Sodium iodide-based detectors
Thallium-doped sodium iodide (NaI(Tl)) has two principal advantages:
1. It can be produced in large crystals, yielding good efficiency, and
2. 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.
Back
Scatter
Compton
edge
Photo
peak
Figure 1: Sodium iodide gamma spectrum of cesium-137 (137Cs)
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):
1. low energy x radiation (due to internal conversion of the gamma ray),
2. backscatter at the low energy end of the Compton distribution, and
3. a photopeak (full energy peak) at an energy of 662 MeV
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.
Figure 2: Sodium iodide gamma spectrum of cobalt-60 (60Co)
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.
Activity
The activity of a radioactive source is the number of disintegrations occurring
within it per second.
The SI unit of activity is the Becquerel(Bq).
1 Bq = 1 count per second = 1 CPS = 1 disintegration per second.
The activity can be measured using the Geiger counter.
Note the activity of a source does not depend on any physical or chemical
conditions. Radioactivity is a random process. It cannot be controlled.
The activity of a source is proportional to the number of atoms present in the
source.
AN
Or
A = constant X N
A=N
Activity (Bq)
s-1
Number of
atoms
 is the decay constant. Each element has its own value. For example the
element Americium has a decay constant  = 5 X 10-11s-1.
Activity and time
As we have already seen, the activity of a radioactive source depends on the
number of atoms in the source. The more atoms you have, the greater the
number of decays per second.
But as time passes more and more of the atoms disintegrate. So the amount of
radioactive material present in the source is always decreasing over time.
Number
of atoms
Graph never quite
reaches the time axis.
Time (seconds)
If the number of radioactive atoms is decreasing then the activity must also be
decreasing since A =  N.
Activity
A (s-1)
Graph never quite
reaches the time axis.