Emergence of Cluster Formation in Light Nuclei

Abstract

Spherical harmonics form a complete orthonormal basis which allows any function on the sphere to be expanded. The nuclear shape of a given eigenstate can thus be described within Bohr's quasi-molecular model by a coordinate transformation from a randomly oriented ellipsoid in space to a coordinate system aligned with the ellipsoid's principal axes. This transformation (Eq. 4) is characterized by three Euler angles and two deformation parameters, $β$ (quadrupole) and $γ$ (triaxiality), but does not uniquely define the nuclear shape; rotational averaging over equivalent orientations is expected to yield a diffuse nuclear shape. Rotational invariance under $β$ and $γ$ is achieved using three transformation operators, which define a new coordinate system aligned with a single intrinsic configuration (Eq. 6). Here we show that the non-unique coordinate system of Eq. 4 with $β$ and $γ$ deformation parameters extracted from experimental electric-quadrupole matrix elements actually yields the most probable nuclear shape. Only then does cluster formation spatially emerge in light nuclei and the characteristic bowling-pin-like shapes of $^{10}$B and $^{20}$Ne are reproduced, consistent with modern nuclear theory. Both coordinate systems generally exhibit the same shape features for heavier deformed nuclei, where substantial triaxial deformation is empirically observed. However, the approach based on Eq. 4, using empirical $β$ and $γ$ values, provides deeper insight by capturing the superposition of multiple intrinsic configurations that collectively form the nuclear state. This, in turn, offers a physical interpretation of triaxiality.