Geology Reference
In-Depth Information
10
Radiometric surveying
The disintegration is accompanied by the emission of
radioactivity of three possible types.
Alpha particles are helium nuclei 2 He which are emit-
ted from the nucleus during certain disintegrations:
10.1 Introduction
Surveying for radioactive minerals has become impor-
tant over the last few decades because of the demand for
nuclear fuels. Radiometric surveying is employed in the
search for deposits necessary for this application, and also
for non-radioactive deposits associated with radioactive
elements such as titanium and zirconium. Radiometric
surveys are of use in geological mapping as different rock
types can be recognized from their distinctive radioactive
signature (Moxham 1963, Pires & Harthill 1989).There
are in excess of 50 naturally occurring radioactive iso-
topes, but the majority are rare or only very weakly
radioactive. The elements of principal interest in radio-
metric exploration are uranium ( 238 U), thorium ( 232 Th)
and potassium ( 40 K). The latter isotope is widespread in
potassium-rich rocks which may not be associated with
concentrations of U and Th. Potassium can thus obscure
the presence of economically important deposits and
constitutes a form of geological 'noise'in this type of sur-
veying. Figure 10.1 shows a ternary diagram illustrating
the relative abundances of 238 U, 232 Th and 40 K in differ-
ent rock types.
Radiometric surveys are less widely used than the
other geophysical methods as they seek a very specific
target. Probably the most common application of radio-
metric techniques is in geophysical borehole logging
(see Section 11.7).
w
w
-
2
4
4 He
A
Æ+
-
n
n
2
Beta particles are electrons which may be emitted when
a neutron splits into a proton and an electron during cer-
tain disintegrations. The proton remains within the nu-
cleus so that the atomic weight remains the same but the
atomic number increases by one to form a new element:
w
AB
e -
Æ+
+1
n
n
Gamma rays are pure electromagnetic radiation re-
leased from excited nuclei during disintegrations. They
are characterized by frequencies in excess of about
10 16 Hz and differ from X-rays only in being of higher
energy.
In addition to these emissions, a further process occurs
in some radioactive elements which also releases energy
in the form of gamma rays. This is known as K capture
and takes place when an electron from the innermost (K)
shell enters the nucleus. The atomic number decreases
and a new element is formed:
w
-1
w
Ae
B
n
n
Radioactive decay may lead to the formation of a sta-
ble element or a further radioactive product which itself
undergoes decay.The rate of decay is exponential so that
10.2 Radioactive decay
-
t
NN
=
0 e
l
Elements whose atomic nuclei contain the same number
of protons but different numbers of neutrons are termed
isotopes.They are forms of the same element with differ-
ent atomic weights.A conventional notation for describ-
ing an element A in terms of its atomic number n and
atomic weight w is n A. Certain isotopes are unstable and
may disintegrate spontaneously to form other elements.
where N is the number of atoms remaining after time t
from an initial number N 0 at time t = 0. l is a decay con-
stant characteristic of the particular element. The half-
life of an element is defined as the time taken for N 0 to
decrease by a half. Half-lives vary from 10 -7 s for 84
212 Po to
about 10 13 Ma for 82
204 Pb. The fact that decay constants
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