2.2 Radiation Dose and Dose Rate Determinations
15
gammas emitted per second and to use that information to calculate the radiation
dose from that source. In fact, these calculations have been performed for a large
number of gamma-emitting radionuclides—the answer to this calculation is called
the gamma constant and it typically presented in units of Sv hr
−1 for each MBq at a
distance of 1 m.
For example, the gamma constant for Cs-137/Ba-137 m is 7.789 × 10
–5 mSv hr
−1
per MBq at a distance of 1 m. So the dose rate from a 37 MBq source would be
0.00288 Sv/h or 2.88 mSv/h at a distance of 1 m from the source.
2.2.6 Allowable Limit for Intake (ALI)
Just as one can calculate radiation exposure from external gamma radiation, so can
one calculate the radiation exposure from radioactivity that is inhaled or ingested.
Performing these calculations is much more complex as it requires adding in the
energies from beta radiation and understanding the biokinetics of the chemical form
of the nuclide(s) in question. This information has been used to develop the concept
of the Allowable Limit for Intake (ALI); the ALI is the amount of inhaled or ingested
radionuclide that will produce a radiation exposure of 50 mSv (5 rem) to the whole
body or 500 mSv (50 rem) to the most-exposed internal organ. Thus, if the amount
of intake is known, one can calculate the amount of radiation exposure received by
comparing the intake to the ALI. An intake of 1 ALI will produce a dose of 50 mSv,
a dose of 3 ALI will produce a dose of 150 mSv, a dose of 15 mSv will result from
an intake of 0.3 ALI [7].
The greatest difficulty in calculating radiation exposure using the ALI is in quantifying the amount of intake. This can be done in the laboratory—analyzing urine or
fecal samples, performing chest or whole-body counting, or other technique—or a
rough assessment can be made in the field using a quick field assessment technique
[5].
2.3 Calculating Radiation Attenuation
Calculating or measuring radiation exposure is a start, but it does not take into
account how that radiation exposure is affected by changes in distance or when
radiation shielding is put between the radiation source and a person. Understanding
how distance and shielding affect radiation exposure can help to determine the length
of time responders can safely work in a given area, how closely they can approach
a radioactive source, and how to help protect them during response to a radiological
or nuclear emergency. In addition, physicians and other medical responders can
use these principles to help minimize their own radiation exposure when caring for
heavily contaminated patients or those with embedded radioactive materials.
15
gammas emitted per second and to use that information to calculate the radiation
dose from that source. In fact, these calculations have been performed for a large
number of gamma-emitting radionuclides—the answer to this calculation is called
the gamma constant and it typically presented in units of Sv hr
−1 for each MBq at a
distance of 1 m.
For example, the gamma constant for Cs-137/Ba-137 m is 7.789 × 10
–5 mSv hr
−1
per MBq at a distance of 1 m. So the dose rate from a 37 MBq source would be
0.00288 Sv/h or 2.88 mSv/h at a distance of 1 m from the source.
2.2.6 Allowable Limit for Intake (ALI)
Just as one can calculate radiation exposure from external gamma radiation, so can
one calculate the radiation exposure from radioactivity that is inhaled or ingested.
Performing these calculations is much more complex as it requires adding in the
energies from beta radiation and understanding the biokinetics of the chemical form
of the nuclide(s) in question. This information has been used to develop the concept
of the Allowable Limit for Intake (ALI); the ALI is the amount of inhaled or ingested
radionuclide that will produce a radiation exposure of 50 mSv (5 rem) to the whole
body or 500 mSv (50 rem) to the most-exposed internal organ. Thus, if the amount
of intake is known, one can calculate the amount of radiation exposure received by
comparing the intake to the ALI. An intake of 1 ALI will produce a dose of 50 mSv,
a dose of 3 ALI will produce a dose of 150 mSv, a dose of 15 mSv will result from
an intake of 0.3 ALI [7].
The greatest difficulty in calculating radiation exposure using the ALI is in quantifying the amount of intake. This can be done in the laboratory—analyzing urine or
fecal samples, performing chest or whole-body counting, or other technique—or a
rough assessment can be made in the field using a quick field assessment technique
[5].
2.3 Calculating Radiation Attenuation
Calculating or measuring radiation exposure is a start, but it does not take into
account how that radiation exposure is affected by changes in distance or when
radiation shielding is put between the radiation source and a person. Understanding
how distance and shielding affect radiation exposure can help to determine the length
of time responders can safely work in a given area, how closely they can approach
a radioactive source, and how to help protect them during response to a radiological
or nuclear emergency. In addition, physicians and other medical responders can
use these principles to help minimize their own radiation exposure when caring for
heavily contaminated patients or those with embedded radioactive materials.