Hexadecapole strength in the region of the low-energy octupole resonance

The strength distributions of hexadecapole transitions have been measured for the closed-shell nuclei of 48Ca, 50Ti, 52Cr, 54Fe, 58Ni, 90Zr, and 208Pb by using (p,p') reactions at Ep=65 MeV. The peak-by-peak analysis revealed the distribution for the excitation energy up to around 10 MeV. In 208Pb, the strength is distributed into two groups. One is the well-known first 4+ state which exhausts an energy-weighted sum-rule fraction of 8.4%. The other is a bump consisting of clustering L=4 states with a total fraction of 11.2%. The structure of the bump is much the same as those of low-energy octupole resonance, suggesting the existence of a resonance-like structure of hexadecapole strength. In an analysis using a cumulative sum of the strength, a bump of a similar kind is recognized in each nucleus examined. These bumps exhaust 3--11% of the E4 energy-weighted sum rule and the centroids are situated at Ex=4--9 MeV