Chapter IV. Coupling Effects of Two-Particle and Alpha Two-Hole States in<i>A</i>= 18 Nuclei

Abstract

Properties of low-lying positive-parity levels in &lt;it&gt;A&lt;/it&gt; = 18 nuclei are shown to be well understood by considering admixtures of 2&lt;it&gt;p&lt;/it&gt; states and 4&lt;it&gt;p&lt;/it&gt;-2&lt;it&gt;h&lt;/it&gt; states with α-cluster structure (α-2&lt;it&gt;h&lt;/it&gt; states). In order to represent α-2&lt;it&gt;h&lt;/it&gt; states, and α-14N(14C) cluster model is introduced. This model can describe the weak coupling feature of α-2&lt;it&gt;h&lt;/it&gt; states and the coupling between 2&lt;it&gt;p&lt;/it&gt; and α-2&lt;it&gt;h&lt;/it&gt; states properly. It is shown that the α-2&lt;it&gt;h&lt;/it&gt; states exist in the same energy region and also with the same level density as 2&lt;it&gt;p&lt;/it&gt; states, and therefore couplings between these two types of states arise inevitably. Calculated couplings of 2&lt;it&gt;p&lt;/it&gt; and α-2&lt;it&gt;h&lt;/it&gt; states are found to be stronger in &lt;it&gt;T&lt;/it&gt; = 0 than in &lt;it&gt;T&lt;/it&gt; = 1 states. The binding energy of the ground 1+ (&lt;it&gt;T&lt;/it&gt; = 0) state is reproduced by virtue of the coupling effect. Further, a selection rule for the couplings between &lt;it&gt;T&lt;/it&gt; = 0 states is derived, which leads to a simple understanding of the breaking mechanism of the weak coupling structure in α-2&lt;it&gt;h&lt;/it&gt; states. &lt;it&gt;E&lt;/it&gt;2 transitions between α-2&lt;it&gt;h&lt;/it&gt; states are enhanced owing to the α-clustering, in good agreement with experiments. Furthermore, by using additional charges δ&lt;it&gt;e&lt;inf&gt;n&lt;/inf&gt;&lt;/it&gt;2&lt;it&gt;p&lt;/it&gt; = 0.5 &lt;it&gt;e&lt;/it&gt; and δ&lt;it&gt;e&lt;inf&gt;p&lt;/inf&gt;&lt;/it&gt;2&lt;it&gt;p&lt;/it&gt; = 0.2 &lt;it&gt;e&lt;/it&gt; for 2&lt;it&gt;p&lt;/it&gt; states, most of the known &lt;it&gt;E&lt;/it&gt;2 transitions are well reproduced. &lt;it&gt;E&lt;/it&gt;2 transitions between &lt;it&gt;T&lt;/it&gt; = 1 analogue states show interesting coupling effects of 2&lt;it&gt;p&lt;/it&gt; and α-2&lt;it&gt;h&lt;/it&gt; states. The α-widths and single-nucleon spectroscopic factors are also reproduced nicely and are discussed from the viewpoint of couplings of 2&lt;it&gt;p&lt;/it&gt; and α-2&lt;it&gt;h&lt;/it&gt; states.