Geology Reference
In-Depth Information
10.4.1 Geiger counter
The
Geiger
(or
Geiger-Müller
)
counter
responds primarily
to beta particles. The detecting element is a sealed glass
tube containing an inert gas, such as argon, at low pres-
sure plus a trace of a quenching agent such as water
vapour, alcohol or methane.Within the tube a cylindri-
cal cathode surrounds a thin axial anode and a power
source maintains a potential difference of several hun-
dred volts between them. Incoming beta particles ionize
the gas and the positive ions and electrons formed are
accelerated towards the electrodes, ionizing more gas
en route. These cause discharge pulses across an anode
resistor which, after amplification, may be registered as
clicks, while an integrating circuit displays the number
of counts per minute. The quenching agent suppresses
the secondary emission of electrons resulting from
bombardment of the cathode by positive ions.
The Geiger counter is cheap and easy to use. How-
ever, since it only responds to beta particles, its use is lim-
ited to ground surveys over terrain with little soil cover.
40
K
238
U
232
Th
10.4.2 Scintillation counter
The
scintillation counter
or
scintillometer
is used to measure
gamma radiation based on the phenomenon that certain
substances such as thallium-treated sodium iodide and
lithium-drifted germanium convert gamma rays to light;
that is, they
scintillate
. Photons of light impinging upon a
semi-transparent cathode of a photomultiplier cause the
emission of electrons.The photomultiplier amplifies the
electron pulse before its arrival at the anode where it is
further amplified and integrated to provide a display in
counts per minute.
The scintillation counter is more expensive than the
Geiger counter and less easy to transport, but it is almost
100% efficient in detecting gamma rays. Versions are
available which can be mounted in ground transport or
aircraft.
1
2
3
Energy (MeV)
Fig. 10.2
Energy spectra of
40
K,
238
U and
232
Th and their
measurement windows.
10.4 Instruments for measuring radioactivity
Several types of detector are available for radiometric
surveys, results being conventionally displayed as the
number of counts of emissions over a fixed period of
time. Radioactive decay is a random process following
a Poisson distribution with time so that adequate count
times are important if the statistical error in counting
decay events is to be kept at an acceptable level.
The standard unit of gamma radiation is the Roent-
gen (R). This corresponds to the quantity of radiation
that would produce 2.083 ¥ 10
15
pairs of ions per cubic
metre at standard temperature and pressure. Radiation
anomalies are usually expressed in
m
R per hour.
10.4.3 Gamma-ray spectrometer
The
gamma-ray spectromet
is an extension of the scin-
tillation counter that enables the source element to be
identified. This is possible as the spectra of gamma rays
from
40
K,
232
Th and
238
U contain peaks which represent
stages in the decay series. Since the higher the frequency
of gamma radiation, the higher its contained energy, it is
customary to express the spectrum in terms of energy
levels. A form of windowing whereby the energy levels
between predetermined upper and lower levels are
