processing condition that allowed stabilization of the emission current without discharge. As a result, we obtained a cold cathode X-ray tube that could generate high-output X-rays of almost the equivalent to that of the thermionic electron emission X-ray tube. Since this X-ray tube did not have the heater or filament and this characteristic could be utilized in a portable X-ray evaluation, we created a battery-operated portable X-ray generator as shown in Fig. 2. This X-ray generator produces X-rays by storing power temporarily in the power storage circuit using one AA battery as the power source. X-rays are produced as the X-ray tube is activated by high voltage generated in the high-voltage generating circuit when power necessary to generate X-rays is accumulated. The high-voltage generating circuit can generate voltages of ±50 kV or more and X-rays of 100 keV or more. This X-ray tube has extremely high energy efficiency since the energy consumption falls to almost zero when it is not generating X-rays. Also, it is convenient as a portable X-ray source since it does not require warming up and can generate X-rays immediately. It is also portable since the total weight is 5 kg or less including the power source.Figure 3 shows the X-ray transmission image of a table tap shot using the X-ray tube activated on 2 J power from the power storage circuit in Fig. 2 supplied to the high voltage generating circuit. The openness of the electrodes in the outlet can be visualized. However, the resolution is insufficient at 2 J, as noises stand out in the fine structures when the irradiated area is large.Fig. 1 X-ray tube using the carbon nanostructure electron source.Fig. 3 (Top) Device under test: table tap. (Bottom) X-ray transmission image photographed using the battery-operated X-ray source. Input power 2 J.Anode: targetCathode: carbon nanostructure electron sourceVoltageboostercircuitPowerstoragecircuitHigh-voltagegeneratingcircuitX-ray tubeX-rayFig. 2 Diagram of battery-operated X-ray generator.