Isospin and the spacing distribution of low-lying levels of Al-26

The nearest-neighbor-spacing distribution and the cumulative spacing distribution of the low-lying energy levels of Al-26 are analyzed with isospin considered as a good symmetry, taking into account that the pure isospin sequence is not fully chaotic. Levels with isospin zero are found well represented by a formula, recently suggested for systems having mixed regular-chaotic dynamics, with the regular motion occupying 10\% of the classical phase space. When levels with isospins zero and one are together considered, the spacing distribution agrees, within the experimental uncertainties, with that of a superposition of two sequences of independent levels without introducing isospin mixing.