Lifetime measurement of $^{12}$Be(2$^+_1$)

We have measured the lifetime of the first $2^+$ state of $^{12}$Be in flight via the Doppler-shift attenuation method (DSAM). This is the first application of DSAM to the intermediate-energy unstable nucleus beam. The lifetime is inversely proportional to the $B$($E2$) value, which relates to the quadrupole deformation of charge distribution. It was suggested that the magic number of $N$=8 disappeared in $^{12}$Be. Besides, the large matter deformation was inferred from the proton inelastic scattering. The measurement of $B$($E2$) value would provide further understanding of the exotic structure. The detail of the experimental method and the result will be presented.