Agriculture Reference
In-Depth Information
γ-radiation consists of high-energy photons, from a few keV to less than 3 MeV.
Their penetrating power is greater than the previous particles, and they require
several centimetres of lead to be absorbed. This type radiation is frequently emitted
immediately after α- or β-radiation.
40
K,
137
Cs, and
226
Ra, among others, are also γ-
emitting radionuclides.
Systems to detect radionuclides are different for each type of radiation, because they have
different characteristics. Due to the low penetrating power of α and β particles, it is necessary
to perform radiochemical separation procedures for their proper assay. In the case of γ-
radiation, however, such radiochemical separations are usually unnecessary. The
determination of α-emitting radionuclides is usually by means of silicon detectors within a
vacuum chamber. Gas-flow proportional counters and LSC (liquid scintillation counting) are
used to determine both β-particles and α-particles. The determination of γ-emitting
radionuclides is by means of NaI(Tl) and HP(Ge) detectors.
S
OURCES OF
R
ADIONUCLIDES IN THE
E
NVIRONMENT
The radionuclides occurring in the environment can be classified into two main groups:
natural and anthropogenic (or man-made). The naturally occurring radionuclides comprise
those belonging to the natural radioactive decay series,
40
K, and cosmogenic radionuclides,
which are formed by the interaction of high-energy cosmic rays with atomic nuclei in the
atmosphere. There are three natural radioactive series present in nature: thorium, uranium,
and actinium, also denominated A = 4n, A = 4n+2, and A = 4n+3, where A is the mass
number of the radionuclide. These natural series begin with
232
Th (thorium series),
238
U
(uranium series), and
235
U (actinium series), and finish with
208
Pb,
206
Pb, and
207
Pb,
respectively. The half-lives of these radionuclides are usually very large, 4.468·10
9
, 7.04·10
8
,
and 1.405·10
10
y for
238
U,
235
U, and
232
Th, respectively. Figure 1 shows the decay chain of the
uranium series as a way of example. The isotope
40
K (T
½
= 1.277·10
9
yr) is also a naturally
occurring radionuclide with a long half-life, and constitutes 0.012% of potassium. The
distribution of these radionuclides in the geosphere depends on the distribution of the
geological media from which they derive and the processes which concentrate them at a
specific location in specific media (IAEA, 2003). They can also be enhanced locally by
human activities, and may be a cause for significant radiological concern because of NORM
(Naturally Occurring Radioactive Material) industries. These industries use raw materials
with high contents of natural radionuclides, and their bioavailability in their products, by-
products, and residues may be enhanced due to physicochemical changes or to the form in
which they are managed. Mining industries (uranium, iron, copper, …), the production of
fertilizers from phosphate rocks, and burning non-nuclear fuels (coal, oil, gas) are some
examples of NORM activities (IAEA, 2003).
Anthropogenic radionuclides were first introduced into the environment as a consequence
of the atom bomb blasts during World War II. During the 1950s and 1960s, there were a great
number of atmospheric nuclear weapons tests, which released huge amounts of a multitude of
radionuclides into the atmosphere. Due to atmospheric circulation, they became distributed
worldwide and ultimately were deposited onto the soil (UNSCEAR, 2000).
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