Note

Half-lives of some fission product nuclides

¹⁶¹Tb has potential applications in targeted radionuclide therapy and nuclear forensic science. However, the half-lives of ¹⁶¹Tb in previous studies show a discrepancy. In this study, ¹⁶¹Tb samples were produced by irradiating ¹⁶⁰Gd₂O₃ with thermal neutron flux. A series of procedures were applied to extract a pure ¹⁶¹Tb solution and three solid samples were prepared. The half-life of ¹⁶¹Tb has been measured with a high-purity germanium (HPGe) detector. The time-dependency of the ¹⁶¹Tb activity was followed by assessing the count rate of their characteristic gamma-ray emissions at 48.9 keV and 74.6 keV over a period of 33–43 days. The experiment and uncertainty budget are discussed in detail. Different uncertainty propagation equations were applied for random uncertainties, medium-frequency deviations and potential systematic errors. The result for the ¹⁶¹Tb half-life of 6.967 (11) d was determined by the weighted mean of half-lives from three samples, which confirms that the half-life is longer than the of the current evaluated half-life of 6.89 (2) d. From all available quoted experimental values, a recommended half-life of 6.934 (14) d was determined by the power-moderated method (PMM). Based on recent four published half-life values, a half-life of ¹⁶¹Tb of 6.9582 (33) d was determined by the PMM analysis.

The experimental results published before August 2022 from the various reaction and decay studies leading to nuclides of Z=51 to Z=67, ¹⁴¹Sb, ¹⁴¹Te, ¹⁴¹I, ¹⁴¹Xe, ¹⁴¹Cs, ¹⁴¹Ba, ¹⁴¹La, ¹⁴¹Ce, ¹⁴¹Pr, ¹⁴¹Nd, ¹⁴¹Pm, ¹⁴¹Sm, ¹⁴¹Eu, ¹⁴¹Gd, ¹⁴¹Tb, ¹⁴¹Dy, ¹⁴¹Ho in the A=141 mass chain have been reviewed and the data are presented together with adopted level schemes and properties. This work supersedes the previous evaluation of the A=141 nuclides by published by N. Nica in Nuclear Data Sheets 122, 1 (2014).

Experimental nuclear structure and decay data are evaluated for all the 17 known nuclides of mass 149 (Xe, Cs, Ba, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb). Detailed compiled and evaluated spectroscopic information is presented for each reaction and decay dataset, and recommended values are provided for level properties, α, β and γ radiations, and other spectroscopic parameters, based on an evaluation of all the available experimental data for A=149 isobaric nuclides. Although large amounts of nuclear spectroscopic data are available for nuclides of A=149, yet large gaps in knowledge exist, as described below. For the lowest atomic number nuclide ¹⁴⁹Xe, only the isotopic identification has been made, with no data for its ground-state half-life. For ¹⁴⁹Cs, ¹⁴⁹Tm and ¹⁴⁹Yb information is available for only the respective ground states. For ¹⁴⁹Ba, ¹⁴⁹La and ¹⁴⁹Er, limited data exist for excited states. Many of the decay schemes of radioactive nuclei of A=149 are considered as incomplete, either due to large energy differences between the highest observed excited states in daughter nuclides and the respective Q-values, or due to the lack of confirmed γ-ray data, as listed below: ¹⁴⁹Cs → ¹⁴⁹Ba, ¹⁴⁹Ba → ¹⁴⁹La, ¹⁴⁹La → ¹⁴⁹Ce, ¹⁴⁹Ce → ¹⁴⁹Pr, ¹⁴⁹Pr → ¹⁴⁹Nd, ¹⁴⁹Tb(4.17 min) → ¹⁴⁹Gd, ¹⁴⁹Ho(21.0 s and 56 s) → ¹⁴⁹Dy, ¹⁴⁹Er(4 s and 9.6 s) → ¹⁴⁹Ho, and ¹⁴⁹Tm → ¹⁴⁹Er. No data exist for the decay of ¹⁴⁹Yb to ¹⁴⁹Tm. Data for half-lives of the excited states in this mass chain are generally lacking as given below by the number of excited levels of known half-life / approximate number of known levels in a nuclide: 2/17 for ¹⁴⁹Ba, 0/18 for ¹⁴⁹La, 3/53 for ¹⁴⁹Ce, 3/44 for ¹⁴⁹Pr, 17/110 for ¹⁴⁹Nd, 9/90 for ¹⁴⁹Pm, 10/210 for ¹⁴⁹Sm, 2/125 for ¹⁴⁹Eu, 6/270 for ¹⁴⁹Gd, 5/200 for ¹⁴⁹Tb, 3/80 for ¹⁴⁹Dy, 3/90 for ¹⁴⁹Ho, and 3/14 for ¹⁴⁹Er. This work supersedes earlier evaluations of A=149 nuclides published by 2004Si16, 1994Si18, 1985Sz01 and 1976Ho17.

