Determination of the activity and nuclear decay data of Tb.

Terbium-157 was radiochemically extracted from an irradiated tantalum target. Since the resulting material contained a significant impurity of Tb, Tb was isotopically purified using laser resonance ionization at the RISIKO mass separator in Mainz and then implanted on an aluminum (Al) foil. The implanted Tb was measured by two different calibrated gamma-ray spectrometers to determine photon emission rates. After dissolving the Al foil, a high purity Tb solution was obtained. The corresponding activity concentration was determined with a low relative uncertainty of 0.52% through a combination of liquid scintillation counting using the TDCR method and 4π(X,e)(LS)-(X,γ)(CeBr) coincidence counting. By combining the results from all measurement techniques, emission intensities for K X-rays and gamma-rays were derived and found to be 16.05(31)% and 0.0064(2)%, respectively. The probability for K electron capture of the first forbidden non-unique transition to the ground state was determined to be 17.16(35)%. The probabilities for the electron-capture branch to the excited level and the ground state were found to be 0.084(4)% and 99.916(4)%, respectively. A Q value of 60.23(18) keV was estimated based on simplified BetaShape calculations, assuming an allowed transition.