Ultrasensitive Analytical Method for Direct Search of Primordial 244Pu in Bastnaesite

Plutonium-244 could be the heaviest primordial nuclide existing on earth since the early formation of the solar system. However, the controversial results on the detection of 244Pu in bastnaesite samples require a reassessment. An extremely sensitive analytical method has been developed for the analysis of total 244Pu in completely digested bastnaesite samples by accelerator mass spectrometry. As no true signal of 244Pu was observed, an upper limit for the 244Pu content in the bastnaesite concentrate sample from the Bayan Obo rare-earth deposit (China) was estimated to be 5.4 × 10–19 g/g at 99% confidence level. This result does not agree with the detection of 244Pu at a concentration of 1.0 × 10–18 g/g in bastnaesite reported by Hoffman et al. [Nature1971,234, 132–134] but confirms the latest measurement for an upper limit of 1.5 × 10–19 g 244Pu/g bastnaesite by Lachner et al. [Phys. Rev. C: Nucl. Phys.2012,85, 015801]. Thus, the direct search for primordial 244Pu on earth still remains an open question.