Physics and Engineering of Radiation Detection

496 Chapter 8. Signal Processing

intensity. Such measurements are performed using two devices called single channel
and multi channel analyzers, which will be discussed in some detail now.

A.1 SingleChannelAnalyzer(SCA)................

In a large number of applications one is interested in counting the number ofinter-
estingpulses. The definition of an interesting pulse depends on what one intends to
look at. For example one might be interested in looking at all the pulses generated in
the detector by the radiation. Or, since the height of a pulse depends on the energy
deposited by the radiation, one might want to count pulses corresponding to some
energy range of interest. No matter which pulses are to be analyzed, the remaining
pulses must be filtered out or blocked so that acleandataset could be obtained. This
is necessary even if one intends to look at all radiation induced pulses since there
are always low level noise and high level spurious pulses. This filtration requires
a two-level discriminator, which we have already discussed in the previous section.
An instrument that discriminates the pulses based on set thresholds and produces
logic outputs is called a single channel analyzer. Heresinglesignifies the fact that
all the pulses are discriminated using only one set of thresholds. The block diagram
of a typical SCA is shown in Fig.8.6.2. Here LLD and ULD stand for upper level
discriminator and lower level discriminator respectively. In most modern commer-
cial single channel analyzers the thresholds can either be set through potentiometer
knobs available on the front panel or through an external port.

ULD

LLD

Discriminator

Logic Out

Amplifier/Shaper

Signal In

Figure 8.6.2: Block diagram of a simple single channel analyzer.

After the pulse has been discriminated and a logical output has been generated,
it must be counted. Most commercially available SCAs do not have counting func-
tionality and therefore one must use a separate counting module for the purpose.
Single channel analyzers are most suitable for discriminating pulses within a
certain height range. A common application is measuring intensity of an x-ray
peak from an x-ray machine. Recall that the x-ray peak from an x-ray machine is
superimposed on a bremsstrahlung continuum. Hence if one knows the energy of
the peak, it can be easily discriminated by setting SCA thresholds around it.
Now, suppose we are interested in determining the whole x-ray spectrum of an
x-ray machine. This can be done counting pulses at different threshold windows such
that the whole energy range is spanned. However it is apparent that this is quite a