What does the Z value represent in the PMT signal processing?

• A. Time of arrival
• B. Amplitude or energy of the original photon
• C. Position along X-axis
• D. Number of photons detected

Answer: (B)

Z represents the pulse amplitude, i.e., the height or integrated charge of the PMT output pulse. When a photon is detected, the PMT converts light into electrons and multiplies them, producing a voltage pulse. The size of that pulse scales with the energy (or intensity) of the original photon after amplification, so Z effectively encodes the energy/amplitude information.

Time of arrival is determined by timing circuitry, not pulse height; position along an axis requires a position-sensitive readout; and counting photons is a rate or total count over time, not the per-event pulse amplitude.