14.3 Instruments and Their Maintenance
157
What this means is that any attempt to interdict a nuclear weapon—improvised
or otherwise—will be heavily dependent on having appropriate radiation detection
instruments and using them properly, as well as properly maintaining them.
Given the relatively weak signal from a uranium weapon, large-volume gamma
detectors are most likely to be successful in detecting a nuclear weapon, if only
because alpha and beta radiation have such a short range and the neutron signature is
so small. A plutonium weapon might be easier to detect via both gamma and neutron
emissions; however the need for a more complex implosion design for plutonium
weapons makes this design less likely for a terrorist group fabricating a weapon [10].
Maintaining one’s instruments is as important as is having the correct detectors,
and any agency that is purchasing radiation instruments must include these costs in
their budget in addition to the costs of instrument replacement and operator training.
Instrument maintenance includes both preventative and corrective maintenance as
well as initial and periodic calibrations to ensure that the meaning of various instrument readings is clear and can be interpreted properly. Consider, for example, the
problem discussed in Sect. 12.3, in which instrument readings were used to help to
quantify the activity of a number of interdicted sources. In order for such calculations
to be accurate, there must be a way for the person performing the calculations to know
the detection efficiency of their radiation detectors for the nuclide(s) in question at
a variety of distances—the counting efficiency at a distance of, say, 10 m will be
very different from that at an altitude of 150 m or more due to the increased distance
as well as to the attenuating effects of 150 m of air. For this reason, it might be
necessary to calibrate airborne instruments by making repeated measurements of the
same source or the same area at a variety of altitudes, comparing airborne readings
with those made on the ground using properly calibrated instruments.
In addition to performing these initial calibrations, instruments that are used regularly should also be checked or calibrated periodically to ensure they are operating
properly. One reason for this is to ensure the instrument is responding properly to
radiation, to ensure that alarms sound at the proper radiation dose rate, and (in some
jurisdictions) for evidentiary reasons as well; to be able to demonstrate to a judge
and/or jury (for example) that elevated radiation readings are most likely the result
of the presence of radioactive materials rather than a malfunctioning instrument.
There are several levels of periodic checks that should be performed on radiation
instruments with varying intervals:
• Bump checks use a low-activity radioactive source to confirm that the radiation
detector responds to the presence of radiation. This need not be a calibrated or
characterized source, it only need be capable of causing the instrument to respond.
To check a sodium iodide (gamma) detector, one may use a low-activity (a few
tens of kBq) gamma source, a radioactive mineral sample, thoriated welding
electrodes, and so forth (a neutron instrument will require a neutron source),
provided the source used emits enough radiation to cause the meter to deflect or
to register more than a few standard deviations higher than normal background
levels. Bump checks should be done daily or weekly when an instrument is in use.
157
What this means is that any attempt to interdict a nuclear weapon—improvised
or otherwise—will be heavily dependent on having appropriate radiation detection
instruments and using them properly, as well as properly maintaining them.
Given the relatively weak signal from a uranium weapon, large-volume gamma
detectors are most likely to be successful in detecting a nuclear weapon, if only
because alpha and beta radiation have such a short range and the neutron signature is
so small. A plutonium weapon might be easier to detect via both gamma and neutron
emissions; however the need for a more complex implosion design for plutonium
weapons makes this design less likely for a terrorist group fabricating a weapon [10].
Maintaining one’s instruments is as important as is having the correct detectors,
and any agency that is purchasing radiation instruments must include these costs in
their budget in addition to the costs of instrument replacement and operator training.
Instrument maintenance includes both preventative and corrective maintenance as
well as initial and periodic calibrations to ensure that the meaning of various instrument readings is clear and can be interpreted properly. Consider, for example, the
problem discussed in Sect. 12.3, in which instrument readings were used to help to
quantify the activity of a number of interdicted sources. In order for such calculations
to be accurate, there must be a way for the person performing the calculations to know
the detection efficiency of their radiation detectors for the nuclide(s) in question at
a variety of distances—the counting efficiency at a distance of, say, 10 m will be
very different from that at an altitude of 150 m or more due to the increased distance
as well as to the attenuating effects of 150 m of air. For this reason, it might be
necessary to calibrate airborne instruments by making repeated measurements of the
same source or the same area at a variety of altitudes, comparing airborne readings
with those made on the ground using properly calibrated instruments.
In addition to performing these initial calibrations, instruments that are used regularly should also be checked or calibrated periodically to ensure they are operating
properly. One reason for this is to ensure the instrument is responding properly to
radiation, to ensure that alarms sound at the proper radiation dose rate, and (in some
jurisdictions) for evidentiary reasons as well; to be able to demonstrate to a judge
and/or jury (for example) that elevated radiation readings are most likely the result
of the presence of radioactive materials rather than a malfunctioning instrument.
There are several levels of periodic checks that should be performed on radiation
instruments with varying intervals:
• Bump checks use a low-activity radioactive source to confirm that the radiation
detector responds to the presence of radiation. This need not be a calibrated or
characterized source, it only need be capable of causing the instrument to respond.
To check a sodium iodide (gamma) detector, one may use a low-activity (a few
tens of kBq) gamma source, a radioactive mineral sample, thoriated welding
electrodes, and so forth (a neutron instrument will require a neutron source),
provided the source used emits enough radiation to cause the meter to deflect or
to register more than a few standard deviations higher than normal background
levels. Bump checks should be done daily or weekly when an instrument is in use.