Geology Reference
In-Depth Information
rapidly in collisions with other electrons. In solids or liquids the average
range of a beta particle is measured in centimetres.
4.1.3 Gamma radiation
At the high frequencies involved, the electromagnetic 'gamma rays' are
best treated as consisting of particles, known as
photons
, with energies,
measured in electron-volts (eV), proportional to frequencies. The boundary
between gamma photons and the less energetic X-rays, which are emitted by
electrons orbiting the atomic nucleus, occurs at about 0.1 MeV (a frequency
of about 0.25
×
10
20
Hz).
Because they are electrically neutral, photons penetrate much greater
thicknesses of rock than do either alpha or beta particles and are consequently
the most geophysically useful form of radiation. Even so, approximately
90% of the gamma radiation detected above bare rock will come from
within 20 to 30 cm of the surface, and even above soil only 10% will come
from below about 50 cm. A metre of water will absorb about 97% of the
radiation travelling through it. Attenuation in air is frequency dependent
and (for once) it is the higher-frequency, higher-energy radiation that has
the greater penetrating power. Half of a 1-MeV flux is absorbed by about
90 m of air but it takes about 160 m of air to absorb half of a 3-MeV flux.
Either figure implies that atmospheric absorption can be generally ignored
in ground surveys.
4.1.4 Radioactivity of rocks
Gamma rays provide information on the presence of unstable atomic nuclei.
The average number of decays in a given time will be directly proportional to
the number of atoms of the unstable element present. The rate of decrease in
mass of a radioactive material therefore obeys an exponential law governed
by a
half-life
(see Section 1.2.6).
Elements with short half-lives occur in nature because they are formed
in decay series that begin with very long-lived isotopes, sometimes termed
primeval.
The principal primeval isotopes are
40
K,
232
Th,
235
Uand
238
U,
which are mainly concentrated in acid igneous rocks and in sediments
deposited as evaporites or in reducing environments. Other primevals, such
as
48
Ca,
50
Vand
58
Ni, are either rare or only very weakly radioactive.
4.1.5 Radioactive decay series
The main radioactive decay schemes are shown in Table 4.1. The isotope
40
K, which forms about 0.0118% of naturally occurring potassium, decays
in a single stage, either by beta emission to form
40
Ca, or by electron capture
(K-capture) to form
40
Ar. The argon nucleus is left in an excited state but
