Physics and Engineering of Radiation Detection

8.6. Discrimination 497

laborious and time consuming task, not to mention inefficient and prone to uncer-
tainties related to time variation of radiation flux. Therefore for such measurements
single channel analyzers are not generally used.

A.2 MultiChannelAnalyzer(MCA)

A multi channel analyzer can be thought of as a modified version of single channel an-
alyzer and a counter since it essentially performs the same task albeit with a number
of threshold windows. Thus it eliminates the need to count pulses at each threshold
window individually, making the process faster and less vulnerable to uncertainties
related to time variations in radiation flux. In terms of internal functioning, multi
channel analyzers are quite different from single channel analyzers since they actu-
ally digitize the input signal for analysis as opposed to the all-analog processing by
single channel analyzers. Fig.8.6.3 shows a simple block diagram of a multi channel
analyzer. The analog detector pulse is first amplified and shaped. The height of the
amplified pulse is then digitized. Since the ADC output is linearly related to the
input analog voltage, therefore the counts thus obtained correspond to the energy
deposited by the radiation. After digitization, a count in the corresponding memory
bin is incremented.

Memory

Display

Control and

Amplifier/

Shaper

Signal In Converter

Analog−to−Digital

Figure 8.6.3: Block diagram of a simple multi channel analyzer designed for

pulse height analysis.

Generally, multi channel analyzers have a number of channels, such as 512, 1024,
or more, giving the user some choice in selecting the appropriate resolution. Each
of these channels actually corresponds to memory locations that can be addressed
by the corresponding ADC counts and thus incremented.. The system can be tuned
such that the whole dynamic range is covered.
Most multi channel analyzers can be operated in two different modes. One is the
usual pulse height analysis mode that we just discussed and the other is the multi
channel scalar mode. In this mode the pulses are counted with respect to some
other parameter. The process starts with a logic pulse that starts incrementing the
counts in the first memory location. This counting continues until another logic
pulse arrives that move the counting to the next memory location. The logic pulses
can correspond to any parameter of interest. It could even be time, in which case
the system simply measures the total intensity.