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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. X-ray applications. X-ray sources find application in a wide range of applications including medical diagnostics, electronics inspection, food security and border control.

Conventional X-ray sources are pervasive and the integration of carbon nanomaterials has the potential to complement the market dominance of traditional thermionic technologies. The strategies for developing new X-ray sources are based on criteria driven by the needs of current applications. Despite the apparent maturity of the technology, many critical challenges remain, including; rapid beam pulsing, dose reduction, improved image contrast, and enhancement of the spatial and temporal resolution. In this review we discuss the use of carbon nanotubes CNTs as a platform for emerging novel field emission X-ray sources.

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. Thermionic and Field X-ray emission technologies. Not the active anode cooling. As a result, electrons are excited and pass over the potential barrier. PE electron sources have a low efficiency. Much of the incident optical radiation is absorbed in the bulk material of the emitter with only a small proportion of the photon population contributing to direct emission. Although PE sources have the potential to achieve extremely fast response rates, and correspondingly high bandwidths, PE has gained very little traction in most electron emission applications as only very low emission currents are possible.

In contrast, TE can derive appreciable current densities that are capable of stimulating X-ray emission. Significant current densities, and consequent heating therein, necessitate rotating anode and active cooling systems in TE sources. Since the emission is intimately dependent on the filament temperature [ 15 ] — as increasing the emitter temperature allows for much of the electron population to pass over the surface barrier - such tubes enable analogue control over the magnitude of the emission current.

In TE sources this beam current is controlled by monitoring the anode current and adjusting the inferred filament temperature using a closed-loop control system. The intrinsic finite thermal inertia of the heated coil, when coupled to the lagging response of such feedback control results in a comparatively slow time response, often several hundreds of milliseconds. In addition, care must be taken to limit the filament drive current to prevent excessive power dissipation, with subsequent damage or destruction of the filament. Issues with severe electromigration can be a significant challenge.

A key design functional parameter is the focal spot size, which is related, in part, to the dimensions of the electron emission area. This requires the adoption of techniques to provide first order focussing. Some TE X-ray tubes are fitted with two filaments, allowing a choice of focal spot sizes, though this is at the expense of maximum available beam current and hence photon output. In order to achieve further reduction in focal spot size, electrostatic and magnetic focussing techniques are often employed, though at the expense of source complexity and size.

An extended analysis of the background theoretical emission mechanisms can be found in [ 19 ]. Various refinements to this model have been applied [ 20 ] to take account of the potentially elevated temperatures during emission and the extreme curvature at the nano emitters apex when whisker-like one dimensional nanomaterials are used as the field emission source. Indeed, the high electric field that is required for field emission is closely associated with the curvature and aspect ratio of the emitting material — this field enhancement factor depends upon a number of geometrical factors such as the precise form and orientation of the emitter; in general sharp whisker-like tips emit at significantly lower potentials compared to the same material that has adopted a planar morphology [ 21 ].

Such geometry-based arguments are the leading rationale for the use of high aspect ratio nanowires in field emission applications. However, it remains challenging to attain such perturbed, high aspect ratio emitters. Following early work by Dyke [ 22 , 23 ], in Spindt published details of a new method of fabricating FE arrays based on Mo conical structures. These structures were 1.

During the subsequent years, the use of Spindt emitters was widely adopted. They have since become common place in many electron emission systems. Spindts can be found in field emission displays [ 26 , 27 , 28 ] to high speed radio frequency devices [ 18 , 29 , 30 ], such as travelling wave tubes [ 31 , 32 ]. Nevertheless, the issue of developing suitably high current densities with low turn-on voltages has remained an on-going challenge, principally due to demanding requirements on the emitting material.

At the time, low attainable aspect ratios that Spindt emitters offered, though better than their planar counterparts, limited emitter performance. A new material capable of forming extremely high aspect ratios was required. Self-assembly via chemical vapour deposition and the emergence of nanowires and nanotubes allowed for such high aspect ratios to be fabricated over large areas.

The geometry of these new emitters allowed for a corresponding amplification in the field enhancement factor and subsequent reduction in turn-on voltage, typically by an order of magnitude. Comparison of common field emitting materials. Adapted from [ 34 ]. The discovery of carbon nanotubes CNTs is generally credited to Iijima in the early s [ ], although it is clear that there had been activity in this area for a considerable time prior to this [ ].

