In the case of proportional counters , scintillators or solid state detectors we know that the pulses leaving the detector have amplitudes proportional to the energy which the particles or photons deposit in the detectors. If these pulses are sorted according to their height, this would be the equivalent to sorting the particles or photons according to their energy. Electronic systems which do this are available and are called pulse height analyzer , or PHA's.
Single channel PHA's only count pulses of a given amplitude (equivalent to
particles or photons of a given energy) called a `channel', at a time. For
example if we wished to single out
I (gamma energy of 159 keV) in the
presence of other gamma emitters , the instrument would
be set to count only those pulses in the a narrow range around 159 keV.
Thyroid monitors and
monitors for airborne radioiodine
are set up in this way.
Multichannel analyzer (MCA's) can scan a
whole energy range and record the number of pulses they count in each of the
channels. Fig. 6.12 
shows the
spectrum of 
Co taken with a high-purity germanium
detector using an
MCA. These instruments are used for identifying gamma emitters and their
concentration in a sample and are often referred to as gamma
spectrometers .
Figure 6.12: MCA Spectrum of
Co