Final Report Summary - AXIS (Advanced X-ray source based on field-emitting carbon nanotube cold cathode)
Executive summary:
The project develops an innovative X-ray source based on the emerging technology of field-emitting carbon nanotubes (CNTs). This kind of source has several advantages with respect to traditional ones such as:
(a) higher intrinsic brilliance;
(b) the possibility to work in pulsed and continuous mode;
(c) minor power consumption;
(d) modularity of beam size;
(e) good stability and longer life time.
The research and development (R&D) activity has been focused on four major blocks:
(i) production of cathodes made of well aligned CNT arrays and meshes;
(ii) design and fabrication of a CNT-based electron gun, constituted by an electron extractor and by focusing optics;
(iii) design and fabrication of an open-type X-ray microfocusing source based on the CNT cathode and e-gun;
(iv) studies of integration of the CNT-based source in two X-ray systems devoted to specific applications: a tomographic system for biomedical applications and an advanced system for material metrology applications.
The objectives of the project have been substantially reached, and a working X-ray source based on CNT ordered arrays has been fabricated and tested. The project has generated basic knowledge on behaviour of CNTs as field emitters, and has found highly innovative technological routes for new types of structures with superior performance. These indicate excellent opportunities for commercial exploitation, but development and engineering phases are still required.
The potential exploitation goes far beyond the specific applications addressed in the project: X-ray sources are routinely used in medical radiography, in security, in industrial quality control, in advanced research, in environmental issues, in cultural heritage, etc. The availability of innovative sources with improved features with respect to the standard ones can have a great impact not only on Europe's competitiveness in this field but also on societal aspects such as health, security, product quality, sustainability.
Project context and objectives:
X-rays constitute one of the most traditional tools for the characterisation of materials. The everyday applications of X-ray methodologies are under our eyes: medical radiography, security in sensitive sites, quality control of industrial products, new drugs discovered thanks to the help of protein crystallography, are only few examples of the pervasive use of x-rays. The technology to generate X-rays dates back to the beginning of the 20th century: a cathode, constituted by a hot filament, emits electrons by thermionic effect. The electrons are accelerated by high voltage to the anode, generating X-ray radiation. The electrons can be focused to the anode by electrostatic or electromagnetic lenses, in order to reduce the focal spot from which x-rays are generated. Presently, microfocus sources are commercialised with spot size from few to few tens of microns, allowing to significantly improve the spatial resolution of radiography and tomography.
The thermionic cathode is intrinsically a random source of electrons, with limited brilliance, with difficult control on electron emission and with no access to pulsed mode of operation. An alternative to hot cathodes is represented by field-emitting cold cathodes, in which the electron emission is controlled by the electric field. CNTs are carbon structures with exceptional mechanical and electrical properties, and have proven to be excellent field emitters, with an order of magnitude higher brilliance than any other electron source. CNT-based field emitter cold cathodes are therefore the natural candidates for improved X-ray sources. There is an increasing interest in the world to fabricate CNT-based X-ray sources, and already there are examples of commercially available CNT cathodes. In these cathodes, the CNTs are randomly deposited by electrophoresis, with no specific control of size and dimensions. AXIS brings substantial innovation with respect to the mentioned cases, fabricating X-ray sources based on regular arrays of CNTs, with precise control of their emitting properties. Implementing advanced electron focusing system together with robust anode and cooling system, AXIS project aims to achieve unprecedented features concerning current density, stability, modularity, time structure (i.e. pulse width and frequency) and brilliance.
The main objectives of the project are:
(a) the development of a cathode made of a well-aligned CNT array, capable of delivering high-current electron density in continuous and pulsed mode;
(b) the fabrication of a CNT-based electron gun which combines the CNT cathode with electron focusing optics;
(c) the integration of the e-gun in stationary and rotating anode x-ray sources, and fabrication of an X-ray microfocusing source with characteristics of high brilliance, continuous and pulsed operation modes, easy control of beam size and power;
(d) the integration of the microfocusing source in two X-ray systems devoted to specific applications: a tomographic system for biomedical applications and an advanced system for material metrology applications. The implementation of phase contrast imaging methodology with this innovative source will be studied as well.
