154 Ecology and Applied Environmental Science
The decay rate of a radioactive isotope is calculated indirectly using
half-life, i.e. the time required for half the original mass of the isotope to
be decayed, whereupon the intensity of emission of the radioactive radiation also becomes half of what it was originally. The half-life is of great
importance regarding health impacts, because a short half-life means a
short lifetime of the isotope but a greater original intensity of radioactivity
per unit of mass, whereas a long half-life means the opposite. The intensity
of radioactivity corresponds to the number of decays per unit of time and
has nothing to do with the kinetic energy of emission of the radioactive
radiation, which is given and constant for every radioactive isotope. The
half-life is a characteristic measure of each isotope and varies widely. It may
be anything from a fraction of a second to billions of years. Frequently
encountered isotopes U-238, U-235, Pu-239, Cs-137, Sr-90 and I-131 have
half-lives of 4.5 billion years, 710 million years, 24,000 years, 30 years,
28.5 years and 8 days respectively (Table 7.2).
Half-life is different from biological half-life (BHL), i.e. the time a living
organism needs to eliminate from its body, through ordinary biological
Table 7.2 Radioisotopes (Radionuclides) That Are Present in
Organic Matter or Are Common Products of Fission
Radionuclide
Half-Life
Radioactivity
Argon
Ar 41
1827 hours
β –
Calcium
Ca 45
163 days
β –
Carbon
Ca 14
5570 years
β –
Cesium
CS 137
30 years
β – ,γ
Hydrogen (Tritium) H 3
12 years
β –
Iodine
I 131
8 days
β – ,γ
Iron
Fe 59
45 days
β – ,γ
Krypton
Kr 85
11 years
β –
Oxygen
O 15
122 seconds
β +
Phosphorus
P 32
14 days
β –
Plutonium
Pu 239
24,000 years
α, γ
Polonium
Po 210
138 days
α
Potassium
K 40
1.2 billion years
β – ,γ
Potassium
K 42
12 hours
β – ,γ
Radium
Ra 226
1600 years
α
Radon
Rn 222
4 days
α
Sodium
Na 24
15 hours
β – ,γ
Strontium
Sr 90
29 years
β –
Uranium
U 235
704 million years α
Uranium
U 238
4.5 billion years
α
Xenon
Xe 133
5 days
β – ,γ
The decay rate of a radioactive isotope is calculated indirectly using
half-life, i.e. the time required for half the original mass of the isotope to
be decayed, whereupon the intensity of emission of the radioactive radiation also becomes half of what it was originally. The half-life is of great
importance regarding health impacts, because a short half-life means a
short lifetime of the isotope but a greater original intensity of radioactivity
per unit of mass, whereas a long half-life means the opposite. The intensity
of radioactivity corresponds to the number of decays per unit of time and
has nothing to do with the kinetic energy of emission of the radioactive
radiation, which is given and constant for every radioactive isotope. The
half-life is a characteristic measure of each isotope and varies widely. It may
be anything from a fraction of a second to billions of years. Frequently
encountered isotopes U-238, U-235, Pu-239, Cs-137, Sr-90 and I-131 have
half-lives of 4.5 billion years, 710 million years, 24,000 years, 30 years,
28.5 years and 8 days respectively (Table 7.2).
Half-life is different from biological half-life (BHL), i.e. the time a living
organism needs to eliminate from its body, through ordinary biological
Table 7.2 Radioisotopes (Radionuclides) That Are Present in
Organic Matter or Are Common Products of Fission
Radionuclide
Half-Life
Radioactivity
Argon
Ar 41
1827 hours
β –
Calcium
Ca 45
163 days
β –
Carbon
Ca 14
5570 years
β –
Cesium
CS 137
30 years
β – ,γ
Hydrogen (Tritium) H 3
12 years
β –
Iodine
I 131
8 days
β – ,γ
Iron
Fe 59
45 days
β – ,γ
Krypton
Kr 85
11 years
β –
Oxygen
O 15
122 seconds
β +
Phosphorus
P 32
14 days
β –
Plutonium
Pu 239
24,000 years
α, γ
Polonium
Po 210
138 days
α
Potassium
K 40
1.2 billion years
β – ,γ
Potassium
K 42
12 hours
β – ,γ
Radium
Ra 226
1600 years
α
Radon
Rn 222
4 days
α
Sodium
Na 24
15 hours
β – ,γ
Strontium
Sr 90
29 years
β –
Uranium
U 235
704 million years α
Uranium
U 238
4.5 billion years
α
Xenon
Xe 133
5 days
β – ,γ