Double Internal Bremsstrahlung in the Beta Decay of Y90

The continuous coincidence $\ensuremath{\gamma}$-ray spectrum discovered in the process of searching for two-photon decay in ${\mathrm{Zr}}^{90}$ is found to be double inner bremsstrahlung originating in the 2.26-MeV $\ensuremath{\beta}$ decay of ${\mathrm{Y}}^{90}$ to the ${\mathrm{Zr}}^{90}$ ground state. The angular correlation, energy dependence, and total transition intensity of the coincidence spectra are compared with recent theoretical calculations.