Biomedical Engineering Reference
In-Depth Information
The alkali halides are good scintillators. In addition to its efficient light yield,
sodium iodide doped with thallium [NaI(Tl)] is almost linear in its energy response.
It can be machined into a variety of sizes and shapes. Disadvantages are that it is
hygroscopic and somewhat fragile. NaI has become a standard scintillator material
for gamma-ray spectroscopy. CsI(Na), CsI(Tl), and LiI(Eu) are examples of other
inorganic scintillators. Silver-activated zinc sulfide is also commonly used. It is
available only as a polycrystalline powder, from which thin films and screens can
be made. The use of ZnS, therefore, is limited primarily to the detection of heavy
charged particles. (Rutherford used ZnS detectors in his alpha-particle scattering
experiments.) Glass scintillators are also widely used.
Two examples of scintillator probes are displayed in Fig. 10.29. In addition to the
detector material, each contains a photomultiplier tube, which is reflected in its
size. The unit on the left is used for gamma surveys. It has a cylindrical NaI(Tl)
crystal with a height and diameter of 2.5 cm. It operates between 500 V and 1200 V.
The unit on the right uses ZnS and is suitable for alpha/beta surveys. It operates
in the same voltage range. The window area is approximately 100 cm
2
.
Specialized scintillation devices have been designed for other specific purposes.
One example is the phoswich (
=
phosphor sandwich) detector, which can be used
to count beta particles or low-energy photons in the presence of high-energy pho-
tons. It consists of a thin NaI(Tl) crystal in front coupled to a larger scintillator
of another material, often CsI(Tl), having a different fluorescence time. Signals
that come from the photomultiplier tube can be distinguished electronically on
the basis of the different decay times of the two phosphors to tell whether the
light came only from the thin front crystal or from both crystals. In this way, the
low-energy radiation can be counted in the presence of a high-energy gamma-ray
background.
Figure 10.30 shows a pulse-height spectrum measured with a
4 × 4
in. NaI(Tl)
scintillator exposed to 662-keV gamma rays from
137
Cs. Several features should
be noted. Only those photons that lose all of their energy in the crystal contribute
to the total-energy peak, also called the photopeak. These include incident photons
that produce a photoelectron directly and those that undergo one or more Compton
Fig. 10.29
Examples of scintillation probes: (left) NaI(Tl) for
gamma surveys; (right) ZnS for alpha/beta monitoring.
(Courtesy Ludlum Instruments, Inc.)
