Measurement of the background activities of a Mo-100-enriched powder sample for an AMoRE crystal material by using fourteen high-purity germanium detectors

Abstract

The Advanced Molybdenum-based Rare process Experiment (AMoRE) in its second phase will search for neutrinoless double-beta (0 nu beta beta) decay of Mo-100 in 200 kg of molybdate crystals. To achieve the zero-background level in the energy range of the double-beta decay Q-value of Mo-100, the radioactive contamination levels in AMoRE crystals should be low. Enriched molybdenum trioxide powder, (MoO3)-Mo-100Enr, enriched to 95.73% abundance of the Mo-100 isotope is used to grow the AMoRE crystals. A shielded array of fourteen high-purity germanium detectors with 70% relative efficiency each was used for the measurement of background activities in a sample of 9.6-kg powder. The detector system named the CAGe located at the Yangyang underground laboratory was designed for measuring low levels of radioactivity from natural radioisotopes or cosmogenic nuclides such as Ac-228, Th-228, Ra-226, Y-88, and K-40. The activities of Ac-228 and Th-228 in the powder sample were 0.88 +/- 0.13 and 0.669 +/- 0.089 mBq/kg, respectively. The activity of Ra-226 was measured to be 1.19 +/- 0.42 mBq/kg. The activity of Y-88 was 0.101 +/- 0.016 mBq/kg. The activity of K-40 was found as 36.0 +/- 4.1 mBq/kg.