First Study of the PIKACHU Project: Development and Evaluation of High-Purity Gd₃Ga₃Al₂O₁₂:Ce Crystals for ¹⁶⁰Gd Double Beta Decay Search

Uncovering neutrinoless double beta decay (0ν2β) is crucial for confirming neutrinos' Majorana characteristics. The decay rate of 0νββ is theoretically uncertain, influenced by nuclear matrix elements that vary across nuclides. To reduce this uncertainty, precise measurement of the half-life of neutrino-emitting double beta decay (2ν2β) in different nuclides is essential. We have launched the PIKACHU (Pure Inorganic scintillator experiment in KAmioka for CHallenging Underground sciences) project to fabricate high-purity Ce-doped Gd₃Ga₃Al₂O₁₂ (GAGG) single crystals and use them to study the double beta decay of ¹⁶⁰Gd. Predictions from two theoretical models on nuclear matrix element calculations for 2ν2β in ¹⁶⁰Gd show a significant discrepancy in estimated half-lives, differing by approximately an order of magnitude. If the lower half-life estimation holds true, detecting 2ν2β in ¹⁶⁰Gd could be achievable with a sensitivity enhancement slightly more than an order of magnitude compared to prior investigations using Ce-doped Gd₂SiO₅ (GSO) crystal. We have successfully developed GAGG crystals with purity levels surpassing previous standards through refined purification and selection of raw materials. Our experiments with these crystals indicate the feasibility of reaching sensitivities exceeding those of earlier studies. This paper discusses the ongoing development and scintillator performance evaluation of High-purity GAGG crystals, along with the anticipated future prospects of the PIKACHU experiment.