Speaker
Description
Radioisotopes with half-lives (t$_{1/2}$) in the order of millions of years play an essential role in the description of galactic events, as well as in the nuclear dating of extraterrestrial samples. Among the different benchmark radionuclides for astrophysics and geophysics studies, the radio-lanthanides $^{146}$Sm, $^{148}$Gd and $^{154}$Dy are of especial interest. In fact, through the pure α-decay sequence $^{154}$Dy → $^{150}$Gd → $^{146}$Sm → $^{142}$Nd, these radionuclides directly influence both the abundance of $^{146}$Sm, as well as the natural isotopic composition of stable Nd, thus contributing to the Sm/Nd chronometer. The latter is one of the most powerful tools to date silicate differentiation and primitive crust formation events occurred during the first hundreds of million years (My) of the Solar System [1, 2]. Furthermore, photodisintegration processes on $^{148}$Gd, the rate of the $^{144}$Sm(α,γ)$^{148}$Gd reaction, and competing nuclear reactions like $^{154}$Dy(α, γ)$^{158}$Er, $^{154}$Dy(γ, α)$^{150}$Gd, and $^{154}$Dy(γ, n)$^{152}$Dy [3-5], just to mention a few, are essential for a better understanding of the nucleosynthetic processes that led to the present isotopic abundances in the lanthanide region. It is apparent that an accurate knowledge on the nuclear properties of the above-mentioned radionuclides is crucial. However, the currently available data for the half-life of $^{146}$Sm, $^{148}$Gd, and $^{154}$Dy are inconsistent, or affected by uncertainties even up to 50% [6-8]. Recently, the re-evaluated half-life data of a considerable number of radionuclides showed a significant disagreement with previous values, as in the case of $^{146}$Sm [9]. The shorter measured half-life for this radio-lanthanide implies a higher abundance of $^{146}$Sm in the early Solar System, and thus, planetary events dated with the $^{146}$Sm-$^{142}$Nd chronometer converge now to a shorter time span than previously estimated. Reasons for these imprecise nuclear data lie in the difficulty of obtaining samples of the isotopes of interest in sufficient amounts and purity, together with inherent problematics in performing such demanding measurements. In this work, which belongs to the initiative “ERAWAST - Exotic Radionuclides from Accelerator Waste for Science and Technology” [10], we could obtain sufficient amounts of $^{146}$Sm, $^{148}$Gd, and $^{154}$Dy by reprocessing irradiated Ta materials available at the PSI accelerator-facilities. The re-determination of the half-lives of the above-mentioned radio-lanthanides proceeded by successively applying the “direct” or the “decay” method, depending on the order of magnitude of the expected t$_{1/2}$. A scheme for both methods is presented in the Figure.
Here, in the framework of the ERAWAST project, preliminary results on the measured half-lives of $^{148}$Gd and $^{154}$Dy, together with the first steps towards determining the decay constant of $^{146}$Sm, will be presented.
Figure: Process followed for the determination of half-lives below 150 years (a) and the determination of half-lives above 150 years (b).
Acknowledgement
This project is funded by the Swiss National Science Foundation (SNF grant no 200021-159738).
References
1 Bouvier, A., and M. Boyet. Nature 537, no. 7620 (2016): 399-402 [2] Harper, C. L., and Stein B. Jacobsen. Nature 360, no. 6406 (1992): 728-732 [3] Scholz, P., H. Wilsenach, H. W. Becker, A. Blazhev, F. Heim, V. Foteinou, U. Giesen et al. Physical Review C 102, no. 4 (2020): 045811. [4] Rauscher, T. Physical Review C 73, no. 1 (2006): 015804. [5] Woosley, S. E., and W. M. Howard. The Astrophysical Journal 354 (1990): L21-L24. [6] Khazov, Yu, A. Rodionov, and G. Shulyak. Nuclear Data Sheets 136 (2016): 163-452. [7] Nica, N. Nuclear Data Sheets 117 (2014): 1-229. [8] Reich, C. W. Nuclear Data Sheets 110, no. 10 (2009): 2257-2532. [9] Kinoshita, N., M. Paul, Y. Kashiv, P. Collon, C. M. Deibel, B. DiGiovine, J. P. Greene et al. Science 335, no. 6076 (2012): 1614-1617. [10] Schumann, D., and J. Neuhausen. Journal of Physics G: Nuclear and Particle Physics 35, no. 1 (2007): 014046.