13.7 Conducting Interdiction Surveys
141
13.7.1 Survey Speed
When surveying for radiation the general rule of thumb is that slower is better; a
radioactive source must be within range of the detector long enough to be “seen.” If
a detector moves too quickly over a source it might only detect a handful of counts
that might simply look like a brief fluctuation in background count or dose rates.
In addition, the further one is from a radioactive source the more slowly one must
survey because radiation levels drop with the inverse square law. For this reason,
aerial surveys using high-volume sodium iodide detectors are typically done at an
altitude of about 100 m and a ground speed of 60–100 knots (about 100–200 km
hr
−1 ). If a possible source is detected from a helicopter the pilot can slow down
or even hover to try to identify the radionuclide(s) present and to narrow down the
possible location; fixed-wing aircraft, unfortunately, cannot hover and can only slow
down to about 100–150 km hr
−1 without falling out of the sky. This can place limits
on the lower activity of a source that can be detected by a fixed-wing aircraft.
Performing a survey from a vehicle also requires a relatively modest survey speed;
at the same time, driving too slowly can slow down traffic and can even be dangerous,
so speed limits and traffic patterns can interfere with survey quality—performing a
survey from a vehicle on the highway can be even more difficult. Surveying on foot
is unlikely to proceed so quickly as to miss a radioactive source. However, it is not
uncommon for personnel who are doing a walk-over survey to slow considerably
when they first see elevated radiation dose rates or receive an alarm. Unfortunately,
given the large distances at which a moderate- or high-activity source can be detected,
this can lead to an excessive delay in reaching the location of the actual source.
Consider, for example, a police officer who slows to a very slow speed once their
radiation detector alarms…on a source that is over 1 km distant…compared to a
police officer who continues walking at a normal pace as they watch radiation dose
rates increase and then drop again when they pass the source. In the first case it might
take hours to reach the source while, in the second, the approximate location will be
found in less than a half hour.
13.7.2 Survey Pattern
In the majority of cases the most certain way to find a source is to conduct a survey
according to a plan, and that plan will typically involve surveying according to a
pattern. Following that survey plan—and survey pattern—will usually take time, but
is certain to ultimately find any detectable source in the search area.
The easiest search pattern to follow is a simple grid search—traveling parallel
search lines spaced such that the lowest-activity source of concern can be detected
from halfway between the search lines. So, for example, the 500 GBq Ir-192 source
noted earlier in this chapter, with a detection distance of 882 meters, would call for
search lines as much as 1750 meters apart. In reality, however, it is reasonable to
141
13.7.1 Survey Speed
When surveying for radiation the general rule of thumb is that slower is better; a
radioactive source must be within range of the detector long enough to be “seen.” If
a detector moves too quickly over a source it might only detect a handful of counts
that might simply look like a brief fluctuation in background count or dose rates.
In addition, the further one is from a radioactive source the more slowly one must
survey because radiation levels drop with the inverse square law. For this reason,
aerial surveys using high-volume sodium iodide detectors are typically done at an
altitude of about 100 m and a ground speed of 60–100 knots (about 100–200 km
hr
−1 ). If a possible source is detected from a helicopter the pilot can slow down
or even hover to try to identify the radionuclide(s) present and to narrow down the
possible location; fixed-wing aircraft, unfortunately, cannot hover and can only slow
down to about 100–150 km hr
−1 without falling out of the sky. This can place limits
on the lower activity of a source that can be detected by a fixed-wing aircraft.
Performing a survey from a vehicle also requires a relatively modest survey speed;
at the same time, driving too slowly can slow down traffic and can even be dangerous,
so speed limits and traffic patterns can interfere with survey quality—performing a
survey from a vehicle on the highway can be even more difficult. Surveying on foot
is unlikely to proceed so quickly as to miss a radioactive source. However, it is not
uncommon for personnel who are doing a walk-over survey to slow considerably
when they first see elevated radiation dose rates or receive an alarm. Unfortunately,
given the large distances at which a moderate- or high-activity source can be detected,
this can lead to an excessive delay in reaching the location of the actual source.
Consider, for example, a police officer who slows to a very slow speed once their
radiation detector alarms…on a source that is over 1 km distant…compared to a
police officer who continues walking at a normal pace as they watch radiation dose
rates increase and then drop again when they pass the source. In the first case it might
take hours to reach the source while, in the second, the approximate location will be
found in less than a half hour.
13.7.2 Survey Pattern
In the majority of cases the most certain way to find a source is to conduct a survey
according to a plan, and that plan will typically involve surveying according to a
pattern. Following that survey plan—and survey pattern—will usually take time, but
is certain to ultimately find any detectable source in the search area.
The easiest search pattern to follow is a simple grid search—traveling parallel
search lines spaced such that the lowest-activity source of concern can be detected
from halfway between the search lines. So, for example, the 500 GBq Ir-192 source
noted earlier in this chapter, with a detection distance of 882 meters, would call for
search lines as much as 1750 meters apart. In reality, however, it is reasonable to