A Sodium-Iodide Scintillator Array for Gamma-Recoil Coincidence Spectroscopy in BeEST

The Beryllium Electron capture in Superconducting Tunnel junctions (BeEST) experiment employs the decay-momentum reconstruction technique to measure the recoil energy spectrum of ⁷Be → ⁷Li electron capture decays from radioisotopes implanted in superconducting tunnel junctions (STJs). BeEST's high-resolution recoil spectroscopy enables the precise reconstruction of weak decay kinematics, enabling new probes of physics beyond the Standard Model. A small branching fraction (10.44%) of ⁷Be decays results in a population of the short-lived excited nuclear state ⁷Li*, which de-excites via. emission of a 478 keV γ-ray. Tagging the 478 keV γ-ray enables exceptionally selective measurements of the ⁷Li* recoil spectrum by time-correlating signals between the STJ and auxiliary detectors. Here we describe progress toward commissioning a new sodium iodide (NaI) scintillator array for γ-ray tagging in BeEST. We will present characterization efforts, including detector-specific energy spectra and background rate measurements, alongside system integration work to optimize the NaI array layout relative to the cryogenic STJ environment to maximize coincidence efficiency. This setup will be deployed in future recoil spectroscopy measurements to improve our understanding of nuclear recoils in superconducting media and enable more sensitive searches for new physics with BeEST.