We expect the source to have several advantages over traditional tubes:
(a) high intrinsic brilliance and high peak power;
(b) possibility to work in both continuous and pulsed mode;
(c) modularity of the emitting spot size due to properties of the CNT cathode;
(d) high intrinsic stability.
There are three particular outputs of the project, all having their unique features:
- CNT cathode: variable emitting area up to approximately 1 mm2, current density of about 1 A / cm2,
- Electron gun: variable anode spot size from few µm2 to 100 x 1 000 µm2,
- Microfocus X-ray tube: up to 60 kV, 100 W at about 30 µm spot.
Project Results:
The activity started with the definition of the specifications for the three main outputs mentioned before, i.e. the CNT cathode, the e-gun and the X-ray microsource. A contingency plan which takes into account the potential risks and the related counter-actions has been defined as well.
Cathode preparation and test
Regular CNT arrays with high uniformity in CNT thickness and height, on areas up to 1 mm2 have then been grown on Si and other substrates. A novel kind of structure has been also synthesised, constituted by vertically aligned CNT meshes (VACM). A honeycomb structure made of VACM is a promising structure for high current and large cathodes and is based on optical lithography and relatively low growth temperature (450 degrees of Celsius).
Design and fabrication of CNT-based e-gun
An electron gun, composed of an electron extractor for control of CNT field emission, and of electron focusing elements, has been designed and fabricated. Initially thorough electron optics simulations have been carried out, which have shown that it is possible, adopting all electrostatic lenses, to obtain a de-magnified image of the cathode at the anode, at least for final spot size at the anode larger than few microns. For smaller spot sizes, aberrations could occur that would eventually be eliminated by combination of electrostatic and electromagnetic lenses. The simulations have also indicated the control of the angle divergence of the beam at the cathode as the most critical issue. This parameter strongly depends on the CNT current angle distribution, which is dramatically affected by the extracting grid characteristics.
Several prototypes of extractors have been fabricated, which allowed characterisation of the emission properties of CNT, i.e. extraction field, maximum current, current density, time stability, angular distribution. To measure total current the extractor was closed, instead to measure angular distribution a transmitting grid has been used, and the transmitted current measured with a Faraday cup. Maximum current up to 1.2 mA has been measured, from cathodes where CNTs were grown in an area of about 0.3 x 0.3 nm2. Current density exceeding 1 A / cm2 were thus obtained.
Several solutions for the transmitting grid have been tried, making use of nanofabrication tools.
The whole e-gun, made of several electrostatic lenses at different potential, has been designed and fabricated, to integrate it in the open type x-ray microfocusing source.
Design and fabrication of an X-ray microsource
An open type microsource, composed of a vacuum chamber, vacuum pumps, e-gun with high-voltage connector and water cooled anode has been designed and fabricated.
One of the main limit to maximum power is given by thermal load at the anode. To the purpose to optimise the anode design, thermal load simulations have been carried out, both in continuous and in transient mode. This led to innovative design of the anode, capable in principle to stand high power. The chamber for the microsource has been designed and assembled, including a retractable water cooled anode. The mechanical parts which hold the electron gun has been produced as well. In this case, the main problem is the high-voltage insulation, and the several connections for the electrostatic lenses. An important part of the activity has been dedicated to the power supply. A commercial X-ray power supply has been taken as the starting point, but considerable work has been carried out to make it suitable to control field emission from cold cathodes. Remote control has been also implemented.
Integration of e-gun and X-ray microsource on specific applications
A CNT-based e-gun compatible with an existing rotating anode system has been designed. The integration of the microfocusing source with a tomography system, and implementation of phase contrast imaging method have been studied as well.
