Ultrasensitive search for long-lived superheavy nuclides in the mass rangeA=288toA=300in natural Pt, Pb, and Bi

Abstract

Theoretical models of superheavy elements (SHEs) predict a region of increased stability around the proton and neutron shell closures of $Z$ $=$ 114 and $N$ $=$ 184. Therefore a sensitive search for nuclides in the mass range from $A$ $=$ 288 to $A$ $=$ 300 was performed in natural platinum, lead, and bismuth, covering long-lived isotopes of Eka-Pt (Ds, $Z$ $=$ 110), Eka-Pb ($Z$ $=$ 114), and Eka-Bi ($Z$ $=$ 115). Measurements with accelerator mass spectrometry (AMS) at the Vienna Environmental Research Accelerator (VERA) established upper limits for these SHE isotopes in Pt, Pb, and Bi with abundances of 2\ifmmode\times\else\texttimes\fi{}10${}^{\ensuremath{-}15}$, 5\ifmmode\times\else\texttimes\fi{}10${}^{\ensuremath{-}14}$, and 5\ifmmode\times\else\texttimes\fi{}10${}^{\ensuremath{-}13}$, respectively. These results complement earlier searches for SHEs with AMS at VERA in natural thorium and gold, which now amounts to a total number of 37 SHE nuclides having been explored with AMS. In none of our measurements was evidence found for the existence of SHEs in nature at the reported sensitivity level. Even though a few events were observed in the window for $^{293}\mathrm{Ek}$a-Bi, a particularly strong pileup background did not allow a definite SHE isotope identification. The present result sets limits on nuclides around the center of the island of stability, essentially ruling out the existence of SHE nuclides with half-lives longer than \ensuremath{\sim}150 million years.