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circle, i.e. within smaller circles. Note that, for a particular
percentage of the in
nite-source yield, the size of the
circle-of-investigation increases with increasing detector
height; so reducing the height means that the individual
survey readings need to be closer together to avoid under-
sampling the distribution of radioactivity.
For the case of a moving detector, the circle-of-
investigation is replaced by a strip-of-investigation whose
centre is directly beneath the detector (
Fig. 4.9c
). The
length of the strip in the survey line direction depends
upon the integration period and the speed at which the
detector is moving, plus a contribution from the semicir-
cular ends of the strip. Radiation emanating from parallel
strips on both sides of the survey line will contribute
equally to the measurements, with the greatest contribu-
tion from the strip immediately beneath the detector, and
progressively less from those offset to either side, neglect-
ing the end effects of the strips.
Figure 4.9d
shows the
relationship between the width of the strip-of-investigation
and the detector height for various percentages of the
in
nite-source yield, con
rming the bias toward radio-
active material located near the centre of the strip. For a
given detector height, the width of the strip-of-
investigation is considerably smaller than the diameter of
the circle-of-investigation because the moving detector
produces overlapping sample-areas along-line. To under-
stand why, it is useful to think of the strip as being com-
posed of a series of overlapping circles-of-investigation (cf.
Fig. 4.9c
). Each radioactive source will lie within more than
one circle and will be closer to the centre as the detector
moves past them, so those sources closer to the centre line
contribute most to the measurement. This results in a
greater bias from sources closer to the detector (the survey
line) than for the case of a stationary detector.
It is clear from
Figs. 4.9b
and d that detector height is
the most important parameter in determining the area of
the circle-of-investigation, and the strip-of-investigation
a)
Stationary
detector
D i a
m
e t e r of f c ir c l e
b)
12
Typical airborne
survey heights
10
8
6.3
6
% of infinite
source yield
90%
4
75%
50%
2.2
2
25%
60 m
0
0
50
100
150
200
Detector height (m)
c)
Moving
detector
W i d t h of f
s tri p
e l o cit y x i n t e g r a ti o n ti m
e
d)
6
Typical airborne
survey heights
5
4.0
4
% of infinite
source yield
3
90%
2
75%
1.25
1
50%
Figure 4.9
Fields of view. (a) The field of view for a stationary
γ
-ray
detector. (b) The relationship between the diameter of the circle of
investigation (expressed as a multiple of detector height) and height
of a stationary detector for selected percentages of the in
25%
60 m
0
0
50
100
150
200
Detector height (m)
nite-source
yield. (c) The
-ray detector. (d) The
relationship between the width of the strip-of-investigation
(expressed as a multiple of detector height) and height of a moving
detector for selected percentages of the in
field of view for a moving
γ
e)
Survey line
Survey line
Survey line
200 m
nite-source yield. (e) The
relative contributions of strips of ground parallel to the survey line
for a moving detector located 60 m above the ground on survey lines
spaced at 200 m apart. (b) and (d) redrawn, with permission, from
50%
7.5%
12.5%
7
5 m
240 m
Location
0
100
Metres
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