4.5 Radio-Isotopic Identification (RIID)
35
4.5 Radio-Isotopic Identification (RIID)
Because the number of scintillation photons produced is frequently proportional to
the amount of energy deposited in the scintillation medium, this type of detector can
be used to identify radionuclides.
Scintillation-type detectors are most commonly used for isotope identification
because they are relatively inexpensive and operate at room temperature. However,
scintillation detectors tend to lack good energy resolution and can fail to correctly
identify closely spaced gammas. A classic example of this is
235 U versus
226 Ra,
whose gamma energies are 185 and 186 keV respectively. To a scintillation detector,
these energies cannot be distinguished; as a result, it is not uncommon for natural
radioactivity to be identified as highly enriched uranium. This can also be used to
mask threat nuclides by packing them in the same container as innocuous radionuclides whose gamma energies are close to those of the threat nuclide(s) and that are
strong enough to hide the gammas from the threat (Fig. 4.4).
To some extent, performing a longer count can help to overcome these effects,
by giving the detector (and the software) enough information that it can begin to
tease out the weak signal of a masked radionuclide. Unfortunately, under routine
circumstances it might not be possible to hold a vehicle or a person for an extended
period of time. In addition, there are some gamma energies that are simply so close
that even a long count with a scintillation detector will be unable to differentiate the
energies.
Scintillation detectors can make mistakes
Another known property of scintillation detectors is that they can be sensitive to
changes in temperature; the energy determination can change as the detector
warms or cools. Newer instruments compensate for this by using a known
gamma energy as a reference point to stabilize the instrument; older devices
do not always do so.
As one example, the author was performing a radiological interdiction
survey using a vehicle-mounted sodium iodide system in Midtown Manhattan
on a cold day. At one point, the instruments began alarming with a
60 Co identification. Looking at the spectrum, it was clear that there was only one gamma
peak (compared to the double peak of this nuclide), yet the “identification”
continued to occur. The operator surmised that when the detectors had been
initialized they were at the ambient temperature (about −2 °C) and that, as
the vehicle warmed up, so did the detectors. As they warmed they began
to “drift” until the 1.46 meV gamma from natural
40 K appeared to be the
1.33 meV gamma from
60 Co. To test this hypothesis, the instrument was turned
off and then turned back on; it performed an automatic energy calibration at the
new, higher temperature, and the
60 Co “identification” vanished. That particular instrument performed the start-up energy calibration using a small
137 Cs
35
4.5 Radio-Isotopic Identification (RIID)
Because the number of scintillation photons produced is frequently proportional to
the amount of energy deposited in the scintillation medium, this type of detector can
be used to identify radionuclides.
Scintillation-type detectors are most commonly used for isotope identification
because they are relatively inexpensive and operate at room temperature. However,
scintillation detectors tend to lack good energy resolution and can fail to correctly
identify closely spaced gammas. A classic example of this is
235 U versus
226 Ra,
whose gamma energies are 185 and 186 keV respectively. To a scintillation detector,
these energies cannot be distinguished; as a result, it is not uncommon for natural
radioactivity to be identified as highly enriched uranium. This can also be used to
mask threat nuclides by packing them in the same container as innocuous radionuclides whose gamma energies are close to those of the threat nuclide(s) and that are
strong enough to hide the gammas from the threat (Fig. 4.4).
To some extent, performing a longer count can help to overcome these effects,
by giving the detector (and the software) enough information that it can begin to
tease out the weak signal of a masked radionuclide. Unfortunately, under routine
circumstances it might not be possible to hold a vehicle or a person for an extended
period of time. In addition, there are some gamma energies that are simply so close
that even a long count with a scintillation detector will be unable to differentiate the
energies.
Scintillation detectors can make mistakes
Another known property of scintillation detectors is that they can be sensitive to
changes in temperature; the energy determination can change as the detector
warms or cools. Newer instruments compensate for this by using a known
gamma energy as a reference point to stabilize the instrument; older devices
do not always do so.
As one example, the author was performing a radiological interdiction
survey using a vehicle-mounted sodium iodide system in Midtown Manhattan
on a cold day. At one point, the instruments began alarming with a
60 Co identification. Looking at the spectrum, it was clear that there was only one gamma
peak (compared to the double peak of this nuclide), yet the “identification”
continued to occur. The operator surmised that when the detectors had been
initialized they were at the ambient temperature (about −2 °C) and that, as
the vehicle warmed up, so did the detectors. As they warmed they began
to “drift” until the 1.46 meV gamma from natural
40 K appeared to be the
1.33 meV gamma from
60 Co. To test this hypothesis, the instrument was turned
off and then turned back on; it performed an automatic energy calibration at the
new, higher temperature, and the
60 Co “identification” vanished. That particular instrument performed the start-up energy calibration using a small
137 Cs