2.1 What Radiation and Radioactivity Are
11
• Since there are about 3.156 × 10
7 s in a year, this comes out to about 4.2
× 10
13 decays per second, or about 42 TBq.
There are a number of measurements whose change can be calculated in this same
manner. Contamination levels, for example, are a measure of the amount of radioactivity per unit area (e.g. decays per second or decays per minute in a 100 cm
2 area);
as the contaminating radionuclide decays, contamination levels will drop steadily.
In a medical setting, for example, it is not uncommon to see contamination from
Tc-99 m, a nuclide with a half-life of only about 6 h. With so short a half-life it
often makes sense to simply lock the door to the room and let the contamination
decay away over the space of a few days than it does to devote resources towards
decontamination.
Example: calculating the decay of a Co-60 source
A radioactive materials licensee was cleaning out storage cabinets and found a
25-year-old Co-60 source with an original activity of 500 mCi (18.5 GBq). The
licensee, not wanting to overpay for the source’s disposal, needed to calculate
the decayed activity of the source.
A t = A 0 × e
−λt
We know from the previous example that the decay constant for Co-60 is
0.1315 yr
−1 (from the previous text box) so the calculation is:
A t = 18.5 GBq × e
−(0.1315yr
−1
)×(25yr)
The decayed activity was calculated to be 0.691 GBq, or 691 MBq—considerably lower than the original activity and much less expensive to dispose
of.
Similarly, as we will see in the next section, the radiation dose rate from a beta or
gamma source depends on the amount of radioactivity present; as these source decay
to lower activities the radiation dose rate drops as well. Thus, the radioactive decay
equation can be used to calculate the reduction in radiation or contamination levels
over time.
2.2 Radiation Dose and Dose Rate Determinations
When considering the health effects of radiation exposure the single most relevant
fact is the amount of radiation dose a person, animal, or organ has received. Some
11
• Since there are about 3.156 × 10
7 s in a year, this comes out to about 4.2
× 10
13 decays per second, or about 42 TBq.
There are a number of measurements whose change can be calculated in this same
manner. Contamination levels, for example, are a measure of the amount of radioactivity per unit area (e.g. decays per second or decays per minute in a 100 cm
2 area);
as the contaminating radionuclide decays, contamination levels will drop steadily.
In a medical setting, for example, it is not uncommon to see contamination from
Tc-99 m, a nuclide with a half-life of only about 6 h. With so short a half-life it
often makes sense to simply lock the door to the room and let the contamination
decay away over the space of a few days than it does to devote resources towards
decontamination.
Example: calculating the decay of a Co-60 source
A radioactive materials licensee was cleaning out storage cabinets and found a
25-year-old Co-60 source with an original activity of 500 mCi (18.5 GBq). The
licensee, not wanting to overpay for the source’s disposal, needed to calculate
the decayed activity of the source.
A t = A 0 × e
−λt
We know from the previous example that the decay constant for Co-60 is
0.1315 yr
−1 (from the previous text box) so the calculation is:
A t = 18.5 GBq × e
−(0.1315yr
−1
)×(25yr)
The decayed activity was calculated to be 0.691 GBq, or 691 MBq—considerably lower than the original activity and much less expensive to dispose
of.
Similarly, as we will see in the next section, the radiation dose rate from a beta or
gamma source depends on the amount of radioactivity present; as these source decay
to lower activities the radiation dose rate drops as well. Thus, the radioactive decay
equation can be used to calculate the reduction in radiation or contamination levels
over time.
2.2 Radiation Dose and Dose Rate Determinations
When considering the health effects of radiation exposure the single most relevant
fact is the amount of radiation dose a person, animal, or organ has received. Some