Decay of Terbium-157

Samples of ${\mathrm{Tb}}^{157}$ have been prepared by irradiating enriched ${\mathrm{Dy}}^{156}$ in the Materials Testing Reactor. After irradiation, the ${\mathrm{Tb}}^{157}$ samples have been prepared by ion exchange separation and the use of the Bohr Institute's electromagnetic isotope separator. The $L_{\mathrm{I}}$-to-$K$ electron-capture ratio has been determined as 2.65±0.20 using a beryllium-windowed sodium iodide spectrometer. This value is comparable to previous determinations. It implies a $Q$ value for electron capture to the ground state of ${\mathrm{Gd}}^{157}$ of 66±6 keV which also permits electron capture to the first excited state at 54 keV. A search has been made for photons of this energy using a germanium semiconductor detector having an energy resolution of 0.75 keV. Photons having 54.35±0.05 keV have been observed in ${\mathrm{Tb}}^{157}$ sources. Using the measured photon intensity and the intensity of the $K$ x rays, a branching ratio of (0.34±0.2)% is calculated for electron capture to the first excited state.