Demonstrators
Demonstrators of CNT cathode, e-gun and X-ray microsources have been completed and tested.
Potential impact:
The results obtained so far demonstrate a working X-ray source based on an ordered array of CNTs and electrostatic lenses for refocusing of the cathode on the anode. To optimise the source, and bring it to a competitive position in the market, an engineering phase is still necessary. The work performed, and the experience acquired, make us confident that, through the routes envisaged to solve the several problems encountered in the course of the project, the initial objectives can be fully obtained. These are:
(a) The fabrication of a CNT cathode with a variable size of the CNT emitting area, capable of providing stable current in both continuous and pulsed mode, with a current density of the order of 1 A / cm2;
(b) the fabrication of an e-gun capable of providing a demagnified image of the cathode at the anode, in order to excite X-ray beams with a spot size variable from few microns to several tens of microns;
(c) the fabrication of a high-brilliance X-ray microsource capable of working both in continuous and in pulsed mode;
(d) the integration of the e-gun in a rotating anode X-ray source.
The source, after the engineering phase, will prospectively represent a real innovation with respect to the existing ones, and will therefore help the small and medium-sized enterprises (SMEs) involved in the project to improve their competitiveness in the market. Indeed the source will have several innovative aspects, here summarised:
(i) Improved brilliance: The AXIS source in its complex is designed for high brilliance operation. This will have important positive consequences: either reduced exposure (or counting) time, either better spatial resolution, for ex. in medical imaging or non-destructive testing.
(ii) Pulsed mode operation: Standard sources are not designed for pulsed operation, because the thermionic cathode has long response time. Instead, field emission can be switched on and off at high frequency. Therefore the CNT based source is suitable for pulsed operation. This can be very important in transient phenomena, or in case where exposure must be synchronised with periodic events, such as, for example, hearth beat or respiration in medical imaging. This would reduce significantly blurring.
(iii) Cold-field emitting cathodes are inherently more brilliant than hot cathodes, and allow more compact packaging. Furthermore, they do not need heating the filament, and have therefore lower power consumption. They are therefore perfectly suited to better, more compact and more powerful portable X-ray sources. This can be particularly important for industrial and medical applications, environment, and cultural heritage. All the advantages described before concerning the time structure of these sources extend obviously to portable sources.
For these reasons the AXIS source is expected to find interesting applications in a number of different areas:
- X-ray tomography for biomedical applications,
- industrial non-destructive testing,
- industrial metrology,
- security applications,
- cultural heritage,
- environmental sciences,
- space.
List of websites:
Participants and contact details
Coordinator: Stefano Lagomarsino, CNR
email: stefano.lagomarsino@ cnr.it
Website of AXIS: http://www.axisproject.eu
The consortium is constituted by four SMEs and 5 research and technological development (RTD) performers. Here a short profile of the participants:
SMEs:
York Probe Sources Ltd.
Contact person: Prof. Mohamed El Gomati
Website: http://www.yps-ltd.com/
DELONG INSTRUMENTS AS
Contact person: Vladimir Kolarik
Web ite: http://www.dicomps.com/
XENOCS S.A.
Contact person: Evi Dova
Website: http://www.xenocs.com/
SCANCO MEDICAL AG
Contact person: Stefan Hammerle
Website: http://www.scanco.ch/
RTD partners:
CNR
Contact person: Stefano Lagomarsino
Website: http://www.ifn.cnr.it/Groups/XRay/Home.htm
CAMBRIDGE UNIVERSITY
Contact person: Bill Milne
Website: http://www-cape.eng.cam.ac.uk/
CZECH TECHNICAL UNIVERSITY (CTU)
Contact person: Alexandr Jancarek
Website: http://xuv.kfe.fjfi.cvut.cz/
UNIVERSITY ROMATRE
Contact person: Gianni Stefani
Website: http://www.fis.uniroma3.it/
D'APPOLONIA
Contact person: Raimondo de Laurentis
Website: http://www.dappolonia.it/
The project develops an innovative X-ray source based on the emerging technology of field-emitting carbon nanotubes (CNTs). This kind of source has several advantages with respect to traditional ones such as:
(a) higher intrinsic brilliance;
(b) the possibility to work in pulsed and continuous mode;
(c) minor power consumption;
(d) modularity of beam size;
(e) good stability and longer life time.