This work, however; lead to a heightened interest which gave rise to some of the first studies on FE using CNTs in [ , ]. By the start of the following decade a range of applications had been identified to utilise these novel field emitters, including high resolution electron beam microscopes [ , , , ], flat panel displays [ 27 , 28 , , , ], RF devices [ 18 , 32 ], electron beam lithography [ , , ] and X-ray cathode emitters [ , , , ].

An early review [ ], provides a concise overview of the state-of-the-art prior to Here we provide a condensed review of the progress, as it pertains to X-ray sources, since then.

Applied Optics

CNTs have some of the highest attainable aspect ratios, high thermal conductivity, low chemical reactivity in non-oxidising atmospheres, highly parallelised en masse fabrication, a low sputtering cross-section, a low secondary electron coefficient, and an insensitivity to direct ion-bombdarment. CNTs are becoming increasingly inexpensive with the release of new, ever larger growth reactors. This limits their practical application as the material platform on which the emitters are fabricated largely dictates the tip robustness towards poor or compromised vacuum conditions which result in aggressive local ionization.

In the case of the metallic Spindts, poor vacuum conditions caused tip degradation. As a result, much of the published work has been accomplished using demountable systems, which incorporate vacuum pumps to maintain the performance, although there are some notable exceptions [ , ]. A useful summary on this was published in [ ]. It is the group of applications associated with CNT-based X-ray FE emitters [ ] that is the subject of this paper, as the properties of the electron emitting CNTs offer many functional and performance advantages over conventional TE X-ray sources [ , ].

The way in which this review is structured details: shaped cathodes, micro focal sources, pulsed sources, multi-pixel sources, and miniaturised emitters. The review concludes by summarising the commercial progress to date. Severe problems with CNT electron source fabrication, reliability, time stability, spatial uniformity, and reproducibility have beset the technology and have prevented its wide spread adoption, particularly in high beam current applications. Here they grew vertically aligned forests of CNTs on Co coated W wire, with the electron beam controlled by a counter electrode mounted 0.

This setup produced a beam current of just 1. The lifetime of the cathode assembly was little more than one hour and the images exhibited severe noise artefacts. Nevertheless, this work did convincingly demonstrate the potential of CNTs in X-ray applications whilst simultaneously highlighting critical functional issues such as fluctuations in emitted beam current that needed to be addressed.

Another early example of a functioning X-ray tube was produced by Haga et al. Catalytically synthesised CNTs and carbon nanofibres CNFs grown on a Pd wire were used as the FE source, although there was no counter electrode, or gate used to extract the electron beam. Clear images were acquired, though requiring a long and technologically unacceptable integration time of the order of minutes.

Many more examples have now been published on the use of similar triode configurations [ , , , , ]. All X-ray tubes require the generation of a beam of electrons. This electron beam is directed towards the anode, which subsequently liberates X-rays when impacted. It is the cathode assembly that provides the source of electrons and it is the cathode design and materials which dramatically influences the resultant performance.

The commercial field of vacuum electronics is now well established and was initially based on Spindt-like emitters, a comprehensive review on which is provided by Temple et al. The incorporation of CNTs within the cathode is the focus of this review as it is currently the subject of intense research in an attempt to enhance the electron emission, and hence, X-ray emission performance.

Common carbon nanotube thin film deposition techniques. CNT thin films are readily patterned using a variety of techniques. There are numerous aspects which are yet to be well developed in the literature. Nevertheless, the attainable pattern resolution is intimately related to the orientation of the constituent CNTs within the thin film and the patterning technique employed. Wet processed thin films — such as dropped, cast, vacuum filtrated, sprayed, and screen printed — are fundamentally wet chemistry deposition techniques that can be patterned by either post deposition etching or shadow masking to protect zones from being coated with the CNT inks during the deposition process.

Without the application of external driving forces, such as magnetic or electric fields, in such patterns the CNTs are always misaligned with respect to one another. Nevertheless, again, the CNTs are disordered and misaligned. In these additive approaches there remain significant challenges in preventing the nozzle from clogging during the deposition or the nanolithography tip becoming deformed.

As a result inkjet printing and dip-pen nanolithography necessitates the use of very short CNTs, which dramatically compromises their usefulness in FE applications. Micro-contact printing is another additive approach to cathode patterning. Here a polymeric stamp is inked with a CNT solution and then placed in contact with a substrate which has an engineered surface hydrophobicity to ensure the CNTs adhere. Interfacial engineering marks the broadband application of this technique and often necessitates the use of self-assembled monolayers which act as adhesion promoters.

