Geoscience Reference
In-Depth Information
4.2.5
Sources of radioactivity in the natural
environment
contributing to the measured spectrum depend on the
nature of the radioactive source and the height of the
detector above it, with the proportion of scattered
-rays
increasing as the opportunities for Compton scattering
increase. For example, aerial radiometric surveys con-
ducted at higher altitude will detect more scattered
γ
There are over 50 naturally occurring radioactive elements,
but terrestrial radiation is dominated by the emission
products from just three elements: potassium (K), uranium
(U) and thorium (Th). The half-lives of their radioactive
isotopes are of the same order as the age of the Earth
(5
γ
-rays
than those conducted closer to the ground because there is
a greater thickness of air between source and detector
within which Compton scattering can occur.
spectrum of the emitted
10
9
years) and are sufficiently long that they remain
comparatively abundant. The other naturally occurring
radioactive elements are too rare and/or too weakly radio-
active to be of signi
-rays for this series, and the
equivalent spectrum incorporating Compton scattering,
γ
cance.
Several estimates of the abundances of these elements in
the continental crust are presented in the literature, but
they differ owing to the assumptions upon which they are
based. The
figures quoted here are from Krauskopf and
component of the continental crust, averaging about
25,900 ppm, but only the
40
K isotope is radioactive, with
a half-life of 1.31
-ray
respectively. It is important to note that neither
238
U nor
232
Th emits
γ
γ
-rays itself, and so it is necessary to detect
γ
-rays emitted by their daughter products to infer their
presence (see
Section 4.3.2.2
). Also, except in the case of
the highest-energy
-rays produced by the decay series, the
energy peaks in the spectra associated with particular
γ
10
9
years. It comprises just 0.012% of
K in the natural environment, representing an average
crustal abundance of about 3.1 ppm. Thorium-232, the
only naturally occurring isotope of Th, has an average
crustal abundance of about 7.2 ppm. Uranium, having an
average crust abundance of 1.8 ppm, has two naturally
occurring isotopes,
238
U and
235
U, both of which decay
via series to isotopes of lead (Pb). Uranium-238 decays to
206
Pb and accounts for 99.275% of naturally occurring
uranium. We will not further discuss
235
U since it repre-
sents only 0.72% of naturally occurring uranium, and the
γ
-ray
emissions include a contribution from Compton-scattered
γ
γ
-rays of originally greater energy.
4.2.5.1
Non-geological radioactivity
In addition to radiation from terrestrial sources, there are
several forms of non-terrestrial radiation, notably radiation
from radiogenic radon (
222
Rn) in the atmosphere, cosmic
radiation and man-made radiation. These are a form of
environmental noise (see
Section 2.4.1
) that must be
removed in order to resolve the radiation of terrestrial
origin of interest in mineral exploration.
Atmospheric
222
Rn, radon gas, is originally of geological
origin, being created from the decay of
238
U. It escapes
into the atmosphere and is very mobile so, obviously, the
presence and abundance of atmospheric Rn bears no rela-
tionship to radioactive materials in the ground below.
Since mapping the ground is the ultimate objective of a
radiometric survey, atmosphere Rn is considered here to be
of non-geological origin.
Gamma-radiation of cosmic origin is produced by
primary cosmic radiation reacting with the atmosphere.
Man-made radiation is the fallout from nuclear accidents
and explosions. For reasons outlined in
Section 4.4.4
,the
most signi
cant product is caesium (
137
Cs), which emits
γ
-rays associated with its decay series are of low energy and
not useful in radiometric surveying.
Changes in atomic number and neutron number
associated with radioactive decay are described using the
In 89% of cases, decay involves
-emission and the creation
of radiogenic calcium (
40
Ca). The remaining 11% is more
signi
cant in that it involves electron capture (K-capture)
and emission of a
β
γ
-ray, with energy of 1.46 MeV. The
daughter product is radiogenic argon (
40
Ar). In practice,
the
-rays due to K detected in the natural environment do
not have a single energy, but instead have a range of
energies with the energy spectrum having a peak, known
as a photopeak, at 1.46 MeV. The spectrum is a result of
unscattered rays and a continuum of
γ
-rays of lower
relative amounts of scattered and unscattered
γ
-rays with energy of 0.662 MeV. It has a half-life of 30
years and is present in much of the Earth
'
snorthern
γ
-rays
hemisphere.
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