$\gamma$-ray spectroscopy of the high-spin states of the neutron-deficient nucleus ${}^{187}\mathrm{Pb}$ has been conducted with the ${}^{155}\mathrm{Gd}{(}^{36}\mathrm{Ar},4n)$ reaction. A cascade of three transitions was deduced from $\gamma -\gamma$ coincidence data gated by detection of recoiling evaporation residues in a gas-filled recoil separator. In an earlier, separate experiment, two of these $\gamma$ rays were positively identified with ${}^{187}\mathrm{Pb}$ by recoil-$\gamma$ coincidence measurements with a high-resolution, recoil mass spectrometer. From comparison with similar sequences in heavier odd-$A$ lead isotopes, the cascade in ${}^{187}\mathrm{Pb}$ is associated with the sequence of three $E2\mathrm{}$ transitions from the yrast ${\frac{25}{2}}^{+}$ level to a low-lying ${\frac{13}{2}}^{+}$ isomer. The variation of excitation energy with mass number of the levels concerned suggests that their structure can be associated with weak coupling of an odd $i_{13/2}$ neutron to states in the spherical well. However, the possibility that they are influenced by mixing with states in the prolate-deformed well cannot be discounted.

• Received 2 July 1998

DOI:https://doi.org/10.1103/PhysRevC.58.2671