Well counters are available with automatic sample changers having pro-
visions of counting as many as 500 samples. Most counters are program-
mable with computers and provide printouts with various information on
counting. The major advantage of the well counter is its high detection
efficiency due to increased geometric efficiency, which approaches almost
100% depending on the volume of the sample. The detection efficiency of
a well counter decreases with increasing photon energy and decreasing
detector size. Typically, the overall detection efficiency is close to 100% for
140-keV photons of 99mTc and 30% to 90% for 364-keV photons of^131 I,
depending on the detector size.
Thyroid Probe
The thyroid probe is a counter commonly employed to measure the uptake
of^131 I or^123 I in the thyroid gland after the oral administration of a^131 I-NaI
or^123 I-NaI capsule. It consists of a NaI(Tl) detector, 5 cm in diameter by
5 cm in thickness, and other associated electronics, as in a well counter.
The operation of the probe is similar to that of a well counter.
One of the differences between the well counter and the thyroid probe
is that the latter requires a collimator, which limits the field of view on the
thyroid. The collimator is a 20- to 25-cm long cylindrical barrel made of lead
and covers the detector as well as the PM tube (Fig. 8.13). This reduces the
background activity from the g-radiations from areas outside the thyroid
reaching the detector.
The efficiency of a thyroid probe varies inversely with the square of the
distance between the detector and the thyroid. The probe is initially cali-
104 8. Scintillation and Semiconductor Detectors
Fig. 8.13. A schematic diagram of a thyroid probe. PHA, pulse-height analyzer; PM,
photomultiplier.