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period of time the daughter product will be decaying as
rapidly as it is being produced, so its concentration
remains constant. The time taken to restore equilibrium,
if a member of a decay series is disturbed, is about 5 times
the half-life of the disturbed member (the time required to
form 97% of the daughter isotope volume; see
Fig. 4.2
).
The time for the whole decay series to reach equilibrium is
governed by the longest half-life in the series; so it would
be about 7 times the half-life of the longest-living daughter
element.
a)
Energy range
for
-ray surve
ys
γ
100
Photoelectric effect
dominant
Pair production
dominant
80
60
40
Compton scattering
dominant
20
Common rocks
air and water
0
10
-2
10
-1
10
0
10
1
10
2
Energy (MeV)
b)
4.2.3
Interaction of radiation and matter
Compton e
-
Radiation produced by radioactive decay can pass through
various materials. As the radiation passes through, it is
attenuated as emission products lose energy to the atoms
of the material through scattering, collision and absorp-
tion. The ability to pass through a material is basically a
function of the energy, size and charge of the different
emission products, and the density of the material through
which they pass. From a geophysical perspective this is
important because it controls the depth beneath the
ground surface from which detectable decay products
may originate. It also has a signi
cant in
uence on how
close the detector must be to the radioactive source for the
radiation to be detected.
Alpha-particles are comparatively large and highly
charged and, with the energy levels that occur in the
natural environment, can pass through a few centimetres
of air before being completely absorbed. The smaller and
less charged
Compton-scattered
photon
Atomic e
-
Incident photon
Figure 4.3
Interaction of
γ
-rays with matter. (a) Mechanisms of
energy loss in
-rays of different energies. Redrawn, with permission,
from Minty (
1997
). (b) Compton scattering due to collision of a
photon with an electron.
γ
most important in geophysical exploration, the incident
photon transfers some of its energy to an electron and
the photon continues its motion in a different direction
(
Fig. 4.3b
). The process is known as Compton scattering;
as it occurs, the
-ray loses energy and eventually is
completely absorbed.
The attenuation of
γ
-rays of three different initial
energies by air, water, overburden and rock is shown in
ness is observed, with the denser materials causing
greater attenuation, and the lower-energy
γ
-particles are more penetrating, being able to
travel through a metre or more of air. Importantly, both
types of particles are absorbed by negligible thicknesses of
rock and soil. Gamma-rays on the other hand, having
neither mass nor charge, can penetrate significantly further
than the other emission products. Consequently,
β
-rays being
attenuated more than those of higher energy. It is clear
that only
γ
-rays originating in rock and overburden
located a few tens of centimetres from the surface can
escape into the atmosphere and potentially be detected by
a radiometric survey. A similar thickness of overburden
will absorb the radiation from an underlying bedrock
source. This is a demonstration of the earlier statement
that the radiometric method provides very little infor-
mation about the subsurface. Attenuation in water is less
than in the bedrock and overburden, but is still import-
ant. Any significant body of water will prevent
γ
it
is
almost exclusively
-rays that are recorded in geophysical
radiometric surveys, despite the fact that they are still
rapidly attenuated in the natural environment.
There are three ways in which
γ
-rays interact with
matter and lose energy, depending on the energy of the
γ
γ
-rays, all the energy
of the photon may be absorbed by a bound electron in an
atom, i.e. the
γ
the
γ
detection of
-emissions from the underlying material;
for example, the dark regions in
Fig. 2.35
are due to the
masking of bedrock responses by lakes. In particular,
moisture in the overburden will signi
γ
-ray disappears. This is known as the photo-
electric effect. High-energy photons are absorbed creating
an electron
positron pair in a process known as pair
production. For
-
γ
-radiation of intermediate energy, and
cantly increase its
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