Construction of a spin-polarized positronium time-of-flight measurement apparatus

Maekawa, Masaki; Li, H.; Kawasuso, Atsuo

For the study of surface magnetic phenomena, we have developed spin-polarized slow positron beams. The spin polarization at the first surface layer was determined through 3- annihilation of positronium which is emitted into vacuum. The energy distribution of the emitted positronium is affected by the electron density of state (DOS). By measuring the energy distribution of positronium, one can obtain spin-polarized DOS at the first surface layer. For this purpose, we are constructing a SP-positronium time-of-flight (TOF) apparatus. Spin-polarized positron beam generated from a Ge-68 source (600 MBq), which is made by a high-energy proton irradiation to the metal-Ga target, is transported to the sample position by the electrostatic lenses avoiding depolarization. The TOF spectrum is obtained by measuring time difference of start and stop signals. The start signal is triggered by secondary electrons detected by a through-hole microchannel plate when the positrons pass through a thin carbon film width thickness of 50 nm. The stop signal is annihilation ray of positronium detected by a NaI detector.