Such approaches have proven useful in patterning carbon nanomaterials [ ], though their spatial resolution is again limited to a few micrometers at present. In addition to the additive patterning approaches discussed, there are various subtractive means available. Most common is by way of conventional lithographically patterning the deposited CNT thin film, defining a hard mask, and oxygen plasma etching the exposed CNTs.

Nevertheless, common to all wet chemistry approaches to CNT deposition remains the fact that the CNTs are disordered and unaligned with respect to one another. Indeed, inclusion of plasma during heating and exposure to the gaseous hydrocarbon and atomic hydrogen sources can be used to assist in the catalytic activity and alignment of the CNTs.

Another important consideration when noting the various merits of the available deposition techniques is the effective roughness and area uniformity achieved. The various deposition techniques produce films of varying thickness uniformity. CVD far exceeds the degree of uniformity of all other techniques, followed, in rank order, by casting, vacuum filtration, screen printing, and finally spray and drop casting; the latter of which typically gives coffee-stained like thin films that have a significant spatial variation in CNT density.

Nevertheless, for processing simplicity and related cost reductions rather than functional reasons, most CNT-based FE X-ray sources are based on wet chemistry processed thin films. What follows here summarises the fabrication details of some of the more common CNT thin film fabrication techniques, with particular emphasis on those that have been used in CNT-based FE X-ray sources. This is likely a direct consequence of their low cost and straightforward processing rather than any direct functional enhancements the fabrication techniques allow for.

Drop, cast and spray techniques all require CNT inks. In each case, respectively, these are deposited either by direct dropping of the ink, spin coating or casting of the ink, or spray coating of the ink using a pressurised carrier gas. All these approaches initially require wet processing of the as-grown CNTs and it is this which, commonly to all, limits their usefulness and the consequent performance of the resulting X-ray source.

The required high power sonication and vacuum unstable surfactants needed to form the stabilised, homogenous inks results in compromised temporal stability, deleterious hysteresis, and generally degraded emission. Electrophoresis is a derivative of inking deposition. Electrophoresis involves the motion, placement, and potentially modest alignment of drop or spray deposited CNTs.

Electrophoresis exploits the CNTs native anisotropic charge distribution and its interaction with an applied electric field [ , , ]. Elsewhere it has been widely used as a means of depositing phosphor materials. It has also been applied to the deposition of bundles of SWCNTs on various substrates, such as stainless steel or doped Si [ , , , , , , , , , , , , ].

Composite CNT films deposited by electrophoresis have been fabricated with controlled surface density with good field emission performance, current density and long-term stability under high operating voltages, which has been applied to an electron source for high-resolution X-ray imaging [ ]. Wang et al. As with other solution-based fabrication techniques, in order to reduce the emitters turn-on bias they were taped to activate them; a threshold field of 3.

Such techniques are comparatively low in uniformity and reproducibility, a direct consequence of the simple macro scale processing. Moreover, the common binder matrices are far from pure, making elucidation of the underlying emission mechanisms somewhat challenging, though nonetheless functional. Indeed, as with all other inking deposition techniques, the emitters do require mechanical activation for the cathodes to be of any practical use, though electrophoretic patterning is indeed rapid and simple to implement.

Here CNT solutions are deposited at the macro-scale and form highly disordered, nominally planar, spaghetti-like networks with but a few individual CNTs standing upright, at many unregistered angles relative to one another. In the case of vacuum filtration the CNT ink is poured onto a porous mixed nitro cellulose ester membrane, which has a partial vacuum applied on its opposite side. Such porous membranes have controlled apertures, typically around 0. The reduced pressure stimulates the solvent within the CNT ink to pass through the membrane, whilst the membrane stops the CNTs from passing.

Once the ink reservoir is depleted the thin film is rinsed with de-ionised water to remove much of the deleterious surfactants, leaving a thin CNT film on the membrane. Note, however; that much of the surfactant still remains even after extensive rinsing using deionised water. The membrane is then dissolved, by exposure to acetone or methanol, and the CNT thin film remains.

