Physics and Engineering of Radiation Detection

718 Chapter 13. Data Acquisition Systems

particles as they deposit energy into the active volume of the detector. A good pulse
is generally understood to represent a pulse that has been generated by the incident
radiation. But the definition is somewhat arbitrary since the experimenter might
want to look at the noise pulses only or pulses generated by particles of certain
energy. Whatever the definition of the good pulse is, the objective is to discrimi-
nate the good pulses from all the pulses reaching the electronics. That is, all the
unwanted pulses must be blocked. To accomplish this task, a typical pulse counting
system has a device calleddiscriminator. A typical discriminator discriminates the
pulses based on their amplitudes. To do this it compares the pulse amplitude to
a presetgood pulse amplitude window. The design of this discriminator and other
devices depend mainly on the rate requirements as discussed in the following two
subsections.

A.1 SlowPulseCounting......................

Most of the radiation measurements in laboratory and industrial environments fall
into this category. The reason is that the rate of radiation emitted by typically used
radioactive sources is not very high. However in case of particle accelerators the rate
is too high and might not be handled by a slow counting system.

Detector Preamplifier Amplifier/Shaper Single Channel

Analyzer

Counter/Timer PC

Figure 13.1.1: Block diagram of a simple slow pulse counting system.

The building blocks of a slow pulse counting system is sketched in Fig.13.1.1. The
pulse from the detector is first preamplified. The preamplifier is usually installed
near the detector output. The preamplified pulse is then transported to the main
amplifier/shaper, where the pulse is properly amplified and shaped according to the
input requirements of the next stage. The amplified pulse is fed into a single channel
analyzer (SCA), which generates a digital pulse whenever the input pulse amplitude
is within the preset window. In the next stage this digital pulse is counted. A
counter module simply increments the count by one each time an SCA pulse arrives.
The output of the counter can then be fed into a computer for further analysis and
storage.

A.2 FastPulseCounting

The response time of a single channel analyzer can be prohibitive for its use in fast
pulse counting applications. In such a case, a simple fast pulse discriminator can
be used. Such a device would block all the pulses whose amplitude lies outside
a preset window. The pulses can then be counted by a specially designed fast