Optimization of gain uniformity in thermal bonding Micromegas for the PandaX-III experiment

Micro-pattern gas detectors (MPGDs) are widely utilized in physics experiments owing to their excellent spatial resolution and high-rate capabilities. Within the PandaX-III experiment, which aims to investigate neutrinoless double beta decay, Micromegas detectors serve as charge readout devices. High energy resolution is a critical requirement for the readout plane in this context, and gain uniformity significantly impacts the achievable resolution, primarily because of the extended tracks of primary ionization electrons. However, scaling up MPGDs to larger active areas exacerbates the challenge of maintaining gain uniformity, and effectively controlling the uniformity of the avalanche gap is a key factor in the detector manufacturing process via the thermal bonding method. This study demonstrates that optimizing the thermal bonding films specifically at the detector edges can effectively improve the gain uniformity, achieving a gain uniformity of < 5% over the entire 200*200 mm2 active area in a 1 bar Ar/isobutane (96.5/3.5) gas mixture. Additionally, the gain uniformity of approximately 14% was characterized at high pressures of up to 10 bar, revealing promising potential for high resolution measurements in the PandaX-III experiment and other high-pressure applications.