Even though COSINE-100 will result interesting outputs for DAMA/LIBRA’s annual modulation signal, it is necessary to develop low background NaI(Tl) crystals. It is mainly due to crystal background of COSINE-100 that are 2-3 times higher than DAMA/LIBRA. This may result not to explain all the possibilities to interpret the modulation signals. We are currently developing ultra low background NaI(Tl) crystals and eventually launch next phase COSINE-200 experiment. Basic strategy of the COSINE-200 experiment is to do every process related with low background NaI(Tl) detectors by ourselves that include raw powder purification, crystal growing, and detector assembly.
Powder purification
We have developed NaI powder purification technology based on recrystallization method. One cycle of recrystallization obtained about factor 10 reduction of potassium and lead impurities with approximately 50% recovery efficiency. This facility can load maximum 70 kg of raw powder and obtain 30 kg of purified powder.
Crystal growing
At 2017, we constructed a small grower to prove low background NaI(Tl) crystal technology. This has been installed at CUP, IBS, Korea. Maximum loading of powder is 2 kg and produce approximately 1 kg NaI(Tl) crystal ingot. It takes time to optimize growing parameters but, now we can grow crystals. Above figure shows a few crystals grown with that grower.
We constructed a full size grower that can produce 40 cm diameter and 30 cm height NaI(Tl) ingot. We can provide maximum three full size detectors with one ingot. This grower was recently installed at IBS HQ, Daejeon, Korea. Mechanical operation as well as heat tests are ongoing. At 2019, we will make progress to produce full size and ultra low background NaI(Tl) crystal detectors, stay tuned!!
Detector assembly
Because of high hygroscopic characteristics of NaI(Tl) crystal, it should be well polished and tightly encapsulated in dried air condition. We developed feasible method to polish crystal surface. Also encapsulator design based on pure copper and quartz window was done with a product using a crystal grown by ourselves. We obtained relatively good light collection and found no issues during three month long data taking. We already developed most of technology for NaI(Tl) detector encapsulation, so waiting low background crystals.