If a very high counting rate is provided to the start input while an extremely low counting rate is supplied to the stop input, the TAC will spend a lot of time responding to start pulses that have no associated stop pulse within the selected time range. Starts with no stops will cause excessive dead time in the TAC without producing useful data. Reversing the input assignments so that the higher counting rate is on the stop input will minimize this dead time.

Reversing the start and stop inputs is particularly important in applications where a sample is excited by a periodic pulse and the time spectrum of the reaction products emitted by the sample is to be recorded. Usually, the repetition rate of the periodic pulse is high and the counting rate of the reaction products is extremely low. Logically, one would expect the excitation pulse to be the start pulse and the reaction products to provide the stop pulses. But, this creates too much dead time in the TAC. To reduce the dead time, the reaction products should drive the start input while the excitation pulse is delayed and fed to the stop input. The length of the stop delay should be approximately 90% of the time range selected on the TAC. Fig. 5 is an example of the reversed start/stop technique applied to a fluorescence lifetime spectrometer.

Figure 5.  A Typical Block Diagram for a Fluorescence Lifetime Spectrometer
with Reversed Start/Stop Assignments.