712 Chapter 12. Radiation Spectroscopy
crosses a set threshold. In other words, it simply puts a stamp on the arrival time of
the pulse. The logic pulses thus produced are fed into a time-to-amplitude converter
(TAC). The TAC produces a pulse with an amplitude proportional to the difference
in the arrival times of the logic pulses. The way it is accomplished is fairly simple.
One of the logic pulses is delayed by a set amount. The direct logic pulse, as it
enters into the TAC circuitry, starts charging (or discharging) a capacitor through
some internal or external source. The capacitor keeps on charging (or discharging)
until the second delayed logic pulse arrives. The output of the TAC is therefore a
voltage pulse with an amplitude proportional to the difference in arrival times of the
two logic pulses.
Detector 2Detector 1SourcePreamplifierPreamplifierDelayTAC MCATime PickoffTime Pickoff
CircuitCircuitFigure 12.5.1: A simplified setup for time spectroscopy.Each event recorded by the two detectors gives rise to one TAC pulse. The
distribution of the amplitudes of these pulses is called the timing spectrum. In most
cases one simply hooks up a multichannel analyzer to the TAC to obtain the timing
spectrum. However, for more demanding applications, one could digitize the pulse
and then use a software to perform analysis tasks on the digitized data.
As mentioned earlier, the setup shown in Fig.12.5.1 can also be used for coinci-
dence spectroscopy. Here one is interested in determining whether the two detector
pulses were coincident or not. This is done by looking at the FWHM o f the dis-
tribution (that is, the timing spectrum). A typical timing spectrum for coincidence
events is Gaussian-like as shown in Fig.12.5.2.