Ultrasensitive detection method for primordial nuclides in copper with Accelerator Mass Spectrometry

The sensitivity of rare event physics experiments like neutrino or direct dark matter detection crucially depends on the background level. A significant background contribution originates from the primordial actinides thorium (Th) and uranium (U) and the progenies of their decay chains. The applicability of ultra-sensitive Accelerator Mass Spectrometry (AMS) for the direct detection of Th and U impurities in three copper samples is evaluated. Although AMS has been proven to reach outstanding sensitivities for long-lived isotopes, this technique has only very rarely been used to detect ultra low concentrations of primordial actinides. Here it is utilized for the first time to detect primordial Th and U in ultra pure copper serving as shielding material in low level detectors. The lowest concentrations achieved were (1.5±0.6)·10^-11 g/g for Th and (8±4)·10^-14 g/g for U which corresponds to (59±24) and (1.0±0.5) μBq/kg, respectively.