Coincidence Doppler-broadening Measurements of Positron Annihilation in Transition Metals

Since 1940s, some experimental techniques based on positron annihilation have been widely used to study properties of solid. Among them, Doppler-broadening spectroscopy (DOBS) plays an important role to obtain elemental information on the surroundings of the positron annihilation site. The annihilation radiation contains information on the electron momentum distribution at the annihilation site, as a result of momentum conservation during the annihilation process. A drawback of DOBS is the low positron annihilation probability of the signal with electrons of high momentum. In 1977, Lynn et al. improved the DOBS by adding a second detector in order to observe the second annihilation quantum in coincidence. In coincidence Doppler-broadening spectroscopy (CDOB), the peak to background ratio is drastically reduced by three orders of magnitude. We have constructed a two-detector spectrometer for measurements of Doppler-broadened positron annihilation spectra. In order to identify chemical elements on surroundings of the annihilation site, basically it is necessary to obtain the annihilation signal from the pure material. In this study, we have investigated 8 transition metals (Ti, V, Cr, Fe, Co, Ni, Cu, Zn) and analyzed the CDOB data using the DualNTW and CDB program