and 684 divisions, respectively, thus assigning the center of the photopeak
at 662 divisions corresponding to the 662-keV g-ray. Starting from low
values, the high voltage is increased in small increments for a given ampli-
fier gain until the observed count rate reaches a maximum. The high voltage
at the maximum count rate is kept as the operating voltage for subsequent
counting of photons of different energies. The discriminator dials are then
said to be energy calibrated; for example, each dial unit corresponds to
1 keV at an amplifier gain of 1. Thus, the center of the 140-keV photopeak
of 99mTc can be set at 140 divisions of the discriminator setting, with lower
and upper values set as desired. After calibration, well counters should be
checked regularly for any voltage drift using a long-lived source, such as
(^137) Cs.
Counting in Well Counters
For relative comparison of count rates between samples, the well counter
does not need to be calibrated, provided all samples for comparison have
the same volume. In radioimmunoassays, ferrokinetics, blood volume, red
cell mass measurements, a standard of the same geometry (volume) and
with relatively the same activity is counted along with all samples, and then
a comparison is made between each sample and the standard. However,
when the absolute activity of a radioactive sample needs to be determined,
then the detection efficiency of the counter must be measured for the g-ray
energy of interest using a standard of the radioactive sample of known
activity. The photopeak efficiency is determined from the count rate of the
standard at the appropriate PHA setting divided by the disintegration rate
from the known activity of the standard. The efficiency correction can then
be applied to the count rates of samples of unknown radioactivity when
counted at the same setting as the standard to give the absolute activity. For
102 8. Scintillation and Semiconductor Detectors
Fig. 8.11. A schematic diagram of a NaI(Tl) well counter with a PM, photomulti-
plier tube.