Page 159 - Soil and water contamination, 2nd edition

146 Soil and Water Contamination

energy transfer from the photon to an orbital electron, which is ejected from its atom. This
process tends to predominate at low energies below about 500 keV. In Compton scattering,
the gamma ray collides with an electron, transferring part of its energy to the electron,
while itself being scattered at a reduced energy. This process tends to predominate at higher
energies above 500 keV. In electron–positron pair production, a gamma ray of sufficient
energy (equal or greater than 1.02 MeV, which is the residual mass energy equivalent of the
pair) disappears, resulting in the creation of an electron and a positron. The penetration of
gamma rays is much greater than that of alpha or beta particles. In solid materials, the depth
over which about 99 percent of the initial gamma-ray energy is dissipated is in the order of
a decimetre and is inversely related to the density of the absorbing material and the initial
energy of the gamma rays.
Radioactivity is both natural and man-made. At present, the radioactive elements
which occur near the Earth’s surface include radionuclides having half-lives in the order
of the age of the Earth (about 5 billion years) or longer (e.g. uranium -238 (half-life
10
9
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4.5·10 y), thorium -232 (half-life 1.4·10 y), potassium -40 (half-life 1.3·10 y)) together
with their decay products. These are commonly known as the terrestrial sources of natural
radioactivity. In addition, radionuclides may also be derived from the effects of cosmic
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radiation : for example, the formation of naturally occurring radiocarbon, C. These are
known as cosmogenic radiation. The sum of terrestrial radiation, cosmogenic radiation,
and the portion cosmic radiation that reaches the Earth’s surface is usually referred to as
background radiation . Naturally occurring radiation levels have been anthropogenically
enhanced due to mining of natural uranium and phosphate rocks (used as fertilisers ), fossil
fuel combustion, and the production of man-made radionuclides through nuclear reactions
in particle accelerators, nuclear reactors, and nuclear weapons . The most important source of
man-made environmental radioactivity is the process of nuclear fission of certain elements.
The fission process implies the break-up of heavy elements such as uranium, plutonium , or
americium, into smaller nuclei (e.g. strontium-90, caesium-137, and iodine-131) through
the bombardment with neutrons in a nuclear fission reactor. Nuclear fission is primarily
used in nuclear power plants to generate electricity, but also in nuclear weapons to generate
an enormous explosive power. Another source is the production of radioisotopes through
neutron activation , i.e. the process of capture of neutrons in the nuclei of elements. Some
of these radioisotopes are produced in nuclear reactors for medical, industrial, scientific,
or military purposes; others arise as radioactive waste . In addition, radionuclides may be
produced by bombarding nuclei with charged particles, such as protons and alpha particles.
Man-made radioactivity is dispersed in the environment due to discharges under normal
operation of nuclear reactors, radioactive waste disposal, accidental releases, and nuclear
explosions.
Radioactive material is considered to be an environmental hazard because its ionising
radiation affects the cells of biota, especially animals and humans, mainly because of the
damage it can cause to vital enzymes, chromosomes, and hormones. Typically, the cells most
easily harmed are those that divide most rapidly (e.g. white blood cells, bone marrow, hair
follicles, and foetuses). The magnitude of the damage caused by ionising radiation depends
on the type of radiation, its energy, and whether the exposure is internal or external. The
hazard to humans from exposure to alpha particles is insignificant if the exposure is external.
In this case the particles are absorbed by the outer layer of the skin or clothing. However, if
the exposure is internal due to inhalation or ingestion, alpha radiation may severely damage
sensitive internal organs. In contrast to alpha particles, when considering the potential hazard
of beta particles and gamma radiation to humans it is necessary to take into account both the
internal and external exposure. The exposure to radiation can be measured as the absorbed
dose , i.e. the energy imparted by ionising radiation per unit mass of irradiated material.
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The unit for absorbed dose is gray (Gy): one gray is equal to an absorbed dose of 1 J kg

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