The CNT film thickness, and hence sheet resistance and optical transparency, is controlled by adjusting the amount of CNT ink filtered through the membrane. CNT thin film fabrication by vacuum filtration has been employed for more than a decade. It offers a rapid, low cost way to fabricate CNT thin films. Little to no infrastructure is required and the films can be processed rapidly, over large areas.

Nonetheless, as in the case of drop, cast and spray, such chemi douche processing requires stabilised inks. CNTs experience high inter-tube van der Waals forces; they tend to agglomerate. Though this interaction has been exploited elsewhere to fabricate novel aligned nanostructured membranes [ ], such agglomeration is problematic in producing homogenous CNT inks and solutions. As such, various, often sodium-based, surfactants such as sodium dodecylbenzene sulphate, sodium dodecyl sulphate, and Triton X are required to produce homogenous solutions with the CNTs well-dispersed throughout the solution.

Further aggressive acid treatments and extended durations under high power ultrasonication significantly degrade the length, crystallinity and subsequent electronic character of the CNTs which necessarily limits the electron emission performance. These inks are then transferred to metallic disks using conventional screen printing methods or vacuum coated mixed cellulose ester membranes that are subsequently dissolved in acetone following transfer [ ].

The morphology of these films is highly disordered. They typically lack high aspect ratio protrusions, resulting in poor field emission performance. FEs fabricated in this way have a number of intrinsic problems; chiefly that mechanical taping is required to activate the surfaces and enhance their field emission characteristics.

Taping, using adhesive coated tape, increases the surface roughness of the CNT thin-films. It is a macro-scale process and affords very little reproducibility, Moreover; the necessary surfactants are usually vacuum unstable giving rise to emission profiles that deleteriously drift with time [ , , , ]. CNT inks can often have significant out-gassing [ ] when even modestly heated, compromising the vacuum envelope. The resultant reduction in field emission performance and reduced current density has, as a result, prevented CNT-based pastes and ink from gaining commercial traction, though cathodes fabricated in this way appeal as the reduced reproducibility is off-set by the ease of fabrication.

Such films are also somewhat dynamic and often have weak adhesion to the substrate. Their morphology shifts with time during the application of a high electric field due to the intrinsic torque induction within the CNTs due to the tip or root positioning of the growth catalyst particle.

To obviate issues of weak interfacial adhesion between the substrate and the CNTs, Kim et al. No surfactants were used. Another fabrication problem associated with screen printing is the limitation on the pattern resolution. These can clog the screen printing mesh resulting in a low porosity and inability to print. Formation of stabilised, homogenous inks is central to drop, cast, spray, vacuum filtration and screen printing.

The formation of these inks, as highlighted above, requires deleterious ultra-sonication and aggressive acid treatments, both of which degrade the length and electronic character of the CNTs. Though the necessary wet chemistry approaches provide a facile, rapid and inexpensive route to fabricate the emitter, the constituent CNTs are coated with deleterious vacuum unstable surfactants that can only be removed following high temperature post-deposition treatments which, if not fully removed, would otherwise cause significant out-gassing during FE operation [ ].

Maintaining slurry and ink consistency over time and between batches is difficult, which manifests as a reduction in device-to-device reproducibility. It is also challenging to pattern emitters fabricated in this way though screen printing the migration of CNT inks through patterned apertures within a regular mesh , as demonstrated by Kim et al. Though screen printing is indeed large-area compatible, it is rather low resolution and thus limits the degree of control over the detailed design of the electron source.

Oxygen plasma etching coupled to conventional lithographic techniques is another viable option though significant surface roughness of the CNT thin film can again compromise the maximum resolution. Moreover, plasma etching techniques are only applicable to non-organic substrates. While there have been reports that electrophoresis can produce a degree of alignment [ , ] this is relative only to very randomly orientated screen printing and ink-based processes. Misalignment prevents any fabricated devices from realising the full field enhancement factor of the composite CNTs; in order to achieve this alternative fabrication methods capable of aligning, en masse , the CNTs must be considered, with chemical vapour deposition being the most promising method to date.

The CNTs self-assemble from atomic units in a highly parallelised process, which when coupled with high resolution lithographic techniques to pattern the catalyst material, allows for near nano-scale engineering of the CNTs and CNFs.

OSA | Calculation of Modulation Transfer Functions of X-Ray Fluorescent Screens

CVD techniques mediate the growth of chemically untreated disordered or aligned CNT thin films depending on the substrate, catalyst and growth precursors employed. In a typical implementation, Silicon is coated with a physical vapour deposited metal catalyst which is then patterned via lithographic or masking techniques by either additive or subtractive process, such as magnetron sputtering, or plasma etching, respectively.

