Geology Reference
In-Depth Information
energies, photons may lose energy in removing orbital electrons from atoms
(
Compton scattering
). Eventually, at X-ray energies, photons may be totally
absorbed when ejecting electrons (
photoelectric effect
).
4.2 Radiation Detectors
The earliest radiation detectors relied on the ability of radiation to ionise
low-pressure gas and initiate electrical discharges between electrodes main-
tained at high potential differences. These
Geiger-M uller
counters are
now obsolete. They responded mainly to alpha particles and suffered long
'dead' periods after each count, during which no new events could be
detected.
4.2.1 Scintillometers
Some materials absorb gamma rays and convert their energy into flashes of
light. The most widely used detector is a sodium iodide crystal
activated
by the addition of a small amount of thallium. The light can be detected by
photomultiplier tubes (PMTs) that convert the energy into electric current.
The whole sequence occupies only a few microseconds and corrections for
'dead time', which in some instruments are made automatically, are required
only at very high count rates.
A scintillometer consists of a crystal, one or more PMTs, a power supply
(which must provide several hundred volts for the PMTs), and some counting
circuitry. The results may be displayed digitally, but quite commonly are
shown on the dials of analogue
rate meters
. Some instruments produce
audible clicks each time a gamma photon is detected, or have alarms that
are triggered when the count rate exceeds a predetermined threshold, so that
the dial need not be continually observed.
Radioactive decay is a statistical process. It is impossible to predict when
an individual atom will decay but the
average
number of events observed at
a given point in a given time will be roughly constant, with some variations
about the mean. The continuous averaging of a rate-meter is controlled by a
time constant, and if this is too small, the needle will be in continual motion
and readings will be difficult to take. If it is too large, the response will be
slow and narrow anomalies may be overlooked. Where a digital display is
used, a fixed count-time is selected that must be long enough to produce a
statistically valid result (see Section 4.3.1).
The sensitivity of a scintillometer depends almost entirely on crystal size;
larger crystals record more events. Count rates are thus relative, not absolute,
and many instruments are designed to be compatible with several different
crystals, chosen on the basis of cost, time available for survey work and
accuracy required.
