We have searched for the l-forbidden beta decay of ${}_{}{}^{207}{}_{}{}^{}\mathrm{Tl}$ to the first excited state of ${}_{}{}^{207}{}_{}{}^{}\mathrm{Pb}$ by looking for 570-keV γ rays following the decay of ${}_{}{}^{207}{}_{}{}^{}\mathrm{Tl}$. We find a branching ratio of (2.4±5.6)×${10}^{\mathrm{-}7}$ per ${}_{}{}^{207}{}_{}{}^{}\mathrm{Tl}$ decay. This limit could provide a test for calculations of core polarization, meson exchange, and Δ excitation effects. We also find a branch of (0.54±0.05)% for the l-forbidden M1 transition ${}_{}{}^{207}{}_{}{}^{}\mathrm{Pb}$(898,(3/$2^{\mathrm{-}}$→570,(5/$2^{\mathrm{-}}$) and measure the intensities of γ rays emitted following the decay of ${}_{}{}^{211}{}_{}{}^{}\mathrm{Pb}$.

• Received 16 May 1988

DOI:https://doi.org/10.1103/PhysRevC.38.1370