In situ plasma can also be employed to enhance the catalysis and align the CNTs during growth.

File history

Cole et al. Here the MWCNTs were several microns in length and randomly orientated, with most running adjacent to the substrate; CVD techniques allow for CNT alignment, though random orientation is still possible. Very little control over the type, orientation and area packing density of the CNTs was evidenced with a largely qualitative analysis presented. Rather surprisingly, the robustness of the cathodes toward arcing was clearly shown resulting in an increase in the voltage for a given current following repeated arcing events.

They suggest that either the emitting sites are not completely destroyed or that they are efficiently replaced by other nanotubes within the film. Indeed, disordered films are structurally dynamic when under the influence of a high electric field, which can augment their emission characteristics. Though film reorganisation benefits the long term stability, in that degraded CNTs are in essence replaced, the short term temporal stability is likely to be very poor.

More than cathodes were tested and a poor reproducibility was indeed noted, a probable consequence of the disorder and uncontrolled microscale morphology of the emitter. As is the case for the screen printed and vacuum filtration methods, one potential problem is the degree of adhesion between the CNT and the substrate when exposed to high electric fields. Detailed control of the underlying catalysis has shown that such emitter removal concerns can be solved. Li and Cole et al. Using rapid thermal CVD, Kim et al. Though the emission showed good performance the geometry was not optimised; the dense forest results in significant shielding of the CNTs from the applied electric field — the material appears as bulk - such that the full field enhancement factor of the CNTs was not realised in this instance.

Whilst individual, one dimensional nanowire and nanotube emitters have been empirically evidenced to produce the highest electric fields, the proximity of other emitters will effectively shield the field enhancement [ , ]. In a field emission device, where our interest is in the total available current density, the optimum arrangement will not be that with the highest density of emitters [ , ].

In addition, if the emission pattern of the field emitter is not uniform CVD growth can result in individual CNTs, or structures such as CNT pillar arrays and toroids [ , , , , , ]. Patterns of control electrodes may be grown, in such a way as to focus or concentrate the field from the tip.

The use of CVD to nanoengineer X-ray sources has remained in its infancy due to a number of challenges in explicating the underlying material growth. Nevertheless, following recent advances in the understanding of nanocarbon catalysis [ , , ], the use of CVD-grown CNTs and CNFs in FE X-ray sources appears to be accelerating with it emerging as an emerging as an exciting candidate for viable commercialisation.

The first examples of X-ray emitters using FE electron sources employed diode configurations which comprised only of a cathode and an anode. However, in such devices the emission current was a function only of the anode voltage. As a result such diode configurations gave rather limited control over the magnitude of the emission current for most applications.

To more accurately control the emission current, whilst also providing a degree of protection, it is now standard procedure to introduce a third electrode. This gate electrode gives rise to a triode configuration. Since this naturally produced an anisotropic beam, the control mechanism of choice became a perforated grid, or gate electrode. The purpose of the gate is to create a local electric field that draws the electrons away from the principal emission beam. However, in practice a significant proportion of the emitted electrons will be attracted towards the gate electrode.

The gate must therefore be sufficiently transparent to allow the maximum number of electrons to pass to the anode. However, there has been recent interesting work on improving the transparency of the gate by incorporating graphene layers. Further, the addition of a fourth electrode, mounted just above the gate, has been described [ ]. This electrode had many apertures for each gate perforation, and was mounted approximately 0.

In addition, there are numerous examples of the addition of a further electrostatic focusing electrodes used to reduce the focal spot size. This is described in more detail below. At this stage in the development if FE X-ray sources, many of the published results referenced in this paper relate to tubes constructed in vacuum chambers which can be directly evacuated with turbo molecular, ion, or cryo pumps. Such systems are analogous to the demountable X-ray tube [ ].

There are situations where it can be of considerable benefit to have the ability to open a tube, replace damaged components, and re-evacuate it. In particular, this offers the advantage that it is possible to exchange both the anode — either for one of different target characteristics, or to replace a unit which had been degraded as a result of continuous high beam current — and cathode assembly, following failure or deterioration due to prolonged use.

