A slow positron beam surface analyzer with 370MBq-22Na and electrostatic fields

A slow positron beam surface analyzer with a 10mCi-22Na without using any magnetic field has been developed. The configuration, dimension and potentials of electrodes were optimized by calculation. The beam properties were experimentally examined with use of an electron gun and a positron source with 22Na. The first moderation of energy-spread positrons was performed by a 15 μ m-tungsten foil set in transmission-moderation geometry. The extracted beams were extremely sensitive to terrestrial magnetism and the residual magnetization of materials in and around the electrodes and the moderator. The beam drift due to Lorentz force was suppressed by covering the positron source and the electrodes with a cylinder made of μ-metal. Resultantly, it was ascertained that positrons thermalized with the moderator were accelerated and focused by the electrostatic fields as a beam of designed diameter at the center of exit electrode. The current density of positron beam was measured to be a several thousands a second by detecting the annihilation γ-ray under the preliminary setting of the device. Meanwhile, a new type of manufacturing process of vacuum vessel for UHV was developed. The degassing rate of the device built in various parts was 8.6x10-13 Torr l/s cm2, as low as an aluminium vessel worked by means of the EX process. (author)