The annual modulation signal observed by the DAMA experiment is a
long-standing question in the community of dark matter direct detection. This
necessitates an independent verification of its existence using the same
detection technique. The COSINE-100 experiment has been operating with 106~kg
of low-background NaI(Tl) detectors providing interesting checks on the DAMA
signal. However, due to higher backgrounds in the NaI(Tl) crystals used in
COSINE-100 relative to those used for DAMA, it was difficult to reach final
conclusions. Since the start of COSINE-100 data taking in 2016, we also have
initiated a program to develop ultra-pure NaI(Tl) crystals for COSINE-200, the
next phase of the experiment. The program includes efforts of raw powder
purification, ultra-pure NaI(Tl) crystal growth, and detector assembly
techniques. After extensive research and development of NaI(Tl) crystal growth,
we have successfully grown a few small-size (0.61−0.78 kg) thallium-doped
crystals with high radio-purity. A high light yield has been achieved by
improvements of our detector assembly technique. Here we report the ultra-pure
NaI(Tl) detector developments at the Institute for Basic Science, Korea. The
technique developed here will be applied to the production of NaI(Tl) detectors
for the COSINE-200 experiment.Comment: 12 pages, 12 figure