To date many CNT based sources have been fabricated in what are largely deemed demountable tubes. Clearly, the nature of devices developed for research and design iteration, this is the sensible choice. However some improvements — for example, miniaturisation — have seen the development of a sealed construction [ , , , ].

Performance of CNT-based X-ray sources. Shaped CNT cathodes. Control over the electron beam distribution, and subsequent symmetry of the X-ray beam, can be achieved by shaping the electron emitting areas on the cathode by conventional lithographic techniques. Copyright Institute of Physics. A resist-assisted patterning process was used to produce a cathode assembly consisting of a gate and focusing electrode. This assembly was approximately 0. Ryu et al. Hydrofluoric acid, though cheap and readily available, is rather hazardous to handle and poses a significant health risk.

This somewhat limits the commercial viability of the compaction technique presented. To increase the geometric uniformity of the emitters the team used an electrical aging treatment, which degrades the taller tubes and bundles that would dominate the emission. For a high performing emitter this aging, also termed seasoning, is not desirable and CNTs of very uniform height and diameter are preferred, as evidenced by Teo et al.

Nevertheless, part of the Ryu et al. Detailed analysis and theoretical studies surmising the emission implications of the metallic nanoparticle in the emitter tip are lacking in the literature. High beam current, and therefore current density, is desirable. This gives rise to higher photon flux, and hence shorter detector integration times, in addition to improved signal-to-noise ratios.

Combined with the use of gate control to pulse the X-ray source, high beam currents can be very advantageous to system designers. Recently, micro-fabricated Spindt-like emitters have been used for applications such as static tomography [ ]. These emitters had a measured current capacity greater than that reported for CNTs, though CNTs will almost certainly exceed this once the technology matures.

For current CNT-based sources the emission current density is several orders of magnitude less than that of equivalent area Spindt emitters. Using electrophoretically deposited SWCNTs [ ], high electron beam currents have been achieved using a triode configuration by Yue et al. Some improvement towards increasing the current density has also been achieved by controlling the emitter morphology. Toroidal CNT arrays, which have a central void [ ], are one such example.

By subsequent surface treatments, it is also possible to enhance the native FE characteristics of such arrays by means of the formation of nano tips, tepees and micro cones, as outlined previously [ , ]. The use of emitter forming post-treatments has also been widely investigated.

One leading example is the use of conical CNTs which enhanced the beam current and stabilised the emitter geometry. Another alternative towards higher beam currents is to augment the electronic character of the CNTs, via the dry or wet deposition of various adlayers. Such adlayers adjust the interfacial characteristics at the critical emitter-vacuum interface. This often leads to a decrease in the turn-on voltage and increase the emission current density at a given anode bias. Little work has been reported on the use of adlayers to enhance the emission performance of CNT-based FE X-ray sources.

Nevertheless, significant advantages may be achieved by carefully designing the emitter-vacuum interface to provide high emission currents, limit adverse effects of vacuum leakage, prevent unintentional work function shifts, as well as robustness towards local ionisation and plasma etching, all of which are critical in ensuring long term stability. The failure mechanisms of Spindt and CNT based emitters, which are similar in many respects, have been widely studied [ ], most notably by Bonard et al.

Thermal migration, field sharpening and subsequent avalanche breakdown are perhaps the most common failure mechanism in such whisker-like emission geometries [ ]. CNTs have a high sublimation temperature and high maximum current densities, making them resilient towards arcing events. Pristine CNTs are also largely inert, although when defects are added to the graphitic lattice, often through plasma processing, they are readily damaged and the formed dangling bonds bolster the CNTs emission performance, though often at the expense of the long term stability.

The enhanced emission is due to the augmented surface characteristics. This effect is often noted as a shift to higher turn-on bias during hysteresis studies. Indeed, many plasma and some dopants, such as oxygen when the emitter outgases, can be particularly damaging to the CNTs. Local Ohmic I 2 R heating can also increase the rate of emitter degradation, as can vacuum breakdown.

It has been shown that the axial resistance of CNFs increases with increasing temperature. Interestingly this intrinsic property helps protect the CNFs from degradation during local heating and helps prevent thermal run-away deleteriously increasing the emission current [ ]. Vacuum breakdown results in the emission of extremely high current densities which may cause a plausible local oxygen micro-plasma formation — this etches the CNTs, particularly at their apex.

