Because semiconductors are much denser than gases, they are more effi-
cient for x- and g-ray detection than gas detectors. Also in semiconductor
detectors, each ionization requires only about 3 eV compared to 35 eV in
gas detectors. Thus, almost ten times more ions are produced in semicon-
ductor detectors than in gas detectors for a given g-ray energy, thus yield-
ing a better spectral resolution of g-ray photons of closer energies.
Fabrication of Ge(Li) and Si(Li) detectors is quite time-consuming and
expensive. The size of the detectors is also small, which prevents their use
in gamma cameras.
Thermal noise at room temperature introduces a high background that
can obscure the sample counts, but is reduced at low temperature. There-
fore, these detectors are operated at low temperature usually employing
liquid nitrogen (−196°C or 77°K). A disadvantage of these detectors is that
liquid nitrogen evaporates over time and needs to be replenished periodi-
cally, typically weekly. Nowadays HPGe detectors can be kept at room tem-
perature when not used, and cooled when used for counting by means of
helium-based cryoelectric and freon-based coolers.
Semiconductor detectors are most useful in differentiating photon ener-
gies because of the high-energy resolution, particularly in detecting
radionuclidic contamination. These detectors are not in common use in
nuclear medicine.
Cadmium–Zinc–Tellurium Detector
Cadmium–Zinc–Tellurium (CZT) detectors are another type of semicon-
ductor made of Cd, Zn, and Te metals, and provide very high efficiency for
g-ray detection because of their high atomic numbers. For reasons of high
detection efficiency, these detectors can be made as small as 2 mm thick and
2 mm diameter with almost 100% efficiency for 100 keV photons. The
energy resolution of these detectors is very good for a wide range of g-ray
energies. These detectors are operated at room temperature. The electron-
ics used are similar to those of other scintillation detectors. Different types
of handheld probes have been devised for various purposes. One probe,
called the Neoprobe 1000, is used for the detection of metastatic sites con-
taining radioactivity (e.g.,^125 1-labeled monoclonal antibody) during surgery
for their removal by incision.
Cesium Iodide (CsI(Tl)) Detector
The CsI(Tl) detector has higher density and hence greater stopping power
than the NaI(Tl) detector and also yields more light photons per keV. But
its scintillation decay time is very long (1000 ns) resulting in longer dead
time for the counting system. The crystal is weakly hygroscopic and does
not require hermetic sealing. Unlike NaI(Tl), it can withstand a wide vari-
ation in temperature.
Scintillation Detectors 85