Back to " HEALTH PHYSICS INSTRUMENTS "

Pulse Height Analysis

Pulse Height Analysis may consist of a simple discriminator that can be set above noise level and which produces a standard logic pulse (see Figure 1.14) for use in a pulse counter or as gating signal. However, most data consists of a range of pulse heights of which only a small portion is of interest. One can employ either of the following:

·         Single Channel Analyzer and Counter

·         Multichannel Analyzer

The single channel analyzer (SCA) has a lower and an upper level discriminator, and produces an output logic pulse whenever an input pulse falls between the discriminator levels. With this device, all voltage pulses in a specific range can be selected and counted. If additional voltage ranges are of interest, additional SCAs and counters (i.e. scalers) can be added as required, but the expense and complexity of multiple SCAs and counters usually make this configuration impractical.

If a full voltage (i.e. energy) spectrum is desired, the SCA’s discriminators can be set to a narrow range (i.e. window) and then stepped through a range of voltages. If the counts are recorded and plotted for each step, an energy spectrum will result.

The multichannel analyzer (MCA), which can be considered as a series of SCAs with incrementing narrow windows, basically consists of an analog-to-digital converter (ADC), control logic, memory and display. The multichannel analyzer collects pulses in all voltage ranges at once and displays this information in real time, providing a major improvement over SCA spectrum analysis.

Figure 1.15 illustrates a typical MCA block diagram. An input energy pulse is checked to see if it is within the selected SCA range, and then passed to the ADC. The ADC converts the pulse to a number proportional to the energy of the event. This number is taken to be the address of a memory location, and one count is added to the contents of that memory location. After collecting data for some period of time, the memory contains a list of numbers corresponding to the number of pulses at each discrete voltage. The display reads the memory contents vs. memory locations, which is equivalent to number of pulses vs. energy.

Figure 1.15.
Multichannel Analyzer Components

Copyright © 2004  Sumhar.com  All right reserved