Geoscience Reference
In-Depth Information
The result is that the reliability of the individual measure-
ments may vary signi
cantly across the survey area.
bene
ts of using larger crystals come at the expense of
increased size and weight of the sensor, an important
consideration for weight-sensitive airborne and size-
sensitive downhole instruments.
Hand-held and downhole instruments typically have
crystal volumes ranging from about 0.1 to 0.3 litres, allowing
the instrument to be of an acceptable size and weight.
Instruments for airborne surveying require significantly
larger volumes in order to obtain the sensitivity needed to
measure the lower radiation levels distant from the source.
Fixed-wing airborne surveys typically use crystals of at least
16 litres volume, often 33.6 litres and sometimes larger for
particularly high-sensitivity applications. Helicopter surveys
may utilise smaller crystals to comply with aircraft weight
limitations, although often the lower sensitivity of the sensor
can be compensated by lower survey height (measurements
made closer to the radioactive sources).
4.3.2
Radiation detectors
The popular image of radiation detectors is a Geiger
-
Müller counter emitting a buzzing sound that varies in
intensity according to the amount of radiation present.
These instruments were used in the early era of radiometric
surveying, but they respond primarily to
-radiation and so
could only be used for ground surveying. Furthermore, they
are an extremely inefficient detector of
β
-rays and are
therefore no longer used for geophysical exploration.
The instruments used in modern radiometric surveys
are scintillometers and spectrometers that detect
γ
-rays by
their interaction with matter. They contain crystals of
thallium-activated sodium iodide (NaI), which is quite
dense and therefore effective in absorbing
γ
γ
-radiation.
4.3.2.1
Gamma-ray scintillometer
Scintillation meters or scintillometers are the simplest form
of instrument used for detecting
When the crystal encounters
-
radiation, it luminesces (this property is enhanced by the
thallium), and a pulse of ultraviolet light is generated in a
process known as scintillation. The light pulse is detected
by a photomultiplier tube, optically connected to the crys-
tals, which converts it to an electrical pulse for counting by
ancillary electronics. The intensity of the light pulse and,
hence, the amplitude of the electrical pulse is proportional
to the energy of the incident radiation. The resolution of
the detector, or its ability to resolve incident
γ
-radiation, and also
β
-rays. Those for hand-
held and downhole use are compact, weigh a few kilograms
and are comparatively inexpensive. The total number of
counts from a broad range of energy is recorded and
known as the broadband, or more commonly, the total-
count (TC) response. Hand-held instruments often pro-
duce an audio signal whose frequency is proportional to
the count rate. A threshold capability is incorporated
whereby sound is emitted only if the intensity of the
radiation (the count rate) exceeds a particular (back-
ground) level. They are easy to use and are convenient
for locating and mapping any source of radioactivity.
γ
-rays of
different energies, is measured in terms of the half-width
of a photopeak as a fraction of its maximum energy.
Resolution is about 7 to 8% for a scintillometer, and varies
slightly with the energy of the
γ
-ray; so rather than distinct
lines in the spectra corresponding with each
γ
-ray emission
(
Fig. 4.6
) there are broader peaks. Thermal drift of the
detector severely degrades resolution, i.e. it causes a change
in the relationship between the intensity of the light pulse
and the energy of the incident
γ
4.3.2.2
Gamma-ray spectrometer
A more complex type of instrument is the
γ
-ray spectrom-
eter which can measure the number of
γ
-rays in discrete,
-ray.
Increasing the volume of the NaI crystals increases the
ef
ciency and sensitivity of the detector to
γ
narrow, pre-de
ned energy bands, also referred to as
windows or channels. Spectrometers provide information
about the geochemical nature of the radioactive source, a
distinctive advantage over the limited information obtain-
able from single-channel broad-band scintillometers.
Spectrometers are usually
-radiation.
A detector with a large crystal volume will measure more
counts over a given integration period than one containing
a smaller volume, resulting in lower statistical error in the
measurement. Because the integration period affects the
rate at which measurements can be made, it should be as
short as possible whilst achieving suf
cient count levels to
maintain acceptable measurement error. The higher sensi-
tivity of larger crystal volumes allows a smaller integration
period to be used for counting the pulses. However, the
γ
to the energies of the
radioelements of geological interest. The names and the
internationally agreed limits of these energy windows are
The K, U and Th energy windows are chosen so as to
detect energetic
'
tuned
'
γ
-rays emitted from relevant elements in
parts of the energy spectrum where emissions from the
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