Radiation of X-rays Using Uniaxially Polarized LiNbO 3 Single Crystal
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Radiation of X-rays Using Uniaxially Polarized LiNbO3 Single Crystal Shinji Fukao1, Yoshikazu Nakanishi1, Tadahiro Mizoguchi1, Yoshiaki Ito2, Toru Nakamura3, and Shinzo Yoshikado1 1 Department of Electronics, Doshisha University, 1-3, Tatara-Miyakodani, Kyotanabe, Kyoto, 610-0321, Japan 2 Institute for Chemical Research, Kyoto University, Gokasyo, Uji, Kyoto, 611-0011, Japan 3 Asahi Roentgen Industrial Co. Ltd., 376-3, Kuze-Tukiyamacyo, Minami-ku, Kyoto, 601-8203, Japan ABSTRACT X-rays are radiated due to the bremsstrahlung caused by the collision of electrons with a target metal placed opposite the negative electric surface of a crystal by changing the temperature of a LiNbO3 single crystal polarized in the c-axis direction. The enhancement of X-ray radiation by supplying thermal electrons externally was examined in the low-pressure range below approximately 10-1 Pa where the concentration of both positive ions and electrons generated by the ionization of gas molecules is very low. X-ray intensity became maximum, at which point high reproducibility was obtained, when the optimal number of thermal electrons was continuously supplied while increasing and decreasing the temperature of the crystal. Under this optimal condition, X-ray intensity increased approximately 100 times at the maximum value. The supply of thermal electrons not in the temperature-increasing process but in the temperature-decreasing process contributes to the enhancement of X-ray radiation. Below approximately 5×10-2 Pa, X-ray intensity was almost constant. INTRODUCTION It is reported that X-rays are radiated at pressures of approximately 1 to 10 Pa from a crystal or a target due to the bremsstrahlung caused by the collision of electrons, which are accelerated by an electric field formed by both the electric dipole moments in the crystal and the surface charges generated by changing the temperature of a single crystal, for example, LiNbO3 or LiTaO3, polarized in a single direction, towards a target metal placed opposite the surface of the crystal [1-7]. Both white X-rays (continuous spectra X-rays) and characteristic X-rays peculiar to the element of a crystal or a target are radiated. The relative intensity of the characteristic X-rays to white X-rays is large compared with that in the case of the conventional system, such as the X-ray tube. Furthermore, the convergence of electrons is possible and the convergence point is also expected to function as a point source of X-rays [8-10]. In the intrinsic surface of a pyroelectric crystal, a surface charge of charge density P ⋅ n is generated by poling, where P is the polarization vector of the crystal, n is the unit vector normal to the surface, and P ⋅ n is the inner product of P and n . However, P ⋅ n is electrically neutralized by the adsorption of the ions in atmosphere gas with constant temperature, the net surface charge becomes zero, and no electric field is formed. However, if the temperature of the crystal is changed, the nonzero net surface charge resulting from the difference
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