The charge-state-dependent lattice relaxation of mono-vacancy in silicon is studied using the first-principles pseudopotential plane-wave method.We observe that the structural relaxation for the first-neighbor atoms of the mono-vacancy is strongly dependent on its charge state.The difference in total electron density between with and without charge states in mono-vacancy and its relevant change due to the localized positron are also examined by means of first-principles simulation,demonstrating the strong interplay between positron and electron.Our calculations reveal that the positron lifetime decreases with absolute charge value increasing.
A new simple digital positron lifetime spectrometer has been developed.It includes a DRS4 waveform digitizing board and two scintillation detectors based on the XP2020Q photomultiplier tubes and LaBr_3 scintillators.The DRS4 waveform digitizing can handle small pulses,down to few tens of millivolts,and its time scale linearity and stability are very good.The new system has reached a 206 ps time resolution,which is better than the conventional analog apparatus using the same detectors.These improvements make this spectrometer more simple and convenient in comparison with other spectrometers,and it can be applied to the other scintillation timing measurements with picosecond accuracy.
