Brightness enhancement of a linac-based intense positron beam for total-reflection high-energy positron diffraction (TRHEPD)

Abstract

The brightness of a linac-based intense positron beam was enhanced for total-reflection high-energy positron diffraction (TRHEPD) measurements. The beam initially guided by a magnetic field was released into a non-magnetic region and followed by a transmission-type remoderation. The term “TRHEPD” is a new name of reflection high-energy positron diffraction (RHEPD), which is a technique for the determination of the topmost- and near-surface atomic configurations; the total reflection of the positron beam from a solid surface is a unique superior characteristic. The present system provides the final beam of almost the same quality as the previous one with a 22Na-based positron beam [A. Kawasuso et al., Rev. Sci. Instrum. 75, 4585 (2004)] but much increased flux, i.e., almost the same emittance but much higher brightness. It gave a ∼ 60 times intensified diffraction pattern from a Si(111)–(7 × 7) reconstructed surface compared to the previous result. An improved signal-to-noise ratio in the obtained pattern due to the intensified beam allowed observation of clear fractional-order spots in the higher Laue-zones, which had not been observed previously.