The Global Rare Anomalous Nuclear Decay Experiment (GRANDE) aims to push the frontiers of nuclear and particle physics by investigating rare nuclear decay processes. A key focus of GRANDE is the measurement of extremely rare nuclear transitions and the search for exotic dark matter particles, including axion-like particles, anapole dark matter, and dark photons in nuclear transitions. Based on the extremely rare electron-capture decay branching of isotopes such as ⁵⁷Co,¹³⁹Ce, and ⁴⁴Ti. The Source-as-Detector technique was chosen for radiation detection by embedding a radioactive source within a CeBr₃ crystal scintillator. We present the fabrication process and scintillation performance of CeBr₃:¹³⁹Ce and CeBr₃:⁵⁷Co, a 4π BGO veto system and shielding setup at the Yemilab with a 1000-m-rock overburden. The first experimental run was performed with CeBr₃:¹³⁹Ce at a data acquisition rate of 20 kHz. We present the analysis of 1010 recorded events, including a preliminary branching ratio limit for the missing 165.86 keV M1 gamma transition of ¹³⁹Ce, which could be indicative of axion-like particle signatures.
Kim et al. (Mon,) studied this question.