Cluster structure of C 11 investigated with a breakup reaction

The excitation spectra above the breakup threshold of C11 were measured using the invariant mass method with a 25.0-MeV/nucleon beam bombarding a carbon target for the first time. This measurement was performed with a zero-degree double-sided silicon strip detector together with four CsI(Tl) detectors, which could detect breakup fragments with low relative kinetic energy. Some resonances in C11 were clearly observed in the excitation energy range of 8-14 MeV proceeding through the Be7 + α channel. Among them, a resonance with the largest yield was observed around 8.10 MeV. We assign it as the head of the Kπ=3/2- rotational band, and suggest the 9.38-MeV state as the second member of this band.