We have deduced the emission probability of the 447-keV $\gamma$ ray from the $\varepsilon +{\beta }^{+}$ decay of ${}^{137}\mathrm{Ce}^g$ (9.0 h) relative to that of the 254-keV $\gamma$ ray from the ${}^{137}\mathrm{Ce}^m$ (34.4 h) decay in transient equilibrium. The time-dependent factor in transient equilibrium was applied following the Bateman equation for a radioactive decay chain. The isotope was produced via the ${}^{139}\mathrm{La}\left(p,3n\right){}^{137}\mathrm{Ce}^{m,g}$ reaction by bombarding ${}^{\mathrm{nat}}\mathrm{La}$ with a proton beam from the 88-in. cyclotron at Lawrence Berkeley National Laboratory. $\gamma$-ray intensities were measured using an HPGe detector. The emission probability for the 447-keV $\gamma$ ray deduced in this work is 1.21(3) (that is 1.21 ± 0.03) per hundred parent decays, which differs significantly from an earlier published value of 2.24(10). We identify the source of this discrepancy to be an incorrect use of the time-dependent factor. Additionally, we have deduced the emission probability of the 504-keV $\gamma$ ray from the decay of ${}^{85}\mathrm{Y}^g$ (2.68 h) relative to that of the 232-keV $\gamma$ ray from the ${}^{85}\mathrm{Sr}^m$ (1.127 h) decay in transient equilibrium. The isotope was produced via the ${}^{86}\mathrm{Sr}(p,2n){}^{85}\mathrm{Y}^g$ reaction by bombarding ${}^{86}\mathrm{SrCO}_3$ with a proton beam at the same facility. The study confirms the assumption of the time-dependent correction for recommending the emission probability of the 504-keV $\gamma$ ray in the literature. Our work highlights the importance of explicit description by authors of any time-dependent correction they have made when reporting $\gamma$-ray intensities for nuclides in transient equilibrium. The need and significance of accurate and precise decay data of ${}^{137}\mathrm{Ce}^g$ and ${}^{85}\mathrm{Y}^g$ in basic science and medicine is briefly outlined.

• Received 13 September 2019
• Revised 28 January 2020
• Accepted 11 May 2020

DOI:https://doi.org/10.1103/PhysRevC.101.064619