Physics and Engineering of Radiation Detection

8.3. Pulse Shaping 481

(b)

Pulse Height

Time

Pulse Height

Time

(a) Figure 8.3.2: (a) Pulse pile-up in high rate applica-

tions. (b) Decreasing pulse width eliminates pile-up.

from each preamplifier output pulse. The rectangular output pulse has fast rise and
fall times and can therefore be very conveniently used for pulse shape discrimination
and timing applications. Fig. 8.3.3 shows a typical delay line pulse shaping circuitry.
The step pulse from the preamplifier is inverted, delayed and then added back to
the original pulse to obtain a rectangular pulse having width equal to the delay time
of the delay line.

8.3.B CR-RCPulseShaping

The CR-RC pulse shaping is perhaps the simplest and the most widely used method
of shaping preamplified detector pulses. The shaper consists of two parts: a CR
differentiator and an RC integrator (see Fig. 8.3.4).
The CR differentiator acts like a high pass filter with pulse amplitude given by

Vout=Vin−

Q

Cd

. (8.3.1)

Differentiating this with respect to time gives a differential equation

dVout

dt

−

dVin

dt

+

Vout

RdCd

=0, (8.3.2)

where we have usedVout=RdI=RddQ/dt.
RdCdin the above equation is the time constant of the differentiator which mainly
determines the decay time of the output signal. In detection systems the preamplifier
shaping time is generally larger than this time constant. This implies that the