as to distinguish between radiation of different energies. The single channel principle can be extended
to circuits in which the gate scans through the pulse height range, and feeds a signal to a ratemeter with
an intensity proportional to the number of pulses falling within the gate. In this way a spectrum can be
generated on a chart recorder or displayed on a video screen. This process can be likened to the relative
movement of a slit and dispersed radiations in an optical spectrometer.
Multi-channel pulse height analysers examine the whole range of pulses coming from a detector
simultaneously, by using a large number of channels or gates, typically 8000. The analogue signal from
the detector is examined by an analogue-to-digital converter (ADC), and a series of pulses is generated
by the ADC for each channel. These are used as the basis for the generation of a spectrum, or relayed to
a variety of other data output modes.
Modern pulse height analysers essentially contain dedicated digital computers which store and process
data, as well as control the display and operation of the instrument. The computer will usually provide
spectrum smoothing, peak search, peak identification, and peak integration routines. Peak identification
may be made by reference to a spectrum library and radionuclide listing. Figure 10.15 summarizes such
a pulse height analysis system.
Figure 10.15
Schematic layout of a nuclear spectrometer with a computer to
provide pulse height analysis and data processing.Autoradiography
Special emulsions which are sensitive to nuclear radiations and X-rays are readily available and find
considerable use in some fields of analysis. Some modern films interact selectivity with different types
of radiation, and can be employed to distinguish between different sources. This feature is proving to be
especially valuable in environmental investigations (vide infra). The main use of autoradiography is in
locating the position of a label in a specimen, prior to its determination, when a high degree of spatial
resolution is needed. Many biological tracer experiments fall into this category, as does the assay of
thin-layer or paper chromatograms and electrophoretic gels. For example, as many as sixteen different
amino acids may be located on a single autoradiograph from a thin-layer plate. Trans-