ABSTRACT
Terbium-157 was radiochemically extracted from an irradiated tantalum target. Since the resulting material contained a significant impurity of 158Tb, 157Tb was isotopically purified using laser resonance ionization at the RISIKO mass separator in Mainz and then implanted on an aluminum (Al) foil. The implanted 157Tb was measured by two different calibrated gamma-ray spectrometers to determine photon emission rates. After dissolving the Al foil, a high purity 157Tb 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)(CeBr3) 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.
ABSTRACT
The activity concentration of an 225Ac solution was determined by means of liquid scintillation counting using three custom-built TDCR counters. The efficiency calculation was carried out in the same way as it had been done in an earlier article on 229Th. The computation of the counting efficiency is rather complex and requires a correction to allow for the short-lived 213Po. The experimental deadtime was varied to validate the correction. One of the TDCR counters is equipped with a CAEN N6751C digitizer for data acquisition. In addition, the system comprises a CeBr3 solid scintillator as a gamma detector. The offline analysis was used to obtain a time-difference spectrum, using signals from the 213Po γ-rays at about 440 keV in the gamma channel in coincidence with the preceding beta decay as the start signal, and signals from the subsequent (delayed) 213Po alpha decays as the stop signal. After fitting an exponential function with a constant background, the half-life of 213Po was determined to be 3.709(12) µs, which is in good agreement with the evaluated value. The half-life of 225Ac was determined from long-term measurements using an ionization chamber (IC) and a TDCR system. The combined result was found to be 9.9179(30) d, which is in agreement with the outcome from Pommé et al. (2012).
