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14 Difficulties of Radiological and Nuclear Interdiction
and beta radiation in air; while gamma radiation can be detected up to several kilometers from a sufficiently powerful source, alpha radiation has a range of only a few
millimeters, beta radiation can travel only 6 or 7 m, and neutron radiation can travel
up to several hundred meters in air only if the source is sufficiently high-activity. Any
or all of these types of radiation can be difficult to detect with the types of radiation
detectors that are most commonly used for interdiction.
Many alpha- and beta-emitting radionuclides also emit gamma radiation, or
gamma radiation is emitted by their progeny. Cesium-137, for example, does not
emit a gamma; Cs-137 is a pure beta-emitting radionuclide. But Cs-137 decays
to form Ba-137 m, and Ba-137 m emits gamma radiation with a half-life of only
about 6 min. Thus, while a gamma detector will not detect Cs-137, it is easily able
to detect its Ba-137 m progeny. Similarly, while Sr-90 emits only beta radiation,
the high-energy gamma emitted by its Y-90 progeny is easily detected. Similarly,
alpha-emitting Am-241 and Ra-226 also emit distinctive gammas, as do many other
beta and gamma-emitting radionuclides.
In addition, beta radiation, when passing through material, will sometimes undergo interactions that result in the emission of x-ray radiation called
bremsstrahlung. Even though it is very difficult to identify a radionuclide by the
bremsstrahlung emitted, its very presence will serve to increase radiation levels,
alerting the user to the presence of radioactivity, even if it cannot be identified.
Neutron radiation can be more problematic; there are very few radionuclides that
undergo spontaneous fission and emit neutrons (Cf-252 is the most common). Most
neutron sources use a combination of Am-241 or another alpha-emitting nuclide in
conjunction with beryllium; when the alpha particle interacts with the beryllium a
neutron is ejected. Thus, many neutron sources will also emit the characteristic 60
keV gamma of the Am-241, in addition to gamma radiation emitted when the neutron
is absorbed by a target atom. As with bremsstrahlung, the gamma radiation might not
be readily identifiable, but it can serve to increase the general radiation dose rates.
In addition, many departments tasked with interdiction are equipped with neutron
detectors, increasing the likelihood of interdicting neutron-emitting materials.
14.1.4 Presence of Nuclides with Similar Gamma Energies,
Limited Gamma Libraries
While no two gamma-emitting radionuclides give off gammas with identical energies,
there are some radionuclides that emit gamma rays with very similar energies, and this
can lead to mis-identification, especially when using scintillation detectors to perform
the nuclide identification. The reason for this is that scintillation-type detectors—
especially sodium iodide and cesium iodide—have a relatively low energy resolution compared to high-purity germanium, making it difficult to resolve and identify
closely spaced gamma energy peaks. Figure 14.3 shows the same gamma energy
spectrum as measured by several different detectors—looking at these peaks, it is
14 Difficulties of Radiological and Nuclear Interdiction
and beta radiation in air; while gamma radiation can be detected up to several kilometers from a sufficiently powerful source, alpha radiation has a range of only a few
millimeters, beta radiation can travel only 6 or 7 m, and neutron radiation can travel
up to several hundred meters in air only if the source is sufficiently high-activity. Any
or all of these types of radiation can be difficult to detect with the types of radiation
detectors that are most commonly used for interdiction.
Many alpha- and beta-emitting radionuclides also emit gamma radiation, or
gamma radiation is emitted by their progeny. Cesium-137, for example, does not
emit a gamma; Cs-137 is a pure beta-emitting radionuclide. But Cs-137 decays
to form Ba-137 m, and Ba-137 m emits gamma radiation with a half-life of only
about 6 min. Thus, while a gamma detector will not detect Cs-137, it is easily able
to detect its Ba-137 m progeny. Similarly, while Sr-90 emits only beta radiation,
the high-energy gamma emitted by its Y-90 progeny is easily detected. Similarly,
alpha-emitting Am-241 and Ra-226 also emit distinctive gammas, as do many other
beta and gamma-emitting radionuclides.
In addition, beta radiation, when passing through material, will sometimes undergo interactions that result in the emission of x-ray radiation called
bremsstrahlung. Even though it is very difficult to identify a radionuclide by the
bremsstrahlung emitted, its very presence will serve to increase radiation levels,
alerting the user to the presence of radioactivity, even if it cannot be identified.
Neutron radiation can be more problematic; there are very few radionuclides that
undergo spontaneous fission and emit neutrons (Cf-252 is the most common). Most
neutron sources use a combination of Am-241 or another alpha-emitting nuclide in
conjunction with beryllium; when the alpha particle interacts with the beryllium a
neutron is ejected. Thus, many neutron sources will also emit the characteristic 60
keV gamma of the Am-241, in addition to gamma radiation emitted when the neutron
is absorbed by a target atom. As with bremsstrahlung, the gamma radiation might not
be readily identifiable, but it can serve to increase the general radiation dose rates.
In addition, many departments tasked with interdiction are equipped with neutron
detectors, increasing the likelihood of interdicting neutron-emitting materials.
14.1.4 Presence of Nuclides with Similar Gamma Energies,
Limited Gamma Libraries
While no two gamma-emitting radionuclides give off gammas with identical energies,
there are some radionuclides that emit gamma rays with very similar energies, and this
can lead to mis-identification, especially when using scintillation detectors to perform
the nuclide identification. The reason for this is that scintillation-type detectors—
especially sodium iodide and cesium iodide—have a relatively low energy resolution compared to high-purity germanium, making it difficult to resolve and identify
closely spaced gamma energy peaks. Figure 14.3 shows the same gamma energy
spectrum as measured by several different detectors—looking at these peaks, it is