Environmental Engineering Reference
In-Depth Information
which they travel. Ionizing radiation can travel only so far into matter, depending upon its energy
and character.
In alpha radiation, a whole nucleus of a helium atom, containing two protons and two neutrons,
is emitted. Because two protons of the original nucleus are lost (and consequently, two electrons
must be lost from the electronic orbitals in order to maintain electric neutrality), the daughter
isotope moves two elements backward from the parent element in the periodic table. For example,
the isotope 239 Pu disintegrates into the isotope 235 U with the emission of an
239 Pu
235 U
4 He
+ α(
222 Rn (which is a gas at normal
Another example is the disintegration of an isotope of radon
218 Po. The latter emits another
temperatures) into polonium
particle with the formation of a
stable isotope of lead 214 Pb:
222 Rn
218 Po
4 He
+ α(
218 Po
214 Pb
4 He
+ α(
particles are relatively heavy, their penetration depth into matter is very small, on the
order of a millimeter. A sheet of paper or a layer of dead skin on a person can stop
In beta radiation, an electron is emitted. This electron does not stem from the electronic
orbitals surrounding the nucleus, but from the nucleus itself: a neutron converts into a proton with
the emission of an electron. In that conversion, a parent isotope converts into a daughter isotope,
which is an element forward in the periodic table because a proton has been added to the nucleus.
An example is the decay of strontium 90 Sr into yttrium 90 Y:
90 Sr
90 Y
0 e
+ β(
Because 90 Sr is one of the products of uranium fission, this isotope is a major source of radiation
from the spent fuel of a nuclear reactor. The emitted electron is relatively light; therefore it can
penetrate deeper into matter, on the order of centimeters. To shield against
radiation, a plate of
metal, such as lead, is necessary.
Generally, the emission of
radiation is followed by the emission of gamma radiation . Because
the number of protons or neutrons does not change in that radiation, the radiating isotope does not
change its position in the periodic table. Gamma radiation is essentially the emission of very short-
wave, and hence energetic, electromagnetic radiation. An example is the emission of
from an isotope of cobalt 60 Co:
60 Co
60 Co
+ γ
radiation from 60 Co finds medicinal use, such as destroying cancer cells, and hence provides
a therapy for certain types of cancer. Because
rays carry no mass, its penetration into matter is
very deep, on the order of meters, and very heavy shielding is necessary to protect against it.
The depth of penetration of
radiation increases with the energy of the radiation. For
a 1-MeV energy, the penetration depths in water or tissue are 1E(
, and
3), 0.5, and 33 cm, respectively,
and 1E(
2), 7, and 55 cm for 10-MeV energy. Their penetration depths in air are about a thousand
times greater than in water.
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