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) x 10(-12) relative to Th-232 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) x 10(-11) relative to Th-232 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 x 10(-15) for the abundance of such a species relative to Th-232. For two neutron-deficient thorium isotopes, Th-211 and Th-218, we find upper limits of 5 x 10(-15). For Th-213 and Th-217 we observed a few events - albeit uncertain - which would put their abundance also into the 10-15 range. (C) 2009 Elsevier B.V. All rights reserved.