Based on how successive pulses are processed owing to the dead time,
the counting systems fall into two categories:paralyzableand nonparalyz-
able. In paralyzable systems, each event sets its own dead time, even if it
arrives within the dead time of the previous event and is not counted. Each
event prolongs the dead time induced by the previous event, and thus adds
to the total dead time of the system, whereby a paralyzable system can
become totally unresponsive to process events if the count rate of the
source is very high. On the other hand, in nonparalyzable systems, the
instrument remains insensitive to successive events for a period of time
equal to the dead time, and these events are lost. But unlike paralyzable
systems, the dead time is not changed or lengthened. When the system
recovers after the detection of the first event, only then is the second event
processed and detected. The two types of dead time losses are illustrated in
Figure 8.10.
The paralyzable and nonparalyzable systems can be represented by
mathematical relationships among the observed count rate Ro, true count
rate Rt, and dead time t. For nonparalyzable systems,
Rt=Ro/(1 −Rot) (8.7)and for paralyzable systems,
Ro=Rte−Rtt (8.8)
Different components of a radiation detection system can have either
paralyzable or nonparalyzable dead time. Scalers and pulse-height ana-
lyzers are nonparalyzable systems, whereas radiation detectors themselves
100 8. Scintillation and Semiconductor Detectors
Fig. 8.10. Plot of observed count rates versus true count rates indicating the dead
time loss in paralyzable and nonparalyzable systems.