Geology Reference
In-Depth Information
Similar instruments with similar crystals should read roughly the same in
the same places, but even this must be checked, since radioactive contam-
inants near and within the crystals can cause readings to differ. Different
scintillometers may record different count rates because crystals are usu-
ally shielded so that they detect radiation from one direction only, and even
supposedly identical instruments may have different apertures. If it is es-
sential that comparable data be obtained, portable radioactive sources can
be used for calibration and also to check the extent of shielding. Such com-
parisons are, strictly speaking, valid only at the specific (usually fairly low)
gamma-ray energy of the test source.
4.2.2 Gamma-ray spectrometers
The energy of the gamma photon that produces a scintillation event can be
estimated if a
pulse-height analyser
is incorporated in the PMT circuitry.
Events with energies within certain predetermined energy
windows
or above
preselected energy
thresholds
can then be counted separately, or the entire
gamma-ray flux can be observed at a series of narrow adjoining windows
(channels) to obtain a curve such as that shown in Figure 4.1. Strictly
speaking, the term
spectrometer
should be reserved for instruments with
many channels (typically 256 or more) but in practice it is used for any
instrument that has some degree of energy discrimination. Often there are
only four channels, one for total count and one each for the
208
Tl peak
at 2.62 MeV (for thorium),
214
Bi at 1.76 MeV (for uranium) and
40
Kat
1.46 MeV (for potassium). Typical windows for these peaks might extend
from 2.42 to 2.82 MeV, from 1.66 to 1.86 MeV and from 1.36 to 1.56 MeV
respectively.
Because the characteristics of the measuring circuits vary slowly over
time (and also, more rapidly, with temperature), older instruments require
regular calibration to check the positions of spectrometer windows or thresh-
olds, using portable sources that produce gamma rays of a single energy.
An additional form of calibration is needed if spectrometer results are to
be converted directly into radioelement concentrations. In many countries,
calibration sites have been established where instrument performance can
be checked over concrete pads containing known concentrations of various
radioelements. 'Null' pads allow background to be estimated. If several in-
struments are to be used in a single survey, it is wise, even if they have all
been calibrated, to compare them in the actual survey area before attempting
to reduce all results to common equivalent readings. Bases at which this has
been done should be described for the benefit of later workers. Portable cal-
ibration pads are available but are not easily transported. The most modern
