15.1 The First 100 Min
165
approach to determining the presence of contamination at a nuclear power plant and
other radiological or nuclear facilities. One problem with this approach, however, is
that it needs to be implemented by personnel who have a fairly detailed understanding
of their instruments and what they normally read at their facility; in a large city with
variable background radiation levels, measured with a variety of instruments used
by personnel who are not full-time radiation safety workers such an approach might
be challenging to implement.
Different types of radiation detectors, for example, can have different readings
in the same location; a non-energy-compensated GM, for example, can easily have
a reading that is double the reading of a pressurized ionization chamber, both of
which might read differently than a PRD using a sodium iodide detector. While a
radiation safety professional would have the experience and training to understand
which of these instruments should be considered to be providing a more reliable
“background” reading, most emergency responders lack this training and might easily
not use an appropriate “background” reading to serve as the basis for determining
that a radiological release has occurred.
Consider, for example, a city in which typical background radiation dose rates are
between 0.05–0.010 µGy hr
−1 as measured with a pressurized ionization chamber.
A PRD that uses a cesium iodide crystal will tend to have higher readings, so a
police officer performing a survey using such an instrument might routinely get
readings of 0.10–0.15 µGy hr
−1 in these same areas. Which readings should the
officer use to determine “background” radiation dose rates and what level represents
a radiological release? One could make a good argument that double “background”
readings might be as low as 0.10 µGy hr
−1 or as high as 0.30 µGy hr
−1 , depending on
the instruments used. Further, radiation dose rates next to a granite building or wall
might be twice as high as the measured background radiation dose rates elsewhere in
the city. Lacking a detailed understanding of the differences between various types
of radiation instruments and the manner in which background readings can vary
from place to place in their city, it is entirely possible that a police officer who turns
their instrument on at the beginning of the day, noting a background reading of, say,
0.1 µGy hr
−1 at their precinct station might later find him or herself responding to
a small explosion near the granite wall mentioned earlier with a reading of 0.3 µGy
hr
−1 . Should this be reported as evidence of a radiological attack?
A different approach would be to conduct a thorough survey of the city, noting
the variations in radiation dose rates from place to place and determining not only
the typical radiation dose rates, but also the variability across the city, including “hot
spots” such as the granite wall noted above—and as measured with a number of
different instruments. These readings can then form the basis for determining the
radiation dose rate at which a radiological incident would be declared; given the
readings noted above, it would be reasonable to consider developing a city-wide
procedure under which any dose rate in excess of 1 µGy hr
−1 as measured by any
dose rate instrument would be considered evidence of a radiological incident, with
the circumstances and (possibly) additional information used to determine whether or
not the incident represented an accident, the routine use of radiation or radioactivity,
or an attack of some sort.
165
approach to determining the presence of contamination at a nuclear power plant and
other radiological or nuclear facilities. One problem with this approach, however, is
that it needs to be implemented by personnel who have a fairly detailed understanding
of their instruments and what they normally read at their facility; in a large city with
variable background radiation levels, measured with a variety of instruments used
by personnel who are not full-time radiation safety workers such an approach might
be challenging to implement.
Different types of radiation detectors, for example, can have different readings
in the same location; a non-energy-compensated GM, for example, can easily have
a reading that is double the reading of a pressurized ionization chamber, both of
which might read differently than a PRD using a sodium iodide detector. While a
radiation safety professional would have the experience and training to understand
which of these instruments should be considered to be providing a more reliable
“background” reading, most emergency responders lack this training and might easily
not use an appropriate “background” reading to serve as the basis for determining
that a radiological release has occurred.
Consider, for example, a city in which typical background radiation dose rates are
between 0.05–0.010 µGy hr
−1 as measured with a pressurized ionization chamber.
A PRD that uses a cesium iodide crystal will tend to have higher readings, so a
police officer performing a survey using such an instrument might routinely get
readings of 0.10–0.15 µGy hr
−1 in these same areas. Which readings should the
officer use to determine “background” radiation dose rates and what level represents
a radiological release? One could make a good argument that double “background”
readings might be as low as 0.10 µGy hr
−1 or as high as 0.30 µGy hr
−1 , depending on
the instruments used. Further, radiation dose rates next to a granite building or wall
might be twice as high as the measured background radiation dose rates elsewhere in
the city. Lacking a detailed understanding of the differences between various types
of radiation instruments and the manner in which background readings can vary
from place to place in their city, it is entirely possible that a police officer who turns
their instrument on at the beginning of the day, noting a background reading of, say,
0.1 µGy hr
−1 at their precinct station might later find him or herself responding to
a small explosion near the granite wall mentioned earlier with a reading of 0.3 µGy
hr
−1 . Should this be reported as evidence of a radiological attack?
A different approach would be to conduct a thorough survey of the city, noting
the variations in radiation dose rates from place to place and determining not only
the typical radiation dose rates, but also the variability across the city, including “hot
spots” such as the granite wall noted above—and as measured with a number of
different instruments. These readings can then form the basis for determining the
radiation dose rate at which a radiological incident would be declared; given the
readings noted above, it would be reasonable to consider developing a city-wide
procedure under which any dose rate in excess of 1 µGy hr
−1 as measured by any
dose rate instrument would be considered evidence of a radiological incident, with
the circumstances and (possibly) additional information used to determine whether or
not the incident represented an accident, the routine use of radiation or radioactivity,
or an attack of some sort.