Search for a superheavy nuclide with A = 292 and neutron-deficient thorium isotopes in natural thorianite

In the search for long-lived superheavy elements (SHEs) in nature the claim for the detection of a SHE species with A = 292 in a commercially available thorium solution at an abundance of (1-10) × 10^-12 relative to ²³²Th using ICP-SFMS has been raised very recently by Marinov et al. The same group also claimed the finding of four long-lived isomeric states in neutron-deficient thorium isotopes at an abundance of (1-10) × 10^-11 relative to ²³²Th using the same experimental method. The aim of our study was to examine, whether the ratios mentioned above could also be found in a natural thorium mineral, using AMS as a much more sensitive technique assuring the complete suppression of molecular background. Chemically untreated thorianite (ThO₂) was used for both measurements. In addition, commercial thorium(IV) oxide (thoria) powder was also analyzed in the SHE measurements. No events were observed in the A = 292 SHE search, resulting in an upper limit of 4 × 10^-15 for the abundance of such a species relative to ²³²Th. For two neutron-deficient thorium isotopes, ²¹¹Th and ²¹⁸Th, we find upper limits of 5 × 10^-15. For ²¹³Th and ²¹⁷Th we observed a few events - albeit uncertain - which would put their abundance also into the 10^-15 range.