A concentration of electric-dipole excitations below the particle threshold, which is frequently denoted as the pygmy dipole resonance, has been studied in the semimagic nucleus ${}^{140}\mathrm{Ce}$ in ($\alpha$, ${\alpha }^{\prime }\gamma$) experiments at $E_{\alpha }=136\mathrm{MeV}$. The technique of $\alpha \mathrm{\text{−}}\gamma$ coincidence experiments allows the separation of $E1$ excitations from states of other multipolarities in the same energy region and provides an excellent energy resolution to allow a detailed analysis for each state. The experimental results show that the PDR splits into two parts with different nuclear structure: one part which is excited in ($\alpha$, ${\alpha }^{\prime }\gamma$) as well as ($\gamma$, ${\gamma }^{\prime }$) experiments and one part which is excited only in ($\gamma$, ${\gamma }^{\prime }$).

• Received 5 July 2006

DOI:https://doi.org/10.1103/PhysRevLett.97.172502