Page 158 - Soil and water contamination, 2nd edition

Radionuclides 145

Thus, the number of nuclei present decreases exponentially with time. Usually, for
radionuclides the decay rate is expressed as a half-life T, that is the time required for half the
amount initially present at time t = 0 to disintegrate. It can easily be shown that

T ln( ) 2 . 0 693 (8.3)

Half-lives for various elements can range from milliseconds to billions of years. Elements
with a short half-life are more radioactive than elements with a long half-life. On the other
hand, elements with a long half-life remain radioactive for a long period. Besides the half-
life and associated decay rate, another factor that determines the environmental hazard of
radionuclides is the nature and intensity of the ionising radiation emitted during decay of the
atomic nuclei. The three forms of ionising radiation of principal interest in environmental
pollution are 1) alpha radiation , consisting of positively charged helium nuclei, 2) beta
radiation , consisting of electrons or positrons (i.e. positively charged electrons), and 3)
gamma radiation , consisting of a discrete quantity of electromagnetic energy without mass
or charge. The energy associated with radiation is usually expressed in mega-electron-volts
(MeV). One eV is the energy acquired when a particle carrying unit electronic charge is
-19
accelerated through a potential difference of 1 volt (1 eV = 1.6022⋅10 J).
4
Alpha particles are the nuclei of stable He atoms and are emitted in a process called alpha
decay, which only occurs in the nuclei of heavy elements. In this process, the α emission from
an initial nucleus (Z, A) leads to the formation of a product nucleus (Z – 2, A – 4). In general,
the emission energy of most alpha particles from natural radioactivity is in the range from
4 to 11 MeV. Along their travel path, alpha particles interact with matter by the transfer of
kinetic energy to atomic orbital electrons during collision. As a consequence, one or more
electrons are ejected from their orbit, leaving the atoms as cations . The energy transfer during
each collision is relatively small, so the alpha particle remains in a straight line until its energy
is dissipated. The range or travel distance of an alpha particle is very short and depends on its
initial energy. In gases, the distance travelled is in the order of several centimetres, whereas in
high-density solid materials the range amounts to hundredths of millimetres.
Beta particles are electrons and positrons emitted from nuclei in a process known as beta
-
decay, which occurs both in heavy and light elements. Electron emission (β emission) is
initiated by the conversion of a neutron into a proton inside the nucleus and yields a product
+
nucleus (Z + 1, A). On the other hand, positron emission (β emission) is produced by the
conversion of a proton into a neutron inside the nucleus and gives a product nucleus (Z – 1,
A). The emission energy of beta particles is approximately between 0.05 and 4 MeV. The
principal mechanisms of interaction of beta particles with materials are basically the same
as those of alpha particles. Beta particles also eject electrons from their atomic shield. Their
range depends on the emission energy. In air, the range is about 1 m for particles having
an emission energy of 0.5 MeV and about 10 m for those of 3.0 MeV. Air is, however, an
inconvenient absorbing medium for beta particles. Solids absorb beta particles much more
effectively. The range of beta particles in solid material is in the order of a few millimetres
and does not depend strongly on the absorbing material.
Gamma radiation (or photons) consists of electromagnetic waves with very short
wavelengths and, therefore, a high energy. Gamma radiation is emitted from the nuclei
together with the emission of an alpha or beta particle, or due to the transfer from a higher to
a lower energy state. It can also be produced as a consequence of the collision and subsequent
annihilation of an electron–positron pair. The typical energy range for most photons resulting
from radioactive decay is about 1 keV to 2 MeV, but may be larger. Gamma rays interact with
matter in various ways. The three main mechanisms are photoelectric absorption, Compton
scattering, and electron–positron pair production. Photoelectric absorption involves the

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