Sodium iodide-based detectors
The second most common type of radiation detecting instrument is the scintillation detector. The basic principle behind this instrument is the use of a special material which glows or “scintillates” when radiation interacts with it. The most common type of material is a type of salt called sodium-iodide. The light produced from the scintillation process is reflected through a clear window where it interacts with device called a photomultiplier tube.
The first part of the photomultiplier tube is made of another special material called a photocathode. The photocathode has the unique characteristic of producing electrons when light strikes its surface. These electrons are then pulled towards a series of plates called dynodes through the application of a positive high voltage. When electrons from the photocathode hit the first dynode, several electrons are produced for each initial electron hitting its surface. This “bunch” of electrons is then pulled towards the next dynode, where more electron “multiplication” occurs. The sequence continues until the last dynode is reached, where the electron pulse is now millions of times larger then it was at the beginning of the tube. At this point the electrons are collected by an anode at the end of the tube forming an electronic pulse. The pulse is then detected and displayed by a special instrument.
Scintillation detectors are very sensitive radiation instruments and are used for special environmental surveys and as laboratory instruments
Scintillation detectors use crystals that emit light when gamma rays interact with the atoms in the crystals. The intensity of the light produced is proportional to the energy deposited in the crystal by the gamma ray. The mechanism is similar to that of a thermoluminescent dosimeter. The detectors are joined to photomultipliers that convert the light into electrons and then amplify the electrical signal provided by those electrons. Common scintillators include thallium-doped sodium iodide (NaI(Tl))—often simplified to sodium iodide (NaI) detectors—and bismuth germanate (BGO). Because photomultipliers are also sensitive to ambient light, scintillators are encased in light-tight coverings.
Scintillation detectors can also be used to detect alpha- and beta-radiation.
 Sodium iodide-based detectors
Figure 1: Sodium iodide gamma spectrum of cesium-137 (137Cs)
Figure 2: Sodium iodide gamma spectrum of cobalt-60 (60Co)
Thallium-doped sodium iodide (NaI(Tl)) has two principal advantages:
It can be produced in large crystals, yielding good efficiency, and
it produces intense bursts of light compared to other spectroscopic scintillators.
NaI(Tl) is also convenient to use, making it popular for field applications such as the identification of unknown materials for law enforcement purposes.
An example of a NaI spectrum is the gamma spectrum of the cesium isotope 137Cs—see Figure 1. 137Cs emits a single gamma line of 662 keV. It should be noted that the 662 keV line shown is actually produced by 137Bam, the decay product of 137Cs, which is in secular equilibrium with 137Cs.
The spectrum in Figure 1 was measured using a NaI-crystal on a photomultiplier, an amplifier, and a multichannel analyzer. The figure shows the number of counts (within the measuring period) versus channel number. The spectrum indicates the following peaks (from left to right):
low energy x radiation (due to internal conversion of the gamma ray),
backscatter at the low energy end of the Compton distribution, and
a photopeak (full energy peak) at an energy of 662 keV
The Compton distribution is a continuous distribution that is present up to channel 150 in Figure 1. The distribution arises because of primary gamma rays undergoing Compton scattering within the crystal: Depending on the scattering angle, the Compton electrons have different energies and hence produce pulses of different heights.
If many gamma rays are present in a spectrum, Compton distributions can present analysis challenges. To reduce gamma rays, an anticoincidence shield can be used—see Compton suppression. Gamma ray reduction techniques are especially useful for small lithium-doped germanium (Ge(Li)) detectors.
The gamma spectrum shown in Figure 2 is of the cobalt isotope 60Co, with two gamma rays with 1.17 MeV and 1.33 MeV respectively. (See the decay scheme article for the decay scheme of cobalt-60.) The two gamma lines can be seen well-separated; the peak to the left of channel 200 most likely indicates a strong background radiation source that has not been subtracted. A backscatter peak can be seen at channel 150, similar to the second peak in Figure 1.
Sodium iodide systems, as with all scintillator systems, are sensitive to changes in temperature. Changes in the operating temperature caused by changes in environmental temperature will shift the spectrum on the horizontal axis. Peak shifts of tens of channels or more are commonly observed. Such shifts can be prevented by using spectrum stabilizers.
Because of the poor resolution of NaI-based detectors, they are not suitable for the identification of complicated mixtures of gamma ray-producing materials. Scenarios requiring such analyses require detectors with higher resolution.