Speaker
Description
$^{32}$Si is an extremely rare, naturally occurring radioactive isotope. With its half-life ($\textit{T}$$_{1/2}$) of approximately 150 years $^{32}$Si would be one of the suitable candidates for radiometric dating in the range of 100–1000 years, where an appropriate dating nuclide is still missing (Fig. 1a).
The fact that the application of this nuclide for dating has been very limited so far is due to the imprecise and contradictory data for its half-life (Fig. 1b). With the current best estimate of 153(19) years, a more precise and more accurate determination is absolutely necessary.
The SNSF-funded project SINCHRON ($\textbf{Si}$ – a $\textbf{n}$ew $\textbf{chro}$nometer for $\textbf{n}$uclear dating) aims for an accurate half-life redetermination of $^{32}$Si with a relative standard uncertainty of less than 5% on the basis of several independent measurements. MBq quantities of $^{32}$Si have been successfully produced at the Paul Scherrer Institut (PSI, Switzerland) by exposing metallic vanadium discs to high-energy protons. In order to obtain radiochemically pure $^{32}$Si solutions, a robust chemical separation procedure has been developed (2). Several partners are involved in the SINCHRON-Project covering different tasks of the half-life determination.
Generally, two approaches are employed, while all measurements will be performed using aliquots of the same source material. The first approach is to follow the decay over a given time interval. For such measurements, the long-term stability of both the sample and the measurement device (e.g., ionization chamber (IC)) is essential. The second approach is the so-called direct method, where the $\textit{T}$$_{1/2}$ can be determined from the relationship $\textit{T}$$_{1/2}$ = $\mathit{N}$ $\mathit{ln(2)}$/$\mathit{A}$. Here, inductively coupled plasma mass spectrometry (ICP-MS), and accelerator mass spectrometry (AMS) are utilized for the determination of the number of atoms ($\mathit{N}$). The activity ($\mathit{A}$) is measured using liquid scintillation counting (LSC) with two techniques that are well established in radionuclide metrology: the triple-to-double coincidence ratio (TDCR) method and CIEMAT/NIST efficiency tracing. In addition, a coincidence setup with a plastic scintillation detector and a gamma-ray detector is used to apply another independent efficiency tracing technique. Finally, enough sample material that meets the quality requirements has been produced and the individual measurements are currently ongoing - first, preliminary results of the half-life determination will be presented.
$\textbf{References}$:
(1) Ouellet, C., and Balraj, S. "Nuclear data sheets for A = 32" $\textit{Nuclear Data Sheets}$ 112.9 (2011): 2199-2355.
(2) Veicht, M., Mihalcea, I., Cvjetinovic, Đ., & Schumann, D. (2021). "Radiochemical separation and purification of non-carrier-added silicon-32". Radiochimica Acta.$\textit{Radiochimica Acta}$ (2021, pre-published online).
$\textbf{Acknowledgements}$:
The authors acknowledge the funding through the Swiss National Science Foundation (Grant No. 177229) and additionally from the Marie Skłodowska-Curie Grant (No. 701647).