Abstract
Different factors influencing the sub-barrier fusion enhancement owing to neutron rearrangement with positive Q values are studied. It was found that opposite to the previous opinion the presence of positive Q values is necessary but not sufficient to observe enhancement of the sub-barrier fusion. Rigidity of colliding nuclei with respect to collective excitations plays a crucial role for the sub-barrier fusion enhancement due to neutron rearrangement. Neutron binding energy has a strong impact but only in the case of fusion of light nuclei. 1 Motivation Sub-barrier fusion is one of the intensively studied phe- nomena, that is still far from its complete understanding. The sub-barrier fusion enhancement induced by coupling of relative motion to surface deformations or rotation of heavy deformed nuclei is well understood and properly described within the quantum coupled channel approach (see e.g. (1-3)) and within the empirical coupled chan- nel (ECC) model (4). At the same time there are many experimental evidences of additional enhancement of the sub-barrier fusion cross section due to neutron rearrange- ment with positive Q values. The mechanism of sequen- tial fusion was proposed in (5) which described for the first time an additional sub-barrier fusion enhancement owing to neutron rearrangement with positive Q value at approaching stage. The corresponding model (called the ECC model with neutron rearrangement) is used in this work. The mechanism of influence of neutron rearrange- ment on the sub-barrier fusion is connected with the fact that the spreading of the valence neutron's wave func- tion into the volume of the other nucleus takes place be- fore the colliding nuclei overcome the Coulomb barrier (6, 7), and, therefore, neutron rearrangement at approach- ing stage may really influence the sub-barrier fusion dy- namics giving a gain in kinetic energy of colliding nuclei if it occurs with positive Q value. This effect can be easier observed if one compares the sub-barrier fusion cross sec- tions for two close projectile-target combinations for one of which neutron rearrangement with positive Q values is possible whereas for another one all neutron transfers have negative Q values. The pair of reactions 40 Ca+ 96 Zr and 40 Ca+ 90 Zr is classical example of such kind (see Fig. 1 (a)). Coupling of relative motion to the surface vibra- tions explains the observed cross section for the 40 Ca+ 90 Zr reaction, but it is insufficient to describe additional sub-
Highlights
Note that the model used takes into account neutron rearrangement, reproduces quite well the experimental data and does not predict any sub-barrier fusion enhancement for these specific reactions with positive Q values of neutron transfers
The role of neutron rearrangement channels in near-barrier fusion reactions is studied within the empirical coupled channel (ECC) model
It is shown that the model reproduces available experimental data
Summary
Barrier fusion enhancement for the 40Ca+96Zr caused by neutron rearrangement. Another example of strong enhancement owing to neutron rearrangement is fusion of neutron rich weakly bound nuclei. Several projectile–target combinations were reported (for example, 130Te+58,64Ni [11], 16,18O+76,74Ge [12], 60,64Ni+100Mo [13]) for which the measured fusion cross sections do not demonstrate noticeable enhancement at sub-barrier energies in spite of positive Q values for neutron rearrangement shown in Tab. 1. Only in the case of the 58Ni+130Te reaction some excess in the sub-barrier fusion cross section is visible as compared with more neutron rich system 64Ni+130Te having all negative Qxn-values except Q2n = +0.55 MeV. Note that the model used takes into account neutron rearrangement, reproduces quite well the experimental data and does not predict any sub-barrier fusion enhancement for these specific reactions with positive Q values of neutron transfers. We have to understand what features of These reactions (properties of colliding nuclei) suppress a gain coming from positive Q value neutron rearrangement (clearly visible in many other cases). All the presented calculations have been performed (and can be repeated) with the NRV Fusion code allocated at the web site with free access [3]
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