Shell-cluster transition in Ti48

Background: The existence of α (⁴He nucleus) clustering in the medium-mass region of nuclear systems is a fundamental and intriguing question. However, the recent analysis of the α knockout reaction on ⁴⁸Ti [Phys. Rev. C 103, L031305 (2021)] poses a puzzle: The microscopic wave function gives an α knockout cross section that is two orders of magnitude smaller than the experiment, while basic nuclear properties such as the charge radius and the electromagnetic transition probabilities are well explained.Purpose: The ground-state structure of ⁴⁸Ti is investigated by using proton- and α-nucleus elastic scattering at a few to several hundred MeV, which offers different sensitivity to the region of the nuclear density profiles.Method: Four types of density distributions, the $jj$-coupling shell model and three cluster model configurations, are generated in a single scheme by the antisymmetrized quasicluster model (AQCM). The angular distribution of the proton- and α-⁴⁸Ti elastic scattering cross sections are obtained with a reliable high-energy reaction theory, the Glauber model.Results: The $jj$-coupling shell model configuration is found to best reproduce the proton-nucleus elastic scattering cross section. On the other hand, the trace of the α cluster structure in the tail region of the wave function is embedded in the α-nucleus elastic scattering cross section.Conclusions: Our results suggest that the structure of the nucleus changes as a function of distance from the center, from the $jj$-coupling shell model structure in the surface region to the α+⁴⁴Ca cluster structure in the tail region. This picture is consistent with the finding of the α knockout reaction on ⁴⁸Ti.