The pure LiF crystals were irradiated by 60Co γ-ray with irradiated dose in the range from 0.99×103 to 1.38× 106Gy.The positron lifetime spectra were then measured using a fast-slow coincidence system.The second lifetime component τ2and its intensity difference △I2 were presented. When the irradiated dose D<1 ×104Gy

τ2 is essentially unchanged

while the △I2 is directly proportional to D1/2.Therefore △I2∝NF can be derived since the concentration of F-center NF is also directly proportional to D1/2.As the dose increases futher

the τ2 is increased but the increase of △I2 is very slow till saturation.