The research and development (R&D) activity has been focused on four major blocks:
(i) production of cathodes made of well aligned CNT arrays and meshes;
(ii) design and fabrication of a CNT-based electron gun, constituted by an electron extractor and by focusing optics;
(iii) design and fabrication of an open-type X-ray microfocusing source based on the CNT cathode and e-gun;
(iv) studies of integration of the CNT-based source in two X-ray systems devoted to specific applications: a tomographic system for biomedical applications and an advanced system for material metrology applications.
The objectives of the project have been substantially reached, and a working X-ray source based on CNT ordered arrays has been fabricated and tested. The project has generated basic knowledge on behaviour of CNTs as field emitters, and has found highly innovative technological routes for new types of structures with superior performance. These indicate excellent opportunities for commercial exploitation, but development and engineering phases are still required.
The potential exploitation goes far beyond the specific applications addressed in the project: X-ray sources are routinely used in medical radiography, in security, in industrial quality control, in advanced research, in environmental issues, in cultural heritage, etc. The availability of innovative sources with improved features with respect to the standard ones can have a great impact not only on Europe's competitiveness in this field but also on societal aspects such as health, security, product quality, sustainability.
Project context and objectives:
X-rays constitute one of the most traditional tools for the characterisation of materials. The everyday applications of X-ray methodologies are under our eyes: medical radiography, security in sensitive sites, quality control of industrial products, new drugs discovered thanks to the help of protein crystallography, are only few examples of the pervasive use of x-rays. The technology to generate X-rays dates back to the beginning of the 20th century: a cathode, constituted by a hot filament, emits electrons by thermionic effect. The electrons are accelerated by high voltage to the anode, generating X-ray radiation. The electrons can be focused to the anode by electrostatic or electromagnetic lenses, in order to reduce the focal spot from which x-rays are generated. Presently, microfocus sources are commercialised with spot size from few to few tens of microns, allowing to significantly improve the spatial resolution of radiography and tomography.
The thermionic cathode is intrinsically a random source of electrons, with limited brilliance, with difficult control on electron emission and with no access to pulsed mode of operation. An alternative to hot cathodes is represented by field-emitting cold cathodes, in which the electron emission is controlled by the electric field. CNTs are carbon structures with exceptional mechanical and electrical properties, and have proven to be excellent field emitters, with an order of magnitude higher brilliance than any other electron source. CNT-based field emitter cold cathodes are therefore the natural candidates for improved X-ray sources. There is an increasing interest in the world to fabricate CNT-based X-ray sources, and already there are examples of commercially available CNT cathodes. In these cathodes, the CNTs are randomly deposited by electrophoresis, with no specific control of size and dimensions. AXIS brings substantial innovation with respect to the mentioned cases, fabricating X-ray sources based on regular arrays of CNTs, with precise control of their emitting properties. Implementing advanced electron focusing system together with robust anode and cooling system, AXIS project aims to achieve unprecedented features concerning current density, stability, modularity, time structure (i.e. pulse width and frequency) and brilliance.
The main objectives of the project are:
(a) the development of a cathode made of a well-aligned CNT array, capable of delivering high-current electron density in continuous and pulsed mode;
(b) the fabrication of a CNT-based electron gun which combines the CNT cathode with electron focusing optics;
(c) the integration of the e-gun in stationary and rotating anode x-ray sources, and fabrication of an X-ray microfocusing source with characteristics of high brilliance, continuous and pulsed operation modes, easy control of beam size and power;
(d) the integration of the microfocusing source in two X-ray systems devoted to specific applications: a tomographic system for biomedical applications and an advanced system for material metrology applications. The implementation of phase contrast imaging methodology with this innovative source will be studied as well.
