Shinji Fukao

Radiation of x-rays using polarized LiNbO 3 single crystal in low-pressure ambient gas

The dependence of X-ray intensity on the pressure and type of ambient gas was investigated for LiNbO3 single crystals polarized in the c-axis direction at pressures of approximately 1 to 30 Pa. Ionization of surrounding gas molecules by the electric field generated by the crystal led to the production of both positive ions and free electrons. The electrons were accelerated toward a Cu target, radiating both white X-rays and X-rays specific to the crystal or target material by bremsstrahlung. The integrated X-ray intensity per cycle in the energy range 1 to 20 keV showed a local maximum value at a pressure Pmax. The logarithm of Pmax was proportional to the Boltzmann factor using the first ionization energy of each ambient gas molecule. The value of Pmax was found to be independent of the electrical surface area of the crystal. The integrated X-ray intensity was approximated qualitatively by a quadratic function with pressure, which was upwardly convex. It was found that one of the causes of the reduction in X-ray intensity at pressures P > Pmax is the adsorption of positive ions generated by the ionization of gas molecules on the negative electric surface. It was also discovered that the lifetime of the X-ray radiation device could be improved when the X-ray radiation case was covered with another hermetically sealed decompression case. The gas with the smallest first ionization energy, with a partial pressure of Pmax, was enclosed inside the X-ray radiation case (inner case) and the gas with the largest first ionization energy was enclosed at a suitable pressure between the inner and outer cases.

Relation between x-ray emission mechanism and crystal structure in LiNbO3

x-ray emission in LiNbO3 is confirmed by thermal treatments in a vacuum system by a new cleaning method of the crystal. Detailed single-crystal high-temperature x-ray structure refinements were carried out at 297, 323, 373, 423 and 473 K, far below the phase transition (~1473 K). The unit cell dimensions a and V show a linear increase with temperature; however, the lattice parameter, c shows only a slight linear increase. The length of Nb–O bonds in adjacent octahedra is almost constant below 423 K. It is suggested that the valence electron in Nb changes in the compound. Therefore, x-ray emission induced by charged particles including electrons can be considered to have a close relation to the electric charge of Nb in LiNbO3.

Excitation of X-Rays Using Polarized LiNbO3 Single Crystal

The radiation of X-rays using the electric charge generated by a temperature change of a LiNbO3 single crystal has been investigated. When the LiNbO3 single crystal was placed in a conductive cylindrical pipe made of graphite, aluminum or copper, the photon flux of the radiated X-rays was dependent on the type of conductive material used. Depending on the type of materials on which the crystal was placed, the photon flux of the radiated X-rays increased with decreasing work function of the material.

Mechanism of Electric Charge Emission from LiNbO3 Single Crystal

Mechanisms of electric charge emission from the surface of a single crystal of LiNbO3 by temperature change are investigated. The electric charge emis sion current detected consists of both dc (direct-current) current and pulsed current. The emission rate i s found to be dependent on pressure, atmospheric gas species, and sample stage metal and beca me maximum at a gas pressure of approximately 5 Pa. The direction of current was dependent on the el ectric surface of the crystal from which the electric charges are emitted. The integrated val ue of dc current is found to be proportional to the Boltzmann factor related to the first ionization e nergy of each atmospheric gas for the stainless steel sample holder.

Radiation and Evaluation of X-Rays Using Electric Charge Generated by Temperature Change of LiNbO3 Single Crystal

The radiation of X-rays using the electric charge generated by a temperature change of a LiNbO3 single crystal has been investigated. The X-ray radiation rate became maximum at an oxygen gas pressure of approximately 5 Pa. When the upper crystal surface of the LiNbO3 single crystal was the positive electrical face (+z face), the characteristic X-rays of a target metal were radiated during an increase in temperature and those of the LiNbO3 single crystal were radiated during a decrease in temperature. When the upper crystal surface was the negative electrical face (-z face), an inverse result was obtained. X-ray radiation rate was dependent on the type of gas in the atmosphere, gas pressure, the direction of the electric surface, and the rate of change in temperature.

