Geology Reference
In-Depth Information
instruments are self-stabilising, i.e. they can identify and 'lock-on' to the
main naturally occurring peaks.
4.2.3 Stripping ratios
Provided that they are in equilibrium with their daughter products, concen-
trations of the three parent radioelements can be estimated from spectrometer
readings, but corrections must be made for gamma rays scattered from other
parts of the spectrum. The thorium peak must be corrected for cosmic radi-
ation, and also for the 2.43-MeV radiation from 214 Bi in the uranium decay
chain, which overlaps into the commonly used 'thorium' window. The ura-
nium count must in turn be corrected for thorium, and the potassium count
for thorium and uranium. The correction process is known as stripping .
Stripping factors vary from detector to detector, primarily with variations
in crystal size. They are listed in instrument handbooks and in some cases can
be applied by built-in circuitry so that the corrected results can be displayed
directly, Since abundance estimates will be correct only if equilibrium,
which is assumed when automatic corrections are made, actually exists, it
is generally preferable to record actual count rates in the field and make
corrections later.
4.3 Radiometric Surveys
Ground radiometric surveys can be frustrating. Because of the shielding
effect of even thin layers of rock or soil, radioactive minerals may be hard
to detect in rocks that are only patchily exposed at the surface. Reliance on
stations placed at uniform distances along a traverse may be unwise, and the
field observer must be more than usually aware of his or her environment.
4.3.1 Reading times
Accurate radiometric data can be obtained only by occupying each station
long enough for the statistical variations to average out. What this implies
will depend on the count levels themselves and must be determined by actual
experiment. The percent statistical error is equal to about 100 / n ,where n
is the number of counts, and so is about 30% for ten counts and only 1% for
10 000. A period of time that is fully adequate for total count readings may
not be sufficient for readings on the K, U and Th channels.
If areas where count rates are low are in any case of no interest, there is
little point in wasting time obtaining accurate data and it may be sufficient
to cover the ground at a slow walk listening to an audio signal or waiting for
an alarm to sound. The rate of progress should be such that the narrowest
source of interest would not be crossed completely in a time equal to the
time constant selected.
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