Physics and Engineering of Radiation Detection

480 Chapter 8. Signal Processing

8.3 PulseShaping...............................

The raw signal coming out of a detector is usually very narrow and therefore not
very useful for extracting information such as height of the pulse. This narrow signal
must first be shaped into a broader pulse with a rounded maximum. The broader
pulse is necessary to reduce noise and the rounded peak is needed to measure the
amplitude with precision. In general, there are two objectives for transforming the
detector signal into a well defined pulse,

Increase signal to noise ratio.


Increase pulse pair resolution.

As we will see, these two requirements are somewhat contradictory and therefore
an optimized solution is sought based on the particular application of the detector.

Preamplifier Output Shaper Output

Shaper

Figure 8.3.1: A pulse shap-

ing amplifier can transform

a preamplifier pulse having

sharp peak and small width

into a well-shaped pulse with

rounded maximum and appro-

priate rise and decay times.

Shapers can produce uni-polar

as well as bi-polar pulses.

For detectors used in spectroscopy and precision measurements it is desired that
the signal to noise ratio (S/N) is improved. This can be done by increasing the
pulse width. The shapers designed for this task not only increase the pulse width
but also produce a rounded peak of the pulse (see Fig.8.3.1). The rounded maximum
facilitates the measurement of the pulse height. If the signal level has no effect on
the shape of the pulse, the pulse height is essentially the measure of the energy
absorbed by the detection medium. This proportionality ensures that the energy
spectrum of the incident radiation can be inferred from the pulse height spectrum.
If the rate of incident radiation is very high then increasing the pulse width is
not a good option because consecutive pulses can pile up over one another (see
Fig.8.3.2). In such a situation, the shaping time (or the pulse width) is decreased
such that the signal to noise ratio does not become unacceptable.
A number of pulse shaping methods are available each with its own pros and
cons. The choice of the shaping methodology is therefore highly application specific.
In the following we will look at some commonly used pulse shaping strategies.

8.3.A DelayLinePulseShaping

Delay line pulse shaping is used for detectors with high internal gains, such as,
scintillation detectors. For detectors with low internal gains, the signal-to-noise
ratio with delay line pulse shaping is smaller than the other available methods.
The basic idea behind this kind of pulse shaping is to combine the propagation
delay of distributed delay lines to produce an essentially rectangular output pulse