Background Stability and Annual Modulation Test Using PICOLON Ultra-Pure NaI(Tl) Detector

The dark matter observation claimed by the DAMA/LIBRA experiment has been a long-standing puzzle within the particle physics community. NaI(Tl) crystals with radiopurity comparable to DAMA/LIBRA’s are essential for adequate verification. Existing experiments using a NaI(Tl) target have been hampered by the high radioactivity concentration of NaI(Tl) crystals. The Pure Inorganic Crystal Observatory for LOw-energy Neutr(al)ino (PICOLON) experiment conducts an independent search for Weakly Interacting Massive Particles using the highest-purity NaI(Tl) crystals. In 2020, a PICOLON NaI(Tl) crystal (Ingot#85) reached the same purity level as DAMA/LIBRA crystals. The DAMA/LIBRA group has stressed that verifying their signal requires high-purity NaI(Tl) crystals with long-term stability. Based on a six-month measurement, we have confirmed the long-term stability of its radiopurity. This stability provides a significant advantage for future efforts to adequately verify the DAMA/LIBRA result using NaI(Tl) crystals. In this paper, we present the background stability of purity in the Ingot#94 NaI(Tl) detector, which was produced using the Ingot#85 purification method, along with the first annual modulation search conducted by the PICOLON experiment.