Upper limits for the existence of long-lived isotopes of roentgenium in natural gold

Abstract

A sensitive search for isotopes of a superheavy element (SHE) in natural gold materials has been performed with accelerator mass spectrometry at the Vienna Environmental Research Accelerator, which is based on a 3-MV tandem accelerator. Because the most likely SHE in gold is roentgenium (Rg, $Z$ $=$ 111), the search concentrated on Rg isotopes. Two different mass regions were explored: (i) For the neutron-deficient isotopes $^{261}\mathrm{Rg}$ and $^{265}\mathrm{Rg}$, abundance limits in gold of $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ were reached (no events observed). This is in stark contrast to the findings of Marinov et al. [Int. J. Mod. Phys. E 18, 621 (2009)], who reported positive identification of these isotopes with inductively coupled plasma sector field mass spectrometry in the $(1\ensuremath{-}10)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ abundance range. (ii) Theoretical models of SHEs predict a region of increased stability around the proton and neutron shell closures of $Z$ $=$ 114 and $N$ $=$ 184. We therefore investigated eight heavy Rg isotopes, ${}^{A}$Rg, $A$ $=$ 289, 290, 291, 292, 293, 294, 295, and 296. For six isotopes no events were observed, setting limits also in the ${10}^{\ensuremath{-}16}$ abundance range. For $^{291}\mathrm{Rg}$ and $^{294}\mathrm{Rg}$ we observed two and nine events, respectively, which results in an abundance in the ${10}^{\ensuremath{-}15}$ range. However, pileup of a particularly strong background in these cases makes a positive identification as Rg isotopes---even after pileup correction---unlikely.