Environmental Engineering Reference
In-Depth Information
TABLE 6.1
Some Isotopes in the Nuclear Fuel
Cycle, with Half-Lives and Radiation
Isotope
t
1/2
Activity
Krypton-87
76 min
β
Tritium (
3
H)
12.3 y
β
Strontium-90
28.1 y
β
Cesium-137
30.2 y
β
Xenon-135
9.2 h
β
and
γ
Barium-139
82.9 min
β
and
γ
Radium-223
11.4 d
α
and
γ
Radium-226
1600 y
α
and
γ
Thorium-232
1.4E(10) y
α
and
γ
Thorium-233
22.1 min
β
Uranium-233
1.65E(5) y
α
and
γ
Uranium-235
7.1E(8) y
α
and
γ
α
and
γ
Uranium-238
4.5E(9) y
β
γ
Neptunium-239
2.35 d
and
Plutonium-239
2.44E(4) y
α
and
γ
which is defined as 3.7E(10) Bq, or 3.7E(10) disintegrations per second. The radioactivity of one
gram of radium-233 is 1 Ci, and that of cobalt-60 is 1 kCi.
11
But for mixtures of radioactive
isotopes, such as found in ore samples or spent reactor fuel, the radioactive level measured in
curies cannot tell us the amount or composition of the radioactive components, only their total
disintegration rate.
Because exposure of humans to
radiation can be harmful, we need practical units
of measurement of exposure to them. The SI unit of absorbed dose of radiation is the gray (Gy),
which equals one joule of absorbed energy per kilogram of matter penetrated by the radiation.
Another commonly used unit of absorbed dose is the rad, which equals 1E(
α
,
β
, and
γ
2) Gy.
12
The absorbed energy is not entirely satisfactory as a measure of the harmfulness to humans
of ionizing radiation, because other qualities of the radiation are also important. To take these into
account, a different unit, the sievert (Sv), is used to measure what is called the dose equivalent. Like
the gray, it has the dimensions of J/kg. The Sievert takes into account the quality of the absorbed
radiation. An equivalent dose of 1 Sv is received when the actual dose of radiation (measured in
grays), after being multiplied by the dimensionless factors
Q
(the so-called quality factor) and
N
(the product of any other multiplying factors), is 1 J/kg.
Q
depends on the nature of radiation and
has a value of 1 for X rays,
−
γ
β
α
particles.
N
is a
factor that takes into account the distribution of energy throughout the dose. An alternative unit of
dose equivalent is the rem, defined as 1E(
rays, and
particles; 10 for neutrons; and 20 for
−
2) Sv.
11
The specific radioactivity level of a sample is obtained by dividing the radioactivity level by the mass or
volume of the sample (e.g., Ci/kg or Ci/L).
12
In the treatment of cancer with radiation, the absorbed dose needed to kill cancer cells is of the order of
100 Gy.
