150
14 Difficulties of Radiological and Nuclear Interdiction
radiation levels are due to an increase in general background radiation dose rates or
if the increase is in only one or two energies. An increase in background radiation
levels will show up across the entire energy spectrum while an increase due to a
single radioactive source will appear only in those energy channels associated with
that nuclide—662 keV, for example, if due to Cs-137. Thus, a mobile radiation
system might well identify the presence of a radionuclide (or, at least, might show a
suggestive energy peak) that would escape a person reading a relatively simple PRD.
Having said this, the user should never lose track of the possibility that an apparent
detection might not represent anything other than counting statistics, and the weaker
the apparent “signal” the more likely that this is the case.
14.1.2 Shielding
Shielding can affect detection and identification in two ways: by attenuating the
radiation emitted by a source and by changing the gamma energy spectrum as the
radiation passes through the shielding. The former effect can impair detection, the
latter can impair identification.
One important point to remember regarding radiation shielding is that, while it
can reduce the “signature” of a radioactive source, it is hard to eliminate the radiation
altogether. It is not difficult to reduce radiation dose rates or count rates by an order of
magnitude or more but each additional tenth value layer adds additional weight and
volume to the shield, making it easier to detect by virtue of its physical properties.
For example, an industrial radiography camera that contains a 2 TBq Cs-137 source
will weigh about 20–30 kg and will be about the size of a briefcase or a computer
bag. But this source will still be detectable to a distance of several meters to anybody
with a PRD, and to a much greater distance to a high-volume scintillation detector.
To reduce dose rates further would require additional layers of shielding that would
increase both the size and weight of the shield, with sources of higher activity the
shielding required to hide the source from detection can weigh enough to affect
the way a vehicle rides and handles; characteristics that police are accustomed to
evaluating (Fig. 14.2).
Shielding can also introduce changes to a nuclide’s spectrum, primarily through a
phenomenon known as Compton scattering. For example, consider a Cs-137 gamma
ray photon that is absorbed and re-emitted in a random direction by an electron in the
radiation shielding. When the photon is emitted it will have a slightly lower energy
than the original photon; perhaps it is emitted with an energy of, say, 650 keV instead
of the original 662 keV. This scattered photon can, itself, be absorbed and re-emitted
within the shield, and again and again—each time having a slightly lower energy.
As these photons continue being scattered and emerge from the shield the original
gamma peak will begin to “smear out” into lower energies. As additional shielding
is added the gamma peaks become less distinct, and the software that identifies
radionuclides based on gamma energy finds it increasingly difficult to identify the
radionuclides with a high degree of certainty.
14 Difficulties of Radiological and Nuclear Interdiction
radiation levels are due to an increase in general background radiation dose rates or
if the increase is in only one or two energies. An increase in background radiation
levels will show up across the entire energy spectrum while an increase due to a
single radioactive source will appear only in those energy channels associated with
that nuclide—662 keV, for example, if due to Cs-137. Thus, a mobile radiation
system might well identify the presence of a radionuclide (or, at least, might show a
suggestive energy peak) that would escape a person reading a relatively simple PRD.
Having said this, the user should never lose track of the possibility that an apparent
detection might not represent anything other than counting statistics, and the weaker
the apparent “signal” the more likely that this is the case.
14.1.2 Shielding
Shielding can affect detection and identification in two ways: by attenuating the
radiation emitted by a source and by changing the gamma energy spectrum as the
radiation passes through the shielding. The former effect can impair detection, the
latter can impair identification.
One important point to remember regarding radiation shielding is that, while it
can reduce the “signature” of a radioactive source, it is hard to eliminate the radiation
altogether. It is not difficult to reduce radiation dose rates or count rates by an order of
magnitude or more but each additional tenth value layer adds additional weight and
volume to the shield, making it easier to detect by virtue of its physical properties.
For example, an industrial radiography camera that contains a 2 TBq Cs-137 source
will weigh about 20–30 kg and will be about the size of a briefcase or a computer
bag. But this source will still be detectable to a distance of several meters to anybody
with a PRD, and to a much greater distance to a high-volume scintillation detector.
To reduce dose rates further would require additional layers of shielding that would
increase both the size and weight of the shield, with sources of higher activity the
shielding required to hide the source from detection can weigh enough to affect
the way a vehicle rides and handles; characteristics that police are accustomed to
evaluating (Fig. 14.2).
Shielding can also introduce changes to a nuclide’s spectrum, primarily through a
phenomenon known as Compton scattering. For example, consider a Cs-137 gamma
ray photon that is absorbed and re-emitted in a random direction by an electron in the
radiation shielding. When the photon is emitted it will have a slightly lower energy
than the original photon; perhaps it is emitted with an energy of, say, 650 keV instead
of the original 662 keV. This scattered photon can, itself, be absorbed and re-emitted
within the shield, and again and again—each time having a slightly lower energy.
As these photons continue being scattered and emerge from the shield the original
gamma peak will begin to “smear out” into lower energies. As additional shielding
is added the gamma peaks become less distinct, and the software that identifies
radionuclides based on gamma energy finds it increasingly difficult to identify the
radionuclides with a high degree of certainty.