Coulomb excitation of the exotic neutron-rich nucleus ${}^{26}\mathrm{Ne}$ on a ${}^{208}\mathrm{Pb}$ target was measured at $58\mathrm{MeV}/u$ in order to search for low-lying $E1$ strength above the neutron emission threshold. This radioactive beam experiment was carried out at the RIKEN Accelerator Research Facility. Using the invariant mass method in the ${}^{25}\mathrm{Ne}+n$ channel, we observe a sizable amount of $E1$ strength between 6 and 10 MeV excitation energy. By performing a multipole decomposition of the differential cross section, a reduced dipole transition probability of $B(E1)=0.49\pm 0.16e^2{\mathrm{fm}}^2$ is deduced, corresponding to $4.9\pm 1.6\%$ of the Thomas-Reiche-Kuhn sum rule. For the first time, the decay pattern of low-lying strength in a neutron-rich nucleus is measured. The extracted decay pattern is not consistent with several mean-field theory descriptions of the pygmy states.

• Received 7 March 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.212503