Attempt to detect primordial ²⁴⁴Pu on Earth

With a half-life of 81.1 Myr, ²⁴⁴Pu could be both the heaviest and the shortest-lived nuclide present on Earth as a relic of the last supernova(e) that occurred before the formation of the Solar System. Hoffman et al. [Nature (London) 234, 132 (1971)] reported on the detection of this nuclide (1.0 × 10⁻¹⁸ g ²⁴⁴Pu/g) in the rare-earth mineral bastnäsite with the use of a mass spectrometer. Up to now these findings were never reassessed. We describe the search for primordial ²⁴⁴Pu in a sample of bastnäsite with the method of accelerator mass spectrometry (AMS). It was performed with a highly sensitive setup, identifying the ions by the determination of their time-of-flight and energy. Using AMS, the stripping to high charge states allows the suppression of any molecular interference. During our measurements we observed no event of ²⁴⁴Pu. Therefore, we can give an upper limit for the abundance of ²⁴⁴Pu in our sample of the mineral bastnäsite of 370 atoms per gram (1.5 × 10⁻¹⁹ g²⁴⁴Pu/g). The concentration of ²⁴⁴Pu in our sample of bastnäsite is significantly lower than the previously determined value.