This shaping stimulates the formation of defects in the graphitic lattice, which preferentially emit electrons, thereby enhancing the emission, however; this results in a temporally unstable emission profile. The CNTs can be sharpened with time, which tends to increase the emission current, or can alternatively be entirely ablated, which reduces the emission current due to the reduced number of electron emitters available.

There have been various methodologies proposed which attempt to reduce temporal instabilities. Thermal annealing or electrostatic seasoning, to remove residual surfactants and non-uniform emitter profiles, are perhaps the most common and certainly the most simple and readily implemented [ ].

This out-gases the emitter, removing weakly surface bound chemisobed species. This increases the work function uniformity across the surface of the emitter. Such out-gassing techniques are also useful in emitter recovery following an arcing event. Arcing events stimulate high current flow which heats and subsequently out-gases the emitter. This out-gassing can lead to further transient arcs which, if allowed to continue, will degrade the emitter. If the emitter is initially well out-gassed any local arcs will only marginally increase the cavity pressure and the emitter will stabilise more rapidly.

Annealing is also employed to enhance the pressure of the vacuum cavity, making local plasma formation increasingly unlikely. The emitters are then left emitting for tens to hundreds of hours to increase the surface smoothness of the emitter and hence, stabilise the emission current. Such approaches are critical in achieving intrinsic emitter stability, and though feedback techniques have been employed to artificially control the stability, engineering intrinsic stability remains central to the formation of a long-term stable emitter.

In feedback based systems, in the same way as conventional TE generator designs, the anode current is monitored and the extraction voltage adjusted accordingly to maintain a known, safe, emission current. Though a viable and widely adopted approach to ensuring emitter stability, the slow response times of the feedback loop cannot entirely remove transient effects, such as arcing events, and only careful design of the electron source can facilitate this. Though CVD is certainly coming to the fore as the most reproducible fabrication technique with the finest degree of control over the emitter design, it, like other techniques, is faced with issues of tip-to-tip uniformity.

Poor uniformity in effective surface roughness is known to de-stabilise the temporal stability. Indeed, wet chemistry ink approaches to emitter fabrication result in much greater surface roughness, and this exacerbates and further compromises their temporal stability. Small height variations between tips can instigate preferential emission from a small proportion of the longer CNTs, which consequently burn-out.

It has been shown elsewhere that individual CNTs can in practice pass a current of several microamps [ ]. Currents in excess of this threshold cause the CNT to sublime, which manifests as a temporal instability in the anode current, and subsequent X-ray emission. In the case of CVD-synthesised CNT emitters one solution — originally described for application with Spindt emitters [ ] and initially proposed for general electron emission applications by [ ] — consisted of integrating a ballast resistor micro-fabricated in series with the electron emitter.

Here resistive deposited layers are fabricated in series with the electron emitters. The series resistance ballasts the emission, functioning as a current limiting resistance, preventing emitter sublimation, subsequently enhancing temporal stability. Yet to be applied immediately to the design of an X-ray source, the group of Milne have developed a novel field effect transistor FET ballasted field emission source, where each individual CNF electron source is equipped with its own dedicated FET ballast layer which is capable of limiting, and electronically controlling, the emission current to prevent emitter sublimation and significant temporal instabilities [ ].

Other thin film deposition techniques have also been considered [ , ] though these too have yet to gain any significant interest. Individual CNTs are fragile. Their propensity toward tip or root growth results in an axial asymmetry in their magnetic susceptibility which manifests as rotational torque induction when exposed to high electric fields - an effect which is typically exploited during electrophoretic alignment.

However, during FE this can result in the removal and transfer of the CNTs from the cathode to the anode. This severely impacts the lifetime and stability of any field emission device into which they are incorporated. Several approaches have been taken to improve the robustness of the emitting elements by increasing the degree of adhesion between the CNTs and the supporting substrate.

However, FE preferentially occurs at the periphery of the patterned CNT forest or array [ , ] suggesting that if some CNTs are in fact removed the emission stability may simply be recovered by new CNTs contributing the emission current of those removed; emitter areal design can in part accommodate and engineer out temporal instabilities. Nevertheless, there is a continuing effort to engineer the CNT-support interface to enhance the degree of adhesion and hence enhance the temporal stability.

In the case of ink approaches various solution additives have been exploited, such as glass frits [ ], though the exact implications of such approaches with regards to the turn-on field and maximum current density remain unclear. Elsewhere there has been significant work on the use of surface treatments to stabilise the temporal stability of the electron emission [ , ].

