'Neck' formation and decay in intermediate energy HIC

Under different impact parameters, the collision system of 40Ca(30 MeV/u) + 40Ca is investigated in terms of QMD model. It is shown that the number of IMFs is the largest at impact parameter b =3D 6fm, which is due to the formation of 'neck' in collision process. Meanwhile, it is found that, in the process of formation of 'neck', the growing of the relative density fluctuation of the reaction system is very slow and the saturation value is small, which imply the character of shape instability