It creates a cascade of secondary particles that are detected over an array of sensors. In this case, the shower triggered 23 detectors, which are part of 507

It creates a cascade of secondary particles that are detected over an array of sensors. In this case, the shower triggered 23 detectors, which are part of 507 stations arranged in a square grid. The stations are spaced by 1.2 km, giving a total effective area of 700km^2