Systematic analysis of inelastic alpha scattering off self-conjugate A = 4N nuclei

Abstract

The multipole decomposition analysis (MDA) for the inelastic alpha scattering is very useful to determine the strength distribution, and widely used to study the cluster structures in nuclei. However, the reliability of the MDA was shaken by the puzzle of the missing monopole strength in 12C, where the monopole strength for the Hoyle state determined by the inelastic alpha scattering is much smaller than by the inelastic electron scattering. In the present work, we systematically measured cross sections of the inelastic alpha scattering at Eα=130 and 386 MeV exciting low-lying discrete states in 12C, 16O, 20Ne, 24Mg, 28Si, and 40Ca. In addition, we compared the measured cross sections with the “parameter-free” distorted-wave Born-approximation (DWBA) calculation. All of the adjustable parameters in the DWBA calculation were determined by the electromagnetic transition strengths and the elastic alpha scattering.We found that this puzzle is not specific to the Hoyle state in 12C but universally observed in all of the ΔL = 0 transitions, and the DWBA calculation with the density-independent effective αN interaction at Eα = 386 MeV is better than that with the density-dependent effective αN interaction. The inadequate density dependence in the effective αN interaction causes this puzzle by overestimating the cross sections for the ΔL = 0 transition. However, it is still unsatisfactory to reproduce the experiment. The present data will provide valuable information to develop a new sophisticated interaction which includes the proper density depend.