We expect the source to have several advantages over traditional tubes:
(a) high intrinsic brilliance and high peak power;
(b) possibility to work in both continuous and pulsed mode;
(c) modularity of the emitting spot size due to properties of the CNT cathode;
(d) high intrinsic stability.
There are three particular outputs of the project, all having their unique features:
- CNT cathode: variable emitting area up to approximately 1 mm2, current density of about 1 A / cm2,
- Electron gun: variable anode spot size from few µm2 to 100 x 1 000 µm2,
- Microfocus X-ray tube: up to 60 kV, 100 W at about 30 µm spot.
Project Results:
The activity started with the definition of the specifications for the three main outputs mentioned before, i.e. the CNT cathode, the e-gun and the X-ray microsource. A contingency plan which takes into account the potential risks and the related counter-actions has been defined as well.
Cathode preparation and test
Regular CNT arrays with high uniformity in CNT thickness and height, on areas up to 1 mm2 have then been grown on Si and other substrates. A novel kind of structure has been also synthesised, constituted by vertically aligned CNT meshes (VACM). A honeycomb structure made of VACM is a promising structure for high current and large cathodes and is based on optical lithography and relatively low growth temperature (450 degrees of Celsius).
Design and fabrication of CNT-based e-gun
An electron gun, composed of an electron extractor for control of CNT field emission, and of electron focusing elements, has been designed and fabricated. Initially thorough electron optics simulations have been carried out, which have shown that it is possible, adopting all electrostatic lenses, to obtain a de-magnified image of the cathode at the anode, at least for final spot size at the anode larger than few microns. For smaller spot sizes, aberrations could occur that would eventually be eliminated by combination of electrostatic and electromagnetic lenses. The simulations have also indicated the control of the angle divergence of the beam at the cathode as the most critical issue. This parameter strongly depends on the CNT current angle distribution, which is dramatically affected by the extracting grid characteristics.
Several prototypes of extractors have been fabricated, which allowed characterisation of the emission properties of CNT, i.e. extraction field, maximum current, current density, time stability, angular distribution. To measure total current the extractor was closed, instead to measure angular distribution a transmitting grid has been used, and the transmitted current measured with a Faraday cup. Maximum current up to 1.2 mA has been measured, from cathodes where CNTs were grown in an area of about 0.3 x 0.3 nm2. Current density exceeding 1 A / cm2 were thus obtained.
Several solutions for the transmitting grid have been tried, making use of nanofabrication tools.
The whole e-gun, made of several electrostatic lenses at different potential, has been designed and fabricated, to integrate it in the open type x-ray microfocusing source.
Design and fabrication of an X-ray microsource
An open type microsource, composed of a vacuum chamber, vacuum pumps, e-gun with high-voltage connector and water cooled anode has been designed and fabricated.
One of the main limit to maximum power is given by thermal load at the anode. To the purpose to optimise the anode design, thermal load simulations have been carried out, both in continuous and in transient mode. This led to innovative design of the anode, capable in principle to stand high power. The chamber for the microsource has been designed and assembled, including a retractable water cooled anode. The mechanical parts which hold the electron gun has been produced as well. In this case, the main problem is the high-voltage insulation, and the several connections for the electrostatic lenses. An important part of the activity has been dedicated to the power supply. A commercial X-ray power supply has been taken as the starting point, but considerable work has been carried out to make it suitable to control field emission from cold cathodes. Remote control has been also implemented.
Integration of e-gun and X-ray microsource on specific applications
A CNT-based e-gun compatible with an existing rotating anode system has been designed. The integration of the microfocusing source with a tomography system, and implementation of phase contrast imaging method have been studied as well.
Demonstrators
Demonstrators of CNT cathode, e-gun and X-ray microsources have been completed and tested.
