The continuous coincidence $\gamma$-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 $\beta$ 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.

• Received 24 April 1969

DOI:https://doi.org/10.1103/PhysRevC.1.1036