Measurement of the Background Activities of a ¹⁰⁰Mo-enriched Powder Sample for an AMoRE Crystal Material by Using a Single High-Purity Germanium Detector

The Advanced Molybdenum-based Rare process Experiment (AMoRE) searches for neutrino-less double-beta (0νββ) decay of ¹⁰⁰Mo in enriched molybdate crystals. The AMoRE crystals must have low levels of radioactive contamination to achieve low background signals with energies near the Q-value of the ¹⁰⁰Mo 0νββ decay. To produce low-activity crystals, radioactive contaminants in the raw materials used to form the crystals must be controlled and quantified. ^100EnrMoO₃ powder, which is enriched in the ¹⁰⁰Mo isotope, is of particular interest as it is the source of ¹⁰⁰Mo in the crystals. A high-purity germanium detector having 100% relative efficiency, named CC1, is being operated in the Yangyang underground laboratory. Using CC1, we collected a gamma spectrum from a 1.6-kg ^100EnrMoO₃ powder sample enriched to 96.4% in ¹⁰⁰Mo. Activities were analyzed for the isotopes ²²⁸Ac, ²²⁸Th, ²²⁶Ra, and 40K. They are long-lived naturally occurring isotopes that can produce background signals in the region of interest for AMoRE. Activities of both ²²⁸Ac and ²²⁸Th were < 1.0 mBq/kg at 90% confidence level (C.L.). The activity of ²²⁶Ra was measured to be 5.1 ± 0.4 (stat) ± 2.2 (syst) mBq/kg. The 40K activity was found as < 16.4 mBq/kg at 90% C.L.