Feasibility Study of High Energy and High Intensity X-ray Generation by Pyroelectric Materials

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A novel method of X-ray generation by pyroelectric crystals such as LiTaO3, LiNbO3 or CsNO3 when they were subjected to cyclic heating and cooling in a vacuum chamber was reported by Brownridge in 1992. Based on this result, battery powered, pocket sized X-ray generator has since developed commercially. The fundamental principle of this technique is the pyroelectric effect. Pyroelectric crystals exhibit a change in polarization proportional to the crystal's pyroelectric coefficient times the magnitude of the temperature change, so there is a built up of charges on the crystal surface when the crystal is heated or cooled. These charges were able to produce an electric field estimated to be 1.35x107V/cm although experimental results suggested that the electric field is roughly 2 orders of magnitude lower than this estimate. This electric field can accelerate electrons which always exist in a vacuum chamber to high velocity, which in turn, can ionise more gas molecules to produce more electrons and ions. When these high energy electrons strike a metal target or a pyroelectric crystal, both the characteristic x rays of the target and the x-ray continuum of bremsstrahlung associated with the deceleration of the electrons striking the target are produced. An apparent disadvantage of the above pyroelectric x ray generator is its weak intensity and low power, which hinders its use on applications for example radiography and x ray fluorescence. We propose in this project to use the strong ferroelectric electron emission to produce (typical current density from a few to more than 100 A/cm2) pulse x rays with very high intensity, and propose to use pyroelectric crystals to provide the necessary triggering and the extracting high voltage pulses. The strong FEE is a plasma-assisted electron emission. When a driving voltage pulse is applied to the rear electrode of the ferroelectric material, tangential components as well as the normal component of the applied electron field are created. In the triple points where metal, vacuum, and the ferroelectric material meet the electric field is increased by a factor of er here er is the relative dielectric constant of the dielectric material, as a result field electron emission occurs at the triple junctions. The emitted electrons then multiply as an avalanche traversing the dielectric surface due to the tangential component of the electric field, which leads to the formation of the surface plasma, and this surface plasma provides electrons for the strong FEE. It is believed that this surface plasma can serve as an almost unlimited source of electrons for a strong electron beam current. Based on the same principle metal-dielectric cathodes have been in use for many years. To some extent the current pyroelectric x ray generator is in principle the same as the cold cathode gas tube x ray generator used by Rontgen when he first discovered x ray, and our new design is similar to the Coolidge high vacuum incandescent cathode x ray tubes which are still in use in the majority of the x ray sources today. The techniques developed in this project can be used to develop devices such as miniature pyroelectric voltage pulse generators, miniature high intensity electron guns by pyroelectric effect, and miniature high intensity high energy x ray sources by pyroelectric effect, etc.