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244Pu, with a half-life of 81.1 Myr, that enriched in ancient ore-forming rare earth samples might be the heaviest and longest half-lived r-process nuclides that could be detected. The analysis of the existing abundance of 244Pu in terrestrial samples could verify whether the estimated value of the abundance ratio of 244Pu and 238U in the early formation of the solar system (0.008) is correct, and whether the calculation model of the 244Pu content in rare earth minerals is reasonable. It could also be helpful to establish the calculation model of stellar evolution.
Since recent attempts failed to confirm the discovery of 244Pu signals at a concentration of 1.0 ×10−18 g/g in bastnaesite reported by Hoffman and Lawrence, et al., the measurement of 244Pu in the HCl leachate and residue of 450 g of Bayan Obo bastnaesite concentrate was performed at CIRP using the complete digestion ultra-sensitive analytical method and ultra-sensitive compact accelerator mass spectrometry (AMS), for attempting to verify the initial (244Pu/238U)ESS abundance ratio in the early formation of the solar system. Attogram level of 239Pu was found in the sample target, and the counting rate of 239Pu in the residue was ∼4.5 times higher than that in the HCl leachate, which imply that Pu could preferentially concentrate in its tetravalent state with Th(IV) in the bastnaesite. Since no true signal of 244 Pu was observed, an upper detection limit for the 244Pu content in the bastnaesite concentrate sample was estimated to be 2.1 ×10−20 g/g at 99% confidence level, which is the lowest upper limit obtained so far.
Flow diagram of the sample preparation procedure
Result of the study denies the discovery of 1.0 ×10−18 g/g primordial 244Pu in bastnaesite reported by Hoff-man and Lawrence, et al. The possible upper limit of 244 Pu content in the sample was estimated to be in the range of 1.2 ×10−21 g/g and 6.5 ×10−23 g/g by assuming the same enrichment factor of Pu as cerium or thorium, respectively. It would still be problematic for direct search of primordial 244Pu in terrestrial samples even if a much larger size of this sample is processed, but samples with high concentrations of Th could be the most promising choice for the future study.
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