A vacuum-sealed miniature X-ray tube based on carbon nanotube field emitters
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A vacuum-sealed miniature X-ray tube based on a carbon nanotube field-emission electron source has been demonstrated. The diameter of the X-ray tube is 10 mm; the total length of the tube is 50 mm, and no external vacuum pump is required for the operation. The maximum tube voltage reaches up to 70 kV, and the X-ray tube generates intense X-rays with the air kerma strength of 108 Gy·cm2 min−1. In addition, X-rays produced from the miniature X-ray tube have a comparatively uniform spatial dose distribution.
KeywordsInsertion Position Braze Process Electron Beam Size Spatial Dose Distribution Alumina Ceramic Tube
A miniature X-ray tube is a small X-ray generation device generally with a diameter of less than 10 mm [1, 2, 3, 4, 5]. Because of the feasible installation in a spatially constrained area and the possibility of electrical on/off control, miniature X-ray tubes can be widely used for nondestructive X-ray radiography, handheld X-ray spectrometers [1, 2], electric brachytherapy, and interstitial or intracavitary radiation therapy or imaging with the substitution of radioactive isotopes [3, 4, 5]. Miniature X-ray tubes have been developed mostly using thermionic electron sources [3, 4] or secondary X-ray emission .
Meanwhile, X-ray tubes based on carbon nanotube (CNT) field-emission electron sources have been extensively developed because CNT emitters have several advantages compared with thermionic electron sources. The advantages of CNT emitters include (1) cold electron sources, and hence, little heat is generated inside the tube  which is important for the minimization of an X-ray tube; (2) simplicity and easy controllability in a pulse operation [7, 8]; (3) high current density for electron and X-ray microscopy devices [9, 10]. Several types of X-ray tubes have also been developed using CNT field emitters [11, 12, 13, 14, 15]. However, the miniature X-ray tubes are mostly not vacuum-sealed and thus should be operated in a vacuum chamber or with a vacuum pump. In addition, the maximum operating voltages of the miniature X-ray tubes were less than 30 kV. As a consequence, the X-ray tubes have limited practical applications.
In this paper, we report that we have developed a vacuum-sealed miniature X-ray tube using a CNT field emitter. The miniature X-ray tube can be operated up to 70 kV and produces X-rays with very high intensities and a comparatively uniform spatial distribution.
Fabrication of the miniature X-ray tube
Figure 1c,d show a photograph and an X-ray radiograph (XR) of the fabricated miniature X-ray tube, which show the exterior and the interior of the tube, respectively. The diameter of the X-ray tube is 10 mm, and the total length is 50 mm. The weight of the tube is only 14.5 g. All of the connection parts of the X-ray tube are tightly vacuum-sealed. The both ends of the alumina ceramic tube were vacuum-brazed with a focusing electrode assembly and a connecting anode, respectively. Both electrodes were made of Kovar (Carpenter Technology Corporation, Reading, PA, USA) that has a similar thermal expansion coefficient to alumina. The connecting anode was used to interconnect a ceramic tube and a Be X-ray window that have different thermal expansion coefficients. The connecting anode and the Be window were also vacuum-brazed. All the components of the X-ray tube were baked at 550°C for 10 h, and subsequently, these were brazed through a single-step brazing process at 680°C for 30 min in a vacuum furnace. Before the brazing process, electron emission and transport tests of the X-ray tube have been carried out inside a vacuum chamber. The position of the cathode inside the focusing electrode could be finely controlled through this process. A non-evaporable getter film was installed around the focusing electrode to evacuate the X-ray tube. The getter was activated during the brazing process. The outer part of the sealed X-ray tube, except the target, was covered with a layer of silicone resin to improve high-voltage insulation between the cathode and the X-ray target. We observed that the fabricated X-ray tube was stably operated up to 70 kV without any high-voltage breakdown or discharge at both the inner vacuum side and the outer air side.
Results and discussion
Performance and characterization of the X-ray tube
Figure 2b shows the dose rate of X-ray that is produced from the miniature X-ray tube and the stability of the dose rate with time. The dose rate was measured with an ionization chamber and an electrometer (PTW 34013 and Unidos-E both from PTW, Freiburg, Germany) at 1 cm apart from the X-ray tube in air. The air kerma strength of the X-ray tube operating at 50 kV with the tube current of 252 μA was as high as 108.1 Gy·cm2 min−1, which is approximately 15 times higher than that of a 10-Ci HDR 192Ir radioisotope source  that is widely used for brachytherapy. The fluctuation of the X-ray dose rate was as low as ±2.7%. In addition, the X-ray tube has worked for over 2 months with no significant change in the electron beam current (0.25 mA at 50 kV) and the X-ray dose rate. The X-ray tube has been continuously operated for approximately 1 h/day. Consequently, the developed miniature X-ray tube produces high enough X-ray output and exhibits very good short-term and long-term stabilities.
To further evaluate the performance of the miniature X-ray tube, the X-ray focal spot size was measured following the European standard EN 12543–5. An XR image of a copperplate (thickness 0.1 mm) was taken with a magnification factor of 1.25 (Figure 4b). The image profiles in both the horizontal and vertical directions were analyzed, and from the analysis, the X-ray focal spot size was calculated to be 3.72 mm in the horizontal direction and 3.64 mm in the vertical direction. The focal spot size of the X-ray corresponds to the electron beam size at the X-ray target. Therefore, the measurement results for the X-ray focal spot size suggest that the inner diameter (7 mm) of the present miniature X-ray tube can be reduced, and accordingly, the X-ray tube can be further minimized.
In summary, we have demonstrated a vacuum-sealed miniature X-ray tube based on CNT field-emission electron source. The X-ray tube can be operated up to 70 kV, and high-dose X-rays are generated with a comparatively good spatial dose distribution. Due to the small electron beam size in the X-ray tube, the prototype X-ray tube can be further miniaturized. We believe that such a vacuum-sealed miniature X-ray tube can be used for various industrial and medical diagnostic/therapy purposes.
SHH has a Ph.D. in Nuclear Engineering and is a president of a ventured company. HJK and JMH are MS students. SOC is a professor of Nuclear Engineering.
This study was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (no. 2011–0018418) and the R&D Program of MKE/KEIT (10035553).
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