Physics and Engineering of Radiation Detection

8.1. Preamplification 473

Time

Low Rate High Rate

Preamp Output

Detector Output

Figure 8.1.7: Low rate and high rate

measurements from a resistive feedback

charge sensitive preamplifier. Although

the preamplifier does not exhibit a dead

time even at high rates, but the pulse

pileup can be a serious problem.

Cf

Ss

Sr

Vout

Detector

Figure 8.1.8: Conceptual design of a pulsed

reset charge sensitive preamplifier. Sris the

reset switch that is used to drain the charge

accumulated on the feedback capacitor. Some

preamplifiers are also equipped with a set

switchSsto minimize dead time (see text).

reset preamplifiers provide an alternate mechanism to reset the feedback capacitor
after the pulse has reached its maximum value. The idea is to replace the feedback
resistor with an electronic switch providing a current drain path to discharge the
feedback capacitoron demand. This switch is toggled on and off through an ad-
ditional timing circuitry, which is sometimes connected to the preamplifier output
to generate a trigger signal for the switch. Fig.8.1.8 shows the basic principle of
such a device. A typical timing diagram of the circuit is shown in Fig.8.1.9(a). The
reader should readily realize that the pulses received by the preamplifier during the
reset time are lost. This can of course be minimized by keeping the reset time at
the minimum. A better way, however, is to introduce another switch at the input
of the preamplifier. If this switch is kept open during the reset time (i.e. Ssopen
andSrclose), then the detector pulses would keep on integrating charges on the
combined detector and stray capacitance. ClosingSsafter the feedback capacitor
had completely discharged would transfer these charges to the feedback capacitor,
hence eliminating the information loss. However, there is one caveat to this scheme,
namely the time jitter of the switching pulses, which introduces some uncertainty
in the rising and falling edges of the pules. To overcome this problem, normally a
small sacrifice is made by keeping theSr-open pulse in the middle of theSr-close
pulse (see Fig.8.1.9(b)).
Most of the electronic switches used in commercially available preamplifiers are
transistor based. Timing of the reset operation in these devices is generally done by