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2 Types of Radiation and Their Properties
2.3.1 Attenuation Due to Distance
One can use the gamma constant to calculate radiation exposure at a distance of
1 m (or some other reference distance from a source), but it is not likely that any
emergency responder will be spending a great deal of time at exactly that distance.
Luckily, the manner in which distance affects gamma radiation exposure is wellunderstood and the calculations are not difficult.
Radiation dose rate drops as the inverse square of distance from a source. Thus,
if one doubles their distance from a source the dose rate is reduced by a factor of 4
and tripling the distance reduces exposure by a factor of 9. Moving towards a source
causes dose rate to increase in the same fashion.
So—when flying radiological interdiction missions the recommended altitude is
about 100 m. However, when the author flew such missions over Manhattan’s highrise district it was necessary to fly at an altitude of about 500 m or higher to avoid
flying into the sides of the skyscrapers. This was desirable from the standpoint of
safety, but it reduced the ability to locate weak sources of radiation because the dose
rate was reduced by a factor of 5
2 (25) at the higher altitude.
The inverse square law can also be used during an emergency response. A person
standing one meter from a high-activity (37 TBq) source of Co-60 will expose a
person to a fatal dose of radiation in about 40 min. Moving just one meter—one
large step—away from the source extends that time to nearly three hours. A second
large step increases the time even further, to about five or six hours.
Finally, the inverse square law can be used to determine the location of radiation
safety boundaries. Say, for example, the Incident Commander wishes to establish a
“Hot Zone” boundary at a radiation dose rate of 20 μGy/h, and is measuring a dose
rate of 0.5 μGy/h at a distance of 250 m from the source. The ratio of these dose
rates (20/0.5) is 40 so the Hot Zone boundary should be established at a distance that
is closer by a factor of the square root of 40 (about 6.3). Dividing 250 by 6.3 gives a
distance of just slightly less than 40 m—the Incident Commander should establish
the boundary no closer than about 40 m from the source.
Using the concepts of the gamma constant and attenuation due to distance makes
it possible to construct a table such as the one below, in which the gamma constant
for each of three radionuclides can be used to calculate the dose rate at a distance of
one meter with the inverse square law used to determine the dose rate at a variety of
distances as shown in Table 2.6 [4].
These calculations use gamma constants of 1.14 R hr
−1 for Co-60, 0.301 R hr
−1
for Cs-137, and 0.424 R hr
−1 for Ir-192, all at a distance of 1 m from the source.
Units are in the American system because of the source from which the table was
taken. To convert activity to the SI system, 37 × 10
9 Bq = 1 Ci.
2 Types of Radiation and Their Properties
2.3.1 Attenuation Due to Distance
One can use the gamma constant to calculate radiation exposure at a distance of
1 m (or some other reference distance from a source), but it is not likely that any
emergency responder will be spending a great deal of time at exactly that distance.
Luckily, the manner in which distance affects gamma radiation exposure is wellunderstood and the calculations are not difficult.
Radiation dose rate drops as the inverse square of distance from a source. Thus,
if one doubles their distance from a source the dose rate is reduced by a factor of 4
and tripling the distance reduces exposure by a factor of 9. Moving towards a source
causes dose rate to increase in the same fashion.
So—when flying radiological interdiction missions the recommended altitude is
about 100 m. However, when the author flew such missions over Manhattan’s highrise district it was necessary to fly at an altitude of about 500 m or higher to avoid
flying into the sides of the skyscrapers. This was desirable from the standpoint of
safety, but it reduced the ability to locate weak sources of radiation because the dose
rate was reduced by a factor of 5
2 (25) at the higher altitude.
The inverse square law can also be used during an emergency response. A person
standing one meter from a high-activity (37 TBq) source of Co-60 will expose a
person to a fatal dose of radiation in about 40 min. Moving just one meter—one
large step—away from the source extends that time to nearly three hours. A second
large step increases the time even further, to about five or six hours.
Finally, the inverse square law can be used to determine the location of radiation
safety boundaries. Say, for example, the Incident Commander wishes to establish a
“Hot Zone” boundary at a radiation dose rate of 20 μGy/h, and is measuring a dose
rate of 0.5 μGy/h at a distance of 250 m from the source. The ratio of these dose
rates (20/0.5) is 40 so the Hot Zone boundary should be established at a distance that
is closer by a factor of the square root of 40 (about 6.3). Dividing 250 by 6.3 gives a
distance of just slightly less than 40 m—the Incident Commander should establish
the boundary no closer than about 40 m from the source.
Using the concepts of the gamma constant and attenuation due to distance makes
it possible to construct a table such as the one below, in which the gamma constant
for each of three radionuclides can be used to calculate the dose rate at a distance of
one meter with the inverse square law used to determine the dose rate at a variety of
distances as shown in Table 2.6 [4].
These calculations use gamma constants of 1.14 R hr
−1 for Co-60, 0.301 R hr
−1
for Cs-137, and 0.424 R hr
−1 for Ir-192, all at a distance of 1 m from the source.
Units are in the American system because of the source from which the table was
taken. To convert activity to the SI system, 37 × 10
9 Bq = 1 Ci.