Emission probability of bremsstrahlung in the $\alpha$ decay of ${}^{210}\mathrm{Po}$ was measured in $\alpha$- $\gamma$ coincidence measurements with Si and Ge detectors. It was found that the bremsstrahlung yields are much smaller than those predicted by a Coulomb acceleration model, in which $\gamma$ rays are emitted during acceleration outside the barrier. This suggests that the radiation amplitude in the barrier cannot be neglected, and the discussion based on a quasiclassical approach is given.

• Received 16 September 1996

DOI:https://doi.org/10.1103/PhysRevLett.79.371