Measurement of ultra-low potassium contaminations with Accelerator Mass Spectrometry

Levels of trace radiopurity in active detector materials is a subject of major concern in low-background experiments. Among the radio-isotopes, 40K is one of the most abundant and yet whose signatures are difficult to reject. Procedures were devised to measure trace potassium concentrations in the inorganic salt CsI as well as in organic liquid scintillator (LS) with Accelerator Mass Spectrometry (AMS), giving, respectively, the 40K-contamination levels of ∼10-10∼10-10 and ∼10-13g/g. Measurement flexibilities and sensitivities are improved over conventional methods. The projected limiting sensitivities if no excess of potassium signals had been observed over background are 5×10-145×10-14 and 3×10-17g/g for the CsI and LS, respectively. Studies of the LS samples indicate that the radioactive contaminations come mainly in the dye solutes, while the base solvents are orders of magnitude cleaner. This work demonstrates the possibilities of measuring naturally occurring isotopes with the AMS techniques.