POSITRON-ANNIHILATION STUDIES OF A DECAGONAL SINGLE QUASI-CRYSTAL OF AL70NI15CO15

Measurements of the two-dimensional angular correlation of positron-annihilation radiation and positron lifetime have been carried out for a thermodynamically stable decagonal single quasicrystal of Al70Ni15Co15. The results show almost isotropic electron-positron momentum distributions in spite of the strong anisotropy in the atomic structure of the decagonal phase. Two positron lifetimes, which are much longer than the expected lifetime corresponding to the electron density of Al70Ni15Co15, were observed. These results suggest that the annihilating positrons are trapped by isotropic structural defects.