The excellent, high-resolution γ- and X-ray spectra which can be obtained from semiconductor
detectors make the detectors very important in modern instruments. A typical spectrum is shown in
Figure 10.11(b) which may be compared with the much broader peaks from a scintillation detector
(Figure 10.11(a)). The spectra are not immune from the problem of Compton scattering (p. 461) but a
good quality modern detector will have a photopeak to Compton peak ratio of 50:1 or better. Computer-
aided spectrum analysis also serves to reduce the interference from the Compton effect.
α-particle spectrometry is also carried out using semiconductor detectors whose operation is based on
similar principles.
Some Important Electronic Circuits
Coincidence and anti-coincidence circuits frequently appear in nuclear instruments. Both are concerned
with the elimination of background pulses which may arise from electronic 'noise', cosmic radiation or
other environmental radiation. The coincidence principle is met frequently in scintillation counters,
where pulses, thermally generated within the stages of the photomultipliers, can greatly increase the
background signal. Two photomultipliers are arranged to view the sample so that scintillations are
recorded coincidentally by both tubes. On receipt of these signals a coincidence unit will feed a pulse to
the display or recording unit whilst ignoring noise pulses, which will not be coincidentally received
from the two tubes.
Anti-coincidence units are used to reduce the effects of external interferences. The detector is
surrounded by a second 'guard' detector, so that ionizing radiation from outside (e.g. cosmic rays) must
active the outer detector before penetrating to the inner. A sample placed close to, or inside, the inner
detector will activate that only. The anti-coincidence unit is arranged to accept only the latter non-
coincident pulses from the measuring detector.
Pulse Height Analysis
Pulse height analysis (PHA) is a nuclear technique to which frequent reference has been made. It is
used to distinguish one radiation from another by means of the proportionality between the radiation
energy and the size of the pulses generated. In the simplest cases radiation from a sample may be
distinguished from high-energy cosmic radiation, or from small 'noise' pulses. On the other hand pulse
height analysis can be used to produce a detailed spectrum and thus to resolve a complex mixture of
gamma, or X-rays.
A single channel pulse height analyser utilizes an electronic 'gate' typically 0.1 V wide, which only
accepts pulses between the preset upper and lower limits. Scintillation counters frequently employ such
devices to remove small noise pulses and large pulses initiated by cosmic rays, as well