Abstract
Since the discovery of X-rays over a century ago the techniques applied to the engineering of X-ray sources have remained relatively unchanged. From the inception of thermionic electron sources, which, due to simplicity of fabrication, remain central to almost all X-ray applications, there have been few fundamental technological advances. However, with the emergence of ever more demanding medical and inspection techniques, including computed tomography and tomosynthesis, security inspection, high throughput manufacturing and radiotherapy, has resulted in a considerable level of interest in the development of new fabrication methods. The use of conventional thermionic sources is limited by their slow temporal response and large physical size. In response, field electron emission has emerged as a promising alternative means of deriving a highly controllable electron beam of a well-defined distribution. When coupled to the burgeoning field of nanomaterials, and in particular, carbon nanotubes, such systems present a unique technological opportunity. This review provides a summary of the current state-of-the-art in carbon nanotube-based field emission X-ray sources. We detail the various fabrication techniques and functional advantages associated with their use, including the ability to produce ever smaller electron beam assembles, shaped cathodes, enhanced temporal stability and emergent fast-switching pulsed sources. We conclude with an overview of some of the commercial progress made towards the realisation of an innovative and disruptive technology.
Highlights
Since the discovery of X-rays in 1895 [1], X-ray analysis and diagnostics have been some of the most widely researched areas in science and engineering
The gate assembly was mounted approximately 100 μm adjacent to the Carbon nanotubes (CNT) electron emitter resulting in a device geometry capable of sustaining a beam current of several milliamps, this would likely prove challenging to manufacture in a practical X-ray source
Compared to other electron emission materials, the potential of carbon nanotubes is largely unparalleled. Their attractive morphological and electronic properties are very promising for future electron source technologies
Summary
Since the discovery of X-rays in 1895 [1], X-ray analysis and diagnostics have been some of the most widely researched areas in science and engineering. J.J. Thomson had already established their ionising nature [2] and it was recognised that X-rays had different characteristics, initially termed qualitatively as “hard”, “medium” and “soft” [3]; a classification which related to the relative absorption by soft tissue and bone. Thomson had already established their ionising nature [2] and it was recognised that X-rays had different characteristics, initially termed qualitatively as “hard”, “medium” and “soft” [3]; a classification which related to the relative absorption by soft tissue and bone Even at this early phase the implications for medical diagnostics was clear. We detail the current state-of-the-art in CNT emitter fabrication including the electron source and the gate electrode micro-fabrication, functional enhancements including reduced turn-on electric fields and enhanced stability via the incorporation of adlayers, improved X-ray beam distribution symmetry achieved through cathode shaping, micro focal sources, pulsed emission, multi-pixel sources, and miniaturisation. We conclude by providing an overview of the commercial progress to date
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