When the source and the detector are in close contact, the fgtends to be
about 50% (Fig. 8.9B). In the case of gamma well counters and liquid scin-
tillation counters, the fgapproaches 100% (Fig. 8.9C).
Dead Time
Each counting system takes a certain amount of time to process a radiation
event, starting from interaction of radiation with the detector all the way
up to forming a pulse and ultimately recording it. The counter remains
insensitive to a second event for this period of time, that is, if a second radi-
ation arrives during this time, the counter cannot process it. This period is
called the dead time. When the counter recovers after this period, only then
can a second radiation be detected. Thus the second event arriving during
the dead time is lost. Counts lost during the dead time are called the dead-
time loss. In scintillation detectors two events may be processed simulta-
neously to form a single event of amplitude that is equal to the sum of the
amplitudes of both events. This is referred to as pulse pileup. If one or both
of the events were photopeaks originally, then the combined peak will fall
outside the PHA window setting and be lost. Dead time loss at high count
rates is a serious problem for any counting system and is more so for scin-
tillation cameras due to pulse pileup (see Chapter 11).
The dead time of a counting system may arise from different components
of the entire counting system: detector, PHA, PM tube, scaler, computer
interface, and so on. While Geiger–Müller (GM) detectors have a longer
dead time of 100 to 500 microseconds, the typical values for NaI(Tl) and
semiconductors are of the order of 0.5 to 10 microseconds and for liquid
scintillators, ~0.1 to 1 microsecond (Cherry et al., 2003).
Characteristics of Counting Systems 99Fig. 8.9. Illustration of geometric efficiency,
fg, of a detector Dwith a circular area,pr^2 ,
where ris the radius of the detector. (A)
The detector Dplaced at a distance Rfrom
the point source Shas an fgfour times
greater than the fgwhen the detector is
placed at a distance 2R. (B) When the
source and the detector are in close contact,
the fgis about 50%. (C) When the source is
well inside the detector as in a well counter,
the fgapproaches 100%.