Multichannel analyzer

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A multichannel analyzer is used in physical research or applied areas (e.g. analytics) to measure statistically distributed sequences of electrical impulses (voltage surges) of changing amplitude in order to determine their frequency distribution . To put it clearly, the device “sorts” the impulses, which arrive at random, according to their height into various “compartments”, the channels . This corresponds to the term kicksorter , which is occasionally used in English .

Fig. 1: Gamma spectrum of 137 Cs, measured with a scintillation detector

Most of the time, the impulses come from a particle or radiation detector . The frequency distribution shown as a histogram then represents the spectrum of the radiation examined. The image shows the spectrum of a gamma radiation source recorded with a range of a total of 512 channels (see gamma spectroscopy ).

Components and function

The main parts of a multichannel analyzer are

  • an analog-to-digital converter ( Analog-to-Digital Converter, ADC)
  • a data memory with z. B. 4096 locations ("channels") for storing whole numbers
  • and a (simple) arithmetic unit.

The multi-channel analyzer often consists of a PC with appropriate additional devices. Due to the technical number representation in the dual system, the total number of channels is usually a power of 2, for example 512, 1024, 2048, 4096, ...

The operation of the ADC differs from the devices described under analog-digital converter . These serve, for example in music recordings, for the ongoing digitization of variable voltages. The ADC of the multichannel analyzer, on the other hand, digitizes the pulse height ; it is only active when an impulse arrives and determines the whole number (in the example between 0 and 4095) that comes closest to the value proportional to the impulse amplitude (a certain proportionality factor is selected for this, e.g. the impulse height should 10.0 V correspond to the number 4095). The arithmetic unit adds a one to the content present there in the channel whose address is this number. The channel content at the end of the measurement is then the number of pulses with amplitudes in this range.

This pulse height spectrum can be made visible on a screen, for example, by displaying the channel number (which usually corresponds to a particle or quantum energy) horizontally and the number of pulses vertically.

There are also multi-channel analyzers for two-parameter registration in coincidence experiments. Two ADCs are used and the memory locations are identified by two addresses each like an element of a matrix by row and column numbers. An event is only registered if the two input signals are sufficiently close in time.

A pile-up rejector is often inserted in the signal chain in front of the device in order to relieve the ADC from digitizing faulty pulses.