Experimental Calibration Of Positron Annihilation Lifetime Spectrometer And Positron Annihilation Effect Of Fe-based Amorphous Alloys

Positron lifetime analysis techniques provide a very effective and sensitive tool for the study of materials defects. It can not only detect the size of the defect, but also detect how many kinds of defects in materials and detects the proportion of each kind of defect in materials. This kind of technique is more sensitive than any other kind of conventional technique.The Positron annihilation lifetime spectrometer in our laboratory was installed with two trouble ones. But the60Co time resolution of FWHM is greater than400ps and the resolution curve is not very symmetrical. So it indicates that the lifetime apparatus in our laboratory can not operate normally. In the paper, the modules of positron annihilation lifetime measurement apparatus were selected, reset and tested. We made the energy calibration, the energy window selection, the time calibration, the time resolution measurement and stabilization measurement for the reassembled lifetime spectrometer. Energy calibration results are as follows:start channel, EmVU=0.7082keV/mV,EmVL=1.0685keV/mV, stop channel, EmVU=0.7197keV/mV,EmVL=1.1042keV/mV. The result of time calibration is30.951ps/ch. Energy windows were selected by dynamic method.The final energy window is:start channel,496～1497mV, stop channel,260～420mV. Time resolution of FWHM=277ps at this energy window and time resolution curve is very symmetrical. When the temperature is constant, we made the measurement for time resolution of60Co for48hours. The result indicate that the peak position move0.5ch. Thermal stability testing experiments of the adjusted spectrometer showed that the movement of the peak location of60Co was6.82ps/, less that10ps/℃, It meet the experimental necessity completely.In this paper, the lifetime of four kinds of Fe-based amorphous Fe78B13Si9, Fe73.5Cu1Nb3Si13.5B9, Fe43Co43Hf7B6Cu1, Fe44.5Co44.5Zr7B4was measured with the positron annihilation lifetime measurement apparatus that was reinstalled and optimized. For these amorphous alloys, the major lifetime component τ1is longer than107ps and shorter than 170ps. It is responsible for positron annihilation with free volumes (the size of which is smaller than a mono-vacancy) in amorphous state. The intensity Ⅰ1of τ1is closed to90%. It is according to the matrix of disorder close packing. The amorphous samples were annealed at temperatures,200℃,300℃,400℃and500℃for30min (protected by nitrogen). Then the lifetime of each specimen which was annealed was measured. With increasing the annealing temperature from27℃to400℃, τ1decrease, whereas the intensity Ⅰi of major component τ1stays constant nearly. This phenomenon is a typical structural relaxation in amorphous alloy during annealing. When the annealing temperature arise to500℃, the intensity of second lifetime component τ2increase clearly. With the precipitation of crystallization fabric, many grain boundary and phase boundary appears in the alloys, Which lead to the increase of the Ⅰ2. According to the whole course of experiment, the Positron annihilation lifetime spectrometer can operated normally.