As discussed previously, such low work function adlayers do indeed increase the emission current density, though they also, depending on the adlayer material and means of deposition, hermetically seal the CNTs — which act as a high aspect support structure — thereby increasing the emitters stability [ ]. FE sources have the potential for extremely high spatial modulation.

By controlling, the electron emitter location at the nanoscale, coarse control over the position of the electron beam, and subsequent X-ray beam are possible. A range of focussing techniques has also been developed to reduce the electron beam focal spot size. The size of the electron beam at its source and latterly as it impacts the target material, contributes, in part, to the size of the X-ray focal spot, which itself impacts on the resolution of the resultant X-ray image.

A large electron beam focal spot will create a penumbra effect where the X-ray spot subtends a significant angle at the subject; this blurs the resultant image. Conventional X-ray tubes will use a lensing cup around the heated filament assembly. This, to a first order, electrostatically focuses the electrons onto the centre of the anode. Careful design of the cathode and supporting electrostatic lenses will minimise the size of the focal spot [ , ]. Additionally, further electrostatic rings or focussing coils may also be employed to redirect the flow of electrons emanating from the cathode, to further reduce the spot size, where the focal spot size is determined by measuring the Point Spread Function using standard methods [ ].

In just such a way, a transmission CNT X-ray tube with a solenoid focussing unit was constructed by Heo et al. Liu et al. A similar mini-focus tube, used for small animal CT work, was also described. FE tubes with cathodes constructed from CNFs have also incorporated conventional three stage electrostatic Einzel lenses.

FE sources have the potential for unprecedented temporal modulation. Compared to other one-dimensional nanomaterials, CNTs allow for near ballistic conduction making them ideally suited for such high-speed applications. Pulse X-ray sources are practically beneficial when imaging moving objects. The principle of operation is analogous to stroboscopic lighting in optical imaging. Conventional TE X-ray sources are generally not capable of rapid control; they cannot be pulsed much more rapidly than a few tens of milliseconds, without sophisticated gating mechanisms.

The means of controlling the beam current in a TE source is by adjusting the filament current and hence temperature, which introduces time delays due to the finite cooling and heating time of the filament. Pulse sources are therefore generally implemented by switching the anode voltage, or by means of mechanically controlled shutters, the latter only allowing for relatively course control over the pulse shape, on period, and mark-to-space ratio, and will inevitably create a trapezoidal pulse.

In the case of existing FE and TE sources, the short duration of the pulse must be compensated by increased power levels, resulting in significant engineering design constraints with regards to the resilience of the emitting material; conventional electron emitting materials degrade rapidly when used in such high power applications. In FE X-ray sources often the means of controlling the beam current is via the gate voltage. Depending on the exact emitter geometry, this voltage can be considerably lower than the anode voltage and so at a much lower power level; it can be switched virtually instantaneously.

This lowers the total thermal dissipation, reduces the total amount of emitted radiation — allowing for safer medical diagnostics, and eliminates the need for bulky mechanical components. Pulsed Sources. Note the increased sharpness of the fan blades with increasingly rapid beam pulsing [ ]. Copyright , Elsevier and American Institute of Physics. Copyright , SPIE. Adapted from [ ]. Copyright , Thales Electron Devices. The limiting factor in many pulsed systems is often the capacitance of the gate assembly. Kim et al. However it is worth noting that such a measure does not correlate with the X-ray photon flux [ ].

Certainly electronic control over the pulsing performance has some use. Nevertheless, this is necessarily at the expense of other technological challenges, chiefly the associated RC constant of the vacuum cavity. This RC constant induces intrinsic time delays that are not experienced in the optically stimulated case.

The pulsing performance of arrayed sources has also been considered. The availability of rapidly pulsed sources opens up the potential for high-speed, real-time inspection technologies, capable of coupling high throughput, on-line manufacturing with real-time inspection.

Radiographic Image

Indeed, with controlled high pulse rates the potential to perform medical diagnostics without strict patient restraints is accessed, allowing inspection of dynamic organs without image blurring. Intrinsic motion and the associated quasi-periodic respiration and heart beats, for example, allow one means of gating such high speed pulsed systems. Such motion-induced artefacts can thus be reduced as the X-ray exposure is synchronised with a patients physiological rhythm, or indeed an objects motion [ ].