Throughout this paper, units of e -/pix/s always refer to the detection (output) rate on the sensor, not the incidence (input) rate since the detection rate may be lower than the incidence rate due to coincidence loss. To numerically simulate the number of incident electrons counted per pixel per second (e -/pix/s) during EBEC, we assumed that each electron event impinging on the detector had the potential to activate a cluster of adjacent pixels (Supplementary Figure ). The Apollo detector applies this same logic during event detection, by considering blocks of up to 5x5 physical pixels during centroid-based electron event registration (Supplementary Figures ). When eight adjacent pixels, present side-byside or diagonal, are simultaneously activated, a bounding box is defined with a maximum size of 5x5; the centroid of isolated signal pixels within this box is assumed to represent a single incident primary electron. Each 5x5 block of physical pixels may successfully detect multiple incident primary electrons, provided that the activated pixels from each are not adjacent.