Improvement of the electrostatic positron beam apparatus and observation system for RHEPD experiment

We have observed reflection high energy positron diffraction (RHEPD) using the electrostatic positron beam apparatus, where positrons were accelerated at 20 keV and converged using three einzel lenses and a collimator. To reduce beam energy dispersion and gamma ray background, a 45° electrostatic prism and einzel lenses were newly added to the original apparatus. The positron beam is further collimated to 1 mm diameter. Final beam energy dispersion and angular divergence are less than 0.1 keV and 0.1°, respectively. Any noises arising from the gamma rays are not observed. We also fabricated the positron beam observation system with a dynamic range of approximately 4.3×109. The above improvements enabled to detect the fine structures of diffraction patterns and rocking curves.