Pyroelectric LiNbO 3 Crystals Emit Self-Focusing Electron Beam
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RESEARCH/RESEARCHERS Pyroelectric LiNbO3 Crystals Emit Self-Focusing Electron Beam Pyroelectric crystals—crystals that become electrically charged on heating or cooling—emit self-focusing, spatially stable electron beams with energies of up to 170 keV when heated in the 100–200°C range and cooled to room temperature. According to researchers J.D. Brownridge of the State University of New York— Binghamton and S.M. Shafroth of the University of North Carolina—Chapel Hill, electron beams with focal lengths of 17–22 mm were achieved by varying the thickness of the crystal. Brownridge said that this finding, which was reported in the November 12 issue of Applied Physics Letters, “allows us to replace—in some cases—the radioactive sources that are now used to excite elements in x-ray fluorescence spectrometers.” While pyroelectricity is a well-known phenomenon used to produce sensitive devices for infrared detection and photonics, the emission of a focused electron beam had been overlooked until now. This is probably because at atmospheric pressure, the negatively polarized surface produced by the heating/cooling sequence is quickly neutralized by bombardment with positive ions present in the atmosphere. By maintaining pressures as low as 0.5 mTorr during the process, the surface charge can be maintained for hours, permitting the electron-beam phenomenon to be observed. Surface electrical-field lines are slightly inclined toward the axis of the crystal, which accounts for the focusing phenomenon. Samples were prepared by cutting the LiNbO3 crystal perpendicular to its threefold rotationally symmetric z axis. By resistively heating the +z base of the crystal, the opposite -z base becomes positively charged; upon cooling, the -z base switches to a negatively charged state. When a cylindrical sample 5 mm in diameter and 5 mm thick was heated to 115°C in a
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