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

1992年，布朗里奇报道了一种利用热释电晶体如LiTaO_3、LiNbO_3或CsNO_3在真空室中循环加热和冷却产生X射线的新方法。基于这一结果，电池供电的袖珍X射线发生器后来得到了商业化开发。这项技术的基本原理是热释电效应。热释电晶体的极化变化与晶体的热释电系数乘以温度变化的幅度成正比，因此当晶体被加热或冷却时，晶体表面会积累电荷。这些电荷能够产生一个电场，估计为1.35x107V/cm，尽管实验结果表明，电场大约比这个估计低2个数量级。这种电场可以将一直存在于真空室中的电子加速到高速，这反过来又可以电离更多的气体分子，产生更多的电子和离子。当这些高能电子撞击金属靶或热释电晶体时，既产生了目标的特征X射线，也产生了与撞击目标的电子减速相关的韧致辐射X射线连续体。上述热释电X射线发生器的一个明显缺点是其强度弱、功率低，这阻碍了其在射线照相和X射线荧光等应用中的应用。在本项目中，我们提出了利用强铁电电子发射产生(典型电流密度从几个到超过100A/cm2)非常高强度的脉冲X射线，并建议使用热释电晶体来提供必要的触发和提取高压脉冲。最强的电荷是等离子体辅助电子发射。当驱动电压脉冲被施加到铁电材料的后电极时，产生了所施加的电场的切向分量以及法向分量。在金属、真空和铁电材料与电场相遇的三点处，电场增加了一个Er因子，这里Er是介质材料的相对介电常数，因此在三结处发生场电子发射。然后，由于电场的切向分量，发射的电子以雪崩的形式在电介质表面倍增，导致表面等离子体的形成，而表面等离子体为强电荷提供电子。人们认为，这种表面等离子体可以作为一个几乎无限的电子源，产生强大的电子束电流。基于同样的原理，金属介质阴极已经使用了很多年。在某种程度上，目前的热释电X射线发生器与伦琴第一次发现X射线时使用的冷阴极气体管X射线发生器原则上是相同的，而我们的新设计类似于今天仍在大多数X射线源中使用的柯立芝高真空白炽灯阴极X射线管。该项目所开发的技术可用于研制微型热释电电压脉冲发生器、微型热释电效应高能电子枪、微型热释电效应高强度高能X射线源等。

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.