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Diamond Field-Emission Cathodes and Channeling Radiation System for High-Brightness X-ray Sources for Phase Contrast Soft Tissue Imaging
Description of Technology
Conventional X-ray sources are the result of a century’s worth of engineering improvements to the original Crookes tube used by Roentgen. Conventional X-ray systems have insufficient spectral brilliance to perform phase-contrast soft tissue imaging. Microfocus tubes, including field-emitters, use bremsstrahlung, which is broad band and limits phase-contrast imaging. Synchrotron radiation sources presently offer the highest spectral brilliance in the X-ray regime, but requires electrons in the 10-GeV range to generate hard X-rays. Additionally, Synchrotron sources are too large and expensive for use in medical imaging applications.
An interesting alternative is offered by channeling radiation based approaches. In a crystal, the ions in each crystal plane form a sheet of positive charge. When a relativistic electron travels through the crystal parallel to the crystal plane, Lorentz contraction increases the charge density and the electron oscillates about the crystal plane in quantum states normal to the plane, as shown. Radiation from transitions between the quantum states is called channeling radiation. Compared with a conventional undulator, channeling radiation requires only a 40-MeV electron beam, rather than a 10-GeV beam to reach the hard X-ray region. Compared with a laser undulator, a channeling radiation source comprises a small diamond chip rather than a complex laser system circulating a kilowatt of laser power.
The technology disclosed is an electron gun system that comprises an array of gated field-emitting cathode tips having a diamond pyramid capable of producing an electron beam with exquisitely small emittance (N ~ 2 nm); an accelerator capable of accelerating the emitted electrons to relativistic energies (from ~ 3 Mev to~ 50MeV); and a focusing assembly capable of focusing the accelerated electrons into a focal spot on a diamond crystal to produce hard X-rays.
Inventors have observed more than 10 mA average current from a single field-emitting tip with improved performance expected with changes to other system components. By using a single tip it may be possible to improve the transverse brightness of the electron beam by as much as six orders of magnitude, with a corresponding improvement of the spectral brilliance of the X-ray beam. The absolute emittance of the beam from a single tip, extrapolated to 30 MeV, is 40 pm. When the beam is focussed at the critical angle for channeling radiation in diamond (about 1 mrad), a spot diameter of 40 nm is noted. The spectral brilliance of the X-ray beam is then 1012 photons/s mm2- mrad2-0.1% BW at an average current of 200 nA.
Potential applications of this technology are as portable medical imaging systems with improved soft-tissue imaging capabilities with reduced patient dose. This technology can also be used for non-destructive investigation of structural and material defects in objects including integrated circuits.
Observed spectrum of channeling radiation for transitions in plane of diamond crystal at an electron energy of 14.6 MeV. Red: naturalspectrum; black, monochromatized by Bragg reflection to remove the wings of the CR line and the Bremsstrahlung background.
Intellectual Property Status
A U.S. Provisional Patent Application has been filed.
Publication in Synchrotron Radiation News, Volume 25, Issue 1, 2012 (http://www.tandfonline.com/doi/full/10.1080/08940886.2012.645419)
Inventors:Charles BrauBo ChoiPhilippe PiotJonathan JarvisMarcus MendenhallWilliam Gabella