Performance of a positron annihilation coincidence doppler broadening spectrometer

A positron annihilation CDB (coincident Doppler broadening) spectrometer of high-performance has been developed with modes of external and internal triggers. The external trigger mode, with time resolution of < 7 ns (FWHM) and peak/background ratio of better than 106, is applicable to measurements at high coincident count rate over 200/s. The internal trigger mode has a time resolution of 94 ns and peak/background ratio of >105. It, is not as good as the external trigger mode, but the peak/background ratio can be increased to 106 after pileup signals are rejected by an arithmetic code, so as to be applicable to measurements at 100/s or lower coincident count rates. The internal trigger mode is simple and convenient to use, unlike the external mode. In addition to performance determination of the CDB spectrometer, CDB spectra of pure Cu, Ni, W and A1 were measured and relativistic kinetic energy formula E2total =p2c2 + m2ec4 was validated by observing positron annihilation in flight. The results show that performance of this spectrometer is favorable in time resolution and background restraint. As an application example, chemical surroundings around the intrinsic vacancy-type defects in NiAl alloys at deviations from the stoichiometric compositions were investigated. The results indicate that the spectrometer is highly sensitive to element variations around open-volume defects and has wide application prospect in the field of microstructure investigation of materials. (authors)