N-Z distributions of secondary fragments and the evaporation attractor line

In many nuclear reactions, one or more excited primary fragments are formed which decay by the evaporation of nucleons and light clusters. This evaporation process can substantially alter the proton-neutron asymmetry of the initial primary fragments. In a study of complex fragment emission in fusion reactions, the Z values of the detected secondary fragments were measured @1#. In order to estimate the average mass associated with each fragment Z , statistical model calculations were performed varying the assumed Z , A , and excitation energy of the primary fragments. It was noted that at sufficiently large excitation energies, independent of the assumed Z and A of the primary fragments, evaporation models predict that the average location of the secondary fragments in the chart of nuclides is always close to a particular line. The location of this line is mainly determined by competition between proton and neutron evaporation. For compound nuclei on the neutron-rich side of the line, neutron emission is the most important evaporation mode and this drives the system towards the line. On the neutronpoor side, proton emission is the strongest decay mode and again, this acts to move the decaying system towards the line. This line thus acts as if it is attracting the decaying systems and hence it will be called the evaporation attractor line ~EAL!. The same concept was referred to as the ‘‘residue corridor’’ by Dufour et al. @2,3# who indicated that its position is near the line where the ratio of neutron and proton decay widths (Gn /Gp) is unity. For light systems, the attractor line is coincident with the line of b stability. However, for heavier systems, the larger Coulomb barrier for proton emission pushes this line to the neutron-deficient side of the valley of stability. At the attractor, the neutron and proton driving forces are about equal. A system located on the attractor will tend to follow the attractor down to lower masses until its excitation energy is exhausted. At lower excitation energies, the only other important light particle decay mode is a particle evaporation. The emission of this particle moves the decaying system almost parallel to the attractor line and thus preserves a memory of system’s neutron excess or deficiency. For these low excitation energies, a particle emission is often less probable than neutron and proton evaporation and only slows down the general movement towards the attractor. The calculations and conclusions of Ref. @1# were confined to region of lighter fragments (Z,40). The location of