Fig. 6-3 Reflection high-energy positron diffraction pattern of the [111] surface of the silicon reacted with hydrogen The white points in the photograph are diffraction patterns created by the interference of positrons reflected from the crystal surface. These diffraction spots correspond to positions of atoms on the [111] surface (reciprocal-lattice), and [] surface indicates the incident direction of the positron against the crystal.

Fig. 6-4 Relationship between the intensity of the reflected positron at diffraction spot (00) from [111] silicon surface and the incident angle of the positron In ordinary electron diffraction, the incident electron is drawn to the inside of the crystal by the coulombic force from atomic nuclei. In contrast, in positron diffraction, the permeation of the positron into the crystal can be limited to only 2 or 3 atomic layers by selecting the incident angle, because the positron receives the coulombic repulsion from atomic nuclei. The reflection intensity increased strongly below an incident angle of 2 degrees and the first order Bragg peak was observed at an incident angle of 1.5 degrees. This agrees with the theoretical prediction in positron diffraction.

The positron corresponds to anti-matter of the electron, and is not composed of the material world and only stably exists in the "anti-nature" world. Positrons are emitted from some radioisotopes and can also be produced by using high-energy accelerators. When a material is observed by using a positron, a different image is obtained by using electrons. For instance, the structure just near the surface of materials is not entirely understood using electrons, but more precise information would be revealed by observation using positrons. It is theoretically predicted that the incident positron beam on a crystal surface at a small angle is reflected at the first layer of crystal. This means that the positron is sensitively diffracted, depending on the structure of the crystal surface, and hence analysis of just the near surface structure of the crystal is available at the atomic level. The first successful reflection high-energy positron diffraction (RHEPD) experiment was carried out at JAERI, as the result of the development of a technique for the production of a highly parallel 20 keV positron beam and a method for injection of the beam close to the crystal surface (Fig. 6-3). The relation between the intensity of the reflected positron beam from a crystal surface and the incident angle matches the theoretical prediction and this phenomenon has been proved to be positron diffraction (Fig. 6-4). From high precision total reflection measurements on the crystal surface, disorders of crystal structure to the extent of one atom are revealed just near the surface of the silicon treated by hydrogen gas. It is expected that the total reflection of positrons leads to more precise elucidation about absorption phenomena on material surfaces and structures of the surface, and the surface Debye temperature can be more precisely determined.

Reference A. Kawasuso et al., Reflection High Energy Positron Diffraction from a Si (111) Surface, Phys. Rev. Lett., 81 (13), 2695 (1998).

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