High-brilliance X-ray sources for improved resolution, reduced exposure
In conventional X-ray systems, a hot semiconductor filament acting as a cathode emits electrons when heated to high temperatures (the thermionic effect). High voltage accelerates the electrons, generating the X-ray radiation. Thermionic cathodes emit a continuous stream of electrons with no possibility of pulsed mode operation. In addition, the random source of electrons has limited brilliance. Scientists initiated the EU-funded project 'Advanced X-ray source based on field-emitting carbon nanotube cold cathode' (AXIS) to significantly enhance X-ray technology and, simultaneously, the competitiveness of EU’s small and medium-sized enterprises (SMEs). Field or 'cold' emission is the emission of electrons subjected to a large electric field as a result of the quantum–mechanical tunnel effect. Despite its potential to overcome limitations in thermionic emission, its application has been challenging due to a lack of high-performance electron field emitters. The scene is changing with promising field emission carbon nanotubes (CNTs). CNT brilliance is an order of magnitude higher than any other electron sources and CNTs can operate in both continuous and pulsed mode. Higher brilliance enables higher resolution or reduced exposure time. Pulsed mode operation makes synchronisation with biological events such as heart rate or respiration possible. Finally, the systems are smaller and consume less power, making powerful X-ray sources for portable applications a realistic possibility. CNTs have been employed successfully, although only in random configurations without control over CNT size or orientation. AXIS scientists achieved unprecedented control of emitting properties through the fabrication of electron sources based on well-aligned arrays of CNTs. They then developed a CNT-based electron or e-gun, a system combining the CNT cathode with optics to focus the electron beam. The technology led to fabrication of two different X-ray systems, a tomographic one for biomedical applications and another for material metrology. Advanced X-ray sources based on organised arrays of field emission CNTs have been fabricated within the scope of the AXIS project. Engineering optimisation is expected to lead to unsurpassed brilliance in both pulsed and continuous modes of operation. Commercialisation will put high-power portable X-ray technology where it needs to be for industrial, medical, environmental and even cultural heritage analyses.