Potential impact:
The results obtained so far demonstrate a working X-ray source based on an ordered array of CNTs and electrostatic lenses for refocusing of the cathode on the anode. To optimise the source, and bring it to a competitive position in the market, an engineering phase is still necessary. The work performed, and the experience acquired, make us confident that, through the routes envisaged to solve the several problems encountered in the course of the project, the initial objectives can be fully obtained. These are:
(a) The fabrication of a CNT cathode with a variable size of the CNT emitting area, capable of providing stable current in both continuous and pulsed mode, with a current density of the order of 1 A / cm2;
(b) the fabrication of an e-gun capable of providing a demagnified image of the cathode at the anode, in order to excite X-ray beams with a spot size variable from few microns to several tens of microns;
(c) the fabrication of a high-brilliance X-ray microsource capable of working both in continuous and in pulsed mode;
(d) the integration of the e-gun in a rotating anode X-ray source.
The source, after the engineering phase, will prospectively represent a real innovation with respect to the existing ones, and will therefore help the small and medium-sized enterprises (SMEs) involved in the project to improve their competitiveness in the market. Indeed the source will have several innovative aspects, here summarised:
(i) Improved brilliance: The AXIS source in its complex is designed for high brilliance operation. This will have important positive consequences: either reduced exposure (or counting) time, either better spatial resolution, for ex. in medical imaging or non-destructive testing.
(ii) Pulsed mode operation: Standard sources are not designed for pulsed operation, because the thermionic cathode has long response time. Instead, field emission can be switched on and off at high frequency. Therefore the CNT based source is suitable for pulsed operation. This can be very important in transient phenomena, or in case where exposure must be synchronised with periodic events, such as, for example, hearth beat or respiration in medical imaging. This would reduce significantly blurring.
(iii) Cold-field emitting cathodes are inherently more brilliant than hot cathodes, and allow more compact packaging. Furthermore, they do not need heating the filament, and have therefore lower power consumption. They are therefore perfectly suited to better, more compact and more powerful portable X-ray sources. This can be particularly important for industrial and medical applications, environment, and cultural heritage. All the advantages described before concerning the time structure of these sources extend obviously to portable sources.
For these reasons the AXIS source is expected to find interesting applications in a number of different areas:
- X-ray tomography for biomedical applications,
- industrial non-destructive testing,
- industrial metrology,
- security applications,
- cultural heritage,
- environmental sciences,
- space.
List of websites:
Participants and contact details
Coordinator: Stefano Lagomarsino, CNR
email: stefano.lagomarsino@ cnr.it
Website of AXIS: http://www.axisproject.eu
The consortium is constituted by four SMEs and 5 research and technological development (RTD) performers. Here a short profile of the participants:
SMEs:
York Probe Sources Ltd.
Contact person: Prof. Mohamed El Gomati
Website: http://www.yps-ltd.com/
DELONG INSTRUMENTS AS
Contact person: Vladimir Kolarik
Web ite: http://www.dicomps.com/
XENOCS S.A.
Contact person: Evi Dova
Website: http://www.xenocs.com/
SCANCO MEDICAL AG
Contact person: Stefan Hammerle
Website: http://www.scanco.ch/
RTD partners:
CNR
Contact person: Stefano Lagomarsino
Website: http://www.ifn.cnr.it/Groups/XRay/Home.htm
CAMBRIDGE UNIVERSITY
Contact person: Bill Milne
Website: http://www-cape.eng.cam.ac.uk/
CZECH TECHNICAL UNIVERSITY (CTU)
Contact person: Alexandr Jancarek
Website: http://xuv.kfe.fjfi.cvut.cz/
UNIVERSITY ROMATRE
Contact person: Gianni Stefani
Website: http://www.fis.uniroma3.it/
D'APPOLONIA
Contact person: Raimondo de Laurentis
Website: http://www.dappolonia.it/
