Speaker
Description
At the Center for Underground Physics (CUP) at the Institute for Basic Science in South Korea there are two major scientific directions in the field of low-background physics. The AMoRE experiment is searching for the neutrinoless double beta decay of the Mo-100 isotope using molybdate-based bolometric crystals, and the COSINE experiment aims to scrutinize the annual modulation signals observed by the DAMA/LIBRA experiment using 200 kg of NaI(Tl) crystals with extremely low intrinsic background levels.
The second phase of AMoRE experiment, AMoRE-II, uses an array of 360 ultra-pure lithium molybdate Li2100MoO4 (LMO) crystals operated as cryogenic calorimeters. The crystals have a cylindrical shape with dimensions (D × L) of 5 × 5 cm and 6 × 6 cm with a target intrinsic radiopurity to be below 10 µBq/kg. After several years of mass purification of initial materials and mass production of crystals, crystal growth was successfully completed using the Czochralski pulling technique with CUP-purified precursor materials. In parallel with crystal production and initial material purification, the extraction and re-purification methods for Mo-100 were developed at CUP to recycle the expensive Mo-100 isotope and maximize the overall material balance. Throughout the production process, we tested each batch of purified products and each produced crystal using ICP-MS and/or HPGe to ensure radiopurity.
Moving towards COSINE-200 stage, we have developed and installed an in-house mass-scale NaI powder purification facility to have an initial NaI material for crystal production with a total K level below 20 ppb and 210Pb below 0.5 mBq/kg. At the moment, we have produced and accumulated about 400 kg of the product with such purity. Currently, CUP-produced NaI powder has been confirmed to be a good material for synthesizing ultra-pure NaI crystals using the Kyropoulos and Bridgman methods. However, we are facing an issue with the availability of the source material for purification, as its production has been discontinued and there is no suitable substitute of sufficient purity.
In this report, we summarize our progress in material recycling and assays for various materials at CUP, including ICP-MS assays of LMO crystals produced from different initial materials using various crystallization methods, the purity of the recycled materials used, and a material balance. Recent developments and improvements in NaI production will be given, along with an update on the newly produced NaI crystals at CUP.