Speaker
Description
Understanding nuclear shape is a crucial problem in nuclear physics, significantly impacting our knowledge of nucleon single-particle dynamics and collective nuclear behavior. While the quadrupole deformation parameter $\beta_2$ has been well studied in terms of magnitude, determining its sign $-$whether prolate or oblate$-$ remains a challenging problem because many observables are sensitive only to the square of $\beta_2$. Traditional approaches, such as electric quadrupole moments and Coulomb excitation experiments, provide crucial insight into nuclear deformation including its sign, however, they are often impractical for neutron-rich unstable nuclei. These unstable nuclei are indispensable for understanding shell evolution, emphasizing the importance of properly determining their deformation and its sign.
In this study, we propose a method to determine the sign of nuclear deformation by using low-energy $\alpha$ inelastic scattering [1]. Our approach utilizes the nuclear reorientation effect (RE), which is known as a self-coupling of excited states [2]. Our approach is based on a standard coupled-channel framework within the macroscopic model, enabling us to present how RE modifies cross sections differently for prolate and oblate shapes. We demonstrated the feasibility of this technique with $\alpha$ scattering on a stable target, $^{154}$Sm, at 50 MeV, for which the experimental data are available. Our results show distinct differences in inelastic cross sections for positive and negative $\beta_2$ values, establishing low-energy $\alpha$ inelastic scattering as a promising tool for systematically determining the sign of deformation. In the presentation, we will extend this method to unstable nuclei with the neutron number $N=28$ such as $^{40}$Mg (expected to be prolate) and $^{42}$Si (expected to be oblate), to demonstrate its broader applicability.
[1] S. Watanabe et al., Phys. Rev. C 110, 034618 (2024).
[2] G. R. Satchler, Direct Nuclear Reactions (Clarendon Press, Oxford, 1983).
