A new x-ray tube design made of carbon nanotubes (CNTs) may be the most significant advance in x-ray technology in 100 years, and could lead to portable and miniature x-ray sources for medical and industrial applications, says a group of physicists and material scientists at the University of North Carolina (UNC; Chapel Hill, NC). The novel CNT cold-cathode x-ray tube generates room-temperature emission and a controllable output current and repetition rate.1
Conventional thermionic cathodes are limited by a slow response time, high power consumption, and a high operating temperature (up to 1000°C) that decreases the average lifetime of x-ray filaments to less than a year. The imaging resolution in typical diagnostic x-ray machines is also limited because the distribution of electrons is random.
The cold-cathode CNT device demonstrated by the group readily produces continuous and pulsed x-ray emission with a programmable waveform. The device is also smaller than thermionic cathodes, and exhibits a more focused x-ray pulse and a sharper response time that may help in the tracking of moving objects.
Current progress
Previous cold-cathode x-ray tubes made from diamond or carbon were hindered by unstable or inadequate current needed for imaging (10 to 50 mA for fixed anode and 50 to 500 mA for rotating anode tubes). By optimizing the morphology of CNT films, UNC professor Otto Zhou and colleagues have generated x-ray flux comparable to that of conventional x-ray tubes. Peak emission current of 28 mA from a 0.2-cm2 area of film resulted in x-ray intensity high enough to image detailed bone structure of a human hand (see figure). This emission current, says the team, is an order of magnitude higher than previous macroscopic cathodes.
The macroscopic cathodes were made of purified single-wall CNT bundles produced by a laser ablation method at UNC. The average nanotube diameter was 1.4 nm and the average bundle diameter was approximately 50 nm. The single-wall CNTs were deposited electrophoretically in a single layer on an iron-coated metal substrate. The cathodes were then arrayed in a triode-type field-emission x-ray tube. In the experimental setup, a gated field-emission cathode and a copper target were placed 50 to 200 µm apart. A 100-kHz x-ray beam was generated by the setup; the diameter of the focused beam was approximately 3.2 mm. The energy spectrum of the generated radiation is similar to that of thermionic electrons.
The group is continuing active research to enhance the performance of the field-emission cathodes and will explore their applications in other devices. The x-ray technology has been licensed by a UNC startup, Applied Nanotechnologies (Chapel Hill, NC), of which Zhou is also chairman. The company is developing the technology with end users, he reports.
Reference
- G. Z. Yue et al., Appl. Phys. Lett. 81, 355 (July 2002).