COINCIDENCE TECHNIQUES

There are many applications that require the measurement of events that occur in two separate detectors within a given time interval, or the measurement of the time delay between the two events. These two approaches are used in gamma-gamma or particle-gamma coincidence measurements, positron lifetime studies, decay scheme studies and similar applications, and are titled coincidence or timing measurements.

A coincidence system determines when two events occur within a certain fixed time period. However, in practice it’s not possible to analyze coincidence events with 100% confidence due to the uncertainties associated with the statistical nature of the process. Statistical timing errors may occur from the detection process and uncertainties in the electronics resulting from timing jitter, amplitude walk and noise, which lead to statistically variable time delays between processed events. A simple coincidence circuit solves this problem by essentially summing the two input pulses, passing the resultant sum pulse through a discriminator level, and generating an output pulse when the two input pulses overlap. Figure 1.36 illustrates this process. Note that the period of time in which the two input pulses can be accepted is defined as the resolving time, which is determined by the width of the pulses, ô, such that the resolving time is equal to 2ô.