Radiation of X‐Rays Using Uniaxially Polarized LiNbO3 Single Crystal

The effect of emitted electron from carbon nanotube (CNT), of which shape is easy to emit electrons, on the X-ray intensity radiated using a polarized pyroelectric single crystal of LiNbO3 was investigated at an air pressure of approximately 7×10−3 Pa. The negatively charged surface of the crystal was placed opposite an oxygen-free copper foil target, its temperature was altered, and the electric field was generated. When the crystal is deteriorated by self polarization reversal and can not generate an electric field with high intensity, the X-ray intensity radiated using this crystal drastically increased with increasing the area of CNT. Moreover, X-ray radiation could be stabilized using CNT. For the crystal without deterioration, it was found that the X-ray intensity decreased for small area of CNT and increased again with increasing the pasted area.

Improvement of Compact X-Rays Source Using Uniaxially Polarized LiNbO3 Single Crystal

X-ray radiation using pyroelectric crystal is intermittent and the X-ray intensity is low and unstable compared with a conventional X-ray radiation method, such as X-ray tube. It is expected that the X-ray intensity becomes stable if electric field intensity and supply of electron are stable. In this study, to use X-ray radiation equipment as an electron source, tandem-type X-ray radiation equipment which is composed of two LiNbO3 single crystals polarized in a z-axis is proposed. When the temperature gradient for each crystal was the same, the X-ray intensity became approximately 6 times higher at a maximum. When the temperature gradient for each crystal was reversed, the period of X-ray radiation became approximately two times longer and the X-ray intensity became approximately 20 times higher at a maximum. Moreover, the stability of X-ray radiation for the repetition of temperature could be improved.

X-RAY RADIATION USING POLARIZED LiNbO3 SINGLE CRYSTALS IN A LOW-PRESSURE GAS AMBIENT

In order to develop miniaturized X-ray devices that provide localized X-ray radiation for medical applications, the dependences of the X-ray intensity on the ambient gas pressure and the inner diameter of the case made of stainless steel were investigated for LiNbO3 single crystals polarized parallel to the c-axis in a N2 atmosphere. The smallest inner diameter (16 mm) is slightly larger than the diagonal (14 mm) of the square crystal. The X-ray intensity had a local maximum in the pressure range of approximately 3.9 to 5.4 Pa and it decreased with increasing inner diameter of the case in low vacuums. The X-ray intensity had a local maximum at an inner diameter of 21 mm. The X-ray intensity increased with decreasing pressure in high vacuums.

Radiation of X-rays using uniaxially polarized LiNbO 3 single crystal

The effect of emitted electron from carbon nanotube (CNT), of which shape is easy to emit electrons, on the X-ray intensity radiated using a polarized pyroelectric single crystal of LiNbO 3 was investigated at an air pressure of approximately 7×10−3 Pa. The negatively charged surface of the crystal was placed opposite an oxygen-free copper foil target, its temperature was altered, and the electric field was generated. When the crystal is deteriorated by self polarization reversal and can not generate an electric field with high intensity, the X-ray intensity radiated using this crystal drastically increased with increasing the area of CNT. Moreover, X-ray radiation could be stabilized using CNT. For the crystal without deterioration, it was found that the X-ray intensity decreased for small area of CNT and increased again with increasing the pasted area.

Improvement of the Stability of X-Ray Emission by Thermal Excitation of a Pyroelectric Single Crystal

X-ray emission using pyroelectric crystals is intermittent, and has low intensity and stability. One of the factors for low stability is related to creeping discharge, due to the accumulation of surface electric charges that change in response to the temperature. The time dependence of the net amount of electric charge was investigated by changing the cycle period of the crystal temperature. The stability of the X-ray emission is demonstrated to be strongly dependent on the temperature cycle period.