counting only those pulses falling within preselected voltage intervals or
“channels” and rejects all others (see Fig. 8.1). Pulses corresponding to g-
ray energies of interest are selected by energy discriminator knobs, known
as the lower levelor upper level, or the baselineand window, provided on
the PHA, and are ultimately delivered to the recording devices such as
scalers, computers, films, and so on.
There are two modes of counting using PHAs:differentialand integral.
In differential counting, only pulses of preselected energies are counted by
appropriate selection of lower and upper level knobs (discriminators) or
the baseline and window. In scintillation cameras, however, differential
counting is achieved by a peak voltage knob and a percent window knob.
The peak voltage knob sets the energy of the desired g-ray, and the percent
window knob sets the window width in percentage of the g-ray energy,
which is normally placed symmetrically on each side of the peak voltage.
In integral counting,g-rays of all energies or all g-rays of energies above
a certain energy are counted by setting the appropriate lower level or base-
line and bypassing the upper level or window mechanism.
A PHA normally selects only one range of pulses corresponding to only
one g-ray energy by means of differential counting. Such a PHA is called a
single-channel analyzer (SCA). A multichannel analyzer (MCA) is a device
that can simultaneously sort pulses into many predetermined voltage
ranges or channels, corresponding to different photon energies. By using an
MCA, one obtains a simultaneous spectrum of different g-ray energies from
a radioactive source.
Display or Storage
Pulses processed by the PHA can be displayed on a cathode ray tube (CRT)
or can be counted for a preset count or time by a scaler-timer device. A rate
meter can be used to display the pulses in terms of counts per minute (cpm)
or counts per second (cps). In scintillation cameras, pulses are used to form
the image on a CRT and polaroid or x-ray films. These pulses can also be
stored in a computer or on a magnetic tape or laser disc for processing later.
Gamma-Ray Spectrometry
Pulses are generated by the PM tube and associated electronics after the g-
ray energy is absorbed in the NaI(Tl) detector. Because g-rays interact with
the NaI(Tl) detector by photoelectric, Compton, and pair production mech-
anisms, and also because various scattered radiations from outside the
detector may interact with the detector, a distribution of pulse heights will
be obtained depicting a spectrum of g-ray energies. Such a g-ray spectrum
may result from a single g-ray or from many g-rays in a sample. Different
features of this spectrum are discussed here.
88 8. Scintillation and Semiconductor Detectors