463Chapter 8
Signal Processing
Signal processing plays an extremely important role in extracting useful information
from detectors. Generally speaking two pieces of information are important with
respect to detection and measurement of radiation: the amplitude and the timing of
the output pulse. Amplitude information is important with respect to applications,
such as energy spectroscopy, in which the measurement of the energy deposited
by the incoming radiation is desired. On the other hand there are applications,
such as particle tracking, in which precise timing of pulses is required. In order to
extract such information from the narrow width and low amplitude detector pulses, a
number of analog and digital signal processing steps are required. Broadly speaking
the signal can be either processed entirely through a chain of analog circuitry or it
can be converted to digital form for analysis. We will refer to the former in this
book by analog signal processing. With the advent of cost effective computing,
digital signal processing (DSP) is now gaining a lot of popularity. In fact it is now a
method of choice wherever possible. It should, however, be noted that digital signal
processing does not eliminate the need for analog circuitry from the electronics chain.
Some analog units are always needed to amplify the signal and make it usable for the
analog to digital converters (ADC). For example although the availability of very
fast ADCs (called flash ADCs) has made it possible to replace the pulse shaping step
by a digital process, preamplifiers are still needed. Fig.8.0.1 shows typical analog
and digital signal processing steps adapted in detection systems.
Whether the processing is analog or digital, the detector pulse must almost always
be first preamplified. The reason is that generally the detector pulse has a very low
amplitude and very short duration, making the direct measurements of amplitude
and time difficult and prone to large systematic errors. The preamplification step not
only amplifies the pulse but also increases the pulse width, thus making it suitable for
processing by the next electronic circuitry. The subsequent processing steps highly
depend on the application and data analysis requirements. If the requirements
dictate the use of analog processing techniques then the pulse is further shaped,
amplified, and fed into a pulse height analyzer. On the other hand, if digital signal
processing is desired, then the signal is amplified and then converted to digital counts
through an ADC. It should be noted that the type of processing is highly dependent
on the application and therefore there is no general electronics chain that could be
universally adopted. In this chapter we will visit the most important of the signal
processing methods used in modern radiation detection systems.