Detection of UH3+ and ThH3+ molecules and 236U background studies with low-energy AMS

The performance of the TANDY AMS facility (0.6 MV) at ETH Zurich has been significantly improved for actinides with the installation of an additional 130° magnet on the high-energy side. The background originating from neighboring masses is reduced by three orders of magnitude in the new setup, leading to an abundance sensitivity of 10−10. This now allows 236U/238U measurements down to the 10−12 level. Systematic studies of the possible background for 236U3+ at the compact TANDY system show that actinide-hydride molecules in charge state 3+ do exist. (232Th1H)3+, (235U1H)3+ and (238U1H)3+ can cause a severe background at masses 233, 236, and 239, respectively. The existence of the (232Th1H)3+ molecule was verified unambiguously by analyzing its breakup. The actinide-hydride molecules can be destroyed efficiently using higher stripper pressures. At an ion energy of 1.2 MeV an overall efficiency of more than 10−4 can be achieved using a gas ionization detector. The high efficiency in combination with the low background opens the spectrum of possible applications to both anthropogenic (236U/238U = 10−7 to 10−9) and natural samples (236U/238U < 10−10).