There is significant interest in the use of terbium radioisotopes for applications in cancer therapy and diagnosis. Of these, ¹⁶¹Tb, as a medium energy beta-emitter, is being investigated as a potential alternative to ¹⁷⁷Lu. The relatively high proportion of conversion electron and Auger electron emissions per decay make ¹⁶¹Tb an attractive targeted therapeutic. As a product of nuclear fission, ¹⁶¹Tb is also of importance to nuclear forensics. The standard uncertainty of the current evaluated half-life of 6.89(2) d contributes significantly to the standard uncertainty of any decay corrected activity determination made. Furthermore, the accuracy of this evaluated half-life has been called into question by measurements reported in 2020 at the Institute of Radiation Physics (IRA), Switzerland, who reported a half-life of 6.953(2) d. In the current work, the half-life of the ¹⁶¹Tb ground state decay has been measured at three independent laboratories located in the United Kingdom and the United States of America for a total of six determinations using three independent measurement techniques; gamma-ray spectrometry, ionisation chamber measurement and liquid scintillation counting. The half-life determined for ¹⁶¹Tb of 6.9637(29) d confirms the observed 1% relative increase observed by IRA, though the reported half-lives in this work and at IRA are significantly different (ζ-score = 3.1).

This work represents an update of the previous evaluation of the nuclear data on the A=147 nuclides (2009Ni02). The experimental data from reaction and decay studies leading to the following nuclides have been reviewed: ¹⁴⁷I, ¹⁴⁷Xe, ¹⁴⁷Cs, ¹⁴⁷Ba, ¹⁴⁷La, ¹⁴⁷Ce, ¹⁴⁷Pr, ¹⁴⁷Nd, ¹⁴⁷Pm, ¹⁴⁷Sm, ¹⁴⁷Eu, ¹⁴⁷Gd, ¹⁴⁷Tb, ¹⁴⁷Dy, ¹⁴⁷Ho, ¹⁴⁷Er, ¹⁴⁷Tm. These data are presented, together with the adopted level schemes and their properties. ¹⁴⁷Pm nuclide and half-life of ¹⁴⁷Nd were evaluated by Balraj Singh (McMaster University; [email protected]). All the other nuclei were evaluated by Ninel Nica (Texas A&M University; [email protected]).

The experimental results published before August 2020 from the various reaction and decay studies leading to nuclides of Z=55 to Z=72, ¹⁵³Ba, ¹⁵³La, ¹⁵³Ce, ¹⁵³Pr, ¹⁵³Nd, ¹⁵³Pm, ¹⁵³Sm, ¹⁵³Eu, ¹⁵³Gd, ¹⁵³Tb, ¹⁵³Dy, ¹⁵³Ho, ¹⁵³Er, ¹⁵³Tm, ¹⁵³Yb, ¹⁵³Lu, ¹⁵³Hf in the A=153 mass chain have been reviewed. These data are summarized and presented, together with adopted level schemes and properties. This work is intended to supersede the previous evaluation of the A=153 nuclides by R.G. Helmer (2006He06), which was published in Nuclear Data Sheets 107, 507 (2006).