Attempt to detect primordial ${}^{244}$Pu on Earth

With a half-life of 81.1$$Myr, ${}^{244}$Pu 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 ($1.0\times {10}^{-18}$ g ${}^{244}$Pu/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 ${}^{244}$Pu 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 ${}^{244}$Pu. Therefore, we can give an upper limit for the abundance of ${}^{244}$Pu in our sample of the mineral bastnäsite of 370 atoms per gram ($1.5\times {10}^{-19}$ g ${}^{244}$Pu/g). The concentration of ${}^{244}$Pu in our sample of bastnäsite is significantly lower than the previously determined value.

Figure 1

Example of a TOF-$E_{\mathrm{res}}$ spectrum of a ${}^{244}$Pu run from sample IH with a large number of well-separated background nuclides. Although cleaner spectra were also recorded, this one was chosen to illustrate the variety of well-identified background nuclides in the TOF spectrometer. The signal of the TOF and the silicon surface barrier detector are sufficient for the separation and no condition on the energy loss signals is required.

Figure 2

Example of a TOF-$E_{\mathrm{res}}$ spectrum of a ${}^{244}$Pu run from sample IJ with well-separated background nuclides. The event slightly below the expected range of ${}^{244}$Pu most likely stems from threefold ${}^{133}$Cs${}^{6+}$ pileup and vanishes after the application of the pileup rejection.

Figure 3

Example of a TOF-$E_{\mathrm{res}}$ spectrum of a ${}^{236}$Pu run from sample IJ.

Figure 4

TOF-$E_{\mathrm{res}}$ spectrum of the ${}^{244}$Cm run following the measurement of sample IJ. The counts appear slightly lower in the $E_{\mathrm{res}}$ spectrum than expected for plutonium because of the larger $Z$ of curium and its larger energy loss in the ionization chamber.