are paralyzable systems. Scintillation cameras have both paralyzable and
nonparalyzable components of dead time. As can be seen from Eq. (8.8),
the radiation detectors become totally paralyzed at very high count rates
giving no reading.
Dead time loss is a serious problem for a counting system at high-count
rates. Therefore, either count rates must be lowered or corrections must be
made to the observed count rates. There are several methods to determine
or correct for dead time. An empirical method is to plot the observed count
rates as a function of increasing concentrations of known activity. From the
plot and Eqs. (8.7) and (8.8), the dead time is calculated for the nonpara-
lyzable or paralyzable system. For subsequent measurements of unknown
samples, correction is made to compensate for the dead time loss giving true
count rates. Another method uses two radioactive sources, which are
counted in the counter individually and together. From these three mea-
surements, one can calculate dead time using appropriate equations (see
Cherry et al., 2003). Various techniques, such as use of buffers, in which
overlapping events are held off during the dead time, use of pulse pileup
rejection circuits, and use of high-speed electronics have been employed to
improve the dead time correction.
Gamma Well Counters
The gamma well counter consists of a NaI(Tl) detector with a hole in the
center for a sample to be placed and associated electronics such as a PM
tube, preamplifier, amplifier, PHA, and scaler-timer. Placing a radioactive
sample in the central hole of the detector increases the geometric efficiency
(almost 99%) and hence the counting efficiency of the counter. The NaI(Tl)
detectors have dimensions in the range of 5-cm diameter ×5-cm thick to
23-cm diameter ×23-cm thick. Smaller detectors are used for low-energy
g-rays (less than 200 keV), and larger detectors are used for high-energy
g-rays. Most well counters are shielded with about 8.5-cm thick circular lead
ring to reduce background from cosmic rays, natural radioactivity such as
(^40) K, or background activity in the work area. A typical well counter detec-
tor is shown in Figure 8.11.
Calibration of Well Counters
It is essential that the dial settings of the discriminators on the PHA are
calibrated so that the dial readings correspond directly to the pulse height
(i.e., the energy of the g-ray photon); that is, the dial readings can be read
in units of keV. This calibration is called the high-voltageor energy cali-
brationof the well counter. The energy calibration is carried out by using
the 662-keV photons of^137 Cs. After placing a^137 Cs source in the well
counter, the lower and upper discriminator levels are set at 640 divisions
Gamma Well Counters 101