Deadtime

Dead time is the interval after a detected event during which a detector, counting system, or imaging device cannot register another event. It arises from finite processing times in the preamplifier, shaping circuitry, analog-to-digital converter, or data acquisition software. Dead time reduces the observed count rate relative to the true event rate, and its impact grows at higher activity. The concept is central in nuclear and particle instrumentation, gamma-ray spectroscopy, PET imaging, and many counting experiments, as well as in photography and other photon-counting applications.

Two common models describe how dead time affects measurements. In the non-paralyzable model, an event occurring during the dead interval is simply lost, and the observed rate m relates to the true rate n by m = n / (1 + n τ), where τ is the dead time per event. In the paralyzable model, any event during dead time not only is not recorded but also extends the dead interval, giving m = n exp(-n τ). The two models predict different behavior at high rates; detectors may behave closer to one model or another depending on rate and electronics.

Correction and calibration techniques aim to estimate n from m, or to design systems with known τ to apply corrections. Common methods include using radioactive sources of different intensities, pulser signals, or dedicated dead-time calibration routines. When necessary, measurements are reported with the corresponding dead-time correction or uncertainty.

Dead time is typically very short in modern detectors but remains a critical factor in high-rate experiments, small-sample radiometry, and quantitative imaging, where accurate activity estimates depend on proper dead-time accounting.