Schematic diagram showing the evolution of single detector event spectra and STE spectra. The brown line represents the single detector event spectrum, corresponding to the right-hand vertical axis. From left to right, the two brown arrows indicate delayed background and intrinsic background, respectively. The red, green, blue, and magenta spectral lines correspond to STEs involving 5, 10, 20, and 25 detectors, respectively. Lower panel: Spectral-shape variations of STEs involving 10, 20, and 25 detectors, using the STE-5 spectrum as the reference baseline.