*3.1. Type-II Shell Evolution*

The shell evolution shown in Figure 5b,c are due to the addition of two or four neutrons into the orbit *j* <sup>&</sup>gt;, respectively. Instead of adding, one can put neutrons into the orbit *j* <sup>&</sup>gt; by taking the neutrons from some orbits below *j* <sup>&</sup>gt;, or equivalently by creating holes there, as shown in Figure 5d. If such a lower orbit happens to be the *j* <sup>&</sup>lt; orbit as in Figure 5d, its monopole matrix elements show just the opposite trends compared to the *j* > orbit. However, because holes are created in *j* <sup>&</sup>lt;, the sign of the monopole-interaction effect is reversed, and the final effect has the same sign as the monopole effect form the orbit *j* <sup>&</sup>gt; (see Figure 5d). Thus, the particle–hole (ph) excitation of the two neutrons Figure 5d reduces the proton *j*>-*j*< splitting even more than in Figure 5b. This reduction becomes stronger with the ph excitations of four neutrons, as depicted in panel Figure 5e. Such strong reduction in the spin-orbit splitting produces interesting consequences beyond shell-structure changes. This type of the shell-structure change within the same nucleus is called *type-II shell evolution*.
