Radiometric method - Geophysics for the Mineral Exploration Geoscientist - page 208

4.4.5 Channel interaction

source. The rate of attenuation with increasing height
depends on the energy of the radiation and source geom-
etry, with greater attenuation from narrower sources than
from broader ones. In addition, the varying amount of air
between the source and the detector causes variations in
the rate of attenuation (see Section 4.2.3 ) . Overall, height
attenuation is an approximately exponential decrease in
measured counts with increasing height. It is usually
assumed that topography is subdued: that is, the sources
of radiation are broad and flat.
Acorrectionforattenuationwithheightineachenergy
window is obtained from calibration surveys conducted at
a number of heights over a calibration range, ideally an
area where the elemental distributions in the ground are
known and constant with time. The height attenuation
coef

The spectrometer energy windows ( Table 4.1 ) are chosen so
that they contain unscattered -rays whose energies are

diagnostic of their source element. However, Compton scat-
tering of the higher-energy

γ

-rays causes them to appear in
the energy windows used to infer the presence of particular
elements. The highest energy window, Th, is unaffected
( Fig. 4.6 ), but

γ

γ -rays scattered from the Th decay series will

' the lower-energy U and K windows. The

lowest energy window, K,

contaminate '

is further contaminated by

γ -rays scattered from the U decay series. Errors due to these

interactions between the energy windows are compensated
by the Compton correction coef cients, or channel stripping
ratios (to strip the contamination from the various chan-
nels). The stripping ratios are an estimate of the ratio of
counts due to scattered

cient for each energy window is applied to the
measured counts using the actual measurement height
corrected for the effects of changes in temperature and
pressure, since these affect the density of the air and its
attenuation.

-rays in the relevant lower-energy
channel(s) relative to the counts of unscattered

γ

-rays in the
relevant elemental channel(s). They predict the number of
counts expected in the lower-energy channel(s) for every
count in the higher-energy channel(s).
The stripping ratios can be obtained from pure U and
Th spectra. They are determined by making radiometric
measurements with man-made sources of known elemen-
tal concentration, and depend on detector height and the
intervening material. The effect of source geometry on the
stripping ratios is minor, although sensor height is import-
ant. Commonly, for airborne systems these are concrete
slabs or pads, with dimensions of several metres, located at
air

γ

4.4.7 Analytical calibration

The height-corrected stripped counts in each of the three
energy channels, K, U and Th, are linearly related to the
ground concentrations of their respective elements. The
relationship between counts and ground concentration is
a function of source type and the detector, and is estab-
lished for each radioelement through analytical calibration
of the survey system.
As with the attenuation-with-height correction, surveys
are conducted over a calibration range where radioelement
concentrations of the ground are known. Ideally, ground
radiometric measurements should be made simultaneously
to allow correction for environmental variables such as
moisture content, which is not possible if only geochemical
assay data are available. The relationships between counts
and element concentrations are known as the sensitivity
coef cients and are obtained by comparing the corrected
counts in each measurement channel with the known
elemental concentrations of the calibration range.
The reduced survey data obtained by applying the sen-
sitivity coef cients to the height-corrected stripped data
are the estimated ground concentrations of the radioele-
ments, expressed as a percentage for K and as parts per
million for U and Th. Since the U and Th concentrations
are actually inferred from emissions from Bi and Tl,
respectively (see Section 4.3.2.2 ), they will only be correct

elds. Smaller hand-size samples are used for portable
instruments. Spectra are obtained by locating the survey
spectrometer (the survey aircraft) over the synthetic
sources (the pads), and the effects of different survey
heights simulated by partial shielding of the detector. The
spectra so obtained are converted to pure K, U and Th
spectra based on the known elemental concentrations of
the samples (the pads), and the stripping ratios obtained
for the various simulated survey heights.
The reduced

γ

-ray counts obtained by applying the
dead-time correction, spectral smoothing, cosmic back-
ground correction, atmospheric Rn removal, and the chan-
nel interaction correction to each measurement channel
are known as stripped data.

4.4.6 Height attenuation

The intensity of the -radiation measured at the detector

decreases with increasing height above the radioactive

γ