2.3 Calculating Radiation Attenuation
17
Table 2.6 Distance to the 20 μGy hr −1 boundary for a variety of sources
Distance (meters) to a dose rate of 20 μGy hr −1a
Nuclide Source activity (Ci)
100
200 500 1000 2000 5000
Co-60
250
350 550 350
475
750
Cs-137 125
175 275 175
250
400
Ir-192
150
200 350 200
300
450
a In the US members of the general public are not permitted to have unrestricted access to areas in
which they might receive a dose of 20 μGy in one hour. Thus, it is not uncommon to establish a
boundary at this dose rate
2.3.2 Attenuation Due to Shielding
In addition, gamma radiation is absorbed by material; any material (including air)
between a person and a source of gamma radiation will reduce radiation exposure
somewhat, and some absorbers (e.g. lead) are more effective than are others (e.g.
water). In an emergency it is easier to use distance to reduce radiation exposure than
to install shielding, at the same time, it is often possible to use improvised shielding
or to take advantage of existing objects (e.g. masonry walls or buildings, vehicles,
and so forth). While there is a formal shielding equation, there might not be the time
or ability to perform such calculations during the emergency phase of a radiological
response. For this reason, it can be useful to use the concept of half-value and tenthvalue layers (HVL and TVL). The half-value layer is the thickness of a given material
that will reduce radiation exposure from a particular radionuclide by a factor of 2;
the TVL reduces radiation exposure by a factor of 10.
When using HVL and TVL to determine attenuation it is important to understand
that every combination of radionuclide and shielding material has a unique HVL
and TVL. The high-energy gamma radiation from Co-60 is more penetrating than is
the intermediate-energy Cs-137 gamma; it takes a greater thickness of lead to reduce
exposure from Co-60 than from Cs-137 so these two nuclides will have different HVL
and TVL values. Similarly, concrete is less dense and provides less shielding than
lead; accordingly, the TVL for concrete is greater than is the TVL for lead. Example
values for HVL and TVL for several important gamma-emitting radionuclides are
provided in Table 2.7.
Calculating radiation dose on the far side of the shielding using HVL and TVL
values is straight-forward; radiation exposure is reduced by a factor of 10 for each
TVL (and by a factor of 2 for each HVL) through which the radiation passes. So,
for example radiation passing through one HVL will be reduced by a factor of 2
while two HVLs will reduce radiation exposure by a factor of 2
2 (a factor of 4) and
three HVLs will reduce radiation exposure by a factor of 2
3 (a factor of 8). Similarly,
radiation passing through two TVLs will be reduced in intensity by a factor of 10
2 ;
to only 1% of the unshielded dose rate.
17
Table 2.6 Distance to the 20 μGy hr −1 boundary for a variety of sources
Distance (meters) to a dose rate of 20 μGy hr −1a
Nuclide Source activity (Ci)
100
200 500 1000 2000 5000
Co-60
250
350 550 350
475
750
Cs-137 125
175 275 175
250
400
Ir-192
150
200 350 200
300
450
a In the US members of the general public are not permitted to have unrestricted access to areas in
which they might receive a dose of 20 μGy in one hour. Thus, it is not uncommon to establish a
boundary at this dose rate
2.3.2 Attenuation Due to Shielding
In addition, gamma radiation is absorbed by material; any material (including air)
between a person and a source of gamma radiation will reduce radiation exposure
somewhat, and some absorbers (e.g. lead) are more effective than are others (e.g.
water). In an emergency it is easier to use distance to reduce radiation exposure than
to install shielding, at the same time, it is often possible to use improvised shielding
or to take advantage of existing objects (e.g. masonry walls or buildings, vehicles,
and so forth). While there is a formal shielding equation, there might not be the time
or ability to perform such calculations during the emergency phase of a radiological
response. For this reason, it can be useful to use the concept of half-value and tenthvalue layers (HVL and TVL). The half-value layer is the thickness of a given material
that will reduce radiation exposure from a particular radionuclide by a factor of 2;
the TVL reduces radiation exposure by a factor of 10.
When using HVL and TVL to determine attenuation it is important to understand
that every combination of radionuclide and shielding material has a unique HVL
and TVL. The high-energy gamma radiation from Co-60 is more penetrating than is
the intermediate-energy Cs-137 gamma; it takes a greater thickness of lead to reduce
exposure from Co-60 than from Cs-137 so these two nuclides will have different HVL
and TVL values. Similarly, concrete is less dense and provides less shielding than
lead; accordingly, the TVL for concrete is greater than is the TVL for lead. Example
values for HVL and TVL for several important gamma-emitting radionuclides are
provided in Table 2.7.
Calculating radiation dose on the far side of the shielding using HVL and TVL
values is straight-forward; radiation exposure is reduced by a factor of 10 for each
TVL (and by a factor of 2 for each HVL) through which the radiation passes. So,
for example radiation passing through one HVL will be reduced by a factor of 2
while two HVLs will reduce radiation exposure by a factor of 2
2 (a factor of 4) and
three HVLs will reduce radiation exposure by a factor of 2
3 (a factor of 8). Similarly,
radiation passing through two TVLs will be reduced in intensity by a factor of 10
2 ;
to only 1% of the unshielded dose rate.