Comparison of midvelocity fragment formation with projectilelike decay

The characteristics of intermediate mass fragments (IMFs: 3≤ Z≤ 20) produced in midperipheral and central collisions are compared. We compare IMFs detected at midvelocity with those evaporated from the excited projectilelike fragment (PLF^*). On average, the IMFs produced at midvelocity are larger in atomic number, exhibit broader transverse velocity distributions, and are more neutron rich as compared to IMFs evaporated from the PLF^*. These characteristics of midvelocity fragments are consistent with the low-density formation of the fragments. We present in the different kinematical regions studied, the 〈E_⊥〉 for isotopically identified IMFs. For a given Z,〈E_⊥〉 is either constant or decreases slightly with increasing A, in contradiction with a mass-dependent collective expansion in which all IMFs are emitted on average at the same time. Neutron-deficient isotopes of even Z elements manifest higher kinetic energies than heavier isotopes of the same element for both PLF^* and midvelocity emission. This result may be because of the charged-particle decay of long-lived excited states.