Special Features of Neutron-Rich Nuclei

There are many theoretical arguments suggesting that the nuclei near the neutron drip line represent another form of nuclear life. For instance, some calculations predict [1,37,38,39] that the shell structure of neutron drip-line nuclei is different from what is known around the beta-stability valley. According to other calculations [40], a reduction of the spin-orbit splitting in neutron-rich nuclei is expected. An interesting question to ask in the context of ``islands of inversion" discussed in Sec. 2 is to what extent the ``traditional" explanation in terms of intruder states, proton-neutron-correlations, pairing, etc., needs to be modified in the neutron-rich nuclei, where the quenching of the known magic gaps is expected. Clearly, the reduction of the N=20 and 28 gaps far from stability can lower the excitation energy of the deformed intruder configuration; hence it can enhance the shape transition. However, before one addresses this question in a systematic way by properly taking into account competition between mean field and pairing, one should avoid calling the shape transition around ³²Mg and ⁴⁴S as evidence for the shell gap quenching.

Halo nuclei are the best known examples of possible exotica. They are examples of physics on the threshold of nuclear binding. The predicted phenomena of low-density neutron skins is another, which is topologically similar to a halo, but quite different in microscopic origin [41]. Correlations due to pairing, core polarization, and clustering are crucial in weakly bound nuclei. In a drip-line system, the pairing interaction and the presence of skin excitations (soft modes) could invalidate the picture of a nucleon moving in a single-particle orbit [39,42,43,44,45]. According to theory, the low-$\ell$spectroscopic strength is dramatically broadened when approaching the neutron drip line. Also, in the presence of large neutron excess, strong isovector effects are expected. For instance, some calculations predict nuclear configurations having different proton and neutron deformations [33,34,46]. However, in order to address this particular question properly, pairing correlations need to be properly taken into account. In the following, we carry out simplified calculations which focus on some of the physics aspects mentioned above.