Geology Reference
In-Depth Information
Even when a spectrometer is used, it is usual to record only total count in
the first instance, reserving the more time-consuming spectral readings for
areas of total-count anomaly. There are, of course, risks with this approach,
as the concentration of one radioelement might decrease where another
increased, but this would be unusual.
4.3.2 Radiometric assays
If a bare rock surface is available, a gamma-ray spectrometer can be used for
quantitative thorium, uranium and potassium assays. The rock should be dry,
so that absorption by moisture, either on or below the surface, is not a factor.
Observations must be taken over sufficiently long periods for statistical
fluctuations to be smoothed out, which in practice means accumulating at
least 1000 counts. Each count takes a few microseconds, and at 10 000 cps
the instrument would be 'dead' for several tens of milliseconds in each
second. Corrections are therefore needed for 'dead' time when working
with very radioactive material.
Radioelement concentrations are estimated either by inserting the ob-
served count rates into equations, provided by the manufacturers, that are
specific to the instrument and crystal being used, or by comparison with
'pad' calibrations.
4.3.3 Corrections for background variations
Atmospheric radon, cosmic radiation and radioactive particles attached to
the instrument itself produce background radiation unrelated to survey ob-
jectives. The background contributions are usually less than 10% of the
total count and are often ignored in ground surveys. If corrections are nec-
essary, either because very subtle variations are being observed or precise
assay work is being done, their magnitude can be estimated by taking read-
ings either in the middle of a body of water at least 1 m deep and at least
10 m across or with the detector shielded from the ground by a lead sheet.
Neither of these methods is likely to be very convenient, and sometimes
'background' is defined simply (and possibly unreliably) as the lowest read-
ing obtained anywhere in the survey area. Variations in background, due
mainly to changes in atmospheric humidity (wet air absorbs radiation far
more efficiently than dry), can be monitored using a fixed detector in this
location.
The level of background radiation due to radioactive material in the
detector itself (including the crystal) should be constant over long periods
and can be measured by placing it in a totally shielded environment, but
in practice this is likely to be difficult to arrange. The correction is usually
trivial and it is far more important to ensure that dirt, which might be
contaminated, is not allowed to remain smeared on the detector housing.
